Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 20, 2017
Valid: June 21-25, 2017 (Wednesday-Sunday)

Summary:

A warming trend will begin on Wednesday across the Mid-Atlantic. Skies will be mostly sunny until Friday as a cold front approaches from the west. A dissipating frontal boundary in the SMA on Wednesday will result in scattered showers followed by clearing skies. These showers and clouds will linger long enough to limit ozone formation. In the NMA, mostly clear skies and near normal temperatures will allow for some ozone formation but scattered afternoon/evening thunderstorms will have potential to keep ozone in the Good Range. Risk for an exceedance will be slight for Wednesday. The westward extension of the Bermuda high will build over the Mid-Atlantic at mid-levels on Thursday, resulting in widespread clear skies and slightly above average temperatures. Onshore flow will keep the SMA clean but calm/light westerly winds in the NMA will allow for some convergence to occur in PA and along the I-95 Corridor, promoting the accumulation of ozone in these areas. The 06Z air quality models are showing widespread Moderate ozone and a few isolated areas of USG along the I-95 Corridor, causing risk for an exceedance to rise to Appreciable on Friday.

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 agreement until 18Z Thursday, at which time the GFS begins to diverge from the NAM and EC. This disagreement continues through the end of the medium range period. An upper level trough with its axis currently over the Great Lakes will move eastward, with its axis over the Mid-Atlantic by 12Z Wednesday. This trough will move out of the region with its axis over the Canadian Maritimes by 12Z Thursday, allowing for the westward extension of the Bermuda high to build into the Mid-Atlantic at mid-levels as early as 06Z Thursday. A weak ridge/zonal flow aloft will move over the region as the next trough develops over the upper Great Plains, starting Thursday into Friday. The medium range models diverge beginning Friday. The GFS develops a considerably weaker upper level trough than the EC and NAM. Our discussion gives more credence to the NAM and EC outcomes from 18Z Thursday until 00Z Sunday at which point the GFS and EC have a similar “big picture” set up. The WPC model diagnostic discussion at 0649Z acknowledges this difference and agrees that a blend of the EC and NAM should be used when dealing with this feature as the GFS shows an unsupported representation. By 00Z Friday, a very weak, broad trough will be over most of southern Canada; shortwaves moving over the Pacific Northwest will develop two lobes in this trough the first over the upper Midwest and the other developing over ID/MT by 18Z Friday. The eastward lobe over the upper Midwest, will quickly sweep over the Great Lakes, reaching into the NMA and CMA, by 06Z Saturday and over New England by 12Z Saturday. The westward lobe over ID/MT will deepen and slowly progress eastward, reaching the upper Great Plains by 00Z Sunday. By 00Z Sunday, the GFS has this upper level trough in the same location but much weaker than the EC. The trough will move over the Great Lakes/upper Midwest by 00Z Monday, extending into the southeast and Mid-Atlantic. The strength and exact placement of this trough will determine the timing and strength of an associated cold front moving into the Mid-Atlantic on Sunday or Monday as well as the movement of the tropical moisture associated with the remnants of TD 3.

As mentioned in yesterday’s discussion, TD 3 is currently over the central Gulf of Mexico. At this time the National Hurricane Center (NHC) is giving this system a 90% chance of developing a tropical cyclone in the next 48 hours making landfall early Thursday near the TX/LA border. The most recent model runs are indicating that this system will move northeast after landfall and begin to impact the Mid-Atlantic sometime during the weekend. There is still a lot of uncertainty with this system as the models are not in agreement with its track and strength.
The Dailies:

Humidity will decrease across the SMA on Wednesday, as the frontal boundary will dissipate and weak high pressure moves into the area. Showers and clouds in the morning will clear by 00Z but will linger long enough to keep temperatures slightly below normal. In the NMA and CMA, weak shortwaves will interact with the surface front resulting in scattered thunderstorms developing in the afternoon/evening. Mostly sunny skies and near normal temperatures will assist in the formation of clouds and precipitation as 03Z SREF indicates precipitation is most likely during peak heating hours (18Z-00Z). The 06Z air quality models are showing widespread upper Good ozone with some scattered Moderate across the NMA and CMA. This is most likely shown due to the scattered nature of the expected precipitation in the afternoon/evening. Locations that don’t experience the afternoon precipitation will most likely reach Moderate (especially along the I-95 Corridor and western PA) as skies will be mostly sunny with westerly flow and near normal temperatures. As a result risk for an exceedance On Wednesday will be Slight.

Conditions will begin to warm up across the Mid-Atlantic on Thursday as the westward extension of the Bermuda high builds in at mid-levels. Mostly clear skies will result in temperatures to climb above normal. High pressure over the SMA will result in a strong southerly onshore flow to help mitigate any ozone formation under clear skies. In the NMA however, light/calm winds through the day combined with mostly clear skies and a nearby weak frontal boundary will allow ozone to accumulate along this area of convergence.
The 06Z 13km GFS and 12km NAM are both showing converging/stagnant flow over PA/ I-95 corridor. This solution lines up with the 06Z air quality models where widespread Moderate ozone with a few locations of USG along the I-95 Corridor in the CMA. This will bring risk of an exceedance up to Appreciable on Thursday, with a focus on the I-95 Corridor.

A low pressure system will be approaching the Mid-Atlantic from the west on Friday, placing the region in the warm sector for the morning and afternoon hours. The I-95 will be the area of most concern as back trajectories remain westerly and precipitation is expected to remain west. Clear skies ahead of the precipitation will continue the warming trend, keeping temperatures above normal. The most recent air quality models show the SMA and CMA cleaning out but keeping widespread Moderate ozone across the eastern NMA as precipitation is not expected to progress eastward of I-81. Although temperature and back trajectories support a high ozone solution, very high humidity at low level will increase cloud cover. These competing influences lead us to conclude that the risk of an exceedance for Friday will be Marginal along the I-95 Corridor.

Uncertainty in the forecast remains high for the weekend as the models disagree on the track and strength of the tropical system in the Gulf of Mexico and its interaction with disturbances in the flow aloft. Saturday is expected to be clean across the entire Mid-Atlantic with onshore flow in the SMA and the cold front will push into the CMA by 12Z Saturday. The front will slow to a creep as it reaches into the SMA and up the Atlantic coast by 12Z Sunday. The GFS is more diffuse with the tropical remnants while the ECMWF has a more organized circulation. At this time risk of an exceedance will remain Slight for Saturday and Sunday.

-Enlow/Ryan