Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 13, 2015
Valid: June 14-18, 2015 (Sunday – Thursday)

Summary:

An unsettled weather pattern will limit the chances for USG ozone to the slight range for most of the medium range period. A consecutive series of back door cold fronts will move through the Mid-Atlantic, keeping chances for widespread clouds, rain showers, and convection in the forecast each day. Sunday is the day of most interest, as very light winds and possible clear skies in the afternoon may allow ozone to reach the Moderate range at locations along and east of I-95. The air quality are providing mixed guidance for tomorrow, with one of them (the BAMS-MAQSIP) bringing ozone to the USG range at PHL/TTN/BAL/DC. Given how quickly air quality cleaned out yesterday across the region, with that trend appearing to continue today, USG ozone seems unlikely for Sunday.

Discussion:

The forecast models agree on the main synoptic features for the beginning of the medium range period, but they diverge beginning on Wednesday. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The medium range will be unsettled as consecutive cold fronts reach approximately to the Mason-Dixon Line and stall, acting as a focus for clouds, convection, and rain showers. The period begins on Sunday with a weak frontal boundary roughly bisecting the Mid-Atlantic from northwest to southeast. Aloft, the persistent ridge centered over the Southeastern US will remain in place, with a Canadian ridge axis moving over the Mid-Atlantic by Monday. A series of small shortwaves will crest the Southeastern ridge and move over the northern periphery of the Mid-Atlantic for most of the period. On Monday, a large shortwave moving east in the northern stream flow over southern Canada will eke out a shallow longwave trough that will reach the Great Lakes by 00Z Tuesday. This trough will flatten the Southeastern ridge and develop a zonal flow over the Mid-Atlantic on Tuesday and Wednesday. The trough will also pull the next back door cold front into the Mid-Atlantic on Tuesday, similar to the progression that is occurring with today’s back door cold front. The models are in general agreement on the strength and speed of Tuesday’s front, bringing it to approximately the Mason-Dixon Line by 12Z Wednesday, but they diverge after that. On Wednesday, the GFS brings another large shortwave down in the northern stream flow over MB/ON and digs out another shallow longwave trough over the Great Lakes on Thursday. In contrast, the EC does not have this shortwave at all and instead re-amplifies the ridge along the east coast. The EC keeps the front across the central Mid-Atlantic for the end of the period, gradually pushing it north and east as a warm front late Thursday. The GFS quickly pushes the front northeastward out of the region on Wednesday as a warm front, and brings another cold front into the region from the northwest on Thursday.

On Sunday, slightly cooler and less humid air will build in behind the frontal boundary, roughly encompassing eastern PA, NJ, eastern MD, eastern VA, and DE. The western portion of the front will slowly lift north and east as a warm front, acting as a focus for clouds and precipitation. The main forecast question will be how much afternoon sun will occur along the I-95 Corridor. The 09Z SREF and 12Z mesoscale models are in agreement on rain and thunderstorms west of I-95, with periods of heavy rain possible along the path of the warm front. The models are mixed on how much cloud cover will occur along the I-95 Corridor. Winds will be light and variable in the morning and pick up somewhat in the afternoon. It will still be warm, with highs in the upper 80s °F. Back trajectories are northerly but not fast. Therefore, if there is sufficient afternoon sun, Moderate ozone is likely along the I-95 Corridor. Given how quickly air quality cleaned out yesterday, and how that trend appears to be continuing today, USG ozone seems unlikely on Sunday. The air quality models are providing mixed guidance, however, with the BAMS-MAQSIP bringing ozone to the upper Moderate range along the I-95 Corridor, with pockets of USG in PHL, TTN, BAL, and DC. The NCDENR model is at the other extreme, with Good ozone across the region.

Monday looks cloudy and wet for most of the region as the warm front slowly moves eastward. The SREF shows a high probability of precipitation across the region, and breezy surface winds will help to ventilate the atmosphere. Periods of heavy rain are possible following the progression of the warm front.

Tuesday will be the warmest and most humid day of the period, ahead of the next back door cold front. Conditions associated with the passage of the front on Tuesday appear to be very similar to today, with an early wind shift to the north/northwest and chances for increasing clouds, showers, and thunderstorms from northwest to southeast, for generally Good air quality. The mesoscale models are keeping the bulk of the clouds and precipitation to the north of the Mason-Dixon Line, placing the best chance for Moderate air quality in locations to the south.

The forecast for Wednesday and Thursday is uncertain given the complete divergence of the global models for the end of the period. The WPC is siding more with the EC solution this morning, keeping the frontal boundary quasi-stationary across the central Mid-Atlantic through Saturday. At this point, Wednesday looks to be sunnier and warmer, particularly for the northern part of the region, with a better chance for Moderate air quality.

-Huff