Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 26, 2014
Valid: July 27- 31, 2014 (Sunday-Thursday)

Summary:

Clouds, showers, and thunderstorms on Sunday associated with an approaching low pressure system will limit ozone to the Good range, while PM2.5 will remain in the Moderate range at inland locations. The passage of a cold front on Monday, followed by the arrival of an unseasonably cool and dry air mass, will keep air quality in the Good range through the end of the period, with the exception of Moderate ozone along I-95 on Thursday and possibly Wednesday.

Discussion:

The weather forecast models are in excellent agreement on the main features of the medium range period. On Sunday, a closed low/shortwave over MN/WI will open up and drop southeastward, carving out an upper level longwave trough over the Great Lakes. The trough will deepen as it moves eastward on Monday, and the shortwave will rotate through the base of the trough, over the Mid-Atlantic, pulling a cold front through at the surface. The upper level trough will persist over the eastern U.S. through the end of the medium range period, with the main closed low/vort max spinning over southeastern ON/southwestern QC.

The medium range will be a quiet period for air quality, with Good conditions the rule across the region, with the exception of scattered Moderate PM2.5 on Sunday and the chance for Moderate ozone along the I-95 Corridor on Wednesday and Thursday.

Sunday will be hot and humid as a warm front pushes northward through the Mid-Atlantic and puts the region in the warm sector of the approaching low pressure system. Despite the heat, clouds and scattered showers/thunderstorms will keep ozone in the Good range. The 06Z NOAA, BAMS, and NC (CMAQ C and D) air quality models all develop a band of Moderate ozone along I-95 in northern/central NJ, but this seems overdone considering the predicted cloud cover and sustained southwesterly winds in the afternoon. The 12Z mesoscale models show a series of scattered showers and storms moving over the region, beginning overnight and continuing through the day, and a MCS from the Ohio River Valley may impact the central and northern Mid-Atlantic in the morning. The SPC assigns a slight risk for severe storms to all of the Mid-Atlantic, except for southern NC. PM2.5 will remain in the Moderate range at inland locations, particularly the Susquehanna Valley, western PA, and western NC, due to humid conditions and a modifying air mass.

The first of two consecutive cold fronts will move through the region early on Monday, followed by the second front in the afternoon. Gusty southwesterly winds, turning westerly, will accompany the passage of the fronts. Showers and storms will continue overnight into Monday morning, and possibly linger through the day in the northern Mid-Atlantic. Both ozone and PM2.5 will be in the Good range across the region.

An unseasonably cool and dry Canadian air mass will build into the Mid-Atlantic on Tuesday. A pool of cool air aloft and northwesterly flow will promote Good air quality for most locations for the rest of the medium range period. The exception will be along I-95 on Thursday and possibly Wednesday, where ozone could reach the Moderate range. The air mass will be very clean, but as the center of surface high pressure slowly moves overhead on Thursday, very light recirculating surface winds will pool ozone and its precursors along I-95, allowing for rapidly rising ozone mixing ratios in the afternoon and early evening. This is a pattern we have seen repeatedly this summer, even on relatively cool days (e.g., less than 85 °F). If a sea breeze forms on Wednesday afternoon, which is certainly possible, then ozone may reach the Moderate range at isolated locations on Wednesday as well, particularly in the Philadelphia metropolitan area.

PM2.5 should remain in the Good range Tuesday-Thursday in the clean and dry air mass, although the NAAPS model brings a slug of smoke from the persistent fires in the western U.S. and Canada into the region from the west on Wednesday and Thursday. We will continue to monitor the smoke model guidance and observational analysis through the medium range period.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 25, 2014
Valid: July 26- 30, 2014 (Saturday-Wednesday)

Summary:

A warm front will travel northward through the region on Saturday and Sunday, causing temperatures and dew points to increase. Saturday will see Moderate air quality as the day will be partly sunny and humid. An approaching cold front on Sunday will create clouds, reducing ozone to the Good range for most locations. PM2.5 will remain in the Moderate range due to continued humidity and warm air advection. The cold front will pass through on Monday and bring in a very cool, dry, and clean air mass from Canada, reducing air quality to the Good range through the end of the medium range period.

Discussion:

The weather forecast models are in close agreement for the medium range period with the exception of Tuesday. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with a weak upper level trough passing through and exiting the Mid-Atlantic by Sunday morning. An intense shortwave will continue its gradual progression through southern Canada and will carve out a trough over MB/ND/MN and will move southeastward into the U.S. on Sunday morning. While the GFS and the ECMWF both have similar placement and timing with the developing large upper level trough, the GFS is about six hours faster with the progression of the shortwave than the ECMWF over the Great Lakes on Sunday. This translates into faster movement by the GFS of the surface cold front associated with the upper level trough through the Mid-Atlantic on Monday. As a result, the ECMWF forecast lags that of the GFS in both the progression of the cold front and associated precipitation by about 6 hours. This discrepancy brings uncertainty to Tuesday’s precipitation forecast, with the ECMWF keeping rain across the northern Mid-Atlantic (NMA) for most of the day, while the GFS keeps the region mostly dry. However, given the very clean, cool, and dry Canadian air mass that will arrive in the wake of the frontal passage, these discrepancies in the precipitation forecast for Tuesday should not have any impact – Good air quality is expected regardless.

Today’s weather forecast model runs are trending toward less widespread precipitation over the weekend, which makes Saturday and Sunday the days of most interest during the medium range period. High pressure will sit over the Mid-Atlantic on Saturday until a warm front moves northward through the region later in the day. By Saturday night, the warm front will reach through central VA and eastern NC, which will trigger clouds and a slight chance for precipitation. Temperatures and dew points will rise in the wake of the warm front. Mostly sunny skies and short, localized back trajectories will promote rising ozone levels, but breezy southwesterly afternoon surface winds will disperse pollutants. As a result, ozone is likely to be limited to the mid-Moderate range, primarily along/near I-95 in the NMA. The air quality models are having trouble resolving the competing factors that will impact ozone on Saturday; the NOAA, BAMS, and NC (CMAQ C and D) ozone models all show various scattered areas of Moderate ozone in the NMA, but there is no real consensus on the magnitude or specific location. With increased humidity, PM2.5 will reach the Moderate range for most locations.

By Sunday, the warm front will continue northward through the region and a surface trough will develop in its place, roughly along I-95. The region will be in the warm sector of the low pressure system approaching from the Great Lakes, with high temperatures near 90 °F and dew points in the low 70s °F. The surface pattern will be complex, with a weaker cold front impacting the NMA on Sunday, followed by the passage of the main cold front on Monday. Heat and humidity, along with the approach of the first, weaker cold front, will promote clouds and scattered rain showers throughout the day. The cloud cover will be enough to reduce ozone concentrations to the Good range, although there is a chance for Moderate ozone along the eastern Mid-Atlantic if cloud cover is not as widespread as expected. PM2.5 will remain in the Moderate range due to continued humidity and westerly flow aloft.

The primary cold front will approach the Mid-Atlantic and pass through during the day on Monday. The day will be mostly cloudy with scattered rain and thunderstorms lasting all day. The front will bring in a very dry, cool, and clean air mass in its wake with sustained northwesterly surface winds. This air mass will remain over the Mid-Atlantic through at least Thursday. High temperatures will struggle to reach 80 °F in the NMA on Tuesday-Wednesday. As a result, air quality will improve to the Good range for Monday-Wednesday.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 24, 2014
Valid: July 25- 29, 2014 (Friday-Tuesday)

Summary:

High pressure will build in overhead for Friday, resulting in sunny skies and light winds. Given the clean air mass in place, Good air quality is expected, with the chance for isolated Moderate ozone along I-95. A warm front will travel northward through the region on Saturday and Sunday, resulting in rising temperatures and humidity. There is a chance for ozone to rise into the upper Moderate range both days, mainly along I-95; however, potentially gusty afternoon surface winds on Saturday and mostly cloudy skies on Sunday may limit rising ozone levels. PM2.5 will reach the Moderate range both days in response to humid conditions. A cold front will pass through on Monday, ushering an unseasonably cool, dry, and clean Canadian air mass. Air quality will be in the Good range both Monday and Tuesday.

Discussion:

The high confidence streak still remains for the weather forecast models for the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with a weak upper level trough overhead. It will gradually pass through the region until flow aloft becomes more zonal on Saturday afternoon. A potent upper level shortwave moving eastward through southern Canada from the West Coast will carve out a trough over MB/ND/MN on Sunday. A small but somewhat intense shortwave will travel just ahead of the trough and will impact the Mid-Atlantic region on Sunday night, which will help to trigger evening showers. The upper level trough will move southeastward through the Great Lakes and will approach the Mid-Atlantic Monday afternoon, pushing a surface cold front ahead of it through the Mid-Atlantic. The GFS becomes slightly faster than the ECMWF with the movement of the trough starting Tuesday morning. This small discrepancy will remain insignificant in regards to air quality.

Friday will stay mostly dry as high pressure builds in. A stationary front located off the Southeast coast will result in rain and clouds for primarily the southeastern Mid-Atlantic. There will be early morning stagnation, variable to light winds, sunny skies, and an afternoon sea breeze in the northern Mid-Atlantic (NMA). This setup has been a recipe for rapidly rising ozone along I-95 several times this summer. However, the arrival of the dry and clean air mass from Thursday’s cold front will keep the chance low for upper Moderate ozone. Most likely, ozone will be in the Good/Moderate threshold in the NMA along I-95. The NOAA and BAMS ozone models agree with this analysis and keep ozone in the Good range across the region. PM2.5 will also be limited to the Good range in the very dry and clean air mass.

Saturday and Sunday are the days of most interest in the medium range period. The stationary front along the Southeast will move northward as a warm front to the central Mid-Atlantic by Saturday morning and will continue to move northward throughout the day. Temperatures and humidity will rise with the passage of the warm front. The day will be mostly sunny with sustained southerly surface flow in the afternoon and a slight chance for scattered rain showers. The chance for rain is greatest in the evening through the night for the NMA as the warm front approaches. Warm air arriving aloft and recirculating back trajectories will push ozone into the Moderate range for most locations on Saturday, particularly along the I-95Corridor. The main forecast questions will be the degree of modification of the air mass and the potential impact of sustained southwesterly surface winds in the afternoon, possibly gusting to 15-20 mph, which would act to disperse pollutants. PM2.5 will also rise into the Moderate range at scattered locations due to increased atmospheric moisture and early morning stagnation.

By Sunday, the warm front will have traveled northward up through central NY, and the Mid-Atlantic will be in the warm sector. Back trajectories will originate in the Ohio River Valley and atmospheric moisture will increase. High temperatures will reach into the low 90s °F in the NMA, and surface trough will develop over the region. Depending on how strong the surface convergence is along the trough, ozone has the potential to rise into the upper Moderate range, particularly along the I-95 Corridor. However, a cold front located over the Great Lakes through the Plains will move eastward toward the Mid-Atlantic on Sunday, triggering increasing clouds and rain showers for Sunday evening. Skies look to be mostly cloudy for much of the day, due to the proximity of the warm front and approaching cold front, which may limit any rises in ozone. PM2.5 will remain in the Moderate range in the increasingly humid and modified air mass.

The cold front will pass through the region on Monday, resulting in scattered precipitation and cloud cover. Air quality will be reduced to the Good range. The cold front will move slightly off the coast by Tuesday, allowing an unseasonably cool, dry, and clean Canadian high pressure system to build in, with Good air quality continuing. The upper level pattern heading into the middle part of next week is more reminiscent of winter than summer.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 23, 2014
Valid: July 24- 28, 2014 (Thursday-Monday)

Summary:

Air quality will oscillate over the course of the medium range period. Good conditions on Thursday due to the passage of a cold front will give way to the chance for upper Moderate ozone at isolated to scattered locations along I-95 on Friday and Saturday due to high pressure moving overhead. Conditions will return to the Good range on Sunday and Monday for most locations due to the arrival of the next cold front.

Discussion:

There is high confidence in the weather forecast models for the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with the passing of an upper level trough through the Mid-Atlantic. It will exit the region by Saturday afternoon and transport aloft will briefly take on zonal flow. A potent shortwave passing through southern Canada from the Pacific coast will intensify as it moves eastward, developing a trough over the Great Lakes region by Sunday evening. The trough will gradually move eastward and intensify, reaching the Mid-Atlantic by Monday night. Both the GFS and ECMWF are in excellent agreement with deep amplification of the trough over the eastern U.S. This trough will pull a cold front through the region on Monday and Tuesday next week. A shortwave travelling ahead of the trough will trigger precipitation for Sunday afternoon/evening. The models diverge slightly with the strength of this secondary shortwave, and as a result, the ECMWF has more widespread precipitation on Sunday compared to the GFS.

A surface cold front passing southward through the region on Thursday will trigger scattered rain and thunderstorms from north to south along with mostly cloudy skies. High pressure will build in primarily over the northern Mid-Atlantic (NMA) by the end of the day while the front moves to NC. The front will bring in a dry and cool air mass with northerly flow. Air quality will be limited to the Good range for most locations Thursday.

Friday and Saturday are the days of most interest for the medium range period, with the potential for upper Moderate ozone along the I-95 Corridor. Friday will have enjoyable weather as the center of high pressure moves over the Ohio River Valley. The day will be mostly sunny with light winds. The cold front will move southward to SC and will stall for the remainder of the day, resulting in a chance for rain in the southern Mid-Atlantic (SMA). Ozone will remain in the Good range in the SMA due to precipitation and cloud cover. In the NMA, an afternoon sea breeze and a shift in surface synoptic winds from northerly to south/southwesterly during the day will pool pollutants along/near I-95. This pattern has resulted in rapid increases in ozone at isolated locations in the NMA, along I-95, this summer. The BAMS air quality models keep ozone mostly in the Good range for Friday, most likely due to the northerly flow and recent arrival of a clean air mass. PM2.5 will stay in the Good range for most locations, due to the dry air mass.

By Saturday, the center of high pressure will extend its ridge over the Mid-Atlantic as it moves eastward over the ocean. The stationary front will linger over NC, leaving clouds and a slight chance for rain showers in the SMA, limiting ozone to the Good range for most locations. The NMA will remain mostly dry and sunny, with light southwesterly winds. With a synoptic set-up similar to Friday, and with localized westerly transport aloft, the chance will continue for upper Moderate ozone along I-95. The BAMS ozone models are more aggressive with rising ozone mixing ratios on Saturday, with the BAMS-CMAQ developing isolated areas of USG ozone in the NMA. PM2.5 will remain in the Good range for most locations due to the dry air mass and sustained surface winds.

The stationary front over NC will move northward through the Mid-Atlantic as a warm front on Sunday and a surface trough will develop down the region. The arrival of the shortwave traveling ahead of the developing upper level trough over the Great Lakes, as discussed above, will trigger clouds and rain in the afternoon. This would keep ozone in the Good range, but there is the potential for Moderate ozone in areas that see more sunshine. With an increase in atmospheric humidity, PM2.5 will also rise into the Moderate range for Sunday, except for location that experience heavy rainfall. By Monday, the next cold front will move through the Mid-Atlantic. This will result in all-day rain and thunderstorms and cloud cover. Air quality will be reduced to the Good range.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 22, 2014
Valid: July 23- 27, 2014 (Wednesday-Sunday)

Summary:

The previous chance for USG ozone in the northern Mid-Atlantic on Wednesday has been reduced due to an incoming cold front and the faster arrival of precipitation ahead of the front, but upper Moderate ozone is still probable along the I-95 Corridor. Thursday will be rainy and cloudy as the cold front moves through the region and stalls over the NC/SC border. Air quality will be reduced to the Good range. Friday and Saturday will be sunny with northwesterly flow as high pressure builds into the region. Air quality will remain in the Good range with the exception of scattered Moderate ozone on Saturday due to increased sunshine. The stationary front over the NC/SC border will move northward as a warm front on Sunday, resulting in widespread rain and thunderstorms, allowing air quality to return back to the Good range.

Discussion:

The weather forecast models are in good agreement until the weekend. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with a weak upper level trough over the Great Lakes. It will approach the Mid-Atlantic Wednesday night into Thursday, pushing a surface cold front through the region. The trough will gradually pass through the Mid-Atlantic until it exits overnight Saturday. The weather forecast models generally agree on the development and progression of a secondary upper level trough formed by a shortwave moving through southern Canada from the Pacific coast. The models diverge on the movement of the shortwave starting Saturday morning. The GFS moves it faster southeastward into the U.S. Great Lakes region, and by Saturday night, the ECMWF lags about 12 hours behind. Even though the models are not in agreement regarding the location and progression of the trough, they are both clearly trending toward a trough somewhere over the eastern U.S., so the current model discrepancies are not expected to have too much of an impact on the region for Sunday’s forecast.

The westward extension of a Bermuda High on Wednesday will promote sunny skies and light winds for the first half of the day. A cold front will approach from the northwest and move southeastward into the northern Mid-Atlantic (NMA) throughout the day, triggering scattered showers and cloud cover in the afternoon through the night. By 00Z Thursday, the front will reach roughly from southeastern PA through western NC. Although the mesoscale models and the SREF continue to keep the bulk of precipitation west of I-95 through 00Z Thursday, the progression of the precipitation is trending slightly faster in today’s models runs. The BAMS ozone models picked up on this trend in this morning’s runs and are showing cleaner conditions relative to yesterday’s runs. However, today’s 06Z NOAA and BAMS-RT do develop a narrow band of upper Moderate ozone along I-95 in the NMA. Overall, the chance for USG ozone along the I-95 Corridor is less likely, but areas of scattered upper Moderate are still possible, depending on the development of afternoon convection ahead of the cold front. West of I-95 and in the SMA, ozone will return to the Good range. With dew points rising into the low 70’s ºF, PM2.5 will reach the Moderate range for most locations.

The cold front will continue to move slowly southward into NC on Thursday, resulting in scattered rain and thunderstorms throughout the day for most of the region. The day will be mostly to partly cloudy with sustained northwesterly winds. The cold front will bring in a relatively cool and dry air mass in its wake, reducing levels of atmospheric moisture. High pressure will build in behind the front, making way for a clear Thursday night. Air quality will be reduced to the Good range for most locations.

The center of high pressure will move eastward over the Ohio River Valley on Friday as the cold front moves southward to the NC/SC border. Most of the region will remain sunny and dry with light winds with the exception of cloudier areas closest to the frontal boundary. The cool and dry air mass will keep air quality in the Good range for most locations.

High pressure will sit overhead on Saturday, keeping most of the region dry with sunny skies. The cold front will linger over the NC/SC border as a stationary front, leaving a slight chance for rain mainly in NC. PM2.5 will remain in the Good range yet ozone will have the chance to reach the Moderate range due to increased sunshine and light winds. By Sunday, the stationary front will move northward as a warm front up through VA, triggering widespread rain and thunderstorms. Air quality will be reduced to the Good range depending on how strong the storms are.

-Catena/Huff