Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 26, 2014
Valid: May 27-31, 2014 (Tuesday-Saturday)

Summary:

Mid-upper Moderate PM2.5 concentrations are expected on Tuesday under the influence of warm and humid conditions and a modifying air mass. Cloud cover and thunderstorms in the afternoon generated by an advancing back door cold front will limit ozone to the Moderate range. Air quality will improve from north to south on Wednesday as a Canadian air mass filters into the region. PM2.5 may linger in the Moderate range in the southern Mid-Atlantic, where the impacts of the advancing cold front will be felt the latest. Air quality will generally be in the Good range for the rest of the period, under the influence of onshore flow Thursday and Friday and the passage of another cold front late Friday.

Discussion:

There is not much change today in the forecast guidance for the medium range period. The Mid-Atlantic will continue to be caught between two persistent features: an upper level longwave trough over eastern Canada and a broad upper level ridge centered over the Midwest. On Tuesday, the upper level ridge will extend over the eastern U.S. At the same time, a series of shortwaves will crest the ridge and move across ON toward the upper level trough axis over southeastern Canada. On Wednesday, a more potent shortwave will flatten the ridge over the northeastern U.S. At the surface, a strong Canadian high pressure system will push a back door cold front through the Mid-Atlantic on Wednesday. The progression of the back door cold front has slowed in today’s weather forecast model runs. It is now expected to pass through the northern Mid-Atlantic on Wednesday morning and the southern Mid-Atlantic Wednesday afternoon and evening. Another shortwave will crest the ridge on Thursday and re-amplify the longwave trough over the northeastern U.S. on Friday. At the same time, a cold front will approach from the northwest and move through the region late Friday into early Saturday. Aloft, the longwave trough over the northeastern U.S. will begin to retreat to the east on Saturday as the Midwestern ridge pushes toward the Mid-Atlantic.

With the gradual reestablishment of the longwave trough over the northeastern U.S., the trend in the medium range period is toward improving air quality conditions. On Tuesday, the westward extension of the Bermuda High and an Appalachian lee trough will still be in place, and transport aloft will be westerly, from the Ohio River Valley. These features will promote rising pollutant concentrations, and it will still be warm and humid. However, the back door cold front approaching from the north will spark scattered thunderstorms in the afternoon. If they verify, clouds and convection will limit rising ozone mixing ratios to the Moderate range. All of the weather forecast models predict scattered to widespread clouds and precipitation, including the GFS, ECMWF, 12 km NAM, 4 km NAM, and the 4 km NMM and ARW models, which lends support to a Moderate ozone forecast. The NOAA-EPA, BAMS, and NC air quality models limit ozone to the Moderate range along the I-95 Corridor, with Good ozone conditions elsewhere in the region. PM2.5 concentrations will rise into the mid-upper Moderate range in the humid and modified air mass, with lower concentrations in areas directly impacted by heavy rain from afternoon thunderstorms.

The transition to cleaner air quality conditions on Wednesday will be more gradual than previously expected due to the slower advancement of the back door cold front. It is currently forecasted to reach the PA/MD/DE border by 12Z Wednesday and the NC/SC border by 12Z Thursday. Air quality will improve from north to south on Wednesday, following the progression of the front. Drier and cooler air will filter in behind the front, along with a switch to onshore flow. The front will continue to generate clouds, rain showers, and thunderstorms on Wednesday across the region, but focused more on the central and southern Mid-Atlantic. Ozone will likely improve to the Good range for most locations, while PM2.5 may linger in the Moderate range until the new air mass fully builds in, particularly in the southern Mid-Atlantic, where the impacts of the advancing cold front will be felt the latest.

Thursday will be cool and partly cloudy, with steady onshore flow, both aloft at the surface. The BAMS and NC air quality models have scattered Moderate PM2.5 conditions in the northern and southern parts of the region, but mostly Good air quality seems likely with the Canadian air mass in place and easterly flow.

The next cold front will approach from the northwest on Friday. There may be a rise in PM2.5 concentrations in some locations due to an increase in humidity, but transport aloft will continue to be from the Atlantic Ocean, which should keep air quality in the Good range for most locations. Behind the frontal passage on Saturday, a cool pool aloft will promote ample vertical mixing and brisk northerly winds at the surface, with Good air quality expected.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region 

Issued: Sunday, May 25, 2014
Valid: May 26-30, 2014 (Monday-Friday)

Summary:

A broad upper level ridge will be in place over the Mid-Atlantic region on Monday and Tuesday. Widespread upper Moderate ozone is expected on Monday, with the chance for isolated USG. Showers and thunderstorms associated with an approaching back door cold front on Tuesday will limit the rise in ozone to the Moderate range. PM2.5 will be in the Moderate range on both days. Air quality will improve to the Good range from north to south on Wednesday following the progression of the back door cold front; PM2.5 and ozone may linger in the Moderate range in the southern Mid-Atlantic. Generally Good air quality is expected for Thursday and Friday, but scattered Moderate conditions are possible on Friday ahead of another cold front advancing from the northwest.

Discussion:

The weather models are in very close agreement on the main features of the medium range period. The Mid-Atlantic will continue to be caught between a persistent upper level longwave trough over eastern Canada and a broad upper level ridge centered over the Midwest. Monday and Tuesday are still the days of most interest for air quality considerations. On Monday, the ridge will be the dominant feature, and it will extend across the eastern U.S. Meanwhile, a shortwave will rotate down from interior QC toward New England around the axis of the Canadian longwave trough. The upper level ridge will still extend over the eastern U.S. on Tuesday, while a series of impulses rotate through the axis of the Canadian trough and try to redevelop the trough over the northeastern U.S. The trough will finally begin to amplify over New England on Wednesday, but it is not as deep as it was in yesterday’s model runs. The ECWMF and GFS bring the trough farther south and west than the NAM. At the surface, a strong Canadian high pressure system will push a back door cold front into the northern Mid-Atlantic on Tuesday evening and through the southern Mid-Atlantic on Wednesday. Aloft, a shortwave will move eastward across southern ON on Thursday and finally redevelop the longwave trough over the eastern U.S. on Friday. At the surface, a second cold front will track across the northern Mid-Atlantic on Friday. The GFS is slightly faster than the ECMWF with both the shortwave aloft and the cold front at the surface.

Pollutant concentrations will rise across the Mid-Atlantic region on Monday under the influence of the westward extension of the Bermuda High, an Appalachian lee trough, and sunny skies. Winds will be light and from the west/southwest in the afternoon, with a period of stagnation overnight and in the morning. The transport pattern aloft will shift from northwesterly to westerly. With the ridge aloft, temperatures will increase into the mid-upper 80s °F, and humidity will be on the rise. Ozone mixing ratios are increasing to the west this morning, in Chicago, for example, which suggests the air mass is modifying. As a result, widespread upper Moderate ozone seems likely on Monday, with the chance for isolated USG. PM2.5 concentrations spiked this morning in urban areas in response to a period of stagnation. With rising humidity on Monday, daily average PM2.5 concentrations will rise into the Moderate range at scattered locations across the region, and may spike in the morning again due to another expected period of stagnation.

The upper level ridge will still be in place over the Mid-Atlantic on Tuesday, and the progress of the back door cold front is slower than was expected yesterday. Consequently, the warm, humid, and modified air mass will remain over the region for most of the day, with the front reaching the northern Mid-Atlantic late in the evening or early Wednesday morning. All of the weather forecast models develop scattered rain and thunderstorms on Tuesday afternoon, sparked by the approaching frontal boundary. The 4 km NAM has a series of scattered thunderstorms moving across the entire region in the afternoon, for example, along with increasing cloud cover. Thus, another day of Moderate ozone seems probable, with lower concentrations in areas impacted by convection. The chance for USG ozone is less likely, due to the rain and clouds in the forecast. PM2.5 concentrations will increase higher into the Moderate range in most locations in response to rising humidity and flow aloft from the Ohio River Valley.

Air quality will improve back to the Good range from north to south on Wednesday, following the progression of the back door cold front. A clean, cool, and dry air mass will arrive in the wake of the front, along with onshore flow. The weather forecast models develop scattered showers and thunderstorms in the southern Mid-Atlantic, along the track of the front. Air quality may linger in the Moderate range in the southern part of the region, depending on how fast the front advances.

The back door cold front will stall near the NC/SC border on Thursday. Cool and dry conditions, with easterly flow aloft and at the surface, will provide Good air quality for most of the region. Surface winds will shift southerly on Friday in advance of the next cold front, which will allow temperatures and humidity to rebound and may push PM2.5 and ozone back into the Moderate range at scattered locations.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, May 24, 2014
Valid: May 25-29, 2014 (Sunday-Thursday)

Summary:

Ozone will begin to increase on Sunday as an upper level ridge extends across the Mid-Atlantic before a persistent upper level longwave trough over eastern Canada reasserts itself over the region on Tuesday. Scattered Moderate ozone is expected on Sunday, with upper Moderate to isolated USG on Monday. PM2.5 will increase more gradually, given the slow rise in atmospheric humidity through Monday. A back door cold front will approach the northern Mid-Atlantic on Tuesday and clear the region by Thursday morning. Air quality will improve from north to south, with upper Moderate ozone and a chance for USG remaining in the southern Mid-Atlantic on Tuesday and generally Good air quality across the region for Wednesday and Thursday.

Discussion:

The weather forecast models have come into closer agreement today on the main features of the medium range period. An upper level longwave trough will persist over eastern Canada and reassert itself over the northeastern U.S. by the end of the period. On Sunday, the axis of a broad upper level ridge will slide over the Great Lakes region. A large shortwave/closed low will slowly but steadily rotate down from central ON/QC to southern QC, toward the axis of the longwave trough. The upper level ridge will extend eastward on Monday over the Mid-Atlantic, while the shortwave/closed low will move toward New England and begin to flatten the ridge over the Mid-Atlantic. On Tuesday, the western edge of the Canadian longwave trough will extend over the Mid-Atlantic; the ECMWF pushes the trough the farthest south and west, but all the models show the redevelopment of the trough over the eastern U.S. At the surface, a back door cold front will move into the northern Mid-Atlantic on Tuesday and continue through the southern Mid-Atlantic on Wednesday. The upper level trough will slowly retreat eastward on Wednesday and Thursday as the persistent upper level ridge over the Midwest tries to move back over the eastern U.S.

Temperatures will begin a warming trend on Sunday as the upper level ridge moves toward the Mid-Atlantic and the Canadian longwave trough temporarily retreats. At the surface, the center of high pressure will slide over the Mid-Atlantic and off of the Atlantic coast, providing mostly sunny skies across the region. Flow aloft and at the surface will turn from northwesterly to westerly. Humidity will still be relatively low, and little to no precipitation is expected. The air quality forecast models are quite conservative for ozone through the medium range period. They keep ozone in the Good range on Sunday, but it seems likely that ozone will reach the Moderate range at scattered locations, under the influence of sunny skies and light winds. PM2.5 will likely remain in the Good range in most locations in the dry air mass.

It will be warmer on Monday, with high temperatures in the mid-upper 80s °F, although humidity will still remain relatively low. The westward extension of the Bermuda High will be in place, along with an Appalachian lee trough. Westerly flow aloft and at the surface will bring plenty of warm air into the region. With the upper level ridge over the Mid-Atlantic, it will be mostly sunny with little to no precipitation. Ozone will rise into the Moderate range across the region, with the chance for upper Moderate to isolated USG still possible. The main forecast question will be how quickly levels of ozone and its precursors increase regionally (i.e., how quickly does the airmass modify?). PM2.5 will increase more gradually, given the slow rise in dew points, but scattered Moderate conditions seem likely.

Tuesday’s forecast remains uncertain due to the reassertion of the Canadian longwave trough aloft and the approach of a back door cold front from near the NY/Canada border. There will be a clean, cool, and dry Canadian air mass behind the cold front, as well as a shift to northeasterly flow. The front is forecasted to approach the northern Mid-Atlantic in the afternoon and evening. Clouds and thunderstorms ahead of the front may improve ozone to the Good range in the northern Mid-Atlantic. With westerly back trajectories from the Ohio River Valley, a lee trough, and light west/southwesterly winds to at least begin the day, another day of upper Moderate ozone, with a chance for USG, seems possible for the southern Mid-Atlantic. Humidity will finally begin to increase as winds at the surface turn southwesterly, so Moderate PM2.5 seems probable across the region.

The back door cold front will move through the southern Mid-Atlantic on Wednesday and stall just south of the region on Thursday. Temperatures and humidity will fall from north to south as a Canadian air mass filters into the region behind the front. The change in air mass and a shift to onshore flow will improve air quality to the Good range at most locations.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 23, 2014
Valid: May 24-28, 2014 (Saturday-Wednesday)

Summary:

Generally Good air quality is expected on Saturday a cool and dry air mass fully builds into the region. A rising trend in temperatures and ozone will begin on Sunday, as an upper level ridge builds over the Mid-Atlantic region. PM2.5 is slower to rise as humidity remains relatively low until late Monday. Widespread Moderate and isolated USG ozone is possible on Monday. There is uncertainty in the outlook for Tuesday and Wednesday as the weather models are gathering consensus for a cleaner air quality solution. The chance for isolated USG ozone still remains for Tuesday, with widespread Moderate PM2.5 likely. Air quality will improve from north to south on Wednesday in the wake of a back door cold front.

Discussion:

The weather forecast models are in general consensus about the main features of the medium range period, but some differences exist in the details. On Saturday, the Mid-Atlantic will be between a broad upper level ridge centered over the Plains and a long wave trough slowly departing to the east. The axis of the upper level ridge will slide over the Great Lakes region on Sunday as a shortwave/vort max spins down from interior ON/QC toward southwestern QC. The upper level ridge will extend over the eastern U.S. on Monday, while a second shortwave moves eastward along southern MB and ON. The ECMWF phases the two shortwaves on Tuesday over southern QC and digs a longwave trough over New England. The GFS does not phase the shortwaves but still develops a longwave trough in roughly the same place as the ECMWF, although it is not as amplified. By Wednesday, both models have a longwave trough over the Mid-Atlantic, with the axis to the east over southeastern Canada. At the surface, a back door cold front will move south from roughly the Canada/NY border to the Central Mid-Atlantic on Wednesday.

Saturday still looks like a great start to the holiday weekend, with a mix of sun and clouds, northwesterly winds at the surface and aloft, and seasonably cool temperatures. A clean and dry Canadian air mass will fully build into the Mid-Atlantic region. Isolated showers and thunderstorms may develop in the afternoon and evening, mainly in the northern Mid-Atlantic, as small shortwaves move overhead. Generally good air quality is expected across the region. The high-res version of the NAM are picking up on a sea breeze along the NJ and DE coasts in the afternoon, which may increase ozone higher into the Good range than might otherwise be expected along the convergence zone inland, near the I-95 Corridor.

A warming trend will begin on Sunday as the upper level ridge builds eastward. There will be a period of stagnation overnight and into the morning as the center of surface high pressure moves over the region and settles of off the southeastern U.S. coast. Although temperatures will be increasing, humidity will still be relatively low. As a result, PM2.5 will likely remain in the Good range for most locations, with isolated Moderate conditions possible. Plenty of sunshine and light winds will push ozone into the Moderate range, particularly along the I-95 Corridor.

Monday will be warmer but humidity will remain relatively low until dew points begin rising later in the day. The westward extension of the Bermuda High will be in place, and the upper level ridge will extend over the Mid-Atlantic. Winds at the surface will turn westerly, but flow aloft will remain northerly through the first part of the day. The air quality models limit increases in ozone to the Moderate range along the I-95 Corridor, but more widespread Moderate conditions seem likely, with the chance for isolated USG. The BAMS and NC air quality models both increase PM2.5 into the Moderate range, but BAMS is much more aggressive, with Moderate PM2.5 across the entire region, while NC keeps Moderate PM2.5 concentrated along the I-95 Corridor. A more gradual increase in PM2.5 concentrations across the region seems probable, given the slow rise in dew points.

There is uncertainty in the outlook for Tuesday and Wednesday. The weather models seem to be coming into consensus regarding a cleaner air quality solution, with an upper level trough reforming over the Mid-Atlantic on Tuesday and a back door cold front moving through at the surface on Wednesday. Tuesday will have many of the key ingredients for poor air quality conditions, including the Bermuda High, an Appalachian lee trough, westerly back trajectories from the Ohio River Valley, and light southwesterly surface winds. The weather models are predicting widespread clouds and rain showers, however, which would limit rising ozone concentrations. Thus, Moderate ozone seems likely for Tuesday, with the chance for either USG or Good conditions in areas that see less or more clouds/rain, respectively. PM2.5 will likely be in the Moderate range across the region in the humid air mass. Wednesday’s forecast will hinge on the progression of the back door cold front. If it reaches the central Mid-Atlantic, as currently predicted, areas to the north will see improving air quality conditions, while Moderate PM2.5 and possibly Moderate ozone will linger in the southern part of the region.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 22, 2014
Valid: May 23-27, 2014 (Friday-Tuesday)

Summary:

A cool and dry Canadian air mass will filter into the region on Friday and Saturday, leading to Good air quality for most locations. An upper level ridge over the Central U.S. will begin to build eastward on Sunday. Temperatures and dew points will start to rise on Sunday, leading to scattered Moderate ozone and PM2.5. Monday and Tuesday will be warm and humid, with widespread Moderate PM2.5 and the chance of USG ozone both days, but the extent of rising ozone will be limited by the possibility of widespread precipitation on Tuesday.

Discussion:

The weather forecast models are in better agreement today regarding the main features of the medium range period. A shortwave will rotate down from southern ON and amplify the developing longwave trough over the Mid-Atlantic region on Friday. At the surface, a slowly-moving cold front will push through the southern Mid-Atlantic in the morning. A broad upper level ridge over the Midwest will begin to extend toward the eastern U.S. on Saturday, but its progress will be retarded by the longwave trough, which will persist over southeastern Canada through the medium range period. By Sunday, the axis of the upper level ridge will be over the Great Lakes region. The GFS still spins a vort max down from interior QC to southern QC, but it is faster than yesterday’s run and stays farther north. The ECMWF also has this feature, but keeps the vort max farther north, closer to central QC. The upper level ridge fully extends over the eastern U.S. on Monday, but both the GFS and ECMWF push a second shortwave across southern ON, which will phase with the QC shortwave on Tuesday and flatten the ridge over the eastern U.S., pushing the persistent longwave trough back down over the northeastern U.S.

A cleaner, cooler, and drier air mass will filter into the Mid-Atlantic on Friday. There will be a mix of sun and clouds, with a chance for showers, mainly in the northern Mid-Atlantic as a vort max rotates through the axis of the upper level trough. Good air quality is expected for most locations. The exception remains the southeastern Mid-Atlantic, where PM2.5 may linger in the low Moderate range due to the slow-moving cold front.

It will be seasonably cool and dry on Saturday, with north/northwesterly flow. The center of surface high pressure will settle over the western Great Lakes. It will be mostly sunny, with a chance for showers in the afternoon, mainly in the northern Mid-Atlantic. Good air quality is expected across the region.

As the center of surface high pressure moves to the southern Mid-Atlantic on Sunday, and the upper level ridge builds eastward, temperatures and humidity will begin to increase. Flow aloft will turn from northwesterly to more westerly in the afternoon, with warm air advection. Little to no precipitation is expected. The air quality models keep ozone and PM2.5 in the Good range, except for the BAMS-CMAQ, which pushes PM2.5 into the Moderate range for areas south of I-76. With light surface winds and a more westerly component in the flow aloft, scattered Moderate ozone seems likely. PM2.5 may also reach the Moderate range in scattered locations, but with a mostly dry air mass in place, concentrations may rise more slowly than predicted by the BAMS-CMAQ.

Monday and Tuesday will be warm and humid, with temperatures rising into the 80s °F across the region. The westward extension of the Bermuda High will be in place, and an Appalachian lee trough is analyzed for Tuesday. Light west/southwesterly winds at the surface and transport aloft from the Ohio River Valley will promote rising pollutant concentrations. Monday looks clear, but both the GFS and ECMWF have rain showers and thunderstorms across the region on Tuesday. Thus, widespread Moderate ozone is likely on Monday, with a chance for isolated USG. The chances for USG ozone on Tuesday will depend on the precipitation forecast. PM2.5 will increase into the Moderate range across the region both days, in response to rising humidity.

-Huff