Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 21, 2015
Valid: May 22-26, 2015 (Friday-Tuesday)

 

Summary:

There is an appreciable chance for Code Orange/Red air quality in the Mid-Atlantic region late in the medium range period. A back door cold front will move through the region on Friday and Saturday, bringing a surge of cool, dry, and clean Canadian air, for Good to low Moderate air quality. Sunday will be a transition day to a potential period of deteriorating air quality. An upper level ridge will build over the Mid-Atlantic, resulting in rising temperatures and humidity through the end of the period. Model differences regarding the location of clouds and rain associated with a weak frontal boundary across the northern Mid-Atlantic on Monday and Tuesday make the potential for rising regional ozone uncertain, with the best chance for upper Moderate to USG ozone across the central and southern Mid-Atlantic.

Discussion:

The weather forecast models agree on the main synoptic features for the first part of the medium range period, but they diverge slightly late in the period. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Friday, a closed low rotating around the axis of the upper level trough centered over the eastern US will push a back door cold front through the Mid-Atlantic. WPC places the front across central VA and southern DE by 00Z Saturday and across southern NC by 12Z Saturday. The closed low will continue to rotate eastward on Saturday, reaching northern New England by 12Z and the Canadian Maritimes by 00Z Sunday. At the same time, an upper level ridge will amplify over the Plains. This ridge will extend eastward over the Mid-Atlantic on Sunday and remain in place through the end of the medium range period. A series of shortwaves dropping down from northern Canada and rotating around the eastern Canadian trough axis will suppress the ridge slightly, but today’s runs of the global models are showing less suppression than yesterday’s runs. The EC still has a slightly broader, less amplified trough over the Mid-Atlantic through Tuesday compared to the GFS and CMC. This translates into precipitation location differences on Monday in particular and Tuesday to a lesser extent. One of the Canadian shortwaves will push another, weaker back door cold front into the northern Mid-Atlantic (NMA) late Sunday into Monday, and this weak front will stall somewhere across the NMA, acting as a focus for clouds and rain on Monday before possibly slowly moving north as a warm front on Tuesday. The GFS and CMC, with their more peaked upper level ridges, keep the rain to the north, across NY and northern PA/NJ (GFS) or NY/southern ON (CMC). The EC continues to place this frontal boundary farther south than the other models, although not as far south as yesterday, keeping the rain across all of PA/NJ and into parts of MD and DE on Monday. The uncertainty continues on Tuesday, with all of the models generating scattered precipitation, generally west of the I-95 Corridor. Thus, with the ridge building across the Mid-Atlantic beginning Sunday, and hooking up with the Bermuda High in the Atlantic, temperatures and humidity will rapidly increase late in the medium range period. This pattern is conducive for rising regional ozone, but there is uncertainty in the amount and location of rain/clouds, particularly in the NMA, which will limit ozone production.

Friday will be mostly sunny with breezy northwesterly winds, average or slightly below average temperatures, and low humidity. The weather models, to varying degrees, are predicting scattered clouds and rain showers in the afternoon across the NMA, ahead of the frontal boundary. The 03Z SREF and 06Z 4 km nested NAM show a band of rain moving through the NMA in the afternoon, reaching roughly to the Mason-Dixon Line by 21-00Z. In contrast, the 00Z 4 km NMM/ARW have only isolated clouds/rain showers associated with the frontal passage. Regardless of the chances for any clouds/precipitation, given a clean air mass in place and ample atmospheric ventilation, generally Good to low Moderate air quality is expected.

Behind the back door cold front, a surge of cool, dry, and clean Canadian air will briefly build into the Mid-Atlantic on Saturday. Surface high pressure will move over the central and southern parts of the region, allowing winds to stagnate. Full sun and very light to calm winds may allow for isolated Moderate ozone, but generally Good air quality is expected given the clean air mass.

Sunday looks like a transition day to a potential period of deteriorating air quality. As the ridge builds overhead and surface high pressure moves offshore, winds will shift southwesterly and bring warmer and more humid air into the Mid-Atlantic. Some recirculation aloft will allow for rising ozone and particles. Generally scattered Moderate conditions are expected, with ozone as the leading pollutant, given the air mass will still be relatively clean and dew points will only reach the 50s °F. Memorial Day weekend emissions will be a consideration as well.

Monday and Tuesday are the days of most interest in the period, but they have the most uncertainty associated with clouds and precipitation. The central and southern Mid-Atlantic look generally dry and clear, making them the most likely locations for ozone rising to the top of the Moderate range. Particles will also likely reach the Moderate range in the increasingly humid and modified air mass. Rising ozone across the NMA will be limited by the location of a possible warm front sagging across PA/NJ and associated rain/clouds. The EC has the wettest solution for the NMA, while the GFS and CMC solutions have more promise for clear skies and thus higher ozone.

-Huff