Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 11, 2015
Valid: June 12-16, 2015 (Friday – Tuesday)

Summary:

A high chance for USG ozone continues for Friday in the Mid-Atlantic, due to hot and humid conditions and the persistent influence of smoke from Canadian wildfires. Possible factors limiting USG ozone on Friday include stronger afternoon surface winds along and east of I-95, faster and more southerly back trajectories, and the chances for afternoon clouds and convection along an Appalachian lee trough. A stronger back door cold front will slowly move into the northern Mid-Atlantic on Saturday; more widespread clouds and convection ahead of this front lower the chances for USG ozone to appreciable on Saturday. Uncertainty in the forecast for the end of the period continues today, related to the strength and location of the cold front, which will stall over the central part of the region and slowly move north and east on Monday as a warm front before dissipating on Tuesday as another back door cold front approaches the NMA. In the absence of a substantial air mass change in the region, marginal chances for USG ozone will continue through the end of the period, with scattered Moderate air quality most likely.

Discussion:

The weather forecast models are in relatively close agreement on the all of the synoptic features during the medium range period. The 06Z NAM, 06Z GFS, 00Z ECMWF, 00Z CMC were consulted for this analysis, with the main emphasis on the NAM, GFS, and EC. The period begins with the persistent mid/upper level ridge in place, centered over the southeastern US. The models are still in agreement today that remnants from Hurricane Blanca will dig out a small upper level trough over the Great Lakes on Friday. At the same time, a shortwave dropping down into ON will help to amplify this trough as it moves eastward, reaching southern QC/New England on Saturday. Today, the models do phase some of the Blanca shortwave energy with the ON shortwave, but the bulk of the Blanca energy will stay slightly farther south, moving through the northern Mid-Atlantic (NMA) on Saturday. The Canadian trough will pull a stronger back door cold front into the NMA on Saturday, with a pool of cooler air aloft. The weather models are still not in consensus regarding the strength and southward extent of the cold front. Today, the EC is back to having the strongest front, bringing it into central VA by 00Z Sunday and to the VA/NC border by 12Z Sunday. The NAM and GFS are slower and do not extend the cooler air as far south and west into the Mid-Atlantic as the EC does. By Monday, the weather models all briefly re-amplify the mid-level ridge and push the cold front slowly north and east as a warm front, before it eventually dissipates by the end of the period. Aloft, a large shortwave moving eastward across the southern Canadian Plains in the northern stream flow will reach ON and QC on Tuesday, flattening the ridge and developing more of a zonal flow over the Mid-Atlantic. This shortwave will also pull the next back door cold front into the NMA on Tuesday.

Friday is still the day of most interest in the period. It will be the second consecutive hot and humid day, with high temperatures reaching into the 90s °F. A weak frontal boundary will be draped along PA/MD border in the morning and will move northward during the day. Winds will be light, variable, and converging in the vicinity of this front, particularly in southern PA in the morning. This period of morning stagnation will promote NOx titration in the air mass, setting the stage for rapidly rising ozone in the afternoon. Some of the limiting factors for USG ozone on Friday include faster back trajectories originating from the vicinity of SC, running along I-95, and more breezy S/SW winds in the afternoon along coastal locations (e.g., DE, southeastern PA/southern NJ). An Appalachian lee trough will be in place along central PA/western MD/central VA. The mesoscale models are still developing scattered thunderstorms and clouds in the afternoon along the lee trough. This may be overdone, since typically a strong ridge aloft will suppress lifting air along a lee trough, but it is still a consideration. The air quality models this morning are responding by pushing areas of USG and upper Moderate ozone northward, into northern and central NJ (NOAA) and southern NY and CT (BAMS-MAQSIP). Canadian smoke is still quite prevalent over the region this morning; it is pushing PM2.5 concentrations to the mid and upper Moderate range in PA, VA, MD, NJ, and DE, stretching back into IN, KY, and OH, and northeastward into southern NY and New England. The NRL NAAPS model indicates the smoke will continue to impact the Mid-Atlantic on Friday, but to a lesser extent. Thus, for Friday, a high chance for USG ozone continues, with the most likely locations shifting to the northern part of the I-95 Corridor. The main questions will be the impact of more southerly and faster back trajectories, possible afternoon showers/convection associated with the lee trough, and the continuing impact of smoke.

On Saturday, a stronger back door cold front will move into the NMA in the morning, reaching approximately to central PA/ NJ by 18Z and into northern MD and DE by 00Z Sunday. A better chance for more widespread clouds and convection associated with the approach of the front will lower the chances for USG ozone to appreciable. Back trajectories will also be fast on Saturday, originating from the Mississippi River Valley. The air quality models continue their trend today of essentially cleaning out the region on Saturday through Monday, due to the approach and then stalling of the cold front. Saturday will be another hot and humid day, however, and for areas ahead of the front that will not experience an air mass change, isolated USG ozone is certainly possible given sufficient afternoon sunlight.

There is some uncertainty in the forecast for Sunday through Tuesday, centering around the location, strength, and impacts of the cold front. WPC places the front across the central Mid-Atlantic through the end of the period, pushing the western portion of it northward and eastward on Monday, as a warm front. Clouds and precipitation in the vicinity of the front appear likely each day, with the NMA mostly dry on Sunday as the front sags to the south, but rain and clouds west of I-95 on Monday as the warm front lifts eastward. Tuesday appears to have a higher chance for more organized precipitation, moving from northwest to southeast into the region, as the next back door cold front approaches. It will still be relatively warm and humid at the end of the period, with a slight drop in dew points on Sunday and Monday for locations behind the cold front. Thus, continued scattered Moderate air quality is possible across the region, expect for areas that see heavy or slow moving rain.

-Huff