Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, August 8, 2015
Valid: August 9-13, 2015 (Sunday – Thursday)

Summary:

Overall Good air quality is expected throughout most of the medium range period as a strong cold front approaches on Monday and passes through the region on Tuesday. Mostly sunny skies and light northeasterly surface winds on Sunday will allow for Moderate ozone at the usual areas along and downwind (west) of the I-95 Corridor and in southwestern PA. Although lower Sunday emissions should keep rising ozone in the Moderate range, there is a Marginal chance for isolated USG ozone just to the west of I-95; as we saw yesterday, light northeasterly surface winds can promote isolated pockets of locally high ozone production in the afternoon. Particles will linger in the Moderate range in the western Mid-Atlantic, where easterly winds will be the lightest and humidity will be the highest. Monday will be an increasingly cloudy and breezy day as a warm front lifts northward, with the bulk of the pre-frontal rain sweeping eastward through the region on Tuesday. A seasonably cool, dry, and clean air mass is expected to arrive behind the front for Wednesday and Thursday, for Good air quality.

Discussion:

The weather models have come into closer agreement since yesterday, and are in very close consensus throughout the medium range period. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large upper level ridge is well established with its axis through the Mississippi River Valley, and its northeasterly edge is reaching into the Mid-Atlantic. The ridge will swiftly recoil back out of the Mid-Atlantic as areas of disorganized shortwave energy crest the ridge and impact the Northeast, keeping scattered clouds and showers in the forecast for Sunday. Large shortwaves over northern ON will begin to dig out a large upper level trough on Sunday afternoon, reflected at the surface via a widespread area of low pressure. As this low pressure gradient strengthens and moves eastward, it will establish an extensive frontal boundary that will stretch from the James Bay to IA by Monday morning. Clusters of small short waves in the base of the trough will promote some cloud cover and showers in the Mid-Atlantic on Monday as a warm front moves northward at the surface. More widespread, heavy, pre-frontal showers will begin later in the day as the cold front associated with the Canadian low approaches. As the trough slowly moves eastward, shortwave energy in its base will continue to promote widespread clouds and rain along the frontal boundary on the east coast through Wednesday morning. Skies will clear quickly behind the front on Wednesday as cooler, drier Canadian air filters into the region on northwesterly winds.

Sunday will be a day of scattered cloud cover throughout the Mid-Atlantic as an unsettled area of dilute shortwaves moves overhead, with the thickest clouds expected along a north-south axis through central PA to central VA. The high-res weather models are resolving areas of light scattered rain showers in areas of central PA, MD, and VA later in the day. The usual areas of concern along and downwind of the I-95 Corridor and in southwestern PA will continue to see mostly sunny skies, however, and therefore Moderate ozone. Light easterly surface winds will push the bulk of the emissions just to the west of I-95, placing the areas of highest ozone around the DC and Baltimore metropolitan areas, which is supported by the air quality models. Due to the lower Sunday emissions, ozone is not expected to rise higher than the Moderate range, although there is a Marginal chance for isolated USG ozone just west of I-95, given the third day in a row of light northeasterly surface winds and mostly sunny skies. Particles will follow a similar pattern, with Moderate concentrations likely in the western part of the Mid-Atlantic due to increasing humidity and lighter inland winds.

A southerly wind shift on Monday as a warm front lifts northward through the region ahead of an approaching cold front will increase humidity, and therefore PM2.5, west of the I-95 Corridor. Ozone, on the other hand, will be inhibited due to mostly cloudy skies moving from west to east. Along and east of I-95, where the clouds will reach last, the southerly winds should be sufficiently breezy and onshore to hinder ozone formation and buildup. Faster, northerly back trajectories lend confidence to the Good ozone forecast.

The cold front moving through the region on Tuesday is substantial, and will generate widespread clouds and precipitation. Despite analyzed areas of Moderate ozone in the air quality models in southern MD and the surrounding areas on Tuesday, the long-lasting and widespread nature of the clouds and precipitation suggests this guidance is overdone. The rain is expected to move gradually westward across the region starting late Monday night and continue through early Wednesday morning. This leaves little opportunity for ozone to form, especially considering the onshore looping back trajectories. In areas where only light rain falls, the high pre-frontal humidity will allow pockets of Moderate PM2.5 to form, particularly in the western Mid-Atlantic.

Wednesday will be a calm and sunny post-frontal day in the CMA and SMA, while clouds may return to the NMA after a brief period of morning sun. The Canadian air mass behind the front is expected to be cool, dry, and clean, limiting ozone to the Good range across the region. By 12Z Wednesday, the southern extent of the cold front is expected to be draped over the eastern Carolinas. Along and just behind this frontal boundary, ozone will be limited by the clouds, while particles will linger in the Moderate range until the front fully clears the region.

By Thursday, the upper level trough will dip to the FL/GA border, with the ridge in the west reaching well into AB. A large area of high pressure will build eastward into the Mid-Atlantic, with abundantly clear skies expected over the entire region. Despite this, air quality likely will rise just barely into the Moderate range – if at all – due to the continuing clean air mass, dry air, below average temperatures, and fast back trajectories from the James Bay.

– Eherts/Huff