Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 21, 2017
Valid: July 22-26, 2017 (Saturday-Wednesday)

Summary:

Warm and humid conditions will last through the weekend before a strong cold front brings a much more pleasant air mass into the Mid-Atlantic on Tuesday. Uncertainty among the weather models regarding the timing of the cold front will keep the risk for an ozone exceedance High to Appreciable though Monday. Although unsettled conditions are expected across the NMA on Saturday, the westward push of precipitation is in question, with mostly sunny skies likely through the late afternoon along I-95. Temperatures for most of the region will reach the low-to-mid 90s °F under partly cloudy skies. There is some question as to how far south a weak frontal boundary will reach before moving northward as a warm front later in the day. Locally high ozone today combined with light and recirculating winds on Saturday will keep the risk of an exceedance High. The Mid-Atlantic will be in the warm sector of the next storm system on Sunday as the warm front continues to lift northward, possibly reaching NY State. Hi-res weather models are showing scattered showers and thunderstorms across the NMA and northern CMA in the morning and afternoon hours before giving way to more widespread thunderstorms in the late afternoon/evening hours. If there is sufficient sun available along I-95, however, ozone formation may be enhanced by smoke transport from MN/WI. As a result, risk of an exceedance will be Appreciable on Sunday. Uncertainty enters the forecast on Monday in regards to the timing of the next cold front. Despite disagreement between the models with the timing of the front, there is not much precipitation expected during the day with the passing of this front. As a result, the risk of an exceedance will remain Appreciable, given the chance that the slower GFS solution verifies, keeping a modified air mass over the I-95 Corridor for a final day. The cold front will clear the NMA and SMA on Tuesday, bringing relief from the extended period of heat and humidity as temperatures drop about 10 °F across the Mid-Atlantic. A few scattered showers are possible along the front in the SMA but most of the region should remain rain free with mostly sunny skies. Risk of an exceedance will drop to Slight on Tuesday. There is some uncertainty back in the forecast for Wednesday. WPC keeps Tuesday’s front along the SMA, but the weather models show a ridge quickly building back in at upper and mid-levels, with flow aloft shifting westerly to southwesterly by the end of the day. Surface high pressure will settle over eastern PA, which can enhance ozone formation despite a presumably clean air mass in place. In addition, the high pressure system moving into the Mid-Atlantic will be coming from the Canadian Prairies, so it may contain dilutee smoke. As a result, the risk of an exceedance will increase to Marginal on Wednesday.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The NAM continues to be an outlier at upper levels in regards to the strength and placement of shortwaves throughout its model run. The GFS and EC have come into closer agreement with the synoptic setup later in the period but they still differ with the timing and position of an upper level trough over the Northeast U.S./southeastern Canada and associated cold front. Although the NAM appears to be an outlier, WPC recommends a blend of the operational models, since none of them handle the synoptic features in the short term particularly well. WPC indicates that “bits and pieces of each model have utility at varying times” regarding the next upper level trough and stronger cold front arriving on Monday. To complicate matters further, none of this morning’s operational or hi-res models initialized well regarding precipitation. So we have much less confidence in the medium range forecast than usual.

At 12Z Saturday, a strong shortwave in the northern stream flow over MB will begin to dig out a longwave trough as it drops into the upper Midwest/Great Lakes region by 06Z Sunday. This feature will develop a surface low over WI, moving into MI on Sunday and strengthening, which will depress the mid-level ridge southward by 12Z Sunday. This will result in zonal flow across the Mid-Atlantic on Saturday and much of Sunday, after which the trough will push into the Ohio River Valley/NMA by 06Z Monday. As mentioned above, the GFS and EC deal with this feature similarly, whereas the NAM has a much weaker and less organized primary shortwave across the Great Lakes and develops shortwaves across the Ohio River Valley by 12Z Sunday. This outcome results in a deeper, but less organized trough over the Mid-Atlantic by 06Z Monday. It appears the WPC is leaning towards a blend of the GFS and EC solutions; the main difference is that the NAM has a much wetter forecast on Monday than the GFS/EC, especially for the I-95 Corridor. For the rest of the period, the main model difference is that the EC is much faster with the movement of the trough and associated cold front, while the GFS is slower but has a stronger front. At 12Z Monday, the EC has the trough dipping into the CMA/SMA as a shortwave spins through the axis over the NMA, whereas the GFS has a more organized trough that briefly develops a closed circulation over southern ON/western QC between 18Z Monday and 06Z Tuesday. The EC moves the axis of the trough across the Mid-Atlantic quicker, between 12Z Monday and 00Z Tuesday, as the trough is pushed by a pinched upper level ridge moving into western ON behind the trough. The GFS is stronger and slower with the axis moving over NY/NMA between 06Z and 18Z Tuesday. As the upper level trough moves over New England and out of the Mid-Atlantic, the GFS and EC are in close agreement that a weak upper level ridge will pass over the region between 00Z and 18Z Wednesday before the next upper level trough pushes into the Great Lakes by 00Z Thursday.
The Dailies:

Saturday’s forecast is tricky, with several competing factors adding uncertainty. A combination of an incoming mesoscale convective system (MSC) and a lifting frontal boundary will bring unsettled conditions to parts of the NMA and CMA on Saturday. There is some question as to how far south this front will make it on Friday before beginning to move back northward on Saturday. The MCS dropping across the Midwest and Ohio River Valley in the early morning hours will enter the western NMA between 12Z and 18Z Saturday. The hi-res models show the system becoming less organized as it moves eastward to the I-95 Corridor by 00Z Sunday, but scattered thunderstorms are expected throughout the region. Also, the stalled frontal boundary draped across northern VA/southern MD will slowly lift northward into PA/NJ throughout Saturday, enhancing chances for thunderstorms in the CMA and SMA. The 03Z SREF shows the highest probability for precipitation around 21Z across the CMA and western half of the NMA. Temperatures for most of the region will reach the low-to-mid 90s °F under partly cloudy skies, with clouds increasing in the afternoon along the I-95 Corridor as the MCS approaches. Flow aloft for I-95 is westerly from OH, but a large MCS is moving through there today, which will likely limit available ozone in the residual layer for Saturday. Surface winds will be light and recirculating, however, and given locally high ozone today (Friday), may be sufficient to exceedances despite potentially less ozone transport from upwind. The 06Z air quality models do highlight the I-95 Corridor as a potential issue on Saturday, with the NOAA model developing isolated USG ozone in PHL/TTN and BAL. The BAMS models have isolated Unhealthy ozone in the same locations, as well as USG ozone in the Lehigh and Susquehanna Valleys of PA, DC, and most of NJ. Elsewhere, the 06Z air quality models are responding to the precipitation in the NMA as upper Good/low Moderate ozone develops to the west of I-81. In the SMA, the BAMS models keep widespread low Moderate ozone while the NOAA model has a strip of Moderate ozone along western NC. With the uncertainty of precipitation not pushing eastward to the I-95 Corridor, along with locally high ozone from Friday and stagnation, risk of an exceedance will be High on Saturday.

The Mid-Atlantic will be in the warm sector of the next storm system on Sunday as the warm front continues to lift northward, possibly as north as NY State. Hi-res weather models are showing scattered showers and thunderstorms across the NMA and northern CMA in the morning and afternoon hours before giving way to more widespread thunderstorms in the late afternoon/evening hours. The 03Z SREF shows slight chance for precipitation across the NMA at 15Z, gradually increasing across the NMA and CMA by 21Z when precipitation is most likely. Cloud cover is expected to increase with precipitation, as partly sunny skies in the morning hours become mostly cloudy by 18Z across the NMA and CMA. In the SMA, mostly sunny skies in the morning hours will give way to partly cloudy skies in the afternoon, allowing temperatures to reach the upper 90s °F once again. Despite breezy southwesterly surface wind, westerly flow aloft and will be favorable for transporting ozone into the region. The 06Z BAMS air quality models keep a swath of mid-to-upper Moderate ozone across central NC in response to these conditions. In the NMA, despite the widespread precipitation that is expected, the BAMS models keep ozone in the mid-to-upper Moderate range with a few isolated areas of USG ozone in northeastern PA/northern NJ. If there is sufficient sun available, ozone formation may be enhanced by smoke transport from MN/WI. As a result, risk of an exceedance will be Appreciable on Sunday.

Uncertainty enters the forecast on Monday in regards to the timing of the next strong cold front. The EC is favored by the WPC with this feature and brings the front into the NMA around 12Z Monday and to the eastern part of the region by 00Z Tuesday. The GFS is about 18 hours slower and slightly stronger than the EC, bringing the front into the NMA around 06Z Tuesday, pushing to the Atlantic Coast by 00Z Wednesday. Despite the difference in the arrival of the front, there is not much precipitation expected with the passing of this front during the day on Monday; the bulk of the precipitation associated with the front will be overnight Sunday into Monday morning. Since the EC shows the front moving into the NMA on Monday, it develops mostly cloudy skies in the western NMA between 12Z and 18Z Monday. The rest of the Mid-Atlantic will be under mostly sunny/partly cloudy skies allowing temperatures to reach the upper 80s °F in the NMA and mid-to-upper 90s °F in the SMA. Breezy to gusty surface winds will limit the accumulation of ozone in the SMA as the 06Z BAMS air quality models lower ozone into the upper Good/low Moderate range in NC. The BAMS seems to be siding with the GFS on the frontal passage as low-to-mid Moderate ozone remains across the NMA. If the GFS solution verifies, the I-95 Corridor will be mostly sunny and hot on Monday, with light westerly winds, which certainly could make for a final day of high ozone at isolated locations. Given the uncertainty in the frontal passage, the risk of an exceedance will remain Appreciable on Monday.

Tuesday should be post-frontal for most of the region. Relief front the extended period of heat and humidity will arrive as temperatures drop about 10 °F across the Mid-Atlantic. The cold front that will enter the region on Monday evening/Tuesday morning will push into the SMA by 12Z Tuesday before stalling along the coast. A few scattered showers are possible along the front in the SMA but most of the region should remain rain free with mostly sunny skies. The 06Z air quality models respond to this long awaited cool down as widespread Good ozone blankets the Mid-Atlantic. A few areas of Moderate ozone remain in the SMA due to westerly flow and near 90 °F temperatures. Risk of an exceedance will drop to Slight on Tuesday.

There is some uncertainty back in the forecast for Wednesday. WPC keeps Tuesday’s front along the SMA, but the weather models show a ridge quickly building back in at upper and mid-levels, with flow aloft shifting westerly to southwesterly by the end of the day. Surface high pressure will settle over eastern PA, which can enhance ozone formation despite a presumably clean air mass in place. In addition, the high pressure system moving into the Mid-Atlantic will be coming from the Canadian Prairies, so it may contain dilutee smoke. High pressure building over the NMA will promote mostly sunny skies across the NMA and CMA while the stalled frontal boundary in the SMA will allow for partly sunny skies. Risk of an exceedance will increase to Marginal due to questions about air mass characteristics and the center of high pressure settling overhead.

-Enlow/Huff