Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 27, 2015
Valid: May 28-June 1, 2015 (Thursday-Monday)

Summary:

The chances for ozone reaching the USG range in the Mid-Atlantic have increased to appreciable for Thursday and Friday; these are the days of most interest in the period. On Thursday, convergence along a weak frontal boundary across the central part of the region will set the stage for locally rising ozone, assuming there is sufficient afternoon sun. Clearing across the northern part of the region on Friday, in conjunction with very light onshore winds, will place the chances for rising ozone at locations north and west of I-95. A substantial cold front will slowly approach the Mid-Atlantic from the northwest on Saturday and will likely stall across the region on Sunday and Monday. Uncertainty remains regarding the timing and location of pre-frontal clouds and convection, with precipitation likely impacting the western Mid-Atlantic on Saturday and spreading to most of the region on Sunday and possibly Monday.

Discussion:

The weather forecast models are in close agreement on the main features that will impact air quality for the first part of the medium range period, but diverge late in the period, particularly on Monday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Thursday, a shortwave moving northeastward across southern ON into southern QC and northern New England will pull a weak cold front or surface trough into the northern Mid-Atlantic (NMA). This feature is weak, and the global and mesoscale models have had difficulty resolving it. It is expected to reach the vicinity of southern NJ/central DE/northern VA late Thursday afternoon. In the wake of the Canadian shortwave passage, the upper level ridge will re-amplify over the Mid-Atlantic on Friday, leading to clearing skies and stagnating winds for most of the region. On Saturday, a strong shorwave/vort max rotating down from central Canada along the axis of the semi-permanent upper level trough over eastern Canada will move a cold front toward the Mid-Atlantic from the northwest. Timing differences among the global models remain regarding the speed of the cold front; the GFS is still the fastest, as per its usual bias, bringing the cold front into the Mid-Atlantic on Saturday evening to Sunday morning. The EC and CMC are slower by about 12 hours. The models generally agree that the front will stall somewhere in the vicinity of the central Mid-Atlantic (CMA; i.e., northern VA/central DE) on Sunday. The global models diverge wildly on Monday. The GFS rebuilds high pressure over the Mid-Atlantic, pushes the cold front slightly farther south, and keeps a strong shortwave/closed low over the lower Mississippi River Valley (MRV). The EC brings this shortwave energy northeast into the Ohio River Valley (ORV) and keeps the cold front stalled over the CMA. The CMC splits the difference, bringing the shortwave energy slightly farther north than the GFS, into the upper MRV. The main surface impacts for air quality translate into differences in precipitation for Monday; the GFS keeps the region basically dry, while the EC and CMC have much higher chances for precipitation associated with the stalled front and shortwave energy approaching aloft.

The chances for ozone reaching the USG range in the Mid-Atlantic have increased for Thursday and Friday; these are the days of most interest in the period. We saw yesterday (Tuesday) that despite onshore transport aloft, given normal weekday emissions, ample afternoon sunshine, and light winds at the surface, ozone can reach into the upper Moderate range at isolated to scattered locations (e.g., Padonia, north of Baltimore; York, PA). Similar conditions have the potential to be in place for locations in the Mid-Atlantic on Thursday and Friday.

On Thursday the main question will be the impact of the weak frontal boundary/surface trough. The surface wind field forecast from the mesoscale models (e.g., 4-km NAM) shows the front reaching roughly to DE/southern NJ/northern VA in the late afternoon. If the front is weak enough, it may not generate widespread clouds or thunderstorms. The mesoscale models (4-km NAM and NMM/ARW) are not showing many clouds or showers/thunderstorms associated with the front, but the 12-km NAM does have more widespread cloud cover. If there are pockets of afternoon sun in the vicinity of the front, converging winds may allow for locally rising ozone, possibly to the top of the Moderate or low USG range. The BAMS air quality models picked up on this low-level convergence in yesterday’s 12Z runs and developed an area of upper Moderate to USG ozone along the DC-BAL corridor, and they have it again in today’s 06Z runs. They also have a swath of Moderate ozone along central NJ. The NOAA model follows this same trend, but with less magnitude regarding ozone concentrations. Thus, expected conditions tomorrow afternoon in the vicinity of the front/trough – both to the north and south – will need to be carefully assessed. Also of note on Thursday is the shifting of back trajectories away from the coast, to the southern ORV.

Skies on Friday look mainly clear for the NMA, while chances for clouds and thunderstorms increase for the southern part of the region. The 09Z SREF has a high probability of afternoon precipitation for roughly western MD and VA southward. Surface winds will be light and onshore (southeasterly). Assuming sufficient afternoon sun, the onshore winds will push emissions from major highways like I-95 to the north and west, increasing the chances for rising ozone in areas like the Susquehanna and Lehigh Valleys of PA and north/central NJ.

The next substantial cold front will approach from northwest on Saturday. We will side with WPC and the EC and favor the slower frontal passage. Back trajectories shift southerly/onshore again on Saturday, making chances for rising ozone less likely, regardless of the speed of the cold front. Organized convection ahead of the front is expected to remain to the west of I-95, keeping the highest chances for Moderate ozone to the NJ/southeastern PA area.

The slow-moving cold front should reach the CMA on Sunday. The global models generate widespread clouds and precipitation ahead of the front, which should return ozone to the Good range for most locations. Particles may linger in the Moderate range, depending on how quickly the air mass changes. On Monday, the GFS favors clear skies with onshore (northeasterly) surface winds around strong surface high pressure over southern ON, while the EC keeps the front stalled over the CMA for continued clouds/rain. Either solution promotes generally Good air quality, with the GFS solution leading to higher chances for a quicker return to Moderate ozone beyond the end of the medium range period.

-Huff