Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 23, 2017
Valid: May 24-28, 2017 (Wednesday-Sunday)

Summary:

Unsettled weather, and thus mostly Good air quality, is again the rule for the medium range period. A series of fronts and associated lows will impact the region, with a strong upper level trough dominating through early Friday. There will be a brief period of clearing across the northern Mid-Atlantic on Wednesday, before heavy rain and possibly thunderstorms move through on Thursday. A mid-level ridge will attempt to build northward into the region on Friday and Saturday, leading to clearing skies from south to north, especially on Friday. The weather models are now showing scattered precipitation across central and southern parts of the region on Saturday, however, due to an advancing warm front associated with the next low. This next system will bring more widespread clouds and precipitation to the region on Sunday.

Discussion:
The weather models are in agreement with placement and timing of key features until late in the medium range forecast period. Models that were analyzed for this forecast were the 06Z GFS and NAM and the 00Z ECMWF. The vorticity maximum at the center of the upper level trough dominating the central and eastern CONUS has split into two vorticity lobes as expected. The main lobe has dropped south and will continue to strengthen the upper level trough and allow it to expand south and east over the central and eastern CONUS. By 12Z Wednesday, the center of this trough will be located over the Central Plains. At the same time, a new vort max/closed low moving into southern BC/AB will continue to slowly progress eastward. A weak ridge will build over the Great Plains between these two troughs. The eastern trough will contract and move farther eastward on Thursday, with the center of the circulation located over the central Mid-Atlantic/Ohio River Valley at 00Z Friday. This trough will continue to contract quickly on Friday, moving over New England while the weak ridge flattens and progresses eastward, with its axis reaching the Great Lakes during the day. At the same time, the western Canadian closed low will continue to slowly progress eastward, reaching SK/MB by 00Z Saturday. The upper level ridge will weaken as it moves over the Mid-Atlantic on Saturday, with the GFS keeping the ridge slightly stronger than the EC. The models disagree slightly on the strength and progression of the Canadian closed low on Sunday. The EC keeps the low over ON, whereas the GFS weakens the low and drops it down farther south into the northern Plains/western Great Lakes. These differences translate into slightly different strengths and positions of a developing low pressure system that will affect the region beginning on Sunday.

Weak high pressure centered over eastern Pennsylvania on Wednesday morning will allow areas in the northern Mid-Atlantic (NMA) to see breaks in clouds for most of the day, with only a chance for precipitation. Despite the mostly sunny skies in the afternoon, ozone is not expected to rise past the Good range due to a very clean air mass in place. In the southern Mid-Atlantic (SMA), weather conditions will be similar to Tuesday, with mostly cloudy skies and precipitation due to a nearby warm front and its associated surface low. Good air quality will persist.

On Thursday, a double-barrel low will progress northeastward and pull a cold front across the region. The heaviest rain and possibly thunderstorms are expected in the early morning hours across the NMA, giving way to more scattered showers in the late afternoon through evening. As the storm moves away from the region, some clearing is expected across the SMA. Both ozone and particles will be in the lower end of the Good range across the region.

Drier conditions are expected for most of the Mid-Atlantic region on Friday due to the weak upper level ridge building over the Ohio River Valley and a mid-level ridge building northward from the Southeast. Friday, at this point, looks like the sunniest day of the week, especially for the central and southern parts of the region. There is a chance for lingering clouds and wrap-around showers in the NMA due to the departing low. Breezy winds and a likely clean air mass will keep pollutants relatively low, with a chance for scattered Moderate ozone in the SMA.

Saturday will bring mostly clear conditions for the Mid-Atlantic as the weak upper level ridge moves overhead. There are some differences in today’s model runs, however, which are pointing toward more unsettled weather for the central and southern parts of the region. A warm front from the next low pressure system will lift into the Mid-Atlantic from the west, and a short wave moving through aloft may trigger scattered clouds and rain in the afternoon. This is different from yesterday, when the models were featuring widespread clearing across the region. Despite the uncertainty in the precipitation forecast, continued Good to low Moderate conditions are expected.

As mentioned in the model discussion, there is some uncertainty regarding the precipitation forecast for Sunday as well, due to questions about the strength and location of the upper level closed low over the upper Mid-West. Regardless, the trend is toward another round of pre-frontal clouds and precipitation due to a developing low pressure system to our west. Persistent Good air quality is expected.

-Enlow/Huff