Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 20, 2014
Valid: May 22-26, 2014 (Thursday-Monday)

Summary:

A slowly-moving cold front will lead to Good air quality for most locations on Thursday; the exception will be the southern and eastern portions of the region, where PM2.5 may linger in the Moderate range. Generally Good air quality is expected for Friday and Saturday, as a cool and dry Canadian air mass builds into the region. Temperatures will begin to rise on Sunday as an upper level ridge extends eastward over the Mid-Atlantic, leading to scattered Moderate ozone. Monday may be a transition to poor air quality, with a chance for isolated USG ozone, but significant uncertainty exists due to differences in the upper air forecast guidance.

Discussion:

The weather forecast models are in agreement on the main features of the beginning of the medium range period but diverge beginning on Sunday. A shortwave/closed low over southern ON will dig out a weak longwave trough over the eastern U.S. on Thursday. A shortwave will rotate though the axis of the trough and amplify it on Friday. At the surface, a weak cold front will slowly move through the Mid-Atlantic on Thursday and clear the southern part of the region on Friday morning. A broad upper level ridge over the Midwest will push the weak trough eastward over the Canadian Maritimes on Saturday. By Sunday, the models begin to diverge. The GFS is faster in extending the broad upper level ridge eastward, while the trough over the Canadian Maritimes retards the progression of the ridge in the ECMWF run. On Monday, the GFS spins a vort max down from interior QC that closes off and flattens the eastern extend of the ridge over the Mid-Atlantic. The ECMWF doesn’t have this feature, and instead pushes the axis of the ridge over the eastern Great Lakes. The GFS solution is wetter on Monday, while the ECMWF solution keeps the Mid-Atlantic dry. These differences impact the chances for a transition to poor air quality on Monday.

There is uncertainty regarding the speed of the cold front through the Mid-Atlantic on Thursday and Friday. Model guidance and WPC’s analysis have the front slowing down compared to yesterday’s analysis. The front is expected to move through the western and northern Mid-Atlantic by Thursday night, but not clear the southern Mid-Atlantic until Friday morning. With a humid and modified air mass in place, the speed of the front will impact the PM2.5 forecast. The air quality models are in agreement that both ozone and PM2.5 will be in the Good range for the entire region on Thursday, but it seems likely that Moderate PM2.5 will linger along the eastern and southern parts of the region, where the impact of the frontal passage will be felt latest. Scattered rain showers and thunderstorms across the region will help to clean out the atmosphere, particularly in the northern Mid-Atlantic. Ozone will be in the Good range for most locations, due to cloud cover associated with the frontal passage.

A Canadian high pressure system will be building into the region on Friday. There is a chance for scattered showers triggered by a vort max that will rotate through the axis of the upper level trough. With the arrival of a cleaner and drier air mass, air quality will be in the Good range for most locations. The exception will be the southern Mid-Atlantic, where PM2.5 may linger in the Moderate range until the slow-moving cold front clears the region and the new air mass can build in.

The center of surface high pressure will settle over the southern Great Lakes on Saturday. With seasonably cool temperatures, low humidity, and northerly flow, it will be a nice day to begin the holiday weekend. Good air quality is expected across the region.

Temperatures will begin an increasing trend on Sunday as the eastern extent of the upper level ridge moves over the Mid-Atlantic. Northwesterly flow will continue on Sunday, which will keep dew points low. Ozone will likely climb into the Moderate range at scattered locations, while PM2.5 will lag in the upper Good range at most locations in the dry air mass. Scattered Moderate PM2.5 is possible along the I-95 Corridor.

Monday may possibly be a transition to poor air quality. The westward extension of the Bermuda high will be in place, and the air mass over the region will have begun to modify. Surface winds will shift westerly, with warm air advection aloft. Temperatures will be rising into the 80s °F across the region. Although humidity will still be relatively low, due to northwesterly flow aloft, it will begin to rise later in the day. PM2.5 will likely reach the Moderate range at scattered locations, while upper Moderate ozone is likely, with the chance for isolated USG. The extent of rising ozone and PM2.5 concentrations will depend on the evolution of the upper level pattern; the GFS’s wetter solution, with a closed low over southern QC, will lead to cleaner air quality conditions, while the ECMWF solution is more conducive to poor air quality.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 20, 2014
Valid: May 21-25, 2014 (Wednesday-Sunday)

Summary:

PM2.5 will be in the Moderate range at the start of the period in a humid and modified air mass. Air quality will improve to the Good range on Thursday as a cold front moves through the region. A slowly-moving upper level trough over the eastern U.S. will keep air quality in the Good range for most of the remainder of the period, with a chance for scattered Moderate ozone on Sunday as an upper level ridge moves in from the west.
Discussion:

The weather forecast models remain in relatively close agreement on the main features of the medium range period. A broad upper level ridge will extend across the central and eastern U.S. on Wednesday, but a shortwave/closed low over southern ON will begin to flatten the ridge and dig a longwave trough over the eastern U.S. The upper level trough will become more pronounced over the eastern U.S. on Thursday, and it will pull a cold front through the Mid-Atlantic during the morning. The upper level trough will linger over the northeast through Sunday, moving slowly eastward, and a series of shortwaves will rotate through the axis of the trough, leading to chances for precipitation mainly on Friday and Saturday.

A warm front will move across the region on Wednesday. It will be warm and humid, with scattered showers and thunderstorms, mainly in the afternoon. Back trajectories are short and from the Ohio River Valley, a source region for ozone and PM2.5 precursors. Overcast skies will limit ozone to the Good range, although isolated Moderate ozone is possible in areas that see fewer clouds. PM2.5 will increase into the Moderate range at most locations in response to the rising humidity, with the highest concentrations along the I-95 Corridor.

A cold front will move across the region on Thursday morning. There are slight differences between the weather forecast models regarding the timing of the front, but the air quality models agree that air quality will be in the Good range for both ozone and PM2.5. Scattered showers and thunderstorms are likely as a series of shortwaves rotate through the axis of the upper level trough over the eastern U.S. PM2.5 may linger in the Moderate range in some locations, especially in the southern and eastern Mid-Atlantic, until the new air mass fully builds in on Friday.

The cold front will be well clear of the region on Friday, and a Canadian high pressure system will move in, bringing cooler and less humid conditions. The upper level trough will remain over the Mid-Atlantic, and a series of shortwaves will rotate through, increasing the chances for rain showers, particularly in the northern Mid-Atlantic. Good air quality is expected.

The slow-moving upper level trough will keep the chances for precipitation in the forecast for Saturday, with clear skies expected on Sunday. Saturday will be seasonably cool, but temperatures will begin to increase on Sunday ahead of an upper level ridge that will arrive from the west. Particles will likely remain in the Good range in the dry air mass, while ozone may increase into the Moderate range by Sunday.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 19, 2014
Valid: May 20-24, 2014 (Tuesday-Saturday)

Summary:

Temperature and humidity will increase on Tuesday and Wednesday as an upper level ridge briefly settles over the Mid-Atlantic region. PM2.5 will rise into the Moderate range at scattered locations on Tuesday and become widespread across the region on Wednesday. Scattered Moderate ozone is also possible on Tuesday, but ozone will be limited by cloud cover associated with a warm front on Wednesday. A cold front will move through the region on Thursday, and a cool and dry air mass will arrive in its wake, leading to Good air quality for Thursday through Saturday.

Discussion:

The weather forecast models are in close agreement on the main features of the medium range period. An upper level ridge will move eastward from the Plains on Tuesday as a closed low over New England departs. The axis of the upper level ridge will be over the Mid-Atlantic region on Wednesday, but the ridge will begin to flatten as a closed low over southwestern ON moves eastward. By Thursday, the closed low will be located over southeastern ON/southwestern QC; this feature will pull a cold front through the Mid-Atlantic early Thursday. The closed low will linger over New England and the Canadian Maritimes on Friday and Saturday, keeping a longwave trough over the East Coast and a pool of cold air aloft, which will allow for the chance of isolated showers, particularly on Saturday.

Temperatures and humidity will increase on Tuesday as surface high pressure moves south and east and a warm front approaches from the west. Surface winds will be light northwesterly for most of the day, shifting southwesterly in the late afternoon. PM2.5 will respond to the rising humidity and increase into the Moderate range at scattered locations, particularly along the I-95 Corridor. Isolated Moderate ozone is also possible, as skies will be mostly sunny, but northwesterly flow aloft and vigorous vertical mixing will limit ozone to the upper Good range in most locations.

The warm front will move across the region on Wednesday, putting the Mid-Atlantic region in the warm sector of an approaching low pressure system. It will be warmer and more humid, with southerly winds. Back trajectories are short and from the Ohio River Valley. PM2.5 will be widespread across the region, with mid-to-upper Moderate conditions likely across the I-95 Corridor. Clouds and scattered rain showers associated with the warm front will keep ozone in the Good range in most locations, but isolated Moderate ozone is possible in areas that experience afternoon sun.

A cold front will move across the region on Thursday morning. The models are in slight disagreement on the timing of the front, with the GFS the fastest, as usual. Scattered rain showers are expected in the morning associated with the frontal passage. Cloud cover will keep ozone in the Good range, while PM2.5 will linger in the Moderate range in some locations, mainly in the southern Mid-Atlantic, until the new air mass fully builds in on Friday.

Friday and Saturday will be seasonably cool and dry as Canadian high pressure arrives behind Thursday’s cold front. Northwesterly to northerly flow and vigorous vertical mixing promoted by the cool pool aloft will keep air quality in the Good range.

-Huff