Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 14, 2015
Valid: June 15-19, 2015 (Monday – Friday)

Summary:

A repetitive synoptic pattern of weak cold fronts stalling across the central Mid-Atlantic and lingering as warm fronts will keep the chances for USG ozone slight during the medium range period. The exception is Wednesday, when clearing skies and light winds may allow for rising ozone along the I-95 Corridor. Back trajectories from the west/central Great Lakes should limit ozone on Wednesday to the mid-to-upper Moderate range. Otherwise, the arrival of weak cold fronts on Tuesday and Friday/Saturday, above average temperatures, and humid conditions will keep chances for clouds, rain, and scattered thunderstorms in the forecast for Monday-Tuesday and Thursday-Friday, for generally Good ozone and scattered Moderate PM2.5.

Discussion:

The forecast models agree on the main synoptic features for the beginning of the medium range, but the GFS and EC continue to diverge at the end of the period. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The synoptic pattern is a repetitive one, with a persistent ridge centered over the Southeastern US and a series of larger shortwaves moving through the northern stream flow across southern Canada bringing periodic weak cold fronts into the Mid-Atlantic, to roughly the Mason-Dixon Line, where they stall. The next cold fronts are expected to move through the northern Mid-Atlantic (NMA) on Tuesday and Friday/Saturday. The PHL/Mt Holly NWS discussion this morning summed up the impacts of this pattern nicely: “This pattern favors persistent above normal temperatures in our area and periods of thunderstorms in the ring of fire around the [northern] edge of the ridge and along the southern boundary of the stronger westerlies to the north.” The only variation in the pattern begins on Wednesday, when shortwave energy over TX ejects northeastward; the models each handle this shortwave energy differently. The NAM quickly merges the TX energy with the northern stream flow over the Great Lakes on Wednesday evening and carves out a weak longwave trough. The EC spits out lobes of energy from the TX shortwave and moves them quickly to the Mid-Atlantic, but keeps the bulk of the energy over TX. The EC keeps this stronger shortwave together and slowly brings it northeastward, reaching the western Mid-Atlantic (WMA) by 00Z Sunday. The GFS shears out the TX shortwave energy across the Mississippi River Valley and Ohio River Valley on Wednesday and Thursday, and brings the tailing end across the central Mid-Atlantic (CMA) on Friday as a more concentrated shortwave. Thus, the EC and GFS both have a similar, concentrated remnant of the TX shortwave crossing the CMA, but the GFS is 24 hours faster than the EC. The GFS also continues to have a stronger and farther southward extending northern stream QC shortwave on Friday, which pushes the end of the week cold front into the NMA faster and brings it farther south than the EC does. The WPC seems to be following a blend of guidance for the end of the week front but leaning toward the EC solution for the TX shortwave.

Given the persistent humid air over the region, the lingering frontal boundaries, and the series of shortwaves moving through aloft, cloudy skies, rain showers, and convection remain in the forecast for at least some part of the region each day during the period, limiting the chances for USG ozone to slight. The exception is Wednesday, when clearing skies and light winds along the I-95 Corridor offer the chance for mid-to-upper Moderate ozone, similar to today. The air quality models essentially clean out the region Monday-Tuesday and Thursday-Friday, keeping Wednesday as the only day of slight interest, with Moderate ozone along the I-95 Corridor and south of the Mason-Dixon Line.

Of note this morning are some slightly elevated PM2.5 concentrations across SC (mid-Moderate) and NC (low Moderate), probably due to smoke from local agricultural fires. The NRL NAAPS model shows smoke from these local fires impacting eastern NC and VA through Thursday. The possible impact of the local burning will need to be monitored in the coming days to see if smoke does indeed contribute to rising PM2.5 concentrations along the southern Mid-Atlantic (SMA).

Monday and Tuesday will be very warm and humid. The warm front over the region today will get hung up to the northeast on Monday, over northern NJ. The mesoscale models are in agreement on mostly cloudy skies and periods of rain and thunderstorms throughout the day for most of the region, with the heaviest storms overnight (Sunday into Monday morning) and in the late afternoon/evening. Periods of heavy rain are likely. Tuesday will also be wet and partly to mostly cloudy, following the track of the next weak cold front. Ozone should remain in the Good range across the region, with lingering Moderate PM2.5 possible in areas that do not experience heavy rainfall or are impacted by smoke from agricultural fires in the SMA.

Wednesday is the day of most interest, but only because there is a chance to reach the Moderate range for ozone in a period of otherwise Good conditions. Wednesday looks similar to today, with slightly cooler and drier air filtering into the NMA and clearing skies. Back trajectories are from the Great Lakes and are fast enough to limit any rising ozone to the mid-to-upper Moderate range.

The forecast for Thursday and Friday will depend primarily on the speed and strength of the next cold front, which may arrive as early as Friday (per the GFS). Thursday looks like another cloudy and wet day for much of the region, with Tuesday’s front lifting northward as a warm front. Scattered thunderstorms and rising temperatures and humidity on Thursday suggest Good ozone and scattered Moderate PM2.5. Friday may see a chance for Moderate ozone, depending on cloud cover and the timing of the next cold front. The GFS, with its faster front, has more rain on Friday in the NMA and CMA, while the slower EC keeps the NMA dry and pushes precipitation to the SMA.

-Huff