Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 18, 2015
Valid: June 19-23, 2015 (Friday – Tuesday)

Summary:

A persistent unsettled weather pattern will continue through the medium range period, keeping the chances for USG ozone slight. The passage of weak cold fronts on Friday and Monday and the arrival of the remnants of TD Bill overnight Saturday through Sunday afternoon will keep clouds, rain, and convection in the forecast for much of the first part of the period. Morning and early afternoon sun in the northern Mid-Atlantic on Saturday may allow for isolated Moderate ozone. Compared to previous analyses, by the time the remnants of Bill reach the Mid-Atlantic, they will exist only as a surface wave, making widespread subsidence ahead of the system on Saturday unlikely. Post-frontal clearing on Monday and Tuesday may permit more scattered Moderate ozone at the end of the period.

Discussion:

The weather models are in generally good agreement for the beginning of the medium range period, but they diverge regarding the southward extent of a cold front arriving on Monday. The 06Z NAM, 06Z GFS, 00Z EC, and 00Z CMC were consulted for this analysis, with an emphasis on the NAM, GFS, and EC. An upper level ridge centered over the southeastern US will gradually weaken on Friday as TD Bill continues moving northeastward around the crest of the ridge. The weather models are in close consensus with placing TD Bill over the MO/AR border on Friday and shearing off short waves in the upper level flow towards the Mid-Atlantic. In addition, a shallow upper level trough currently over central ON will move eastward into the northeastern US, dragging a cold front into the northern Mid-Atlantic (NMA) from the northwest on Friday morning. A weak mid-level ridge arriving behind this cold front will help to advance this cold front southward to the VA/NC border by 12Z Saturday. The weather models agree on moving the remnants of TD Bill into the Ohio River Valley (ORV) by approximately 18Z Saturday. By Saturday evening, the forecast models weaken Bill to more of a surface wave. 700 mb vertical velocity guidance shows essentially no subsiding air over the Mid-Atlantic on Saturday ahead of Bill’s arrival. In addition, back trajectories for Saturday no longer have much subsidence either. As a result, widespread subsidence ahead of Bill’s remnants no longer looks like a major concern for the Mid-Atlantic on Saturday. Widespread clouds and heavy rain are expected on Sunday as the remnants of TD Bill move over the Mid-Atlantic and out to sea Sunday night. By Monday, the GFS and EC agree to move a shallow trough eastward into the northeastern US, dragging another cold front into the NMA. At this point, the GFS and EC diverge. The GFS brings this front into northern PA on Monday and quickly pushes it back northeast as a warm front. In contrast, the EC brings the cold front to the Mason-Dixon Line by 12Z Monday and essentially stalls it there into Tuesday as an upper level ridge in the southwestern US expands and connects with the southeastern US ridge, covering the entire southern US. The CMC bring the cold front even further south than the EC. We have seen these discrepancies among the global models with almost every recent weak front, but the models have been coming into agreement 1-2 days before the actual frontal passages.

Friday looks less like a day of interest due to the early arrival of the cold front, which will shift surface winds to the northwest in the morning. The cold front will start moving through PA tonight, northwest to southeast, promoting overnight and early morning convection on Friday along its axis. Scattered showers and widespread cloud cover will persist throughout the day through southern PA, MD, and VA as the front continues southeastward. Frontal passage through the eastern shore of MD and DE will increase cloud cover in the Delmarva and intensify surface winds in the northeastern Mid-Atlantic for the afternoon. Skies will clear behind the front as high pressure builds into the northern Mid-Atlantic (NMA), helping to disperse clouds in central and northern PA throughout the evening. The passage of a cold front, cloudy skies, the chance for showers, and relatively fast back trajectories from IA/MO will inhibit ozone and PM2.5 formation and keep concentrations of both in the generally Good range on Friday.

Saturday is one of the days in the medium range period when ozone has the chance to reach the Moderate range as high pressure persists in the NMA. The main forecast question will be how clean the air is behind Friday’s frontal passage. A brief surge of slightly cooler and drier air overnight into Saturday morning looks likely. Surface high pressure will help to suppress clouds in the NMA, while the frontal boundary in the central Mid-Atlantic (CMA) will act as a focus for cloud formation along and south of the PA/MD border. The continuing ridge over the southeastern US will prevent yet another front from exiting through the southern Mid-Atlantic (SMA), pushing the boundary back north towards the Mason-Dixon Line (MDL) as a warm front Saturday afternoon. Clouds associated with this front, as well as those as a result of tropical moisture and heat from Bill, will build throughout the entire Mid-Atlantic on Saturday night. Depending on the amount afternoon sunshine in the NMA, the strength of convergence along the warm front, and the air mass characteristics, Saturday will most likely see ozone and PM2.5 concentrations in the upper Good range with isolated locations along the I-95 Corridor possibly reaching the Moderate range.

As the remnants of TD Bill move over the Mid-Atlantic overnight Saturday through Sunday afternoon, the entire region can expect overcast skies and widespread rainfall. Additionally, the front with nowhere to go will be stalled in the vicinity of the MDL, providing a focus for increased rainfall along the track of Bills’ remnants, primarily across southern PA, MD, DE, and NJ. These factors, along with breezy winds and the clean tropical air mass, will make for a below average air quality day with ozone and PM2.5 concentrations well within the Good range.

Monday is also a potentially interesting air quality day, due to the fact that it will likely be the sunniest day of the period. The EC and CMC agree that the next cold front will approach from the northwest and move southeastward through PA, slowing in southern PA around 18Z, and stalling in the vicinity of the MDL soon after. The 13km GFS as well as the global models are showing no convection or cloud cover along this front, making it a prime area for converging winds under clear skies to cause increased ozone and PM2.5 concentrations. However, the rain from Sunday along with the clean tropical and post-frontal air masses in place will probably keep ozone levels in the high Good/low Moderate range. The models agree that a period of rain will move eastward through the Mid-Atlantic overnight Monday into early Tuesday morning, but have yet to reach a consensus on when the rain will reach the area. Solutions regarding when the heaviest rainfall will start range from 00Z – 12Z, any of which will help to clean out the atmosphere and make for a good air quality day on Tuesday.

-Huff/ DeBoe/Eherts