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

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 22, 2017
Valid: May 23-27, 2017 (Tuesday-Saturday)

Summary:
It will be a quiet week for air quality across the Mid-Atlantic region courtesy of a strong, winter-like upper level low that will dominate the weather for most of the medium range period. Good air quality will be the rule, with a chance for isolated low Moderate ozone on Wednesday (Northern Mid-Atlantic) and on Friday/Saturday (Southern Mid-Atlantic). The weather will be unsettled, with clouds and rain possible every day through Friday. The upper level low will expand eastward on Tuesday and Wednesday and then elongate over east coast on Thursday. This feature will allow a strong low to form at the surface on Wednesday, which will pull a slow-moving cold front through the Mid-Atlantic late Wednesday through Thursday evening. By Friday, the upper level trough will have contracted into New England, allowing a Southeastern ridge to push northward. This in turn will allow skies to clear and temperatures to rebound to around or slightly above average for the first part of the holiday weekend.

-Enlow/Huff

Discussion:
For the medium range forecast, models are in close agreement with the major features but have some disagreements with placement later in the forecast period. Models that were analyzed for this forecast were the 06Z GFS and NAM and the 00Z ECMWF. The period begins with a strong upper level trough across most of the CONUS, with a closed low at the center of its circulation, centered over the western Great Lakes/southern ON. On Tuesday, the closed low’s vorticity will split into two lobes, with one lobe moving way to join a trough in east/central Canada, while the other lobe sinks southward and reinforces the Eastern U.S. upper level trough. The trough will strengthen on Wednesday and move south- and eastward. At the same time, a new upper level closed low will move into the Pacific Northwest/southern Canadian Prairies. A weak ridge will form between these two upper level lows on Thursday. There are some minor discrepancies between the GFS and EC versus the NAM on Wednesday and Thursday regarding the strength and shape of the eastern trough, but overall pattern is similar. We focus on the EC and GFS, which have a broad closed low stretching over the entire eastern U.S. on Thursday – more of a winter-like than summer-like pattern. By Friday, the trough quickly contracts, with the main circulation moving northeastward into New England and the weak ridge progressing into Great Lakes region before reaching the Mid-Atlantic on Sunday.

The slow-moving cold front will reach the I-95 Corridor early Tuesday morning, bringing precipitation that could linger throughout the day. The front will clear the Northern Mid-Atlantic (NMA) but stall across the Southern Mid-Atlantic (SMA). There is some model disagreement as to exactly where and when precipitation associated with the front will occur, but it is most likely across the SMA, following the track of the front. Regardless, it will be a mostly cloudy and unsettled day across the region. Pollutant concentrations are currently low, and there is clean air behind the cold front. Therefore, Good air quality is expected.

The big story on Wednesday is the strong low forming over the central Mississippi River Valley. This low will strengthen quickly and gradually progress northeastward, pulling along a cold front that will bring clouds and periods of heavy rain to the western and southern portions of the region. Skies should remain clear along the northern I-95 Corridor, however, which will allow temperatures to briefly rebound to around average. The 06Z air quality models respond to this clearing and develop an area of Moderate ozone along the Mason-Dixon Line (MDL) and northern extent of I-95. This seems a bit overdone, given the likely very clean air mass in place, but certainly scattered Moderate ozone is possible. Elsewhere, the clouds and rain will limit ozone and particles to the Good range.

The slow-moving cold front will continue its progression through the Mid-Atlantic on Thursday. The system will bring periods of heavy rain to the region. Precipitation is most likely during the morning and afternoon, with rain tapering off in the evening as the low pulls away toward the Northeast. This system will ensure another day of Good air quality.

Skies will clear on Friday as the low moves into New England. The Southeastern ridge will begin to push into the SMA, with clearing occurring from south to north across the region. Temperatures will begin to rise as a result, but only up to around normal for this time of year. Continued Good air quality is anticipated, with the cleanest conditions across the NMA, where the influence of the retreating low will be felt the longest.

On Saturday, the trend is for the Southeastern ridge to continue progressing northward, bringing mostly sunny skies to the entire region. The GFS is a bit stronger with the ridge, but both it and the EC show a similar placement, with the northern extent of the ridge only reaching as far north as roughly the MDL. Temperatures will continue to moderate, but they will remain around or slightly above average. As a result, only scattered Moderate ozone is expected.

-Enlow/Huff