Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 1, 2014
Valid: July 2- 6, 2014 (Wednesday-Sunday)

Summary:

A pre-frontal trough will develop ahead of an approaching cold front from the west on Wednesday resulting in afternoon rain and thunderstorms. Air quality will be limited to the Good range with the exception of Moderate ozone along the I-95 Corridor in PHL and MD. The cold front will move to the Mid-Atlantic and stall Thursday, calling for all-day rain and thunderstorms. Tropical Storm Arthur has formed off the coast of Florida and will move northward to SC. Air quality will remain in the Good range. The front will pass through Friday as the tropical system moves northward to Cape Hatteras. Good air quality is mostly likely although there is a slight chance for Moderate ozone as the tropical system will result in subsidence and calm weather for inland locations. The weekend will be beautiful and sunny as high pressure moves into the area and remains until Sunday. Northerly flow will keep air quality in the Good range.

Discussion:

There is high confidence in the weather forecast models for the medium range period. The 06Z NAM, 06Z GFS, and the 00Z ECMWF are in phase with each other regarding the eastward movement of the upper level longwave trough. The trough is positioned over the Great Plains at the beginning of the period and will pass over the Mid-Atlantic Friday night into Saturday morning. The development of an upper level ridge will follow the trough across the U.S, but will flatten out by Saturday morning over the Great Plains becoming more zonal for the weekend. Tropical Storm Arthur has formed off the coast of Florida and will travel along the East Coast to Cape Hatteras Friday morning and move northeast into the Atlantic by Saturday morning. The GFS keeps it closer to the coast on Saturday than the ECMWF. The impact on air quality remains uncertain at this forecast length.

As a cold front approaches from the west on Wednesday, a pre-frontal trough will develop just west of the I-95 Corridor, sparking widespread rain and thunderstorms, primarily in the second half of the day. Wednesday will be hot and humid with above average temperatures and scattered clouds throughout the day. Mixing from diurnal heating and cleaner air transported from southwesterly winds and late day convection will keep air quality in the Good range, with the exception of Moderate ozone along and west of the I-95 Corridor, mainly in PHL and MD.

By Thursday, the frontal boundary will move and stall over the Mid-Atlantic. A tropical system will develop off the coast of Florida and travel northward up to the shores of SC. All-day heavy rain and thunderstorms are expected with overcast skies. The front will bring in northwesterly flow, although southwesterly flow will reside in locations ahead of the front. Air quality will remain in the Good range.

TS Arthur will move northward up the coast to Cape Hatteras until 12Z Friday and will then travel northeastward into the Atlantic. Arthur’s impact on mid-Atlantic air quality will be strongest on Friday. At that time, however, a frontal boundary will be crossing the mid-Atlantic and the interaction between these two features will determine regional air quality. Tropical storms typically have a zone of subsidence at the periphery of their circulation and this can, in the right circumstances, result in poor air quality. In this case, however, synoptic scale upward motion, along with clouds and rain, from the advancing frontal boundary will limit the impact of this subsidence effect and we expect good air quality Friday.

As the cold front moves off the coast and the tropical system moves northeastward into the Atlantic, high pressure will move into the Mid-Atlantic and remain overhead for Saturday and Sunday. Northerly flow will bring in dry, clean air for both days, limiting air quality to the Good range for the weekend.

-Catena/Ryan