Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 14, 2017
Valid: June 15-19, 2017 (Thursday-Monday)

Summary:

Unsettled conditions and clean onshore flow aloft will combine to keep the chance for an exceedance Slight across the Mid-Atlantic through the medium range period. A frontal boundary will be stalled across the region on Thursday. Onshore flow behind the front and precipitation along and to the west/south of this front should keep conditions generally clean on Thursday. Similar to yesterday, however, the BAMS air quality models are developing a lot more ozone than we expect based on meteorological conditions. The 06Z BAMS models have isolated USG ozone around RIC, with scattered mid to upper Moderate ozone following the line of the stalled front. This seems overdone, but it is worth consideration depending on the track of the front today and how quickly ozone cleans about in the wake of the front. Unsettled weather will spread across the region on Friday as the frontal boundary begins to lift northeastward as a warm front. Temperatures and humidity jump back up for the weekend, but southerly onshore/coastal back trajectories should limit any rising ozone in sunny locations. By Sunday, the next strong but slow moving cold front will approach the region from the west. An organized line of convection ahead of the front will reach the western Mid-Atlantic on Sunday afternoon, with scattered instability thunderstorms likely at eastern locations. The front will progress through the region on Monday, with organized convection following suit, reaching the I-95 Corridor by the early afternoon.
Weather Model Discussion:

The weather models are hand in hand agreement on the placement and timing of major atmospheric features for the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. A back door cold front that was expected to reach northern VA by 12Z today (Wednesday) stalled across central PA around 04Z where it remains now (12Z Wednesday). The front is expected to make its way southward today, reaching roughly southeastern VA by 00Z. By 12Z Thursday, shortwaves will interact with this boundary forming a wave of surface low pressure of the VA/NC coast. The boundary will extend from the VA/NC coast to the northwest through western PA and into the Great Lakes. At 500 mb, an upper level ridge will continue to be pinched between two upper level troughs; one to the east over the Canadian Maritimes and the other to the west over MB. This ridge will move eastward and over the Mid-Atlantic on Thursday. As it does, it will begin to flatten as the trough to the east pulls farther away, and the MB trough depresses the western edge. By 12Z Friday, the trough to the west will slowly spin eastward, with its closed center over western ON. As the eastern trough pulls away to the northeast and the ON closed low moves eastward, the frontal boundary draped across the Mid-Atlantic will begin to move northeastward very slowly as a warm front, with the leading edge reaching central PA by Friday night. On Saturday, shortwaves moving in the northern stream flow, along the U.S./Canadian border from the west, will amplify the upper level trough over the upper Great Plains. The trough will quickly deepen and progress eastward, with its axis moving over the western Great Lakes by 06Z Sunday. As the upper level trough amplifies, it will develop a surface low in the western Great Lakes, placing the Mid-Atlantic in the warm sector of this approaching storm system by 12Z Saturday. The center of the trough will close off over the Great Lakes by 18Z Sunday, with the trough covering much of the upper Midwest. The associated surface low will follow the center of upper level circulation as it moves to the northeast over northern ON/QC by 12Z Monday, pulling a cold front to the western edge of the Mid-Atlantic. At this time, the front is expected to push well eastward into the Mid-Atlantic by 00Z Tuesday. The GFS and EC have slight timing differences, which is typical, especially this far out from the frontal passage.

The Dailies:

The frontal boundary across the western NMA will bring increasing clouds and precipitation throughout the day, whereas in the SMA a surface trough extending along the Appalachian Mountains will trigger scattered showers with isolated thunderstorms in the late afternoon, likely enhanced by the nearby frontal boundary. The 09Z SREF has a high probability of precipitation for locations roughly along and west of I-81. Eastward of the precipitation, including much of the I-95 Corridor, skies are expected to remain dry due to a weak overhead high pressure centered in QC/ME. It will be partly cloudy, however, due to steady onshore surface winds and strong onshore flow aloft. Similar to yesterday, the BAMS air quality models are developing a lot more ozone than we expect based on meteorological conditions. The 06Z BAMS models have isolated USG ozone around RIC, with scattered mid to upper Moderate ozone following the line of the stalled front. This seems overdone, but it’s something to consider, depending on how quickly conditions clean out today behind the front. Despite this uncertainty, the risk of an exceedance will lower to Slight for Thursday, given the widespread precipitation expected along the western and southern Mid-Atlantic, with onshore flow in the northeast/I-95 Corridor.

As the frontal boundary begins to slowly creep northeastward as a warm front on Friday, clouds and precipitation will follow. The 09Z SREF has a high probability of precipitation for most of the region, with the highest along and east of I-95. A passing weak shortwave will bring unsettled conditions to the eastern NMA with scattered showers developing in the evening hours. Continued onshore flow along with increasing clouds throughout the day will keep conditions clean in the NMA. In the SMA, clearing through the afternoon and dew points in the upper 60s °F will trigger afternoon/evening thunderstorms. The 06Z BAMS air quality models continue to keep ozone in the Moderate range at inland locations, especially across the SMA. At this time we think that the BAMS is not picking up on convection that is likely to occur across the SMA, and is instead reacting to slow back trajectories. Risk of an exceedance will be Slight for the Mid-Atlantic on Friday.

The Mid-Atlantic will be in the warm sector on Saturday as the next storm system develops over the Great Lakes. Temperatures will rise slightly above normal as flow at the surface and aloft shifts southerly and strengthens. Back trajectories turn southerly, but they run along the Atlantic coast, keeping a clean flow of maritime air into the region. Morning clouds will clear in the early afternoon, but daytime heating and dew points nearing 70 °F in the CMA and SMA will allow afternoon/evening thunderstorms to develop. These storms are expected to be scattered in nature so there may be sufficient sunlight available for ozone formation at some locations. As expected with slightly above normal temperatures and clear skies, the 06Z air quality models are developing areas of Moderate ozone across the region, particularly near densely populated areas. The BAMS models are showing an area of upper Moderate/USG in southeastern PA/MD, but there is no supporting evidence to be found for this feature, so we are discounting it for now, especially in light of very clean back trajectories. The risk of an exceedance will remain Slight for another day.

A strong but slow moving cold front will approach the Mid-Atlantic from the west on Sunday. Convective showers and thunderstorms are expected to form ahead of the front, moving from west to east, starting around 18Z Sunday. This convection will not make it much farther east than I-81. However, persistent southerly flow across the region will allow for another day of above average temperatures and dew points near 70 °F, making scattered afternoon convection likely in the eastern portions of the region. The flow aloft will remain predominantly southerly, keeping back trajectories clean. A combination of onshore flow in the NMA and southerly flow in the SMA, along with clean back trajectories, will keep the risk of an exceedance Slight for Sunday.

The cold front will be passing through most of the Mid-Atlantic on Monday. There are some discrepancies between the GFS and EC in the timing of the front. Regardless of the timing, widespread clouds and precipitation are expected across the entire Mid-Atlantic, moving from west to east. Precipitation will start around 00Z Monday in the western Mid-Atlantic and reach the Atlantic coast by 00Z Tuesday. This line of strong convection will allow a Slight risk of an exceedance to persist for Monday.

-Enlow/Huff