Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 2, 2018
Valid: August 3-7 (Friday-Tuesday)

Summary

Widespread unsettled conditions will keep a Slight risk of an ozone exceedance on Friday before Bermuda high pressure building westward at mid-levels will increase the risk to Appreciable for Sunday and Monday before a return to unsettled weather drops the risk back to Marginal on Tuesday. The Bermuda high to the east and weakening trough to the west will keep a northward flowing conveyor belt of moisture moving over the region on Friday, maintaining a Slight risk of an ozone exceedance. Most of the Mid-Atlantic will begin to dry out on Saturday as mid-level high pressure builds westward. Near average temperatures, mostly sunny skies, weak surface winds, and weak southwesterly flow will be favorable for ozone formation across the western NMA and CMA, but persistent strong southerly flow and lingering precipitation will likely keep ozone in check across the eastern half. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, and how quickly the air mass across the NMA and CMA will be able to modify. Given these conditions, the risk of an ozone exceedance will rise to Marginal with a focus on locations across the western and central NMA. Conditions will be oppressive across the entire Mid-Atlantic on Sunday as temperatures rise above average values under mostly sunny skies in a very humid air mass. Surface and mid-level high pressure centered over the Southeast U.S. will promote conditions favorable for ozone formation as flow aloft shifts slowly westerly and surface winds diminish across the region. The primary forecast question on Sunday will be how quickly the air mass in place can modify, given the previous week of very clean transport and lower Sunday emissions. The risk of an ozone exceedance will rise to Appreciable with a focus on the I-95 Corridor from DC to NYC. Monday continues to be a day of concern as ozone conducive conditions will continue under surface and mid-level high pressure. There still remains some uncertainty in regards to precipitation across the region, but any showers will likely be isolated – the trend is toward a drier forecast. Light westerly surface winds, another day of slow westerly flow aloft and above average temperatures under mostly sunny skies will be favorable for further ozone formation to take place in areas that remain dry. Similar to Sunday, westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants along and east of I-95. These conditions will keep an Appreciable risk of an ozone exceedance with the highest ozone expected along and east of I-81, particularly along I-95. Unsettled conditions will likely return to the NMA and CMA on Tuesday. Despite disagreement between the weather models at upper levels, both models bring scattered showers and thunderstorms into the NMA and CMA beginning on Tuesday afternoon. The coverage and timing of this precipitation will be the primary forecast question for Tuesday as continued westerly flow and southwesterly surface winds along the I-95 Corridor will otherwise be favorable for ozone formation. The threat for an ozone exceedance will shift to the SMA as high pressure aloft will promote mostly sunny skies, weak surface winds, and westerly flow aloft. The risk of an ozone exceedance will drop to Marginal, with a focus primarily on the SMA and uncertainty for the I-95 Corridor, depending on the coverage of precipitation.

NWP Model Discussion

Although the weather models remain in relatively close agreement, there remains several small discrepancies in regards to the development and progression of upper level features impacting the Mid-Atlantic throughout the medium range period. The upper level longwave trough currently stalled over the Mississippi River Valley will continue to weaken and lift northeastward through Friday. Weak upper level troughing will remain over the northern half of the Mid-Atlantic by 12Z Saturday as lingering shortwave energy across the Ohio River Valley prevents the trough from completely filling in. The weak, low amplitude upper level trough will pass across the northern half of the Mid-Atlantic throughout Saturday, with its axis moving off the New England coast by 00Z Sunday. As this occurs, the northern edge of the Southwest U.S. ridge will be broken off of the parent ridge by a strong shortwave trough traversing the Rockies and into the Great Plains Friday and Saturday. This low amplitude ridge will quickly push eastward ahead of the strong shortwave trough, pushing into the western Mid-Atlantic by 00Z Sunday. The westward edge of the Bermuda high will build westward, around the departing NMA trough, combining with the low amplitude ridge, placing the entire Mid-Atlantic under upper level ridging by 12Z Sunday, with the axis directly overhead. The ridge axis will not remain overhead for long, as the combination of shortwave energy stretched across the Great Plains and a low amplitude longwave trough across the Upper Midwest will sweep across the Great Lakes on Monday, pushing the axis eastward, flattening the ridge over the NMA by 00Z Tuesday. This will result in generally zonal flow across the NMA from about 12Z Monday to 18Z Tuesday, while the SMA remains under the influence of upper and mid-level high pressure. Small differences between the GFS and ECMWC solutions become evident on Tuesday as the models begin to diverge with the timing and development of a longwave trough over the Ohio River Valley/Great Lakes region on Tuesday. The GFS, like yesterday, is quicker than the ECMWF in the development of this trough. The GFS reinforces the longwave trough over the Great Lakes on Tuesday, resulting in broad upper level troughing over the entire Northeast U.S. by 00Z Wednesday. The slower ECMWF still develops this reinforced trough over the Great Lakes but is about 24 hours slower with its push into the NMA. The WPC is favoring the ECMWF solution over the GFS solution as other model guidance (GEFS and UKMET) are more in line with the ECMWF. Despite these differences, the models are generally in agreement that most of the Mid-Atlantic will remain dry on Monday before widespread unsettled conditions return to the region on Tuesday.

The Dailies

Day 1 (Friday): The Bermuda high to the east and weakening trough to the west will keep a northward flowing conveyor belt of moisture moving over the region through Friday, promoting widespread unsettled conditions once again. The threat for heavy rain persists as precipitation will be training from south to north, prompting a Marginal risk for excessive rainfall from the WPC throughout the entire region, with a Slight risk across the eastern NMA and central CMA.
Widespread, sometimes heavy showers and thunderstorms, mostly cloudy skies, and strong southerly flow will keep ozone in check across the entire Mid-Atlantic. The air quality models keep widespread Good ozone throughout most of the region but the various versions of the BAMS and NC models show quite a range in solutions for the western part of the region, varying from Good to Unhealthy. The isolated USG and Unhealthy predictions across the western NMA seem overdone, considering the likely clean air in place. In fact, most versions of the NC models seem to have been poorly initialized in the 06Z runs this morning, making them probable outliers. Continued southerly flow and a presumably clean air mass will keep a Slight risk of an ozone exceedance on Friday.

Day 2 (Saturday): Most of the Mid-Atlantic will begin to dry out on Saturday as mid-level high pressure builds progressively westward. A weak trough setting up along the I-95 Corridor will keep scattered showers and thunderstorms across the eastern Mid-Atlantic at least through the morning and possibly into the early afternoon. This precipitation, along with persistent strong southerly flow aloft for the first half of the day, will keep ozone formation in check. Ozone is more likely to start climbing across the western portions of the NMA and CMA, where near average temperatures, mostly sunny skies, and weak surface winds will combine with weak southwesterly flow aloft. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, and how quickly the air mass across the western NMA and CMA will be able to modify. The air quality models double down on ozone formation on Saturday as the BAMS-MAQSIP develops USG ozone in PIT and the DC-BAL corridor, while the BAMS-CMAQ keeps ozone in the mid-Moderate range. While it is unlikely that ozone reaches the USG range, the models are in agreement that locations across the western Mid-Atlantic, specifically the PIT metro area, will have the highest ozone levels on Saturday. Given these conditions, the risk of an ozone exceedance will rise to Marginal with a focus on locations across the western and central NMA.

Day 3 (Sunday): Sunday is one of the days to watch in the medium range period, as much of the region will remain dry and mostly sunny. Conditions will be oppressive across the entire Mid-Atlantic as temperatures rise above average values in a very humid air mass. Surface and mid-level high pressure centered over the Southeast U.S. will promote conditions favorable for ozone formation as flow aloft shifts westerly and surface winds diminish across the region. In addition to these conditions, the shift to light westerly surface winds will lead to the development of sea/bay breezes along the eastern NMA and CMA that will concentrate ozone along and east of the I-95 Corridor. Southerly flow and scattered afternoon precipitation across the SMA will likely keep ozone in check across most of the southern half of the region. The air quality models respond to ozone-conducive conditions by developing widespread upper Moderate ozone across the NMA and CMA, with USG to Unhealthy ozone along, south, and east of the I-95 Corridor. We have limited confidence in this guidance, however, since the recent trend for the air quality models is to over-forecast ozone. The primary forecast question for Sunday will be how quickly the air mass in place can modify, given the previous week of very clean transport and lower Sunday emissions. Given the many ozone conducive weather conditions and uncertainty in the air quality model guidance, the risk of an ozone exceedance will rise to Appreciable with a focus on the I-95 Corridor from DC to NYC.

Day 4-5 (Monday-Tuesday): Monday continues to be a day of concern as ozone conducive conditions will continue under surface and mid-level high pressure. There still remains some uncertainty in regards to precipitation across the region, but any showers will likely be isolated. Although a surface trough will be in place along I-95, it is still more likely to act as a focus for afternoon convection, given the mostly zonal flow aloft that will be in place, although the trend in the weather models seems to be for a drier forecast on Monday. Light westerly surface winds, another day of slow westerly flow aloft, and above average temperatures under mostly sunny skies will be favorable for further ozone formation to take place in areas that remain dry. Similar to Sunday, westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants along and east of I-95. Throughout the SMA, southwesterly flow should be strong enough to keep ozone in check for one last day. The BAMS models continue to develop areas of USG to Unhealthy ozone along/south/east of the I-95 Corridor and east coast, likely in response to the development of sea/bay breezes, and USG ozone along I-76 through PA, likely in response to weak surface winds. These conditions will keep an Appreciable risk of an ozone exceedance with the highest ozone most likely in the PIT metro area and along, south, and east of I-95.

Unsettled conditions seem likely to return to most of the Mid-Atlantic on Tuesday. Despite disagreement between the GFS and ECMWF at upper levels, both models bring scattered showers and thunderstorms into the NMA and CMA from west to east, beginning on Tuesday afternoon. Currently, the models bring precipitation to the NMA/CMA I-95 Corridor, stretching southwest along the VA/WV border by 00Z Wednesday. The coverage and timing of this precipitation will be the primary forecast questions for Tuesday as continued westerly flow and southwesterly surface winds along the I-95 Corridor will otherwise be favorable for ozone formation. The threat for an ozone exceedance will likely shift to the SMA as high pressure aloft promotes mostly sunny skies, weak surface winds, and westerly flow aloft. With the arrival of precipitation across the NMA, the risk of an ozone exceedance will drop to Marginal with a focus on the SMA and locations along the NMA/CMA I-95 Corridor.

-Enlow/Huff