Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 30, 2015
Valid: July 31 – August 4, 2015 (Friday – Tuesday)

Summary:

Generally Good air quality is anticipated for the medium range period behind today’s cold front. The base of a virtually stationary upper level trough will remain just over the northern Mid-Atlantic, maintaining a zonal flow over the region. This will allow a series of weak cold fronts to move eastward through the region and subsequently stall along the Atlantic coast. These boundaries will allow surface westerlies to prevail through the period, placing the areas of most likely Moderate ozone east of the I-95 Corridor and along the VA coast near the stalled fronts. Relatively consistent back trajectories from areas surrounding the Great Lakes will help to limit overall rising ozone to the Moderate range, with the highest ozone during the period expected on Monday. PM2.5 will follow a similar pattern, with any Moderate concentrations being limited to coastal areas near the stalled frontal boundaries. The next possible substantial air mass change will be on Tuesday, when the next more substantial cold front will arrive in the Mid-Atlantic.

Discussion:

The weather models are in generally good consensus throughout most the medium range period, but begin to diverge on Tuesday. The 06Z GFS and 00Z ECMWF were consulted for this analysis. Currently, a large closed low aloft over the Hudson Bay is digging an upper level trough southward and eastward towards the Mid-Atlantic, forcing a small ridge out of ME and out to sea. The closed low aloft will remain virtually stationary through Monday, with only changes in the trough characteristics impacting Mid-Atlantic weather. By 12Z Friday, with the closed low still over Hudson Bay, the southern edge of the trough will reach just into the central Mid-Atlantic (CMA), creating a zonal flow over most of the region. Today’s cold front will clear the northern Mid-Atlantic (NMA) and CMA by Friday morning but it will stall along the coast of NC. Behind the cold front, a wide area of surface high pressure centered in AR will spread into the Mid-Atlantic and keep skies mostly clear except for the NC coast. The base of the upper level trough will reach its most southward extent on Saturday, reaching into the Carolinas. An area of shortwave energy will skirt just north of the Mid-Atlantic in the flow overnight Saturday into Sunday morning, bringing another weak cold front southeastward through the region. As the front encounters the warm and humid air of the CMA it will slow and catch up to the stalled front along the Atlantic coast, oscillating there through the end of the period. By Sunday, the upper level trough base will lift to northern VA, promoting a zonal flow aloft and allowing another day of relatively calm weather in the Mid-Atlantic. On Monday, the trough starts to really dig towards the Northeast, with some small areas of shortwave energy moving over the NMA and CMA during the day. These shortwaves will be reflected at the surface via a surface wave over the Chesapeake Bay, established along Sunday morning’s stalled front. Model discrepancies become apparent in the upper level pattern by Tuesday morning. The GFS has a more aggressive solution regarding the next cold front that will impact the Mid-Atlantic beginning on Tuesday. The GFS amplifies the upper level trough over the eastern US and brings it much farther south, while the EC maintains the trough’s northward extent. In addition, the GFS brings a large shortwave southeastward through the NMA while the EC keeps the bulk of the energy to the north of the region. The main impact for the Mid-Atlantic is that the GFS brings a much stronger cold front completely through the region Tuesday-Wednesday, but the EC has a weaker front that stalls along the Mason-Dixon Line (MDL) on Wednesday. The WPC currently sides with the EC solution for the end of the period.

The passage of today’s cold front will make Friday a very pleasant day, with continuing temperatures into the low 90s °F but dew points around 60 °F. The front will slow and stall off shore, crossing inland near HAT, promoting cloud cover and scattered showers in coastal areas of the southern Mid-Atlantic (SMA). Clean air filtering in from the northwest behind the front in the CMA and NMA will contain any Moderate ozone to sunny areas near the remnants of the front, east of I-95, from the southernmost reaches of MD to southeastern VA. Scattered Moderate ozone is also possible in the SMA to areas just north and west of the rainy frontal impacts. These trends are reflected in the BAMS, NOAA, and NCDENR air quality models, lending confidence to this forecast. PM2.5 will be in the Good range across most of the NMA and CMA, but a swath of concentrations reaching to the Good/Moderate threshold is expected along the most humid stretch of the frontal boundary in the SMA.

Saturday will see a slight increase in humidity throughout the region as winds shift southwesterly ahead of an approaching weak cold front. A surface trough along the NJ/DE/VA coast will support convergence along the NMA coastline, promoting the buildup of emissions from the I-95 Corridor at areas east of the interstate. There are hints in the mesoscale models that the approaching front and surface trough will promote scattered unsettled conditions in the afternoon across the NMA, with some afternoon pop-up thunderstorms in particular. Aside from these possible scattered storms, Moderate ozone will likely be found along the east coast of the Mid-Atlantic due to back trajectories originating in northern IN and abundant sunshine throughout the region. PM2.5 concentrations will also respond to the precursor-rich back trajectories and I-95 emissions, with high Good to low Moderate concentrations expected near the surface trough in the coastal NMA and the stalled remnants of Thursday’s front in the coastal Carolinas.

The next weak cold front is expected to move southeastward through the entire Mid-Atlantic overnight Saturday, reaching the coast and catching up to Thursday’s stalled front by 12Z Sunday. The passage of Sunday’s front is expected to do little more than bring scattered showers and cause a slight wind shift as it passes through, acting only as a line of convergence and a source of intermittent surface waves as it lingers just offshore. The trend of areas downwind of I-95 seeing the highest ozone will continue on Sunday, placing pockets of Moderate concentrations along and east of the interstate in MD, DE, NJ, and eastern VA. Elsewhere, Sunday emissions and WI-sourced back trajectories will help to limit ozone to the Good to low Moderate range. Particles will likely continue with persistence for another day, barely reaching into the Moderate range at locations near the stalled frontal boundary in the CMA and SMA.

Monday’s weather will be similar to past days as temperatures continue to hover around 90 °F but humidity will increase slightly, with dew points reaching the mid-60s °F. Light westerly winds, mostly clear skies, and a lingering frontal boundary over the coast will increase the chances for more widespread Moderate ozone. The air aloft is expected to originate from the western Ohio River Valley, with the air at 500 m AGL showing localized recirculation. This will promote Moderate ozone in areas already rich in ozone precursors, along and downwind of the I-95 Corridor, especially in metropolitan areas. The SMA is expected to stay mostly in the Good range as afternoon clouds and showers along the frontal boundary limit daily ozone production. Just to the west of these impacts in western NC, post-frontal surface high pressure will promote pockets of possible Moderate ozone on Monday. There is a chance that additional surface waves will form along the persistent stalled boundary along the Atlantic coast, but the global models are not showing much in the way of precipitation for Monday, suggesting that any waves that form will not strongly impact the Mid-Atlantic.

Given the divergence between the GFS and EC in regard to the upper level pattern at the end of the period, there is some uncertainty for the forecast on Tuesday. But both models do bring a southeastward moving cold front into the region on Tuesday, reaching central PA by 12Z. In the following 24 hours, however, WPC analyzes the front moving only a couple hundred miles farther south and stalling along the MDL, per the EC soultion. Although there are model discrepancies regarding the extent and longevity of the resulting precipitation, there is consensus that a widespread area of clouds and at least scattered showers will impact the NMA and CMA on Tuesday. The slowing of the front and placement of the resulting clouds will allow locations in the SMA to see another day of scattered Moderate ozone and PM2.5, regardless of which solution verifies. In the NMA and CMA, ozone levels will drop to the Good range under shrouded skies. There is more uncertainty regarding the particle forecast, however, since the EC keeps the bulk of the clean air behind the front well to our north. This would also bring less rain to the region, allowing PM2.5 to spend another day in the Moderate range. If the GFS verifies, a clean air mass will reach into the NMA accompanied by a period of widespread showers, both of which would clean out particles.

– Eherts/Huff