Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 2, 2016
Valid: August 3-7, 2016 (Wednesday-Sunday)

Summary:

Greater support for sustained onshore transport should keep ozone in check for Wednesday and Thursday, with a stronger and faster moving cold front on Friday-Saturday allowing for increased confidence in lower regional ozone, with only an Appreciable chance for an isolated ozone exceedance. High pressure will settle over the Gulf of Maine by Wednesday afternoon and remain in place though Thursday, promoting sustained onshore transport at the surface and aloft. Though there will be sunshine across the forecast region both days with relatively light surface winds, this persistent influx of clean, maritime air should be sufficient to temper rising ozone along the I-95 Corridor. As a result, the chance for an ozone exceedance will be Marginal for both Wednesday and Thursday. Friday is a day of interest for the medium range period. An approaching cold front will funnel warmth and moisture into the Mid-Atlantic. But today’s guidance shows some key changes, including more southerly (not southwesterly) surface winds, continued onshore transport aloft into Friday morning, and pre-frontal precipitation remaining well west of the forecast region. The first two changes favor lower ozone, while the third favors higher ozone. Given these competing factors, the chance for an ozone exceedance on Friday will rise to Appreciable. Saturday is also a potential day of interest for the Mid-Atlantic, but closer consensus among the deterministic models regarding the track and extent of the cold front and its associated precipitation lend confidence to its ability to limit ozone formation. The front will arrive across northern PA in the morning and push southward, with GFS and EC both developing organized precipitation along and ahead of the front throughout the day. The models bring this front all the way southward to northern VA by 00Z Sunday, which is further south than either model showed in yesterday’s guidance. The main area of interest will be portions of the central Mid-Atlantic which remain ahead of the front, where intense sunshine, the presence of a presumably modified air mass, and the formation of a pre-frontal/lee trough may enhance local ozone formation. As a result, the chance for an ozone exceedance will remain Appreciable for these locations. On Sunday, the cold front will push toward the VA/NC border, with a shift to northwesterly transport aloft and at the surface behind the front. This will temper rising ozone along the I-95 Corridor, but the presence of sunny skies and ample heat will allow the chances for an ozone exceedance on Sunday to drop only to Marginal.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with the GFS and EC coming into general consensus regarding the arrival of a cold front and its associated precipitation on Saturday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A weak upper level ridge currently over the Great Lakes region will shift eastward over the Mid-Atlantic Wednesday morning, before flattening out across NY/New England during the afternoon. Shortwave energy beneath the ridge will traverse the central and southern Mid-Atlantic, with the NAM even developing a weak shortwave trough near the Ohio River Valley beginning 12Z Wednesday. The semi-permanent mid-level subtropical ridge will remain centered over the southeastern U.S. through Friday. At the surface, an area of high pressure centered over ME at 06Z Wednesday will slowly drop southward, reaching the coastal MA by 00Z Thursday. Another weak upper level ridge will re-amplify over the Mid-Atlantic on Thursday ahead of a potent upper level trough barreling across MB/southern ON. The surface high will continue to move eastward into the Gulf of Maine. On Friday, the upper level trough will broaden across ON, pulling an associated cold front into the Ohio River Valley around 18Z. This trough will churn eastward into QC on Saturday but will remain well to the north of the forecast region, with its cold front moving into northern PA at roughly 12Z. The GFS and EC are in closer agreement in this morning’s guidance regarding the southward progression of this cold front, with both models bringing it into the central Mid-Atlantic across northern VA by 00Z Sunday, which is slightly more southward in comparison to yesterday. On Sunday, the upper level trough will lift northeastward toward the Canadian Maritimes, as an encompassing upper level ridge builds across the remainder of the CONUS.

High pressure will settle along coastal MA during Wednesday afternoon, promoting onshore flow both at the surface and aloft across the northern and central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are strongly onshore, northeasterly from the northern Gulf of Maine. This influx of clean, maritime air should limit ozone formation along the I-95 Corridor, and will also act to keep temperatures average or even slightly below average, despite at least partly sunny skies across much of the forecast region. One of the main forecast questions will be how quickly the relatively clean air mass in place will be able to modify, especially given low ozone levels observed Monday and so far this morning due to widespread clouds and scattered precipitation. Another question will be what the exact impact of surface winds. Though the easterly surface winds that are expected generally have a cleaning effect, they are not very strong, suggesting that they might not be sufficient to completely ventilate the atmosphere. If this is the case, these surface winds may instead push highway emissions west of the I-95 Corridor. The 06Z air quality models show hints of this, with all of the models developing Moderate ozone just to the west of I-95. The models also resolve isolated patches of USG ozone, though they differ their exact placement. The BAMS model develops USG ozone in western PA across PIT, while the BAMS MAQSIP-RT model has USG ozone across south central PA. The NCDENR model has this USG ozone further south across northern VA. Though the likelihood of USG ozone is questionable in light of strong onshore transport aloft and the clean air mass in place, light winds and afternoon sunshine may be sufficient for some locations to experience spikes in ozone formation, especially those along and west of the I-95 Corridor. As a result, the chance for an ozone exceedance on Wednesday will be Marginal.

On Thursday, high pressure will move over the Gulf of Maine, promoting another day of mostly sunny skies across the northern and central Mid-Atlantic. The placement of the high will continue to support onshore flow aloft and at the surface. 06Z GFS back trajectories for the I-95 Corridor are more strongly onshore today, in contrast to yesterday’s guidance. Once again, however, surface winds may be light enough to push pollutants west of the I-95 Corridor. Additionally, there will be more abundant sunshine across the forecast region in comparison to Wednesday, which may favor more ozone formation along I-95. The 06Z air quality models, however, are trending toward a clean-out for the end of the period, and they do not develop any USG ozone across the Mid-Atlantic on Thursday. The BAMS models bring areas along and west of the I-95 Corridor down into the upper Good range for ozone, while the NCDENR model develops Moderate ozone in these locations. These solutions actually seem a bit underdone, which seems unusual given the consistent over-forecasting of ozone seen as of lately in the air quality model guidance. However, these solutions may indicate the full impacts that onshore transport will have on ozone formation along the I-95 Corridor during the middle of this week. The full extent of these impacts won’t likely be understood until Wednesday, however, when we can see how the competing effects of onshore flow and local ozone production under sunny skies play out across the Mid-Atlantic. Given these questions, and the presence of light winds and mostly sunny skies, the chance for an ozone exceedance on Thursday will remain Marginal.

Friday is still a day of interest during the medium range period. A cold front entering the Ohio River Valley during the afternoon will shift surface winds more southerly across the Mid-Atlantic, funneling warmth and moisture into the forecast region. Today’s guidance is keeping pre-frontal precipitation farther west, well west of the forecast region. As a result, we expect mostly sunny skies for at least the eastern portions of the forecast region on Friday. Furthermore, today’s guidance keeps surface winds across the region more due south, with southeasterly flow along coastal locations, which is a change from the more southwesterly wind in yesterday’s guidance. Temperatures will rebound to seasonable or slightly above average values, with ample sunshine expected across the region, potentially supporting ozone formation along the I-95 Corridor. 06Z GFS back trajectories for the I-95 Corridor are still onshore, but are much shorter and more localized at 1000m and 1500m AGL. Depending on the impacts of onshore transport the two days preceding, this shortening of trajectories at the upper levels may have a lessened impact on rising ozone along the I-95 Corridor. That said, the 06Z air quality models do not resolve much of an uptick in ozone levels, with the BAMS models keeping locations along and west of I-95 under upper Good/low Moderate ozone. Once again, this seems too conservative, given a third consecutive day of afternoon sunshine and rising temperatures. But we have seen the effects of strong onshore transport in past summers, which makes a cleaner forecast for Friday plausible, despite pre-frontal conditions, hot weather, and mostly sunny skies. Given the possibility that these conditions will be sufficient to promote ozone formation along the I-95 Corridor, the chance for an ozone exceedance on Friday will rise to Appreciable.

Saturday is also a day of interest for the Mid-Atlantic, but there is slightly less support for rapid ozone formation south of the front in this morning’s guidance, due to closer consensus between the GFS and EC with the placement of the front and its associated precipitation. The front has sped up in today’s guidance; it is expected to arrive across northern PA around 12Z, and push south/southeastward throughout the day. The GFS, which yesterday showed barely any clouds and precipitation developing ahead of the front, now develops a more organized line of precipitation, though the EC still resolves slightly more widespread pre-frontal precipitation. Additionally, the GFS and EC are in agreement regarding how far south this front will be able to reach by Saturday evening, with both models bringing it all the way into the central Mid-Atlantic roughly across northern VA by 00Z Sunday. This placement is further south than either of the models resolved in yesterday’s 06Z model runs, suggesting a slightly stronger cold front. The combination of more convincing precipitation ahead of the front, as well as its more southward progression, signals less support for rapid ozone formation across the northern and central Mid-Atlantic. The main area of interest will be portions of the central Mid-Atlantic which remain ahead of the front, where intense sunshine, the presence of a presumably modified air mass, and the formation of a pre-frontal/lee trough may enhance local ozone formation. The 06Z BAMS models do not develop any USG ozone across the Mid-Atlantic, but do bring the I-95 Corridor into the Moderate range for ozone. Therefore, the chance for an ozone exceedance will remain Appreciable on Saturday for locations that remain ahead of the cold front.

On Sunday, the cold front will progress southward toward the VA/NC border. Though there will be clearing across the northern and central Mid-Atlantic behind this front, a shift to northwesterly transport both aloft and at the surface will usher drier, cleaner, and slightly cooler air into the forecast region. This should be sufficient to temper rising ozone along the I-95 Corridor, in spite of afternoon sunshine. The chance for an ozone exceedance will drop to Marginal as a result.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 1, 2016
Valid: August 2-6, 2016 (Tuesday-Saturday)

Summary:

The first half of the medium range will be characterized by the competition between clean onshore regional transport and possible local ozone production under mostly sunny skies. A wave of low pressure will depart offshore and high pressure will be centered over QC/NS on Tuesday. On Wednesday, the high will settle over New England, and then will drift slightly southward on Thursday into the Gulf of Maine. These synoptic features will promote onshore flow aloft and at the surface, as well as seasonable temperatures, across the Mid-Atlantic all three days. Although there may be some local isolated spikes in hourly ozone under sunny skies, the predominant onshore flow should limit rising ozone, for a Marginal chance of an ozone exceedance Tuesday-Thursday. An approaching cold front and the development of high pressure across western NC on Friday will shift surface winds southwesterly, funneling heat and moisture into the forecast region. Rising temperatures and return flow around the high will support rising ozone along the I-95 Corridor, which will remain under sunny skies through the early evening. Therefore, the chance for an ozone exceedance will rise to Appreciable on Friday. There is uncertainty in Saturday’s forecast regarding the extent of clouds and precipitation developing ahead of the cold front, as well as its track into the northern Mid-Atlantic. The GFS does not develop clouds and precipitation ahead of this front, while the EC shows more organized precipitation. The front will likely reach to somewhere around southern PA/NJ or the Mason-Dixon Line by Saturday evening. Locations along and ahead of the front will likely be in a modified air mass under sunny skies, which would promote rapid ozone formation. As a result, the chance for an ozone exceedance on Saturday will remain Appreciable for locations south of the front.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences on Saturday regarding the arrival of a cold front and its associated precipitation. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough will depart the Mid-Atlantic Tuesday morning ahead of a weak upper level ridge, which will move eastward and fully envelop the forecast region by 00Z Wednesday. At the surface, high pressure will gradually move eastward, from southeastern QC/NS on Tuesday, to New England on Wednesday, to the Gulf of Maine on Thursday. The position of this high will promote onshore flow into the Mid-Atlantic during this period. On Wednesday, the upper level ridge will flatten out across PA/NY, returning most of the Mid-Atlantic to zonal flow aloft. This will allow some weak shortwave energy on the underside of the ridge to stream across portions of the northern and central Mid-Atlantic. Concurrently, a mid-level subtropical ridge, which is currently centered over southeastern U.S., will build northward across the entire eastern U.S. An upper level ridge will re-amplify over the Mid-Atlantic and move eastward throughout Friday, ahead of an upper level trough churning across ON, which will pull a cold front into the Great Lakes beginning 12Z Friday. On Saturday, this trough will progress eastward into QC, with the northern Mid-Atlantic remaining under its southern periphery throughout the day. Its associated cold front is expected to move into the Mid-Atlantic from the northwest on Saturday, but the EC and GFS differ slightly on the exact timing of its arrival. The EC is slightly faster, bringing the cold front to the Mason-Dixon Line by 00Z Sunday, whereas the GFS only brings the front to roughly central PA/NJ.

The cold front that has been quasi-stationary across the central Mid-Atlantic since last week will push southward toward the VA/NC border by 12Z Tuesday. Though there will be clearing behind this front, showers circulating onshore around the departing low may bring some morning clouds and precipitation to the northern and central Mid-Atlantic, especially coastal locations. The NAM in particular develops heavy precipitation along the I-95 Corridor through 12Z. In the afternoon, diurnal convection and thunderstorms will bring a second round of precipitation to much of the Mid-Atlantic, especially across VA and NC in the vicinity of the cold front. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m AGL around the low positioned offshore, but short and westerly at both 1000m and 1500m. The combination of this onshore transport at lower levels, clouds and scattered precipitation, and northeasterly surface winds will act to temper ozone formation along I-95. The main question will be if there is sufficient afternoon sunshine for rising ozone to counteract these limiting factors. The 06Z air quality models suggest that there will enough clearing and sunshine, with all of the models developing an area of USG ozone in the vicinity of DC/northern VA. The NOAA model also shows USG ozone across PH and northern DE, but the BAMS and NCDENR models only resolve upper Good and low Moderate ozone in these locations. The air quality models have been consistently over-forecasting for most locations since last week. Therefore, these solutions may be slightly overdone, especially given the number of factors in place that should suppress ozone formation. Given the possibility of clearing skies and possible air mass modification, the chance for an ozone exceedance on Tuesday will be Marginal.

On Wednesday, high pressure will settle over New England around 12Z. Though this will promote more substantially sunny and clear skies, the position of the high will funnel clean, maritime air into the northern and central Mid-Atlantic, both at the surface and aloft. This once again should limit ozone formation. There may still be a slight chance for some weak afternoon convection and showers, but there is much less support in the deterministic models and Hi-Res guidance in comparison to Tuesday. As on Tuesday, the key to the air quality forecast will be determining which of the two battling factors – clean regional transport and local ozone production under mostly sunny skies – will win out. The 06Z air quality models side with the clean transport pattern in place, with the BAMS and NCDENR models resolving only Moderate ozone along and west of I-95. That said, given widespread afternoon sunshine and questions about how quickly the air mass in place will modify on Tuesday, the chance for an ozone exceedance will remain Marginal on Wednesday.

Surface high pressure will drift slightly southward on Thursday, centered off of the MA coast in the Gulf of Maine. The placement of the high will continue to support onshore flow at the surface and aloft, which should temper ozone formation, especially along I-95, for a final day. If onshore surface winds are lighter than expected, they may serve to push I-95 emissions north and west of the highway, promoting ozone formation in those locations. The 06Z air quality models are split regarding the magnitude of ozone levels. The BAMS models are more conservative, showing a solution very similar to that of Wednesday, with Moderate ozone developing to the west of the I-95 Corridor, suggesting that the clean transport pattern and onshore surface winds will be sufficient to temper rising ozone. The NCDENR model, however, develops a large area of USG ozone across PHL and the entire northern half of NJ, which could mean the surface winds will be sufficiently light for ozone to rise rapidly directly along I-95. The NC model solution may be overdone, but it hints at a regional setup that could potentially be conducive to local ozone production. As a result, the chance for an ozone exceedance on Thursday will remain Marginal.

Friday and Saturday are the days of most interest in the medium range period. An approaching cold front will bring clouds and thunderstorms into the western Mid-Atlantic by 00Z Saturday, but these will remain well west of the I-95 Corridor, which will see mostly sunny skies. The pre-frontal set-up will shift winds to the south/southwest, aided by the presence of high pressure over western NC, with its associated return flow pushing warm and moist air into the forecast area. 06Z GFS back trajectories ending at 12Z along the I-95 Corridor are still southeasterly and still onshore, but they are slow, especially at 1000m and 1500m, hinting at a shift to continental transport. The main forecast question is how modified the air mass in place will be after three days of sustained but relatively light onshore flow. There will also be a pre-frontal or possibly weak lee trough in place ahead of the front, which may promote afternoon convection, since there will not be a strong upper level ridge overhead. Interestingly, the BAMS and NCDENR models do not develop any USG ozone, keeping the entire I-95 Corridor in the Moderate range for ozone. The chance for an ozone exceedance will rise to Appreciable, given the favorable synoptic conditions for rising ozone.

On Saturday, the cold front will move into northwestern PA, with clouds and precipitation ahead of the front impacting the northern and central Mid-Atlantic. Though the deterministic models show relatively the same placement and track of this precipitation, they disagree regarding its intensity. The EC develops much more substantial and organized precipitation ahead of the front, while the GFS has only scattered patches of precipitation. Furthermore, the 06Z 13km GFS does not show much cloud cover developing ahead of the front. Additionally, 06Z GFS back trajectories for the I-95 Corridor turn westerly on Saturday. The main forecast question is how far south the front progresses. Locations along and ahead of the front will likely be in a modified air mass under sunny skies, which would promote rapid ozone formation, similar to what occurred in southern Maryland and along the western Chesapeake Bay coastline the first part of last week. As a result, the chance for an ozone exceedance will remain Appreciable on Saturday, mainly for locations which remain ahead of the approaching front.

-Brown/Huff