Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 31, 2014
Valid: August 1-5, 2014 (Friday-Tuesday)

Summary:

A coastal front will bring cloudy skies, periods of heavy rain, and Good air quality to most locations on Friday-Sunday, with the cleanest conditions expected on Saturday. The exception will be PM2.5, which will remain in the Moderate range in the northern Mid-Atlantic on Friday, since precipitation will move from south to north. The forecast for Monday and Tuesday is uncertain and hinges on how long the coastal front lingers, but Good to Moderate conditions are expected.

Discussion:

The weather forecast models are in close alignment on the main features of the medium range period through Saturday, but they diverge beginning on Sunday. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The closed low aloft over ON/QC will retrograde northward on Friday, but shortwave energy dropping down over the Great Lakes/northern Mississippi River Valley on Saturday will reinforce the longwave trough over the eastern U.S. This larger area of shortwave energy will move eastward into the Ohio River Valley (ORV) on Sunday. The forecast models disagree on how fast this larger shortwave will progress through the ORV and into the Mid-Atlantic. The ECMWF is about 12 hours faster with the progression of the shortwave than the NAM, while the GFS is in the middle. The ECMWF brings the shortwave into the northern Mid-Atlantic (NMA) by 00Z Monday. For both the ECMWF and GFS, this shortwave promotes a transition to more zonal flow aloft. Consequently, the ECMWF is faster with the transition to more zonal flow over the Mid-Atlantic on Monday and Tuesday, although the persistent trough over eastern Canada remains. As the upper level trough over the eastern U.S. finally transitions to more of a zonal flow, the GFS is faster than the ECMWF to build in mid-level ridging centered over the Great Lakes on Tuesday. This difference, coupled with the variations in the speed of the larger shortwave over the Mid-Atlantic, results in uncertainty regarding the precipitation forecast beginning Sunday, as discussed below.

The persistent frontal boundary that has been lingering out in the Atlantic, parallel to the coastline, will move back towards the Mid-Atlantic shore as a warm front on Friday. The forcing will come from the gradual expansion of the western Atlantic ridge as the strong Canadian closed low retreats northward. The coastal front will approach the southern Mid-Atlantic (SMA) first, hugging the NC/VA coastline by 12Z Friday. Small shortwaves moving through the eastern side of the upper level trough will trigger waves of low pressure that will ride along the coastal front from south to north. As a result, rain and cloud cover will spread northward on Friday. Overcast skies and widespread rain will keep air quality in the Good range in the SMA. Scattered cloud cover will become mostly cloudy by the afternoon in the NMA, with scattered showers all day, which should keep ozone in the Good range for most locations. Light onshore surface winds and a shift toward onshore flow aloft will promote falling PM2.5 concentrations in the NMA, but continued Moderate conditions are likely, particularly at inland locations west of I-95.

The coastal front will be located along the entire Mid-Atlantic coast on Saturday and Sunday before pulling away from the NMA coast on Monday. Waves of low pressure will ride along the front through the weekend, particularly on Saturday, promoting periods of heavy rain. The forecast models are not in agreement regarding the exact location and timing of the waves, and thus of the heaviest rainfall, especially on Sunday, when the ECMWF dries out the fastest due to its more progressive flow aloft. Regardless of the timing of precipitation, overcast to mostly cloudy skies and onshore flow on both Saturday and Sunday will keep air quality in the Good range, with the cleanest conditions likely on Saturday.

The forecast for Monday and Tuesday is uncertain and hinges on how long the coastal front lingers before surface high pressure builds back in from the west. On Monday, the GFS has the wetter solution, keeping the coastal front intact along the coast and continuing to move waves of low pressure along it. The ECMWF is relatively dry, expect for the SMA, where the front remains. By Tuesday, both models are predicting drier conditions, although a weak cold front will be approaching from the Great Lakes later in the day. The ECMWF develops a line of precipitation along the far NMA, associated with the front, while the GFS does not, likely due to its more enhanced mid-level ridging to the west. Thus, Good to Moderate air quality is expected at the end of the period, depending on the track of the coastal front, with better chances for Moderate conditions on Tuesday as skies clear and temperature reach the upper 80s °F.

-Huff