Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Friday, May 30, 2014
Valid: May 31 – June 4, 2014 (Saturday-Wednesday)
Summary:
Ozone and PM2.5 will be in the Good range for Saturday and Sunday at most locations due to a clean and dry air mass over the region. Temperatures and dew points will increase beginning on Monday as an upper level ridge moves overhead. Ozone and PM2.5 concentrations will rise into the Moderate rage at scattered locations on Monday, mainly along the I-95 Corridor. Uncertainty remains in the forecast for Tuesday and Wednesday, with scattered to widespread Moderate PM2.5 likely and ozone in the Good to Moderate range depending on the chances for clouds, rain, and convection associated with a developing low pressure system over the Great Lakes/southeastern Canada and an advancing cold front from the northwest.
Discussion:
The weather forecast models have come into consensus today regarding an area of low pressure developing over the northern Plains on Monday. On Saturday, the western periphery of a deepening upper level trough over southeastern Canada and its coastal waters will extend over the Mid-Atlantic. This trough will begin to retreat to the east on Sunday as the persistent Midwestern upper level ridge extends over the eastern U.S. On Monday, the axis of the ridge aloft will be over the Mid-Atlantic. At the same time, shortwave energy will close off in the upper level flow over the northern Plains. The 00Z ECMWF is about 3 hours slower than the 12Z NAM and 06Z GFS with closing off the low, but all three operational model runs put the shortwave in roughly the same location over MN by 00Z Tuesday. At the mid-levels, the GFS is slightly faster (3-6 hours) than the ECMWF to build in warmer air over the Mid-Atlantic on Monday as the upper level ridge extends overhead. The closed low aloft will slowly move to the northeast on Tuesday and Wednesday, reaching approximately southeastern QC by 00Z Thursday. The ECMWF develops a more intense upper level disturbance than the GFS on Wednesday at the Canadian border, which leads to uncertainty in the precipitation forecast for Wednesday.
Saturday will be a beautiful sunny day with scattered clouds. The region will be dominated by a cool and dry Canadian high pressure system, resulting in calm weather. Cool air aloft will promote mixing of the boundary layer, and northerly flow from Canada will also clean out the region. As a result, generally Good ozone and PM2.5 conditions are expected across the region. The exception may be the southern Mid-Atlantic, where clouds and rain will linger and PM2.5 will likely at least start the day in the Moderate range.
The center of the Canadian high pressure system will move over the Mid-Atlantic region on Sunday. The high pressure overhead will result in excellent weather conditions leaving little to no chance for precipitation all day. Although temperatures will rise slightly from Saturday, dew point temperatures will stay relatively low, resulting in low humidity again for Sunday. Winds will be light all day, which will allow ozone and PM2.5 concentrations to increase, but they should remain in the Good range. The BAMS-CMAQ air quality model develops an area of Moderate PM2.5 over interior sections of the northern Mid-Atlantic, but with such a dry air mass in place, Good conditions seem more likely.
The center of surface high pressure will settle over the southern Mid-Atlantic/Southeast U.S. on Monday and remain there through the end of the medium range period. This will set up a return flow, causing temperatures and dew points to increase. Air aloft will originate from over New England but recirculate around the I-95 Corridor, which will promote rising pollutant concentrations. As the upper level ridge moves overhead, warm air advection will create stagnant conditions and a relatively low boundary layer. There is still some uncertainty in the models regarding the timing of the arrival of warm air aloft over the region; the GFS moves the warm air in slightly faster, which would lead to a faster increase in pollutant concentrations. Ozone and PM2.5 concentrations will both rise into the Moderate range in scattered locations, particularly in the northern Mid-Atlantic along the I-95 Corridor.
Although the weather models have come into agreement regarding the timing and track of the developing area of low pressure over the Great Lakes and southeastern Canada, there is still uncertainty in the air quality forecast for Tuesday and Wednesday. The advancing closed low aloft will flatten the ridge over the Mid-Atlantic. Flow aloft will be from the west/southwest, with light south/southwesterly winds at the surface, which will favor rising pollutant concentrations. It will be warm and humid both days, which increases the chances for convection, however. Both the GFS and ECMWF develop widespread precipitation across the northern and western Mid-Atlantic on Tuesday. With the low lifting north and east of the region on Wednesday, the models have less precipitation over the Mid-Atlantic, but there chance remains for scattered showers and thunderstorms, particularly with the ECMWF solution, which has a more intense closed low than the GFS. A cold front will approach the region from the west on Wednesday, which will act as a focus for convection. Rising humidity and a modifying air mass suggest that PM2.5 will reach the Moderate range at widespread locations on Tuesday and Wednesday. Areas that do see heavy rain or thunderstorms will observe lower PM2.5 levels. Ozone will reach the Moderate range in locations that do not experience afternoon cloud cover or heavy convection.
-Catena/Huff