Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 18, 2014
Valid: July 19- 23, 2014 (Saturday-Wednesday)

Summary:

Ozone will reach the Moderate range at scattered locations along the I-95 Corridor on Saturday due to the influence of surface high pressure centered over New England. PM2.5 will also be in the Moderate range at inland locations, particularly the Susquehanna Valley and western MD. Sustained onshore flow on Sunday will return air quality to the Good range for most locations. Monday-Wednesday are the days of most interest for the period, as weak ridging aloft will allow the chance for ozone to rise into the upper Moderate/USG range in the northeastern Mid-Atlantic, depending on the chances for daily precipitation, which is uncertain at this time.

Discussion:

The 00Z ECMWF, 06Z NAM, and 06Z GFS start out the period with an upper level shortwave over IN/OH carving out a weak trough over the eastern U.S. This trough will move eastward on Saturday, and several smaller shortwaves will rotate through the Mid-Atlantic, along the base of the trough. The trough will gradually lift through the region until Monday night, when the flow aloft turns zonal. Starting Tuesday morning, weak ridging develops over the eastern U.S. The ECMWF and the GFS have come into agreement today on the development of this ridge. On Wednesday, potent shortwaves moving over the top of the ridge into the Great Lakes will begin to flatten the eastern ridge. The models diverge on the placement and speed of the shortwaves and the subsequent trough they develop, with the GFS featuring faster eastward movement. By Wednesday night, the ECMWF shows a more negatively tilted trough axis, centered roughly over WI/IL, while the GFS has the trough centered over southern ON/MI. This developing trough will eventually pull the next cold front through the Mid-Atlantic at the end of next week. For now, we follow WPC and the NWS and favor the slower ECMWF solution.

Surface high pressure will sit over New England on Saturday as a stationary front lingers along the GA/SC border up through the Southeast coast. Precipitation created by the approaching shortwaves aloft will keep ozone in the Good range in the western Mid-Atlantic. The eastern Mid-Atlantic will remain mostly to partly sunny and dry due to the influence of surface high pressure, with early morning stagnation and light onshore winds in the afternoon. Some clouds may develop due to the passage of shortwaves and positive vorticity advection associated with the weak trough aloft. Whether they impact the air quality forecast will depend on the altitude of the clouds. Guidance suggests they will be mid-to-high level clouds along the eastern Mid-Atlantic, which would allow ozone to reach the Moderate range at scattered locations, particularly along/north/west of I-95. PM2.5 will be in the Moderate range at inland locations, particularly the Susquehanna Valley and western MD. Areas along the western Mid-Atlantic that experience precipitation overnight and through the day on Saturday will see PM2.5 fall into the Good range. Smoke from the central/western Canadian wildfires has spread into the northern Mid-Atlantic today, as analyzed by the NOAA HMS Fire and Smoke team. The remnant smoke doesn’t appear to be impacting surface air quality, but we will continue to monitor it through the weekend.

As the center of surface high pressure pulls away to the Canadian Maritimes on Sunday, a more sustained onshore flow will develop, both aloft and at the surface. The weather models are showing poor run-to-run consistency regarding an area of low pressure that is expected to develop roughly over the Delmarva coastal waters, along the stalled frontal boundary draped along the Atlantic coast, some time on Saturday night into Sunday morning. Most of the guidance keeps precipitation associated with the low offshore, but there is a chance that coastal locations in the NMA could see some rain and clouds on Sunday. Regardless, ozone should be in the Good range for most locations, due to the onshore flow. PM2.5 will remain in the Moderate range at inland locations.

Monday-Wednesday will be the days of most interest during the medium range period, especially Tuesday and Wednesday. As the weak ridging aloft spreads over the Mid-Atlantic, surface high pressure will reestablish itself over the region, and a warming trend will take high temperatures into the low 90s °F by Wednesday, with rising humidity. The persistent stationary front will linger along the Atlantic coast through at least Tuesday, which may keep air quality in the Good to low Moderate range in the southern Mid-Atlantic (SMA), particularly NC. The BAMS air quality models develop a widespread area of high Moderate to USG ozone in the northeastern Mid-Atlantic, along the I-95 Corridor, on both Monday and Tuesday. This seems a bit aggressive at this point; more likely, Monday will be a transition day, given that high temperatures are only forecast to reach the low-to-mid 80s °F, and flow will still be onshore. On Tuesday, surface winds will shift south/southwesterly, with slow and localized westerly transport aloft, giving a greater chance for ozone to reach the USG range at isolated locations. The weather forecast models are generating instability showers/thunderstorms for Tuesday and Wednesday, while there is uncertainty regarding the speed of the developing upper level trough over the Great Lakes, as discussed above. Thus uncertainty in the precipitation forecast makes the air quality forecast uncertain as well. PM2.5 will respond to the rising humidity and should reach the Moderate range across the region by Tuesday and Wednesday.

-Catena/Huff