Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 28, 2015
Valid: May 29-June 2, 2015 (Friday-Tuesday)

Summary:

There is still an appreciable chance for isolated USG ozone in the Mid-Atlantic on Friday, but then chances decrease to slight for the remainder of the medium range period. The upper level ridge over the Mid-Atlantic will re-strengthen on Friday. Light onshore surface winds and westerly back trajectories will allow for rising ozone at scattered locations north and west of I-95. The next substantial cold front will slowly approach the Mid-Atlantic from the northwest on Saturday. A shift back to onshore transport aloft should limit ozone to the Good to low Moderate range across the region. The weather forecast models have come into consensus regarding the timing of the cold front, bringing it to roughly the Mason-Dixon Line by 12Z Sunday, where it will stall and sag southward to NC by Tuesday. Widespread clouds, rain, and convection associated with the stalled boundary will keep air quality generally in the Good range for the end of the period.

Discussion:

The weather forecast models are in close agreement on the synoptic features of the medium range period, except for the CMC, which is considered an outlier after Saturday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Friday, the upper level ridge re-strengthens over the eastern US, and Canadian high pressure centered over ON/QC pushes a weak back door cold front to northern PA/NJ early in the morning. This front reaches much farther south in today’s analysis compared to previous days before dissipating late Friday morning. On Saturday, the forecast models are still in agreement that a strong shortwave/vort max will drop down from west/central Canada and rotate around the axis of the semi-permanent upper level trough over eastern Canada. The base of the shortwave will reach southeastern ON by 00Z Sunday and southeastern QC by 18Z Sunday. This feature will pull a substantial cold front into the Mid-Atlantic on Sunday. The forecast models have come into consensus on the speed of the front, bringing it roughly to the Mason-Dixon Line by 12Z Sunday. As was the case yesterday, the forecast models disagree on the fate of shortwave energy moving eastward from the Plains on Sunday. The GFS is consistent in placing this energy near the lower Mississippi River Valley (MRV), over eastern TX/LA, and closing it off late Monday into Tuesday. The EC has a smaller magnitude shortwave that it keeps slightly farther north and east, over the central MRV, near AR/TN/northern MS. The CMC brings the shortwave energy quickly northeast, to the Ohio River Valley (ORV) on Monday and to New England on Tuesday, and develops a wave of low pressure along the stalled front across the Mid-Atlantic. We consider the CMC solution an outlier and focus on the GFS and EC solutions for Monday and Tuesday. The GFS and EC have another Canadian shortwave dropping down in the upper level flow over southern ON on Monday and moving southeastward into New York State and New England on Tuesday, which builds a weak upper level trough over the Mid-Atlantic on Tuesday. At the surface, the GFS and EC keep Sunday’s cold front stalled roughly across the central Mid-Atlantic (northern VA/southern Delmarva) on Monday and push it slightly farther south into northern NC on Tuesday.

Yesterday (Wednesday), we saw the influence of the maritime back trajectories on air quality across the region, particularly ozone. For example, in Philadelphia, where skies were mostly sunny in the afternoon until a line of thunderstorms arrived around 5:30 PM EDT, daily 8-hr average ozone peaked at 39 ppb, which is the 10th percentile for ozone for this time of year.

On Friday, the re-strengthening upper level ridge and surface high pressure will diminish surface winds and turn them onshore (southeasterly). The mesoscale models differ in regard to cloud cover, with mostly clear skies likely across the northern Mid-Atlantic (PA, NJ). The chances for clouds and isolated thunderstorms increase for the central Mid-Atlantic; the mesoscale models (06Z 4-km NAM and 00Z ARW and NMM) develop scattered clouds and afternoon thunderstorms across VA/MD/southern PA/northern DE. The 09Z SREF has a high probability of precipitation across the western Mid-Atlantic in the afternoon, to the west of I-95. As a result, the best chances for rising ozone will lie across eastern PA and central/north NJ, where light onshore surface winds can transport highway emissions north and west of I-95. The BAMS models pick up on this transport pattern, with Moderate ozone across northeastern PA and northern NJ. The NOAA ozone model has a similar pattern but places the Moderate ozone farther south, across central MD and southeastern PA. Back trajectories will be westerly and relatively slow, from the vicinity of the Ohio River Valley. A limiting factor could be the weak back door cold front that WPC shows dissipating over northern PA/NJ around 12Z Friday. If this front does reach into northeastern PA/northern NJ, clean air behind the front would likely suppress ozone formation.

The next strong cold front will slowly approach the Mid-Atlantic from the northwest on Saturday. Back trajectories shift southward, with an onshore component. Surface winds will also turn southerly, with breezy winds in the afternoon at locations along the coast. This shift in transport may be sufficient to return air quality to the Good to low Moderate range across the region. In addition, clouds and scattered thunderstorms seem likely across the western Mid-Atlantic. The 09Z SREF has a high probability of precipitation across the western Mid-Atlantic beginning at 15Z Saturday. Locations east of I-95 should see mostly sunny skies into the afternoon, making Philadelphia and central/northern NJ the most likely places for Moderate ozone.

The cold front will reach the central Mid-Atlantic on Sunday morning and then stall and sag slightly southward through Tuesday. Widespread clouds, rain, and convection on Sunday across most of the region will promote Good air quality. On Monday and Tuesday, chances for clouds and showers across much of the region will continue, given the location of the stalled frontal boundary, with some clearing in the northern Mid-Atlantic possibly by Tuesday. In addition, surface high pressure centered over southern QC will generate onshore surface winds (east/northeasterly) north of the front, which will further promote Good air quality.

-Huff