Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 24, 2014
Valid: July 25- 29, 2014 (Friday-Tuesday)

Summary:

High pressure will build in overhead for Friday, resulting in sunny skies and light winds. Given the clean air mass in place, Good air quality is expected, with the chance for isolated Moderate ozone along I-95. A warm front will travel northward through the region on Saturday and Sunday, resulting in rising temperatures and humidity. There is a chance for ozone to rise into the upper Moderate range both days, mainly along I-95; however, potentially gusty afternoon surface winds on Saturday and mostly cloudy skies on Sunday may limit rising ozone levels. PM2.5 will reach the Moderate range both days in response to humid conditions. A cold front will pass through on Monday, ushering an unseasonably cool, dry, and clean Canadian air mass. Air quality will be in the Good range both Monday and Tuesday.

Discussion:

The high confidence streak still remains for the weather forecast models for the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with a weak upper level trough overhead. It will gradually pass through the region until flow aloft becomes more zonal on Saturday afternoon. A potent upper level shortwave moving eastward through southern Canada from the West Coast will carve out a trough over MB/ND/MN on Sunday. A small but somewhat intense shortwave will travel just ahead of the trough and will impact the Mid-Atlantic region on Sunday night, which will help to trigger evening showers. The upper level trough will move southeastward through the Great Lakes and will approach the Mid-Atlantic Monday afternoon, pushing a surface cold front ahead of it through the Mid-Atlantic. The GFS becomes slightly faster than the ECMWF with the movement of the trough starting Tuesday morning. This small discrepancy will remain insignificant in regards to air quality.

Friday will stay mostly dry as high pressure builds in. A stationary front located off the Southeast coast will result in rain and clouds for primarily the southeastern Mid-Atlantic. There will be early morning stagnation, variable to light winds, sunny skies, and an afternoon sea breeze in the northern Mid-Atlantic (NMA). This setup has been a recipe for rapidly rising ozone along I-95 several times this summer. However, the arrival of the dry and clean air mass from Thursday’s cold front will keep the chance low for upper Moderate ozone. Most likely, ozone will be in the Good/Moderate threshold in the NMA along I-95. The NOAA and BAMS ozone models agree with this analysis and keep ozone in the Good range across the region. PM2.5 will also be limited to the Good range in the very dry and clean air mass.

Saturday and Sunday are the days of most interest in the medium range period. The stationary front along the Southeast will move northward as a warm front to the central Mid-Atlantic by Saturday morning and will continue to move northward throughout the day. Temperatures and humidity will rise with the passage of the warm front. The day will be mostly sunny with sustained southerly surface flow in the afternoon and a slight chance for scattered rain showers. The chance for rain is greatest in the evening through the night for the NMA as the warm front approaches. Warm air arriving aloft and recirculating back trajectories will push ozone into the Moderate range for most locations on Saturday, particularly along the I-95Corridor. The main forecast questions will be the degree of modification of the air mass and the potential impact of sustained southwesterly surface winds in the afternoon, possibly gusting to 15-20 mph, which would act to disperse pollutants. PM2.5 will also rise into the Moderate range at scattered locations due to increased atmospheric moisture and early morning stagnation.

By Sunday, the warm front will have traveled northward up through central NY, and the Mid-Atlantic will be in the warm sector. Back trajectories will originate in the Ohio River Valley and atmospheric moisture will increase. High temperatures will reach into the low 90s °F in the NMA, and surface trough will develop over the region. Depending on how strong the surface convergence is along the trough, ozone has the potential to rise into the upper Moderate range, particularly along the I-95 Corridor. However, a cold front located over the Great Lakes through the Plains will move eastward toward the Mid-Atlantic on Sunday, triggering increasing clouds and rain showers for Sunday evening. Skies look to be mostly cloudy for much of the day, due to the proximity of the warm front and approaching cold front, which may limit any rises in ozone. PM2.5 will remain in the Moderate range in the increasingly humid and modified air mass.

The cold front will pass through the region on Monday, resulting in scattered precipitation and cloud cover. Air quality will be reduced to the Good range. The cold front will move slightly off the coast by Tuesday, allowing an unseasonably cool, dry, and clean Canadian high pressure system to build in, with Good air quality continuing. The upper level pattern heading into the middle part of next week is more reminiscent of winter than summer.

-Catena/Huff