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