Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 24, 2015
Valid: June 25-29, 2015 (Thursday – Monday)

Summary:

Chances for USG ozone remain slight for the medium range period as an anomalously deep longwave trough over the eastern US will bring cooler conditions, widespread clouds, and precipitation to the Mid-Atlantic region. The exception will be Thursday and Friday, where locations south of a stalled frontal boundary in NC will continue to be impacted by Saharan dust and regional smoke, keeping PM2.5 in the Moderate range. In the northern and central parts of the region, cloud cover and pulses of precipitation associated with an oscillating frontal boundary will keep air quality in the mostly Good range for Thursday and Friday. The weather models have come into close consensus on the precipitation forecast for the weekend. A strong wave of low pressure will form over the Tennessee River Valley on Saturday and move northeastward, promoting widespread clouds and rain showers, and keeping air quality in the Good range. Skies will gradually clear Sunday into Monday, but ample vertical mixing and onshore flow will promote generally Good air quality.

Discussion:

The weather models are in fairly close agreement on the overall synoptic features of the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. WPC recommends a blend of the GFS and EC guidance regarding the anomalous upper level longwave trough that will be developing over the eastern US; as a result, we relied mainly on these two models for this analysis. A frontal boundary is currently draped east to west across southern VA and extends back into the Tennessee River Valley (TRV). This front will oscillate between NC and the Mason-Dixon Line (MDL) through Saturday. On Thursday, a closed low over southeastern QC and a series of small but strong shortwaves over northern Plains and Great Lakes will begin to develop a longwave trough that will extend over the eastern US. The strong shortwaves will trigger a surface wave early Thursday morning along the stalled frontal boundary over IL. This wave will pull the front in the Mid-Atlantic northward to the MDL. On Friday, the upper level trough will continue to rapidly amplify over the eastern US, while an equally strong upper level ridge builds over the western US. By Saturday, the eastern US trough will be anomalously amplified, reaching southward to the Gulf Coast, with the axis along the Great Lakes/Mississippi River Valley. As the longwave trough becomes anomalously strong on Saturday, a wave of low pressure will form along the frontal boundary in the TRV. The EC and GFS have come into closer agreement, compared to previous runs, on the movement of this wave of low pressure. This wave will move northeastward into the northern Mid-Atlantic (NMA) Saturday afternoon into Sunday morning. By 06Z Sunday, the global models develop a closed low at 500 mb over OH at the center of the trough. This 500 mb closed low will strengthen the surface wave as they move northeastward during the day on Sunday. By early Monday, they will become vertically stacked and weaken as they reach eastern QC around 18Z Monday. At this time, a shallow upper level ridge and trough with its axis over the Appalachian Mountains and Mississippi River, respectively, will move eastward into the Mid-Atlantic. The arrival of additional shortwave energy dropping down from Canada will allow the longwave trough to persist over the eastern US beyond the end of the medium range period.

As high pressure moves off the NJ coast on Thursday morning, a wave of low pressure approaching from the Ohio River Valley (ORV) will help to drag a frontal boundary northward from the Carolinas. There is still some question about the exact track of the wave; this morning’s mesoscale model runs show a more northerly track, through primarily PA and NJ. As a result, rain showers associated with this low pressure center will mainly impact the northern Mid-Atlantic (NMA), moving west to east across PA and MD starting around 18Z Thursday. Some convection is expected along the frontal boundary as a small short wave passes overhead, which would strengthen any showers in southeastern PA/eastern MD/NJ into afternoon thunderstorms. Cloud cover will proceed the arrival of the surface wave, reaching the I-95 Corridor by early afternoon. These clouds and showers will limit ozone to the Good range for most of the region. Areas south of the front that observe clear skies may see isolated Moderate ozone, such as western NC. Calm and converging winds in the vicinity of the frontal boundary will promote the buildup of PM2.5 in the morning, ahead of the precipitation. But, the arrival of the rain and possible thunderstorms will limit any rising PM2.5 to the Good to low Moderate range for the daily average. The exception will be NC, where the influence of smoke from local and regional wildfires and Saharan dust is keeping PM2.5 squarely in the Moderate range this morning, and this trend will continue through Friday.

On Friday, the frontal boundary will move back southward as a cold front as the surface wave of low pressure exits off the NJ coast. As this front continues southward towards the Carolinas throughout the day Friday, weak surface high pressure will build in behind this front in the NMA and help to decrease cloud cover. However, another low pressure center associated will a strengthening upper level trough over the Great Lakes will be making its way towards the TRV along the frontal boundary. This feature will bring cloud cover to most of the Mid-Atlantic and scattered showers to areas south of the Delmarva throughout the afternoon and overnight hours, diminishing chances for substantial ozone or PM2.5 production on Friday. Although there will be clearing in the northern part of the region, mainly across PA and NJ, Thursday’s rain will have cleaned out pollutant precursors from the atmosphere, and northerly winds will help to limit any rising ozone or PM2.5 concentrations.

As the upper level trough continues to amplify and dig into the southeastern US on Saturday, the TRV low pressure center will strengthen significantly. Much like a wintery low pressure system, it will promote a comma-shaped area of widespread precipitation in the vicinity of its center of rotation. Its path is expected to be a northeastward one, continuing the widespread rainfall in the central and southern Mid-Atlantic (CMA, SMA) and moving into the NMA during the early afternoon. Due to the large scale cloud coverage, lower temperatures, and brisk surface winds associated with this system, air quality throughout the Mid-Atlantic will stay in the Good range.

Skies will gradually clear Sunday into Monday, but with a pool of cool air aloft promoting vertical mixing, as well as onshore flow, Good air quality will likely be the rule. As the anomalously large low pressure system continues northeastward into New England, Sunday will be a day of slow clearing throughout the Mid-Atlantic. The cold front associated with this system will extend southward along the southeastern coast, keeping the chances for scattered clouds and pop-up showers along the eastern seaboard. Inland, showers will steadily dissipate throughout the day from south to north, allowing clear skies to finally dominate the east coast by Monday morning. As the upper level trough slowly lifts northward out of the southeastern US, another shortwave in its base will enhance a surface low pressure system to the north of the Great Lakes on Monday. Its southward-extending cold front will move into the NMA late Monday night, bringing another round of precipitation and a new air mass to the region on Tuesday.

-Huff/ Eherts/DeBoe