Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, May 31, 2014
Valid: June 1-5, 2014 (Sunday-Thursday)

Summary:

Air quality will be in the Good range at most locations on Sunday, with the chance for isolated low Moderate ozone and PM2.5 along the I-95 Corridor. Temperatures and humidity will increase on Monday as the axis of a weak upper level ridge moves overhead. Mostly sunny skies in the eastern Mid-Atlantic will allow ozone and PM2.5 to rise into the Moderate range, while rain and convection will limit rising ozone to the low Moderate or upper Good range in the western Mid-Atlantic. Uncertainty remains in the air quality forecast for the remainder of the period due to questions about clouds, rain, and convection associated with a weak cold front that will move into the northern Mid-Atlantic on Wednesday and stall near northern VA on Thursday, but Good to Moderate conditions are expected.

Discussion:

The weather models are in relatively close agreement on the main features of the medium range period, but uncertainty remains in the air quality forecast for the end of the period related to the chances for clouds, precipitation, and convection associated with a slowly-moving frontal boundary. On Sunday, a weakening upper level ridge will approach the Mid-Atlantic from the west. At the mid-levels, a cool pool of air over the region will gradually retreat to the east as the ridge builds aloft. The axis of the upper level ridge will be over the Mid-Atlantic on Monday, while shortwave energy will close off in the upper level flow over the northern Plains (i.e., near ND/MN by 00Z Tuesday). The 12Z NAM, 06Z GFS, and 00Z ECMWF operational runs all place the shortwave/closed low in roughly the same location, but the GFS is slightly farther south and east. The closed low will flatten the ridge over the Mid-Atlantic as it moves northeast on Tuesday and Wednesday, reaching south/central ON by 00Z Wednesday and southeastern QC by 00Z Thursday, where the low will open up. This area of low pressure will slowly pull a weak cold front into the northern Mid-Atlantic on Wednesday. The front will stall west-to-east over the central Mid-Atlantic, near northern VA, on Thursday. Additional shortwave energy moving into the Mid-Atlantic from the west along zonal flow aloft will trigger precipitation along the front on Wednesday and Thursday and possibly form waves of low pressure that ride along the front.

Sunday will be sunny and warm, but with low humidity. The center of surface high pressure will move from north to south along the Mid-Atlantic coast, resulting in light winds. The BAMS and NC air quality models keep ozone in the Good range across the region, but with full late May afternoon sun, ozone may rise into the low Moderate range at scattered locations, particularly along the I-95 Corridor. The BAMS-CMAQ air quality model is consistent in developing an isolated area of Moderate PM2.5 over interior sections of the northern Mid-Atlantic, which seems possible given the slackening winds. But with a dry air mass in place, Good PM2.5 seems more likely for most locations across the region.

Monday will be warmer, with rising humidity as winds turn southwesterly in response to the center of high pressure settling over the southern Mid-Atlantic/Southeast U.S. Warm air will build in over the region as the upper level ridge axis extends overhead. Skies will be mostly sunny in the eastern part of the region, with increasing clouds and possibly thunderstorms affecting the western Mid-Atlantic in the afternoon. The 12Z 4 km NAM develops a line of scattered thunderstorms that move across the region from west to east in the afternoon. The 09Z SREF indicates a high probability of precipitation in the western Mid-Atlantic, roughly west of the Appalachian Mountains, in the afternoon and evening, which is supported by the operational models. As a result, Moderate ozone seems likely in the eastern Mid-Atlantic, in areas that will see mostly sunny skies throughout the day. Afternoon clouds, showers, and thunderstorms will limit rising ozone mixing ratios in the western Mid-Atlantic to the Good to low Moderate range. PM2.5 concentrations will increase into the Moderate range at scattered locations in response to rising humidity.

The specific air quality forecast becomes uncertain beginning on Tuesday, but Good to Moderate conditions are expected. Tuesday looks to have the best chances for widespread convection. The Mid-Atlantic will be in the warm sector of the low pressure system centered over southern ON. It will be very warm and humid, with southwesterly flow. The SREF and the operational models all indicate a high chance for clouds, rain, and thunderstorms across the region, except for the southeastern Mid-Atlantic (i.e., coastal VA/NC). The weak cold front moving into the northern Mid-Atlantic on Wednesday and stalling near northern VA on Thursday will act as a focus for showers and thunderstorms. Waves of low pressure may form along the front in response to a series of shortwaves aloft, which would increase the chances for rain and clouds. The air quality forecast will strongly depend on the track and location of the front, along with associated clouds and precipitation. Ozone will reach the Moderate range in locations that do not experience afternoon cloud cover or heavy convection – such as the southeastern Mid-Atlantic on Tuesday. It is not clear to what extent the weak front will displace the warm, humid, and increasingly modified air mass that will be in place over the region. PM2.5 may improve to the Good range on Thursday in the northern Mid-Atlantic behind the front, but locations near and to the south of the front may see continued Moderate PM2.5. Areas that are impacted by heavy rain or thunderstorms will observe lower PM2.5 levels.

-Huff

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

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 29, 2014
Valid: May 30 – June 3, 2014 (Friday-Tuesday)

Summary:

Air quality will be mostly in the Good range across the region through Sunday, with the chance for scattered Moderate PM2.5 in the northern Mid-Atlantic on Friday ahead of an advancing cold front and isolated Moderate ozone on Sunday along the I-95 Corridor. An upper level ridge will build across the eastern U.S. on Monday. Differences in how the weather forecast models handle the track and location of a developing closed low aloft/area of low pressure at the surface on Monday and Tuesday introduce uncertainty in the air quality forecast. PM2.5 will likely be in the Moderate range both Monday and Tuesday. Ozone may be Good to Moderate depending on the location of precipitation associated with the developing area of low pressure.

Discussion:

The weather forecast models are in close agreement on the main features of the beginning of the medium range period, but they diverge starting on Monday, which makes the air quality forecast for the end of the period uncertain. The upper level longwave trough over eastern Canada will deepen southward over the northeastern U.S. and Mid-Atlantic on Friday, amplified by a series of shortwaves rotating through the trough axis. The trough will push a cool pool over the Mid-Atlantic, behind a weak cold front at the surface which will move through the region on Friday afternoon and evening. The upper-level trough will gradually retreat to the east on Saturday and Sunday as the persistent Midwestern upper level ridge builds back over the eastern U.S. By Monday, both the 06Z GFS and 00Z ECMWF agree that the upper level ridge will extend over the Mid-Atlantic. A potent shortwave will move eastward over the Great Lakes region along the crest of the ridge on Monday, developing an area of low pressure at the surface. The GFS is about 12 hours faster than the ECMWF with the progression of this shortwave. By 00Z Tuesday, the GFS has the shortwave over MI, while the ECMWF has it over WI. These differences carry over into Tuesday. Both models close off the shortwave aloft, but by 00Z Wednesday, the GFS has the closed low over southeastern ON, while the ECMWF has it over WI. The closed low will flatten the ridge, but in different locations. The bottom line for air quality is that the GFS solution brings widespread precipitation from west to east into the northern and central Mid-Atlantic on Monday afternoon/evening, while the ECMWF keeps precipitation west and north of the region. On Tuesday, both models develop precipitation across the region in the afternoon/evening, but the ECMWF keeps heaviest precipitation across the northern and western Mid-Atlantic.

Friday will be partly to mostly cloudy, with a chance of rain showers across the southern Mid-Atlantic and possibly a thunderstorm in the afternoon in the northern Mid-Atlantic ahead of a weak cold front. The chance for thunderstorms and showers will keep ozone in the Good range. The front will usher in a reinforcing shot of relatively dry air, which will help to keep PM2.5 concentrations in the Good range for most locations. The NOAA-EPA, BAMS-CMAQ, and NCDENR air quality models call for Moderate PM2.5 concentrations in the interior northern Mid-Atlantic, which suggests that PM2.5 concentrations may rise into the Moderate range west of the I-95 Corridor in advance of the front.

Good air quality is expected at most locations for Saturday and Sunday. It will be mostly sunny with low humidity and little to no precipitation under the influence of a Canadian high pressure system moving down from QC. A cool pool of air aloft will promote ample vertical mixing. Transport aloft will be northerly from Canada on Saturday and shift more northeasterly on Sunday. Winds at the surface will stagnate on Sunday morning as the center of surface high pressure slides southward along the east coast. With strong late May sun in the afternoon and light winds, isolated locations along the I-95 Corridor could see ozone rise into the Moderate range on Sunday.

Uncertainty in the forecast increases for Monday and Tuesday due to the differences in the weather forecast models. With the upper level ridge overhead on Monday, temperatures and humidity will increase. The center of surface high pressure will settle over the southern Mid-Atlantic/Southeast, which will set up a return southwesterly flow. Recirculating back trajectories will promote rising pollutant concentrations. If the ECMWF solution verifies, with the shortwave aloft/surface low farther west, then widespread Moderate air quality is likely in the Mid-Atlantic on Monday. If the GFS solution verifies, then rain and clouds would limit rising ozone to possibly the Good range. Moderate PM2.5 seems likely with either model solution, due to rising humidity. The forecast for Tuesday is even more uncertain. Widespread Moderate PM2.5 seems likely in the humid and progressively modifying air mass, while the ozone forecast will depend on the location of the developing surface low and associated precipitation. The WPC favors the ECMWF solution more at this point, as they keep the developing surface low over the western Great Lakes on Tuesday in their surface analysis, which suggests Moderate ozone for Monday and Tuesday across the Mid-Atlantic.

-Huff/Catena

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 28, 2014
Valid: May 29 – June 2, 2014 (Thursday-Monday)

Summary:

Generally Good air quality is expected across the region through the beginning of the medium range period under the influence of onshore flow on Thursday and the passage of another cold front on Friday afternoon. As an upper level ridge builds eastward over the Mid-Atlantic on Sunday and Monday, pollutant concentrations will gradually rise, with ozone and PM2.5 likely reaching the Moderate range at widespread locations on Monday.

Discussion:

The weather forecast models remain in quite close agreement on the features of the medium range period. The longwave trough over eastern Canada will reestablish itself over New England on Thursday and then deepen southward and westward over the northeastern U.S. and Mid-Atlantic on Friday and Saturday. The NAM extends the upper level trough farther south and west than the GFS and ECMWF on Saturday, which will push a pool of colder air aloft farther over the Mid-Atlantic for the first half of the weekend. At the surface, a cold front will move through the region from the north on Friday afternoon and evening. The persistent Midwestern upper level ridge will build back over the eastern U.S. and Mid-Atlantic on Sunday and Monday, with the axis of the ridge centered over the eastern Great Lakes region on Monday. The return of the upper level ridge will set the stage for rising pollutant concentrations on Monday.

A slowly-moving back door cold front will reach southern NC on Thursday morning, with a cool, clean, and less humid air mass building in behind it. It will be mostly cloudy to overcast, with steady easterly flow. Scattered showers and thunderstorms will develop across the region, mainly in the afternoon. Ozone will be in the Good range region-wide. PM2.5 will also be in the Good range at most locations. The BAMS-CMAQ and NC air quality models keep the chance for Moderate PM2.5 in the southern Mid-Atlantic and the interior north/central Mid-Atlantic (e.g., southern PA/MD/northern VA), due to the slow-moving back door front and the easterly flow pushing pollutant precursors inland from the I-95 Corridor, respectively.

Another cold front will move through the region from the north on Friday afternoon/evening. Winds at the surface will shift from northeasterly in the morning to northwesterly behind the front. There is a chance of scattered thunderstorms in the afternoon associated with the frontal passage, but relatively low atmospheric humidity will limit the chance for widespread convection. Good ozone conditions are expected across the region. PM2.5 concentrations may rise into the Moderate range west of the I-95 Corridor ahead of the front, but transport aloft will continue to be from the Atlantic Ocean, which should keep PM2.5 in the Good range for most locations.

Surface high pressure located over QC will dominate weather conditions on Saturday. It will be a seasonably cool day with low humidity, mostly sunny skies, and northerly flow. The cool pool aloft will promote vertical mixing. Good air quality is expected across the region.

Weather conditions on Sunday will be very similar to Saturday. The difference will be lighter winds as the center of surface high pressure moves over New England, just to the northeast of the northern Mid-Atlantic. Mostly sunny skies and light winds will allow ozone to reach the Moderate range at scattered locations. PM2.5 will remain in the Good range in most areas until humidity begins to rise on Monday.

As the upper level ridge reestablishes itself over the Mid-Atlantic on Monday, the center of surface high pressure will settle over the southern part of the region, setting up a return southwesterly flow. Temperatures and humidity will increase. Recirculating back trajectories aloft and mostly sunny skies will help ozone and PM2.5 increase into the Moderate range across the region.

-Huff/Catena

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 27, 2014
Valid: May 28 – June 1, 2014 (Wednesday-Sunday)

Summary:

PM2.5 concentrations will drop into the Good range from north to south on Wednesday, following the track of a slow-moving back door cold front, with Moderate conditions lingering in the southern Mid-Atlantic. Clouds and convection sparked by the front will keep ozone in the Good range for most locations on Wednesday, with scattered Moderate ozone possible in the southern Mid-Atlantic where fewer clouds are expected. Air quality will generally be in the Good range for the rest of the period, under the influence of onshore flow Thursday and the passage of another cold front Friday afternoon. Ozone may rebound into the Moderate range at scattered locations on Sunday as an upper level ridge builds back over the region.

Discussion:

The weather forecast models continue to be in surprisingly close agreement on the details of the medium range period, with no real discrepancies. The trend is toward a period of Good air quality for the Mid-Atlantic region. The main players are still the broad upper level ridge centered over the Midwest and the upper level longwave trough over eastern Canada and its coastal waters. On Wednesday, a back door cold front will move through the Mid-Atlantic region and reach approximately northern VA/southern Delmarva by the end of the day and continue to southern NC by Thursday morning. Aloft, a shortwave will flatten the ridge over the eastern U.S. on Wednesday. A second shortwave will crest the ridge on Thursday and re-amplify the trough over the northeastern U.S. on Friday. At the surface, another cold front will move through the region from the northwest on Friday afternoon. The upper level trough over the northeastern U.S. will retreat to the east on Saturday and Sunday as the Midwestern upper level ridge pushes toward the Mid-Atlantic. By Sunday morning, the eastern edge of the ridge will be over the east coast of the U.S.; the ECMWF is slightly faster than the GFS with the eastward extension of the ridge.

Wednesday is the day of most interest and uncertainty in the medium range period. The Bermuda High will still be in place to start the day, along with a modified and humid air mass. The back door cold front is still expected to move through the northern Mid-Atlantic during the day, reaching NYC by 06Z and the PA/MD/DE border by 12Z, and then sagging southward toward the southern Delmarva by 00Z Thursday. A clean, cool, and less humid air mass is behind the front, along with a switch to onshore flow. Areas to the north of the front, such as PHL, will experience a 15-20 °F drop in high temperatures from Tuesday. Air quality is expected to improve from north to south, following the progression of the front. The front will spark showers and thunderstorms, which will taper from north to south in the evening, following the track of the front. It will be mostly cloudy in the northern Mid-Atlantic, with scattered to partly cloudy skies in the southern part of the region. Clouds and convection will limit ozone to the Good range in the northern Mid-Atlantic, while scattered Moderate ozone is possible in the south, depending on the extent of cloud cover. PM2.5 concentrations will decrease into the Good range in areas to the north, where the front passes early enough in the day to provide a change in air mass and the influence of onshore flow at the surface. PM2.5 will linger in the Moderate range in the southern Mid-Atlantic, where the impacts of the advancing cold front will be felt the latest.

The Canadian air mass should fully build into the region on Thursday, bringing cool and less humid conditions. It will be mostly cloudy, with scattered showers and thunderstorms and steady onshore flow, both aloft at the surface. The SREF members show a very high probability of precipitation in the southern Mid-Atlantic in the afternoon. Both ozone and PM2.5 should be in the Good range across the region. The exception may be the southern NC, where the BAMS-CMAQ model keeps PM2.5 in the Moderate range in the vicinity of the slowly-moving back door cold front.

Another cold front will move through the region from the northwest on Friday afternoon. Some scattered showers and thunderstorms are possible in advance of the front, but atmospheric humidity will be relatively low, which will limit the chance for widespread convection. Good ozone conditions are expected across the region. PM2.5 concentrations may rise in the Moderate range west of the I-95 Corridor ahead of the front, but transport aloft will continue to be from the Atlantic Ocean, which should keep PM2.5 in the Good range for most locations.

Behind the frontal passage on Saturday, a cool pool aloft will promote ample vertical mixing and northerly flow, with Good air quality expected. As the upper level ridge pushes back over the Mid-Atlantic and the center of surface high pressure moves over the northern Mid-Atlantic on Sunday, ozone may rise into the Moderate range at scattered locations in the northeastern part of the region. PM2.5 will remain in the Good range in the relatively dry and non-modified air mass.

-Huff/Catena