William F. Ryan
The Pennsylvania State University
Department of Meteorology
With assistance from:
Charles Piety
University of Maryland
Department of Meteorology
Additional assistance from many sources, see Acknowledgments
Note (12/12/02): Added Fort Meade profiler images for some episodes, and initial discussion for the NEOPS
IOPS of July 22-23 and July 28-29.
Note: Color-code forecasts are often alluded to in the discussion and follow the EPA 1-hour standard with Code Red > 124 ppbv, Code Orange (100-124 ppbv)
and Code Yellow (60-99 ppbv), see, http://www.epa.gov/airnow.
July 7-9:
Canadian Wildfires
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Summary: A single day spike in O3 associated with the reversing of a weak frontal boundary. Highest O3 concentrations are found on June 11 in a band from northeast of Baltimore into southeastern PA. O3 concentrations fall on June 12 with strong southwest winds and convection developing in central PA>
Background: A short wave crosses quickly through New England on June 9. The cold front associated with this system weakens as it approaches the Mason-Dixon Line by 0600 UTC on June 10 and becomes stationary.
June 10: At 0000 UTC, at cold front is analyzed in northern PA. This boundary drifts slightly south and becomes stationary along a DOV-BWI-PIT line by 1200 UTC. Visible images show clear skies with hints of haze at 1600 UTC with only shallow cumulus developing by 1800 UTC. Surface reports at 1800 UTC have scattered reports of haze along the B-W Corridor and more widespread reports west of the Appalachians. The upper air pattern is conducive to the development of a high O3 period with high pressure at 850 mb centered over KY and a modest ridge west of the region at 500 mb. The forecast back trajectories are consistent with standard high O3 cases with westerly flow. Upstream O3 at 1500 UTC at the source of the back trajectories on June 9 are strongly enhanced – in the 70-80 ppbv range. Widespread Code Orange concentrations are found south of the frontal boundary, generally SW of a line from DTW to northern NJ. One station in Washington DC reached the Code Red range briefly (125 ppbv for 1 hour).
At PHL, variable winds were reported overnight becoming E to NE at 7-13 kts from 1300-1700 UTC then shifting SE after 2000 UTC and remaining generally SE overnight into the 11th. Visibility is briefly below 10 SM from 0900-1200 UTC. Maximum temperature reaches only 84 F.
June 11: The slow moving/near stationary frontal boundary reaches just NE of BWI at 0000 UTC and then pushes quickly north to a location just south of NYC by 1200 UTC. The boundary, now in the nature of a retreating warm front, is well into New England at 1800 UTC with an Appalachian lee trough (ALT) analyzed from BWI to northern NC. A broad ridge at 850 mb stretches from AL to the Delmarva with the 500 mb ridge axis remaining just west of the mid-Atlantic. High O3 concentrations are reported right along the I-95 Corridor with scattered Code Red concentrations from central NC to northern CT. The most concentrated area of high O3 is in a band from just NE of Baltimore into southeastern PA between ILG and LNS. The forecast back trajectories show a shift from west-northwesterly flow to along-Corridor. The 1200 UTC sounding at IAD showed continuing westerly flow through the depth of the boundary layer with a very strong cap at 660 mb. A residual mixed layer, often found in association with high O3 cases, is also seen.
PHL reports SE winds overnight. As the frontal boundary shifts north, dew point temperatures rise from 62 F at 1000 UTC to 70 F at 1800 UTC. Visibility drops below 10 SM beginning 0700 UTC and is generally 6-9 SM through the day with maximum temperature reaching 91 F. Winds shift SW at 1300 UTC and then increase to 10-12 kts after 1600 UTC. Strong SW winds (10-18 kts) continue overnight and into early on June 12. BWI sees the transition to hazy, humid conditions a bit earlier than PHL with visibility dropping below 10 SM at 2300 UTC on June 10 reaching as low as 2 SM at 1000 UTC on June 11. Overnight winds at BWI were easterly with periods of calm then becoming SW at 1200 UTC veering W later in the morning.
June 12: O3 concentrations fall region-wide on June 12 with strong SW winds observed. At PHL, winds are SW overnight at > 10 kts and SW to WSW throughout the 12th with speeds of 13-18 kts. Forecast back trajectories suggest very fast flow backing further to the SW than the previous two days. Convection develops by afternoon across central PA with a substantial cirrus shield moving into the mid-Atlantic in advance of the rain.
Forecast Summary:
Forecast skill was good throughout this episode with mean
absolute error for the June 9-12 period of 10.8 ppbv in PHL, 9 ppbv in
Baltimore and 6.2 ppbv in Washington DC.
The long range forecast issued on Thursday, June 6 identified a higher
ozone period beginning June 10 with peak concentrations likely in the Code
Orange range. This forecast was
repeated on June 7 with the high O3 period noted as June 10-12. Most of the long range forecasts called for
Code Orange concentrations with the forecasts issued on June 8 and 9 calling for a “chance” of scattered Code
Red concentrations at the peak of the episode on June 11. Observed O3 basically followed
the forecasts with Code Orange generally observed with a spike into Code Red on June 11.
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Summary: Portions of the northeastern US are beneath a compact area of high pressure wedged between a disturbance crossing southern Canada and a large upper level low in the Gulf of Mexico that advects cleaner maritime air into the southern mid-Atlantic. The location of peak O3 varies day-to-day in relation to the position and movement of weak frontal boundaries.
Background: During this episode, the mid-Atlantic is sandwiched between two systems. First, a vigorous low that crosses southern Canada on July 20-21 and, second, an upper level low that develops over the southeastern US and then drifts westward with time. A small area of high pressure is wedged between the systems and the highest O3 concentrations are found beneath this.
June 22: The strong system that crossed Canada earlier in the period has now weakened and moved just northeast of ME (500 mb). A sprawling area of high pressure at 850 mb is found from IN east to just south of Long Island. At the surface at 1200 UTC, high pressure is located over WV. The frontal boundary associated with the departing Canadian system is quasi-stationary over New England and northern NY. With the upper level forcing exiting New England, there is little push to this front except at its eastern boundary where it drops south to near BOS at 1800 UTC. Precipitation is found along the frontal boundary over New England and northern NY. Further south, the upper level low over the eastern Gulf of Mexico has resulted in the development of a coastal trough with sustained easterly winds reaching as far north as NC. The O3 pattern this day follows the synoptic situation closely with highest concentrations beneath the ridge over OH, western PA and into CT and good air quality north of the frontal boundary and south across VA and NC where the maritime inflow is strongest. In the Washington DC-Philadelphia corridor, an astonishing range of concentrations is found. In Washington DC, concentrations range from Code Green in the northwest suburbs to upper Code Orange in the near southern suburbs.
Skies are generally clear across the region with with some shallow cumulus developing by afternoon. The 1200 UTC IAD sounding showed light SW winds beneath the surface based inversion with easterly flow aloft and a very strong cap at 820 mb. BWI reported calm conditions overnight with light W winds 1100-1500 UTC then returning to variable/calm winds. The air mass is relatively dry for the season (62-63 F) and haze reports are mainly confined to west of the Appalachians across northern OH and northwestern PA. Similar conditions at PHL, with brief report of haze overnight but visibility at 10 SM after 1000 UTC and moderate SW winds.
June 23: Again, a very wide range in local peak O3. A fairly large area of Code Red concentrations were found across CT as the frontal boundary shifts northward during the day. A broad band of Code Orange O3 is found on both shores of Lake Erie and extending into western PA. Significant haze is also reported along the Great Lakes and into western PA. Areas of good air quality (Code Green) are again found in the Baltimore-Washington Corridor with much higher concentrations located northeast of PHL. The very clean maritime air mass associated with the coastal trough and associated upper level low is seen over VA. The forecast back trajectories show a complex transport pattern with an offshore component at 500 m. Back trajectories based on analysis field are roughly similar with a weaker maritime component. The morning IAD sounding again has an intense inversion based at 850 mb with light and variable winds beneath it.
June 24: By mid-afternoon on June 23, low pressure develops along the frontal boundary that has retreated northward. This system develops quickly so that, by 1200 UTC on June 24, a reinforced cold front has pushed to just north of NYC. By 1800 UTC, the boundary is quasi-stationary across northern NJ and central PA with scattered convection occurring across central PA. This convection was well analyzed by the short range forecast models. Scattered haze is reported south of the frontal boundary in the morning hours, persisting into afternoon. Again, the highest O3 concentrations are organized in an west-to-east band although the northern boundary of this band is suppressed slightly further south. The highest concentrations are found across OH and PA into NJ and MD. Good air quality is found further north in NY and south in VA. Scattered Code Red concentrations are found east and northeast of Baltimore and Washington DC and well east of PHL near ACY. Afternoon haze reports reflect the locations of the highest O3 and are clearly seen in the early afternoon visible image.
The 1200 UTC IAD sounding shows a residual inversion present at 800 mb though much weaker than the two previous days. An elevated mixed layer is evident just beneath the inversion. Winds have now steadied into the NW after several days of high shear. At PHL, temperatures reach 94 F with dew points in the mid 60’s F. Visibility is generally good until 1400 UTC when it begins to fall off though only to 7 SM by 1900 UTC. Winds are variable early becoming WNW 9-13 kts from 1700-1900 UTC but then shifting abruptly to the east at 2200 UTC. Dew points rise dramatically as well fron 63 F at 2000 UTC to 72 F by 0000 UTC on July 25th . Haze is reported as well by 2100 UTC. BWI reports haze and variable winds during the day with the highest local O3 concentrations.
June 25: The frontal boundary that was driven southward on June 24 reaches as far as southern NJ by 1200 UTC before returning northward by mid-afternoon. This movement was well forecast by the numerical forecast models. The highest O3 concentrations are again trapped in a box south and west of NYC and north of VA with widespread 1-hour exceedances in the Ohio River Valley and scattered Code Red concentrations along the Washington-Philadelphia Corridor. The most unusual aspect of the O3 field on this day is the presence of Code Red concentrations over the urban centers of both Baltimore and Washington. Haze reports are widespread within the high O3 box with very persistent reports SW of PHL. Forecast back trajectories are less complex than previous days with standard WNW flow within the boundary layer.
Brief periods of light rain reported overnight at PHL with significant haze (3 SM) developing by 1100 UTC. Winds are variable to NW overnight and remain variable into the afternoon hours with intermittent pulses of S and SE winds. Visibility rises to 9 SM by 1300 UTC but haze reforms after 0000 UTC on July 26. At BWI, haze reports are most consistent with 3-5 SM overnight through 1900 UTC. Winds are calm and/or variable overnight and into late morning before settling into the SE at 2100 UTC. The shift to easterly winds did not, in this case, bring much cleaner air into the region. Visible satellite images show that after several days of light NW flow, there was a large reservoir of hazy, polluted air just offshore.
June 26: The “box” of high O3 over the mid-Atlantic and westward breaks down on the 26th with the band of highest concentrations now oriented SW to NE and stretching from BWI to BOS. The magnitude of peak O3 is considerably lower with only Code Orange peaks reported. The area of good air quality also expands northward reaching as far as Washington DC. The surface analyses show the frontal boundary, which was located just north of PHL at 0000 UTC, moved rapidly northward to CT by 1200 UTC and as far as BOS by 1800 UTC. The haze layer also moves northeast just behind the retreating warm front. The upper level ridge, which had been fairly persistent the past several days, began to flatten. This allowed for increased boundary layer winds and a much more unstable atmosphere. The 1200 UTC IAD sounding was nearly dry adiabatic in the boundary layer with strong SW winds throughout. Surface observations in the IAD-PHL Corridor also showed SW winds in the 10-15 kt range. The unstable air mass results in convection developing along the Blue Ridges by 1900 UTC with widespread convection later in the afternoon.
The high O3 period ends gradually as a trough digs into the upper Midwest. Temperatures remain warm on the 27th but strong boundary layer winds and continued convective overturning keep O3 peaks in the 90-100 ppbv band.
Forecast Summary: A range of forecast skill during this episode. Mean absolute forecast error in PHL is quite good (6.3 ppbv) but skill diminishes further south with BWI error of 12.2 ppbv and Washington DC at 19.5 ppbv. However, Code Red forecasts during the heart of the episode (June 24-25) were accurate in all locations. The main forecast errors were large over-predictions on June 23 in Baltimore and Washington DC and over-predictions in Washington DC on June 26. None of the over-predictions carried a false alarm of Code Red conditions, however, as Code Orange forecasts issued on these days.
The long range forecast issued on Tuesday, June 18, expected an upper level ridge to build over the region beginning June 20 but noted large uncertainties in the forecast models on its strength and timing. This uncertainty was very slow to be resolved. The main forecast problem was the movement of two systems: First, an upper level low with a strong cold pool that parked over the Northeast and, second, an inverted trough along the SE coast. The forecast models had a particular problem with the speed and direction with which the latter system would move east. Since it was advecting very clean maritime air on its northern side, this posed a problem for the mid-Atlantic. The forecast for Wednesday, June 19, forecast westerly flow and Code Orange conditions by June 24 with Code Red to follow June 24-25. Changes in the model forecasts moved the timing up a bit and the June 20 forecast called for Code Orange on June 22 will slightly lower O3 on June 23 followed by a resurgence on June 24. This proved to be accurate. The short range forecast for June 23 correctly forecast Code Red conditions on June 24 and the following day’s foreceast correctly forecast an increase in peak levels. The slow ramp down of the episode on June 26-27 was also well forecast although concentrtations fell in Washington DC on June 26 much faster than expected.
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additional images
Summary: A relatively mild O3 episode with strong local peaks on July 2nd. Peak 1-hour O3 concentrations in the Washington-Philadelphia Corridor were generally in the 114-128 ppbv range with the exception of a peak of 158 ppbv in the Washington area on July 2nd. Widespread one hour exceedances on July 2 were reported near Washington DC, south of Richmond, and east of New York City in Connecticut and Long Island. Throughout the episode, the highest concentrations were found along and east of the I-95 Corridor with lower concentrations further west.
Background: The high O3 event of June 22-26 ended with an upper level trough crossing the northeastern US. The trough exited the region fairly rapidly but the transition to a ridging pattern was complicated by a small “cut off’ low left behind by the trough over New England. This low drifted only slowly eastward to near Nova Scotia by July 2. Surface winds were north to northeast on June 29 becoming southwest early on June 30.
July 1: The 1200 UTC surface analysis shows a fairly standard high O3 pattern. The center of high pressure is located in western VA with a very weak pressure gradient across the region. At 850 mb, high pressure is centered further west over western TN with a weak trough exiting New England. A similar pattern is present at 500 mb although the presence of a lingering closed low just east of Maine suggests that the pattern will remain stationary in the short term not allowing the ridge to build quickly east.
O3 concentrations along the DCA-PHL corridor were high (95-99 ppbv) on the preceding day (June 30), and back trajectories suggested slow transport along and slightly west of the I-95 Corridor. Peak 1-hour O3 concentrations rose into the 114-117 ppbv range in the Washington-Philadelphia Corridor with widespread exceedances of the 8-hour standard. The bulk of the monitors exceeding the 8-hour standard were in the 90-100 ppbv range. The widespread 8-hour exceedances reflects a rising regional O3 load. Background O3 concentrations were forecast, based on observations linked to forecast back trajectories, to be on the order of 65-70 ppbv, and this appears to have occurred with most monitors seeing concentrations near that level as the nocturnal inversion broke up in late morning.
While O3 concentrations were generally uniform across the Northeast, haze reports on the morning of July 1 were limited to the region along and west of the Appalachians with the easternmost haze observation at Harrisburg (MDT). Visibility at PHL remained at 10 SM (statute miles) through the day. At BWI, visibility lowered (to 8 SM) briefly from 1700-2000 UTC.
Temperatures, which were forecast in the lower 90’s F, reached only 89-90 F along the Corridor. Winds in some locations were higher than forecast with PHL observing SW winds at the surface of 10-15 kts. The morning IAD sounding showed a layer of strong southwest winds and the Fort Meade, MD profiler showed steady WSW winds through the day. As reflected in the afternoon visible image, there was significant boundary layer overturning producing widespread, though shallow, cumulus beneath a subsidence inversion based at ~ 770 mb. The 0000 UTC IAD sounding for July 2 shows a residual mixed layer to ~ 800 mb corroborating that deep boundary layer mixing occurred.
July 2: The most unusual aspect of this day was the abrupt decrease in visibility beginning in the late morning and continuing through the afternoon and the speed with which the haze layer moved northeastward into New England. Typically, visibility reaches its maximum with mixing and increased winds in the afternoon and then reaches a minimum just before sunrise. At PHL, however, visibility remained at 10 SM at 1300 UTC and then began steadily falling until reaching 4 SM by 1800 UTC. This occurred in the context of continuing brisk southwest winds with 10-13 kts reported by afternoon. The regional surface wind field was complex as BWI reported variable winds through the early afternoon becoming southeast. The onset of lower visibility began a bit earlier (~ 0800 UTC) at BWI as well.
The upper level analyses at 850 mb and 500 mb are barely distinguishable from the previous day. Of most interest is the position of high pressure at 850 mb that is slightly further north over IL at 1200 UTC. Unlike the standard mid-Atlantic pollution case, however, this continental high has not linked up with the semi-permanent Bermuda High as a lingering trough is present along the Eastern Seaboard. As a result, 850 mb winds reported at IAD and APG remain N to NNW early on July 2. At IAD, this flow pattern is consistent to the base of a strong capping inversion at ~ 800 mb. The IAD sounding at 0000 UTC on July 3 showed the boundary layer was further suppressed during the day with mixing only to ~ 1500 m. As a result, only shallow cumulus develops over the mid-Atlantic during the afternoon with the exception of scattered strong convective activity across western PA south into WV.
Back trajectories, and regional surface observations, suggest transport of pollutants from locations northwest of the I-95 Corridor. Upstream O3 at 1600 UTC across northwestern PA was on the order of 70-80 ppbv. Aircraft spirals near FME at 1600 UTC on July 2nd showed ozone of 60-80 ppbv up to 2 km. Mixing depth at that hour was ~ 2 km. A spiral later in the afternoon near Bedford PA (HMZ) showed a lower PBL height (~ 1500 m) with concentrations in this layer of ~ 80 ppbv with high Bscat.
Additions:
RUC analysis fields for low-level winds and sounding
through the day along I-95 Corridor
July 3: Although temperatures continued very warm with PHL reaching 97 F and BWI 98 F, the characteristics of the air mass appear to change rapidly yet again. O3 concentrations fell across the region with peaks reaching the Code Orange range. Highest concentrations were found over southern and coastal NJ and south of RIC into NC. Significant portions of the Baltimore area reported good air quality. At PHL, visibility, which lowered to 2 SM in haze at 1200 UTC, increased rapidly to 9 SM at 1200 UTC in association with a shift to NW winds. Winds continued WNW through the afternoon particularly after 1800 UTC when speeds of 13-15 kts with gusts from 17-18 reported. Winds at BWI were more variable and the transition to clearer air was less abrupt with visibility of 2 SM at 1100 UTC rising to 8 SM by 1800 UTC.
Forecast back trajectories for July 3 were quite similar to the preceding day and verify well with analysis trajectories. O3 concentrations upstream, however, ran ~ 10 ppbv lower than on the preceding day. The morning sounding for July 3 was more unstable with a much reduced cap from the previous day although only shallow cumulus form during the afternoon hours.
The ALT, which was analyzed along the I-95 Corridor for much of July 2, slips slowly eastward and the region of highest O3 concentrations is roughly aligned with its later afternoon position.
Additions:
Low-level wind and moisture fields
HYSPLIT source of air mass entering region later in
the afternoon
How well did forecast models forecast ALT
Forecast Summary: The first warning of a high O3 period were issued at 96 hours (June 27th) and followed up on successive forecasts. Forecasts expected Code Red on July 1-2 with falling concentrations but still in the Code Orange range on July 3. The short range forecasts over-predicted the extent of high O3 on July 1 and 3 but were reasonably accurate for July 2. For July 1, forecasts were in the 121-130 ppbv range and over-predicted local peaks on the order of 4-16 ppbv. The forecasts for July 2 were generally in the same range and verified well with the exception of the Washington DC forecast which under-predicted the extreme peak of 158 ppbv by ~ 30 ppbv. Code Red forecasts were continued for July 3rd although observations remained in the 116-121 ppbv range (Code Orange).
The NAQP model predicted a local maxima near Washington DC in the >= 135 ppbv range with a small Code Red area in southern NJ on July 2nd. The model did have anomalous spikes in the usual locations: eastern boundary of the Chesapeake Bay, Boston Bay, and near Monmouth County, NJ (SW of NYC). On July 3rd, the NAQP model, as did the statistical models, over-predicted across the region with Code Red forecast along the I-95 Corridor in NJ, Long Island Sound and near Cape Cod.
Over-prediction continued by all models into July 4th.
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Summary: This episode contained some of the highest 1-hour peak O3 observed during the 2002 season. It was associated with several plumes of smoke from wildfires burning in northern Quebec.
There are a number of questions related to this event:
Background: The first hints of the smoke are found mid-day on July 5 and can be seen embedded in a cloud shield west of the upper level low over the Canadian Maritimes. The smoke appears to be fanned by strong winds as the plumes become widespread later that afternoon. The fires appear to have diminished overnight and on the morning of July 6th the main plume is detached from its source and is found over upstate New York. A second distinct plume is found over southeastern Ontario. New plumes, or simply a re-flaming of the same source, are evident by 1615 UTC while the original plume has dropped south as far as eastern PA and NJ. The extent of the new plumes is seen in this close-up image. By Sunday, July 7th, a cold front advances into the fire burning area. This cuts off any further transport of smoke southward. However, there are already three fairly distinct plumes affecting the northeastern US. The first plume is now offshore and over coastal Carolinas. The second pulse is over the mid-Atlantic region and the third plume is over New England. Clouds embedded in the smoke plume are present over MD by 1815 UTC but not over PHL.
July 7: On the morning of July 7th, surface high pressure is located over western PA. It will remain more or less in place through 0000 UTC on the 8th and then slowly drop southward. The back trajectories in the boundary layer terminating this morning originate in southern Quebec. The upstream O3 on the preceding day (July 6) remains quite low, on the order of 33-34 ppbv. PHL reported brisk NW winds (10 kts gusting to 22 kts) late on July 6 but visibility begins falling after 2000 UTC on the 6th and falls to only 4 SM by 2300 UTC. Visibility remains at 4-7 SM into the morning of the 7th and then falls further to 2-3 SM by 1500 UTC finally reaching 1 SM by 0100 UTC on July 8th. Winds remain fairly strong (10-13 kts) from the NW through the day on the 7th with maximum temperature reaching only 85 F. The 1200 UTC IAD sounding showed a residual mixed layer evident from above the surface based inversion to a strong cap around 840 mb with very dry conditions above. Smoke and haze are reported across the northeastern US this morning with visibility of 2-5 SM although haze is not reported west of western PA. O3 concentrations remain in the lower moderate to good range through most of the mid-Atlantic on the 7th although there is a region of southern NJ that shows remarkably high O3 (to 117 ppbv) occurring very late in the day.
July 8: A
band of high O3 concentrations
are found in a large area from just northeast of Baltimore to northern NJ. The highest
concentrations are from Harford County in northeastern MD to central
NJ. Another area of enhanced O3
is aligned from SW to NE across OH. Back trajectories
for early in the day are via western NY where previous day’s O3 was
quite low. Concentrations at1600 UTC on
the preceding day were ~ 45 ppbv, or not much different from mean background
conditions.
The morning IAD sounding showed a shift from
NW to SW winds in the lowest layers with light westerly winds aloft. A cap is still present, though a bit weaker
than July 7 and location a bit higher (~ 805 mb). A thin layer of very dry air is found beneath the cap at 950-900
mb. The critical question is whether
this represents a thin layer of O3-enriched air from further aloft.
The surface pressure gradient across the region is weak with haze or smoke reported as far south as NC and a bit further west into central OH. Limited reports of haze or smoke are found in New England and mainly in ME, eastern MA and CT. By 1600 UTC, the visible image shows that most of the haze over New England has advected offshore with haze still lingering over the mid-Atlantic. Afternoon surface observations have no reports of haze NE of NYC.
Visibility at PHL remains at 1-2 SM this day, rising briefly to 4 SM in early afternoon, with temperatures rising to 91 F. Winds shift gradually from NW to WSW from 0400 to 1300 UTC and then become SW by afternoon. Dew points are still low, ranging through the upper 50’s F. BWI reports SW winds by 1400 UTC with visibility 5-7 SM and a maximum temperature of 92 F.
July 9: Haze and O3 spreads rapidly northeastward into New England on July 9 with highest O3 concentrations again across northeastern MD, southeastern PA and central and southern NJ. As a frontal boundary reaches the eastern Great Lakes, winds increase along the Corridor and an ALT is again analyzed. PHL reported SW winds of 10-15 kts during the afternoon hours and BWI 10-13 kts with gusts to 20 kts. Temperatures rise further to the mid-90’s F. The morning IAD sounding showed strong westerly winds through a deep layer. The very dry layer, associated with the wildfire smoke on previous days, is now displaced further aloft to > 620 mb.
Visibility in the mid-Atlantic remains in the 4-6 SM range into mid-afternoon. There are strong local variations in dew point. PHL is generally in the mid-60’s F but BWI reported several distinct changes from 66 F at 0400 UTC to 57 at 0800 UTC returning to 68 F at 1500 UTC. Widespread haze is reported from MA west into OH and KY. The cloud shield associated with pre-frontal convection reaches just west ofI-95 by 1930 UTC with thunderstorms reported at PHL by 0100 UTC on July 10. The cold front reaches the mid-Atlantic ~ 1200 UTC on the 10th.
Forecast Summary: A colossal forecast failure for this episode. The long range forecast issued July 3rd predicted a cold front passage on July 5 followed, as in the previous week, with an upper level low cutting off near the Gulf of St. Lawrence. This would drive sustained northerly winds and keep air quality in the good to moderate range through July 6th (Saturday). Warming was then expected to begin on Sunday, July 7, as the Bermuda high retrogrades west, leading to ozone in the Code Orange range, by Monday (July 8). This forecast was repeated July 5th with few modifications. A cold front was expected on July 9. The forecast issued Saturday, July 6th, indicated a slower arrival of the front and extended Code Orange concentrations into Tuesday. While the forecast for Sunday expected temperatures in the upper 80’s F, 850 mb temperatures were still forecast in the 12-13 C range with rather deep boundary layer mixing expected and low upstream O3 concentrations.
The forecast issued July 7 noted smoke plumes crossing the region but noted that the regional O3 levels had not budged much so that limited impact of the smoke on air quality expected. This forecast was issued prior to the rapid late day rise in O3 in southern NJ. The forecast for July 8th was in the 108-114 ppbv range. This was generally consistent with the regression guidance although one of the newer PHL algorithms did reach the Code Red range. The NAQP model forecast 105-115 ppbv in NJ but did forecast scattered cells at the Code Red range west of Baltimore.
The forecast issued July 8 for Tuesday (July 9) forecast O3 dropping to the Code Orange range. This was due to increasing boundary layer winds which were expected to push the intense haze and smoke eastward and the chance of afternoon thunderstorms in advance of the cold front.
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Summary:
Background: As was often the case during July, low pressure was located over eastern Canada early in the period and was expected to move east allowing a ridge to build over the mid-Atlantic with high O3 conditions developing. As early as July 15, haze was reported west of the Appalachians with spotty areas of enhanced O3 concentrations. By early on July 16, haze is reported over OH, IN and IL with strong winds. SNP reports 80-90 ppbv overnight and into early on the 16th.
July 17: Widespread Code Orange concentrations reported this day from SC as far north as CT and westward into OH with scattered peaks in the Code Red range along and east of the I-95 Corridor. One of the interesting aspects of this day is the continuation of northerly flow aloft. Back trajectories for early on July 17 are oriented directly north and upstream O3 on the preceding day was quite low in that region. Significant haze is reported south and west of a line approximately from CLE-IAD at 1400 UTC. Visibility at PHL remains at 10 SM through 1900 UTC. A residual back door front is analyzed at 1200 UTC and then dissipates. Winds shift from NW to SSW at PHL by 1500 UTC with dew points rising from 51 F at 1300 UTC to 67 F by 1900 UTC and further to 70 F by 0100 UTC on July 18. Winds are relatively strong along the Corridor with PHL reporting 10-17 kts with gusts.
July 18: At PHL, the strong SW winds of July 17 diminish a bit overnight to the 7-10 kt range. Visibility begins to degrade after 0900 UTC and continues through the day with 6 SM in haze at 1800 UTC reducing to 5 SM by 2200 UTC. Haze is reported across the region on this day and, at PHL, haze is reported continuously through 2200 UTC on July 19. The surface analysis for 1200 UTC shows an Appalachian lee trough (ALT) analyzed from PVM along the eastern seaboard south to Florida. This feature remains in place through 1800 UTC. The ALT is typically associated with a band of high O3 levels and this occurs on July 18 with O3 concentrations enhanced along and east of I-95 but only Edgewood, MD reaches above 125 ppbv in the mid-Atlantic. Back trajectories have swung further west with a classic anti-cyclonic curvature from just north of the Ohio River Valley. Mid-day O3 concentrations on July 17 are much higher and in the 74-78 ppbv range. The modulation in peak O3 appears to result from widespread cloud cover.
July 19: The band of highest O3 is concentrated in the PHL-NYC Corridor with lower concentrations both north and south. Back trajectories suggest weak westerly transport and upstream O3 is again high in the range of 69-77 ppbv. Skies are relatively clear at 1600 UTC with extensive convection developing later in the afternoon across NY, central PA with scattered cells observed in NJ and DE. The morning sounding at IAD showed a very stable boundary layer with consistent NW winds. A cold front, found along a BOS-CLE line at 0000 UTC drops slightly south by 1200 UTC to a line from PVD through NYC to CLE and oscillates about that position at 1800 UTC. South of the boundary, haze is reported. At PHL, visibility drops to 2 SM at 1100 UTC with overnight winds5-8 kts SW. Maximum temperature at PHL is again in excess of 90 F (93 F) with light rain reported by 20000 UTC and strong thunderstorm by 2300 UTC. The cold front finally pushes to PHL by 1400 UTC on July 20 and then dissipates south of DCA by 1800 UTC.
July 20:
Forecast Summary:
Overall, the forecasts were out of phase during this event with
over-predictions on July 15-16 and then under-predictions on July 18-19. In particular, forecasts for July 18 in
Baltimore and July 19 in Philadelphia did not resolve the Code Red conditions
observed. Overall, mean forecast error
was 8.6 ppbv in Baltimore and 15.2 ppbv in Philadelphia.
The long range forecast issued July 15 (Monday) accurately identified the highest O3 period as July 17-18. This forecast was repeated on July 16 although Wednesday’s forecast downgraded to Code Orange based on the likelihood of pre-frontal convection.
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Summary: Although this was a very hot and humid period, strong southerly winds and low regional background O3 kept peak O3 concentrations in the southern mid-Atlantic generally in the moderate range with a few locations reaching the Code Orange range. Highest regional O3 levels, included scattered Code Red observations, were found across central CT and western MA on July 22 and 23. Strong thunderstorms ahead of a cold front ended this episode later on July 23.
Background: A cold front, which ended the previous episode, was located just south of Maryland on the afternoon of July 20. High pressure in its wake passed north of the mid-Atlantic on July 21 (Sunday) with southeasterly winds prevalent across the region. Significant haze is reported across PA, MD, WV and VA with dew points in the lower 70’s F. Scattered Code Orange O3 concentrations are observed in south-central PA with good air quality along coastal NJ. The air quality forecasts for the coming work week predicted rising O3 levels as warm air advection and along-corridor southwesterly winds were expected to develop.
July 22: As expected, temperatures increased on July 22 with PHL reporting a maximum temperature of 93 F and BWI reaching 94 F. Winds were generally south-southwest in the mid-Atlantic and in excess of 10 kts by late morning. Gusty southerly winds are reported along the mid-Atlantic coast and into southern New England. A band of high O3 is found just west of the I-95 Corridor from southern ME back into southern PA with scattered Code Red peaks in western MA. Again there is a wide range in O3 locally with concentrations reaching the upper end of the Code Orange range in Berks Co. PA (northeast of LNS) but good air quality observed at near coastal monitors. While Baltimore did reach the Code Orange range, this occurred at only a single monitor with center city Baltimore reporting good air quality. With a strong low-level inversion in place, and stable conditions aloft, only shallow cumulus clouds form over the mid-Atlantic. The key factors limiting O3 on this day are strong winds and low background O3 levels. The forecast back trajectories show a lingering maritime component to the low level winds and locations upstream of both PHL and BWI reported only 30-50 ppbv O3 at 1600 UTC on the preceding day.
July 23: A narrow band of high O3, similar in extent to that observed on July 22, was now found right along the I-95 Corridor with Code Red observations in central CT and western MA. Again only widely scattered observations in the Code Orange range in Baltimore and Philadelphia with falling concentrations in Washington DC. Forecast back trajectories predicted strong southwesterly flow with air mass origins in western VA and NC. Upstream O3 along this path is again low with 1600 UTC concentrations on the prior day limited to 36-40 ppbv. Strong convection ahead of an advancing cold front is observed along and west of the Appalachians by 1600 UTC with widespread strong thunderstorms along a line from southern NH through POU to just west of HGR by 1900 UTC. Thunderstorms or rain reported at PHL and BWI by 0000 UTC on July 24. Prior to the onset of convective activity, it was another very hot day with maximum temperatures reaching 96 F at PHL and BWI. Although haze was reported across eastern PA, NY and New England early in the day, no haze reports are found south of TTN. Steady south-southwest winds were reported overnight with PHL reporting winds in excess of 17 kts by 1200 UTC with gusts as high as 30 kts by afternoon.
This episode ends as a cold front crosses the region early on July 24.
Forecast Summary: Forecasts tended to over-predict through the July 21-23 period. The extent of over-prediction in the mean ranged from 9.3 ppbv at PHL to 6.3 ppbv at BWI. The worst over-predictions (mean of 16 ppbv) occurred in Washington DC. The low background O3 concentrations, coupled with very high temperatures, acted to push the statistical higher and the forecasts tended to lie within the statistical model guidance.
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Summary:
Background: A stationary front is located along a line from just north of HAT through WV to western PA on July 27. Significant convection occurs throughout the day along the western portion of this boundary particularly across WV. With low pressure just offshore, east to southeast winds are found in the mid-Atlantic and, coupled with considerable cloud cover, O3 concentrations are in the good range. By afternoon, the stationary front begins to slowly move northward as a warm front. Although temperatures only reach the lower 80’s F, dew points are in the low 70’s F making for a very humid day.
July 28: The warm front continues to move slowly northward across the mid-Atlantic, reaching ILG by 0000 UTC and only to PHL by 1200 UTC and just north of TTN by 1800 UTC. Very humid conditions continue with dew points as high as 76 F in PHL by evening. Overcast skies persist into the morning hours from ILG to the north but clear by early afternoon south of TTN although widespread cumulus builds across MD and to the south and west. Regional O3 is generally in the good range with the exception of a band of higher O3 along the I-95 Corridor from Baltimore to Philadelphia with scattered Code Orange concentrations observed northeast (downwind) of Baltimore and Philadelphia.
July 29: Another very hot day on the 29th with maximum temperatures reaching 97 F at PHL and 95 F at BWI. Strong westerly winds are present across the region with PHL reporting west winds of 10-15 kts through the day with gusts to 22 kts. The forecast back trajectories show very fast westerly flow and upstream O3 is again low, in the 36-40 ppbv range at 1600 UTC on July 28. O3 concentrations are generally in the moderate range with widely scattered peaks in the Code Orange range in southern NJ and extreme eastern DE. This appears to be a function of convergence along a lee trough that is analyzed over that region which persists into the afternoon.
Forecast Summary: Forecast skill is good on July 27-28 but over-predictions on the order of 20 ppbv are made through the DC-PHL corridor on July 29-30. This is likely a response to the very warm temperatures driving the statistical guidance.
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Summary:
Background:
July 31: Scattered Code Orange reported in the southern mid-Atlantic and in the vicinity of western PA and OH. Generally moderate O3 reported along the I-95 Corridor. Concentrations well forecast this day by both statistical and numerical models. Forecast back trajectories suggest fairly fast boundary layer flow with the air mass origination in southern Ontario. O3 concentrations at 1600 UTC on July 30th in that region were about average (45-50 ppbv). The cold front, located near ORF at 0000 UTC, drifts a bit further south by 1200 UTC and then dissipates by 1800 UTC. PHL reports NW winds 9-13 kts with gusts to 18 kts from 1600-1700 UTC. Visibility is 10 SM with dewpoints in the low to mid 60’s and maximum temperature reaching 95F. No strong cap is found in the boundary layer.
August 1: O3 concentrations rise region-wide with highest concentrations in central and southern NJ as well as near CLE. Forecast back trajectories show a strong anti-cyclonic curvature from near Lake Ontario through northern NJ. Again, upstream O3 is relatively low (40-50 ppbv) in southern Canada and northern PA but emissions along the path are likely quite high as the air parcels cross the metropolitan New York area. An ALT is analyzed overnight along and east of the I-95 Corridor. By 1200 UTC, high pressure is centered over WV a climatologically favored location for high O3 in the I-95 Corridor. A very strong low-level inversion is observed with NE winds in the layer beneath 800 mb. Limited reports of haze with mostly early morning reports in VA and NC.
August 2: Highest O3 concentrations are reported right along the I-95 Corridor with scattered Code Red observations from IAD to NYC. The highest O3 concentrations are found south and west of Washington DC with lower concentrations near Baltimore. Forecast back trajectories show a good deal of variability. At 1200 UTC, a back door front is analyzed near NYC with surface high pressure still centered near WV. Scattered afternoon convection develops in eastern NY again this day with an isolated cell over Delmarva. As on August 1, the sounding at IAD shows a strong low-level inversion.. Early morning observations show haze along the I-95 Corridor with the most numerous observations in VA and NC. As afternoon mixing occurs, only widely scattered haze reports by afternoon. At PHL, maximum temperature rises to 98 F with haze reported drom 1100-1200 UTC and wind calm or variable becoming W by 2000 UTC and increasing to 12-17 knts. Rapid flucuations in dew point is reported at PHL shifting several times from 65 F to the 72-75 F range from 2000 UTC on August 2 to 1600 UTC on August 3.
August 3: High O3 levels are concentrated along the IAD-NYC Corridor again on August 3 with Code Orange concentrations west in the Ohio River Valley and good air quality reported in eastern NC. The highest peak concentrations are found along and NNW of Philadelphia and Baltimore. The forecast back trajectories are the standard west-northwest flow although mid-day O3 across northern OH on the preceding day was not extreme (47-56 ppbv). This likely reflects difficulties in the trajectory model in the vicinity of a frontal boundary which reaches central PA by 0000 UTC and then just N of PHL by 1200 UTC where it stalls. Scattered haze is reported across PA, MD and VA lingering into the early afternoon hours. At PHL, temperatures reach 94 F with generally good visibility through the afternoon. Winds shift to the ESE at 1500 UTC and increase to 080 degrees at 12 kts by 2000 UTC.
August 4: O3 increases along a band from just north of DCA to just north of NYC with widespread Code Red concentrations in a pattern characterized by re-circulation and stagnation. The cold front that reached into eastern PA on the previous day becomes stationary along a line fron PVM to just north of NYC and then across central PA to near PIT. This boundary washes out by 1200 UTC with surface high pressure remaining in place over WV and an ALT analyzied along the I-95 Corridor at 1800 UTC. PHL reports calm winds overnight with visibility falling to 3 SM by 1200 UTC. Winds then become SW by late morning with dew points in the lower 70’s F. Visibility recovers to 10 SM by 1700 TUC.
August 5: A wide range of O3 values this day with scattered Code Red near Washington DC and VWI and lower concentrations further north in PHL and west into the Ohio River Valley. The presence of significant cloud cover north of the Mason Dixon Line reduces peak O3 in that region. The frontal boundary has washed out over New England with remnants still quasi-stationary over northern PA. The next cold front reaches northwestern PA by 1200 UTC. Again there is considerable vertical shear noted by the forecast back trajectories with southerly flow at the lowest (500 m) layer.
Cold front passes?
Forecast Summary: Overall forecast skill reasonable for this episode with a mean absolute error of 13.2 ppbv in PHL and 14.2 ppbv in Baltimore. Three of the four Code Red days in PHL were accurately predicted with a miss on August 4 (140 ppbv observed and 120 ppbv forecast). In Baltimore, Code Red was predicted on August 1 and 2 but only Code Orange (117-120 ppbv) observed although Code Red was observed in PHL on August 1 and 2 and in DC on August 2.
The SECMEP NAQP had problems locating high O3 in PHL throughout the episode with large under-predictions each day. In Baltimore-Washington, Code Red was correctly predicted on August 2, 3 and 5 although the location was focused west of DC on all but August 5.
The long range forecast issued on July 28 (Sunday) predicted hot and humid weather developing by July 29 though peak O3 limited due to intermittent convection locally and upstream. A weak boundary, in the nature of a dew point front, was expected on July 31 with higher O3 returning on August 1. This forecast updated with essentially the same prognosis on July 29 (Monday) although Code Red conditions were not expected to develop until August 2. The regional O3 load was still low on July 31 but Code Red forecasts were issued in the short range forecast that day as winds expected to be light and variable with “reverse Corridor” or northeast flow. The forecast issued August 1 noted the unusual weather pattern with a surface high located west of the region but no extension of the Bermuda high westward. This resulted in continuing N to NE boundary layer flow. Code Red conditions were expected on August 2 and 3 with a weak front stalling along the B-W Corridor on August 4 likely lowering peak levels. This forecast essentially repeated on August 2 with the expectation that SSE flow, with a maritime component, would lower concentrations on the 4th.
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Summary:
Background:
August 10: High pressure is directly over the mid-Atlantic. With dew points still in the mid-50’s F, the skies are extra-ordinarily clear with 10 SM visibility reported through out the day. Temperatures are moderate with maximum of 87 F at PHL and 88 F at BWI. With high pressure overhead, the forecast back trajectories suggest very light winds and re-circulation. Previous day O3 was low, however, with peak O3 ~ 70 ppbv along the I-95 Corridor and average O3 at 1600 UTC only 44-46 ppbv in the locations of the initial points of the back trajectories. This results in a highly variable O3 field with concentrations ranging from Code Green near Washington DC to one monitor near TTN reaching the Code Red range. In the larger scale, scattered Code Orange peaks are reported along the I-95 Corridor with highest concentrations found across Ohio with locations near CLE and across western Ohio reaching the Code Red range. While the IAD sounding at 1200 UTC did not show a very strong low-level inversion, with 950 mb temperatures only 21 C, there was a very strong cap at 805 mb with absolutely stable conditions above. The presence of a deep residual layer (975-805 mb) suggests the extent of stagnation.
August 11: Surface high pressure drops slowly southeastward across the mid-Atlantic with the center analyzed in western NC at 1200 UTC drifting to coastal SC by 1800 UTC. The upper level ridge has also moved east and is located over the mid-Atlantic at 1200 UTC. Widespread concentrations reaching the Code Orange range in the northeastern US with peak 1-hour concentrations in the Code Red range in southwestern PA, near NYC and also in southern Ontario and extreme northern NY. The O3 map for the Baltimore-Washington Corridor shows another day of highly variable peak O3 although missing data later in the afternoon from the Maryland monitors limits the accuracy of the map. A peak concentration of 120 ppbv occurred along the I-95 Corridor northeast of Washington DC. Again, a very clear day in the mid-Atlantic with 10 SM visbility reported throughout the day although dew point temperatures rose at PHL from 57 F at 2300 UTC on August 10 to 65 F by 1300 UTC on August 11. Winds are generally south to southwest as is reflected in the boundary layer back trajectories. Scattered haze is reported at 1300 UTC in the Ohio River Valley with convection developing later in the day. The key factor driving local O3 peaks appears to be a very stable boundary layer. The 1200 UTC sounding at IAD shows a very strong low-level inversion from 950-900 mb with a deep residual layer beneath a continuing strong subsidence inversion – now based at 760 mb.
August 12: The upper level ridge remains quasi-stationary with its axis over the mid-Atlantic. The center of high pressure at 850 mb is over NC/GA. At the surface, the characteristic Appalachian lee trough (ALT) is analyzed at 0000 UTC and remains in place through 1800 UTC, continuing into August 13. Clear skies remain, although haze is seen in the 1600 UTC visible image. The surface observations show a rapid and widespread decrease in visibility west and northwest of the I-95 Corridor, shifting further east by early afternoon. The 1200 UTC IAD sounding is similar to the preceding in several respects: a strong low level inversion, deeper a bit now from 975-925 mb, a strong cap now at ~ 770 mb. Winds are fairly strong from the NW. This is reflected in the forecast back trajectories that show a shift to westerly transport. The upwind O3 concentrations at 1600 UTC on August 11 in the vicinity of the origin of the forecast back trajectories is enhanced, on the order of 78-86 ppbv. Ozone concentrations fall this day west of the Appalachians but increase markedly across the mid-Atlantic with widespread Code Red observations from NC to BOS. Widespread exceedances are found in the Washington-Philadelphia Corridor.
August 13:
Periods of calm winds are reported overnight with variable winds during
the day at BWI and PHL including periods of southeasterly winds during the
afternoon and early evening. Significant haze has now spread all along
the I-95 Corridor and remains into the afternoon
hours. At BWI, haze is reported as
early as 0900 UTC with visibility 4-6 SM into the early morning hours
increasing briefly to 7-8 SM in SE winds later in the day. PHL reported decreasing visibility after
0800 UTC dropping to 5 SM by 1100 UTC.
The regional O3 map
shows a huge O3 episode along the I-95 Corridor with large areas in
the Code Red range across southern NY into CT and MA. In the mid-Atlantic, the highest
concentrations are found south of BWI and east of DCA and across central
NJ. Skies are again clear with shallow convection developing later in the
afternoon. The 1200 UTC IAD sounding showed a continuing
strong low level inversion with a residual mixed layer to 850 mb ending just
beneath a weak secondary inversion. The
cap aloft has lifted to ~ 630 mb and the sounding is more unstable compared to
previous day’s between the two inversion layers. In addition to a very stable, and hazy, boundary layer, the
surface analyses show that the ALT continues in place from late on August 12, to a position slightly
further east at 1200 UTC and then
backing west at 1800 UTC. As is typically the case, the highest O3
concentrations are found in proximity to this boundary.
August 14: O3 concentrations decrease south and west of BWI and along coastal NJ but widespread Code Red concentrations are reported northeast of BWI all the way to coastal ME. Back trajectories forecast fairly fast flow along the I-95 Corridor although upstream O3 remains high with mid-day concentrations onAugust 13 ~ 90 ppbv. The increase in southerly wind component results as the upper level ridge axis finally moves offshore. On the back side of the upper level ridge, low level southerly winds increase as the Bermuda High circulation pushes maritime air northward. The haze reports corroborate a slow clean out from south to north with morning haze reported north of DCA and east of eastern OH.
This episode ends in a very different manner than the standard high O3 episode. Instead of the passage of a sharp cold front, this episode ends gradually as cleaner air sweeps north, winds increase and the atmosphere steadily destabilizes. On August 15, concentrations fall across the region as low level flow becomes more southeast and the Bermuda high fills in westward. Cloud cover spreads over the region on August 16 with O3 falling further.
Forecast Summary: Forecast skill was quite good throughout this episode with mean absolute errors of 7 ppbv in PHL, 7.4 ppbv in Baltimore and 8.6 ppbv in Washington DC. In PHL, 3 of 5 Code Red days were accurately forecast with under-prediction on the both the first and last days of the episode. In Baltimore, all three Code Red days were accurately forecast with a false alarm Code Red issued on August 14 (118 ppbv observed although both PHL and Washington did reach Code Red). A similar result in Washington DC with a false alarm on August 10 (120 ppbv observed) although Baltimore reached Code Red that day. Unlike most earlier episodes, the statistical forecast guidance was not much help until late in the episode. For August 10-12, forecast guidance was in the 105-122 ppbv range. The numerical model guidance was roughly similar although more successful in increasing ozone concentrations on August 11 and lingering Code Red concentrations on August 14.
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