Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 14, 2017
Valid: July 15-19, 2017 (Saturday-Wednesday)

Summary:

A cold front will move through most of the Mid-Atlantic on Saturday, ushering in a more seasonable and less humid air mass. The front will stall near the NC/VA border, acting as a focus for showers and thunderstorms. This combination of a new air mass to the north and unsettled weather to the south will keep the risk of an ozone exceedance Slight over the weekend. Hot and humid conditions will return next week as a strong upper level ridge builds eastward into the Mid-Atlantic on Tuesday-Wednesday. Several weak fronts and associated lows will linger through the beginning of the period, however, adding uncertainty to the overall forecast, especially in terms of daily chances for showers and thunderstorms. Monday appears to have the best chance for more widespread showers and thunderstorms, centered approximately from I-99 to I-95. Although skies will be mostly sunny to the east of this precipitation, onshore S/SE winds should help to limit any rising ozone, for only a Marginal chance of an exceedance. Lingering shortwave energy along the Mid-Atlantic coast on Tuesday-Wednesday will keep the chance for afternoon/evening showers and thunderstorms along the eastern part of the region. The best chances for rising ozone appear to be in the western Mid-Atlantic, which will feel the impacts of the building ridge first. The main forecast questions will relate to the strength and location of the ridge; the weather models different on the strength and northern extent of the ridge, with the EC’s solution being most conducive for ozone formation. The air quality models are not in consensus beginning Sunday. After the massive over-prediction of the BAMS and NC models yesterday, we are wary of the BAMS’ continued predictions of very high (mid-upper USG) ozone along I-95 and the PIT metro area Sunday-Tuesday. So we are keeping the risk of an exceedance Marginal on Tuesday, rising to Appreciable on Wednesday, with a focus on the western Mid-Atlantic.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in relatively close agreement until 12Z Tuesday, when the GFS and EC diverge with the placement and strength of a large shortwave over the Mid-Atlantic. The NAM is a bit more of an outlier with this feature, keeping it less organized than the EC and GFS on Sunday and Monday. At 12Z Saturday, an amplified pattern will be over much of the U.S. as an upper level ridge builds over north/central U.S., with a weak upper level trough over the eastern U.S. extending into the SMA. A strong shortwave will crest the ridge and drop down over southwestern ON by 06Z Sunday, reinforcing the longwave trough over the Great Lakes. This area of concentrated shortwave energy will move slowly over southern ON on Monday, reaching roughly to the southern ON/QC/NY border by 00Z Tuesday, keeping weak longwave troughing over the Mid-Atlantic. The GFS and EC begin to diverge by 12Z Tuesday. The GFS slowly moves the shortwave southeastward into the NMA as the strong western ridge builds eastward. The GFS has a slightly weaker ridge that is displaced a bit southward compared to the EC. In contrast, the EC keeps the strong shortwave moving eastward into New England on Tuesday, but splits off a lobe of shortwave energy that it brings to the SMA coast and briefly closes off at 00Z Wednesday. The differences continue on Wednesday, with the GFS bringing its shortwave to the Delmarva, with its upper level ridge axis draped across the northern Plains into the Great Lakes. The EC has its shortwave energy displaced southward, over the VA/NC coasts, with a stronger ridge reaching across most of the CONUS.

The Dailies:

A cold front will push through the NMA and CMA by 12Z Saturday, ushering in a slightly cooler and less humid air mass. The cold front is expected to slow down as it reaches the VA/NC border in the morning and very slowly creep southward to southern NC by 12Z Sunday. Temperatures to the north of the VA/NC border will be slightly below or near average, while those in NC will remain slightly above average. Weak high pressure moving into the Ohio River Valley will decrease cloud cover throughout the day, with partly cloudy skies by the afternoon. Sunny skies in the SMA will combine with a humid air mass and temperatures in the mid-90s °F, forming afternoon/evening thunderstorms south of the VA/NC border. The 03Z SREF and the 3-km NAM show that precipitation is likely starting around 18Z Saturday in central NC, and lasting through 00Z Sunday as it progresses southward. The 06Z air quality models show widespread Good ozone with scattered low Moderate across the Mid-Atlantic, mainly around the Chesapeake Bay and east of I-95. The NC and BAMS models are in close agreement with an area of Moderate ozone in central/eastern VA, despite light/breezy surface winds and northwesterly flow aloft. The NOAA air quality has Good ozone across the region with a few isolated areas of upper Good in the CMA and SMA. With near or below average temperatures, the arrival of a new air mass on northwesterly flow aloft, scattered afternoon/evening thunderstorms in the SMA, and periods of clouds and sunshine across the region, risk of an exceedance will be Slight on Saturday.

A cold front approaching the NMA from the northwest and Saturday’s cold front stalled in the SMA will both bring a chance for unsettled conditions in the northern and southern edges of the Mid-Atlantic on Sunday. The 03Z SREF shows precipitation is possible in northern PA and coastal locations in NC starting around 18Z Sunday. The 06Z 13-km GFS is showing the precipitation in PA pushing to central PA by 00Z Monday. The rest of the Mid-Atlantic is expected to have mostly sunny/partly cloudy skies with near average temperatures across the region. Surface winds will become light/calm across much of the CMA and NMA. The air quality models are picking up on these conditions as regional ozone rises, with more areas of Moderate ozone. The BAMS-MAQSIP is the most aggressive and develops more widespread Moderate ozone with USG ozone in PIT and along the I-95 Corridor. This increase in ozone appears similar to what the air quality models did for Thursday’s forecast (7/13/2017) when the air quality models massively over-predicted ozone levels along the I-95 Corridor. Yesterday’s over-prediction suggests that air quality models may have started to shift to more consistent over-prediction of ozone, as they typically do by this time of the year. Given lower Sunday emissions and presumably a still relatively unmodified air mass, rapidly rising ozone on Sunday seems unlikely. Aside from light surface winds, a recent change in air mass with a strong northerly flow aloft, partly cloudy skies, and low emissions on a Sunday are not favorable conditions for USG ozone in the NMA. As a result of these conditions, risk of an exceedance will remain Slight for Sunday.

The synoptic trend for the beginning next week will be a return to hot and humid conditions as a strong upper level ridge builds eastward on Tuesday-Wednesday. Several weak synoptic features linger through the beginning of the period, however, adding uncertainty to the overall forecast, especially in terms of daily chances for showers and thunderstorms. There are also questions about the strength and location of the building ridge. The GFS has a weaker ridge, with the NMA along its northern axis. The EC’s ridge is stronger and farther north, which would be more conducive for rising ozone in the Tuesday-Wednesday period, especially in western parts of the Mid-Atlantic (e.g., PIT and western MD).

A cold front will move into the NMA very slowly on Monday will bring widespread unsettled conditions to the region. The front is expected to stall in the western NMA and CMA between 12Z and 18Z Monday and dissipate by 00Z Tuesday. As the front stalls, the strong overhead shortwave will interact with the front, resulting in clouds and precipitation as early as 15Z Monday and lasting through 00Z Tuesday. The precipitation is not expected to progress eastward throughout the day, instead remaining in the vicinity of the front, resulting in a potentially soggy day for parts of the central NMA and CMA. The 09Z SREF has a high probability of precipitation from roughly I-99 to I-95 on Monday afternoon. The stalled frontal boundary across the SMA will also be a trigger for afternoon showers. Locations not impacted by the precipitation will experience mostly sunny skies and near average temperatures. Light/calm surface winds and a front stalling in western PA may create problems for the eastern NMA. The 06Z air quality models are acknowledging these conditions as the BAMS models develop a large swath of USG ozone in northern VA, MD, eastern PA, and NJ. The NC air quality model is not as aggressive but highlights the same area with low-mid Moderate ozone. Surface winds will shift onshore, S/SE, along I-95, which will help to limit ozone formation. As a result risk of an exceedance will rise to Marginal.

The overhead shortwaves will keep conditions unsettled on Tuesday, with the low along Monday’s incoming cold front possibly phasing with a wave over the initial SMA frontal boundary. Given questions about the exact track of the shortwaves, the locations of precipitation are not certain, but scattered showers are likely to the east of I-79 starting after 18Z Tuesday and lasting through 00Z Wednesday. Locations not impacted by the precipitation will experience partly sunny skies and temperatures slightly above average. The 06Z air quality models are not in agreement, as the BAMS models keep USG in PIT and the eastern NMA, while the NC model has widespread Good with scattered Moderate ozone. With clearing skies to the west as the ridge builds in, PIT and western MD seem mostly like to experience any rising ozone. It will also depend on the strength and location of the building ridge, as discussed above. Risk of an exceedance will remain Marginal on Tuesday due to the uncertainty between the air quality models.

High pressure will begin to build over the SMA on Wednesday resulting in mostly sunny skies across the Mid-Atlantic. Southwesterly flow aloft and light southwesterly surface winds will be favorable for ozone accumulation. Shortwave energy lingering along the Mid-Atlantic coast will keep showers and thunderstorms in the forecast in the vicinity of I-95. The best chances for clear skies will once again be in the western part of the region, which will feel the impacts of the building ridge first. Wednesday definitely is a day to watch. Consequently, the risk of an ozone exceedance will rise to Appreciable. The main forecast questions will relate to the strength and location of the ridge. The EC’s solution would allow for westerly flow aloft, with higher ozone in the residual layer.

-Enlow/Huff