Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 25, 2014
Valid: June 26-30, 2014 (Thursday-Monday)

Summary:

The movement of a cold front to the East Coast by early Friday will trigger clouds and a chance for rain and thunderstorms Thursday and Friday. Thursday will have Good air quality due to cloud cover and northwesterly surface winds, with exception of a chance for Moderate ozone along the East Coast. Friday will have some sunshine with convergence in the afternoon, which will create the potential for ozone to rise into the mid-to-upper Moderate range. PM2.5 will stay in the Good to low Moderate range due to northerly/easterly flow. High pressure will move overhead on Saturday and remain there for the rest of the medium range period. A line of convergence will form inland on Saturday, similar to Friday, which will keep the chance for mid-to-upper Moderate ozone along and west of I-95. Sunday and Monday will both be partly to mostly sunny with a chance for afternoon rain and thunderstorms. Sustained onshore flow should limit the chances for Moderate ozone and PM2.5 to scattered inland locations.

Discussion:

The weather forecast models are in generally good agreement for the medium range period. They show the typical differences in the timing and location of shortwaves that will impact the area. The 06Z NAM, 06Z GFS, and the 00Z ECMWF have similar features until Saturday morning. All of the models develop an upper level ridge over the western Great Lakes by 00Z Friday. The ridge moves progressively eastward during the medium range period. By 12Z Saturday, all of the models roughly have the ridge axis centered over the Great Lakes, but the GFS has a less amplified ridge compared to the NAM and the ECMWF, which pushes an upper level trough, located over the Northeast Thursday and Friday, eastward to the Atlantic faster than the ECMWF and the NAM. This could affect Saturday’s precipitation forecast if the GFS were to verify. Starting 12Z Sunday, the GFS and the ECMWF become slightly out of phase with each other with the progression of the upper level ridge over the eastern Great Lakes/Ohio River Valley. The GFS tilts the axis more westward while the ECMWF is more eastward. The ridge axis shifts eastward to the eastern U.S. by Monday. Over the range of the period, the models agree on the advancement of a huge longwave trough moving through the southern Canadian Plains that will impact the Mid-Atlantic next week.

A cold front will reach the east coast of the Mid-Atlantic region by 12Z Thursday morning. A wave of low pressure will be located along the frontal boundary, over NJ. The lingering front will leave behind scattered clouds and a chance for rain and thunderstorms throughout the rest of the day. There will be northwesterly flow behind the front, with sustained surface winds. Air quality will be limited to the Good range for most locations. Given partly to mostly sunny afternoon skies with strong June sun, however, the northwesterly flow will push ozone precursors from the urbanized I-95 Corridor toward the southeast. As a result, there is a chance for Moderate ozone along the east coast from NJ southward to NC, which is supported by the NOAA and BAMS air quality forecast models.

By Friday, the cold front will be off the Atlantic coast and just south of NC, and surface high pressure will move over southwestern PA. There will be a likely chance for showers in the southern Mid-Atlantic in the late afternoon and evening. Surface flow will transition from northerly to easterly, bringing in cleaner air to coastal locations throughout the day. With the center of high pressure over the northern part of the region, wind speeds will slacken, and a line of convergence will develop inland in the afternoon, west of I-95. Consequently, ozone has the potential to rise into the mid-to-upper Moderate range in locations impacted by the convergence zone. The air quality models are fairly conservative with ozone on Friday, but the history of the summer so far has shown that ozone is quite sensitive to stagnation and convergence. PM2.5 should be slower to respond to the slackening winds, and given the northerly transport aloft, concentrations will remain in the upper Good to low Moderate range for Friday.

Saturday will be mostly to partly cloudy with light onshore flow. Surface high pressure will remain overhead, shifting slightly eastward toward NY. This will create a line of convergence inland, similar to Friday, which will keep the chance for mid-to-upper Moderate ozone along and west of I-95. However, an approaching surface warm front in WV will potentially create rain in the western Mid-Atlantic, which would limit ozone to the Good range in these locations. Light winds will allow PM2.5 to reach the Moderate range across the region, except for the East Coast, where PM2.5 will be in the Good range due to onshore surface winds.

Sunday and Monday will experience sunny weather with a chance of afternoon rain and thunderstorms, as a surface high pressure sits over western VA/NC and also off the coast of NJ. Temperatures and humidity will increase on Monday as the upper level ridge shifts overhead. At the same time, flow aloft and at the surface will become fully onshore, which will bring clean maritime air into the region, particularly on Monday. As a result, the chance for USG ozone seems low, but Moderate ozone may occur west and north of I-95. The forecast will be complicated by the development of an Appalachian lee trough on Monday. The sustained onshore flow, especially on Monday, will limit the chances for Moderate PM2.5 to the northern Mid-Atlantic.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 24, 2014
Valid: June 25-29, 2014 (Wednesday-Sunday)

Summary:

A cold front will move into the Mid-Atlantic region on Wednesday resulting in rain and thunderstorms for the second half of the day, limiting air quality to the Good range. A wave of low pressure will develop over NJ, along the frontal boundary, on Thursday morning. Widespread rain and thunderstorms will impact most of the Mid-Atlantic Thursday morning and continue for much of the day in the northern part of the region, resulting in Good air quality in the northern half and Moderate air quality in the southern half of the Mid-Atlantic. Friday will see a rise in ozone to the Moderate range for most locations due to mostly sunny skies and light winds as a high pressure system moves over New England, while northerly flow will limit PM2.5 to the Good range. Saturday and Sunday will be partly to mostly sunny with a chance of rain and thunderstorms. Humidity and sunshine will allow for Moderate air quality at inland locations.

Discussion:

The weather forecast models are in close agreement with the exception of Thursday night into Friday regarding the development of an upper level trough over the Northeast. This feature was hinted at in yesterday’s model runs. Today, the 12Z NAM has the most amplified upper level trough, followed by the 00Z ECMWF and then the 06Z GFS. These differences keep Friday’s precipitation forecast uncertain. The ECMWF, the GFS, and the NAM show similar predictions of an upper level trough over West Coast, a ridge over the Rockies, and a trough over the Great Lakes at the beginning of the period. Once we get to the extended forecasts, the GFS and the ECMWF show very similar paths for an upper level ridge that will develop over the Mid-Atlantic. The two models are in phase with each other for the rest of the period and into next week. They do have some differences in the timing and location of shortwaves that will impact the Mid-Atlantic for the weekend, which will affect the precipitation forecast.

Wednesday, a pre-frontal trough will develop ahead of an approaching cold font arriving from the northwest. Cloud cover and precipitation will impact the Mid-Atlantic region mainly in the second half of the day. Flow aloft will be southeasterly from the Atlantic ocean, bringing in cleaner air. Ozone and PM2.5 will be limited to the Good range for most locations.

The front will travel to the coast through Wednesday night into Thursday morning and a wave of low pressure will form over NJ. This will create widespread convection and precipitation on Thursday morning for most of the region, which will continue for much of the day primarily in the northern Mid-Atlantic. The southern half of the Mid-Atlantic will see scattered areas of Moderate ozone due to more sunshine, while the northern half will mainly have Good ozone. Southwesterly flow and persistent humidity will increase PM2.5 into the low Moderate range.

The cold front will move eastward off the coast on Friday, but will linger along the NC shore, resulting in mostly dry conditions during the day with the exception of a few scattered rain and thunderstorms. Mostly sunny skies and light winds will allow ozone to reach the Moderate range in scattered locations, whereas northerly dry flow will reduce PM2.5 to the Good range.

Saturday will see partly sunny skies with a chance for scattered rain and thunderstorms as the center of the surface Canadian high pressure system moves over MA. The high pressure will allow for northeasterly flow aloft for Saturday. The surface high pressure will extend its surface ridge down the Mid-Atlantic for Sunday, bringing mostly sunny skies, rising temperatures, and a slight chance for rain and thunderstorms. PM2.5 and ozone will be able to maintain Moderate concentrations for most locations for both days, with the exception of coastal areas, which will remain in the Good range due to the influence of onshore flow.

-Catena

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 23, 2014
Valid: June 24-28, 2014 (Tuesday-Saturday)

Summary:

Clouds, rain, and thunderstorms will spread from the western Mid-Atlantic on Tuesday afternoon to the entire region on Wednesday afternoon ahead of a slowly-moving cold front. Mostly Good air quality is expected, with the chance for Moderate conditions along the I-95 Corridor on Tuesday. Clearing skies and light winds on Thursday for most of the Mid-Atlantic will bring air quality back up to the Moderate range. Good to Moderate conditions will persist for the end of the period, with uncertainty about the development of a potential upper level trough over the Northeast and an upper level ridge over the Southeast.

Discussion:

The weather forecast models are in reasonable agreement on the main features of the medium range period, with some discrepancies, particularly at the end of the period. The Mid-Atlantic will be in the warm sector of a weak and slow-moving extra-tropical cyclone on Tuesday. The associated cold front will progress through the northern Mid-Atlantic on Wednesday and reach roughly central VA by Thursday morning. Canadian high pressure will build into the Mid-Atlantic for the remainder of the period. There is still a question of how far south the cold front will travel, given the quasi-zonal flow aloft over the region on Thursday-Friday. The WPC analysis pushes the front to the southeast over the southern Mid-Atlantic by 12Z Friday and leaves a stationary front along eastern NC and its coastal waters through the end of the period. Any hint of a developing wave of low pressure along the front, seen in yesterday’s model runs, is gone.

The situation is complicated by the 00Z ECMWF, which moves a strong shortwave across the Great Lakes on Thursday, into NY on Friday and then develops a weak upper level trough over NY and New England on Saturday. The 06Z GFS has the same shortwave feature, but keeps it farther north, over southern Canada, and does not develop a trough. The PHL NWS Office isn’t sure how to handle the ECMWF solution at this point, and it’s too early to make a determination, especially since the GFS has the same shortwave, albeit weaker and displaced northwards. The potential northern trough competes with a developing ridge over the southeastern U.S. on Saturday.

There are still some discrepancies in the precipitation forecast for Tuesday between the operational models and the high-resolution versions of the NAM. With the Mid-Atlantic in the warm sector, thunderstorms will likely develop in the afternoon in the warm and increasingly humid air mass. The 06Z NAM, GFS, ECMWF, and 09Z SREF all develop rain and thunderstorms across the western Mid-Atlantic in the afternoon and early evening, roughly west of I-81. The 06Z 4km NAM has some scattered showers overnight and is less aggressive with showers in the afternoon, keeping most of the activity west of the region until after sunset. The 06Z 4km NAM shows relatively poor continuity with the 00Z run, so we are inclined to discount it for now and assume that clouds and precipitation along the western Mid-Atlantic will keep both ozone and PM2.5 in the Good range on Tuesday. Light onshore flow and recirculation aloft to the east will keep the chance for Moderate ozone in the forecast for areas along and west of I-95, similar to Sunday and Monday’s conditions. Stronger afternoon winds on Tuesday (relative to Sunday and Monday), with no morning stagnation expected, should keep any areas of Moderate ozone in the low-to-mid Moderate range. The NOAA and BAMS ozone models limit Moderate ozone to isolated locations, likely due to stronger winds in the afternoon. PM2.5 concentrations are rising to the west today (e.g., southwestern PA), so Moderate PM2.5 is expected in the eastern part of the region on Tuesday, again mainly along and west of I-95.

Wednesday looks cloudy and rainy, with the chance for severe thunderstorms in the afternoon. There are some discrepancies in the operational models, with the GFS and ECMWF wetter than the NAM, but all of the models agree on afternoon clouds and rain. With a humid air mass in place, periods of heavy rain are likely. As a result, both ozone and PM2.5 should be in the Good range for most locations.

Thursday looks mostly clear, with light northwesterly flow. Temperatures and dew points will not fall appreciably behind the front, so it will remain relatively warm and humid. The ECMWF does keep the chance for rain showers across the region, while the NAM and GFS dry things out, except for eastern NC (where the remnants of the cold/stationary front will linger). Assuming mostly sunny or even partly sunny skies, with light flow both aloft and at the surface, Moderate ozone seems likely along the I-95 Corridor. Ozone has been very sensitive to stagnation so far this summer, even given the arrival of a new and presumably clean air mass. The BAMS-CMAQ model brings PM2.5 up to the Moderate range across most of the region, probably in response to light winds.

Friday and Saturday’s forecast will hinge on the development of the potential upper level trough over the Northeast U.S. and/or the upper level ridge building over the Southeast U.S. Both days look to have light winds with morning stagnation, so we will have to watch for rising ozone mixing ratios along the urbanized I-95 Corridor. The ECMWF does spark showers across the region on Friday and Saturday afternoons, which would lead to lower ozone levels, so there is considerable uncertainty in the forecast. At this point, mainly Good to Moderate conditions are expected.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 22, 2014
Valid: June 23-27, 2014 (Monday-Friday)

Summary:

Strong June sun/rising humidity vs. onshore flow will be the competing factors Monday and Tuesday, keeping ozone and PM2.5 in the Moderate range at inland locations along and west of I-95. An approaching cold front will trigger rain and thunderstorms along the western Mid-Atlantic Tuesday afternoon, spreading into the eastern Mid-Atlantic on Wednesday, which will return air quality to the Good range. There is uncertainty in how far south the advancing cold front will reach on Thursday, with the possibility for more rain on Friday associated with a wave of low pressure.

Discussion:

The weather forecast models have come back into closer agreement today regarding the track of the next cold front, which will impact the Mid-Atlantic beginning on Tuesday, but they are still having trouble handling the development of the front’s associated low pressure system. We continue to prefer the 00Z ECMWF solution, which has shown the best run-to-run consistency over the past few days. WPC and the PHL NWS Office also prefer the ECMWF.

At 500 mb, weak ridging over the eastern Great Lakes/Mid-Atlantic on Monday will give way to the development of a weak trough over the Plains/western Great Lakes on Tuesday. At the surface, an area of low pressure will intensify over northern MI by 12Z Tuesday. This low will slide to the northeast and pull a cold front into the Mid-Atlantic on Wednesday. The models agree on the track of the low but differ in its speed; the 12Z NAM and 00Z ECMWF bring the low to the St Lawrence River by 12Z Wednesday, but the GFS is about 6 hours slower. Despite these differences, the operational models all bring precipitation to the western Mid-Atlantic on Tuesday afternoon/evening. At the surface, both the GFS and ECMWF move the cold front to roughly the Mason-Dixon Line by 12Z Thursday. At this point, the models again diverge slightly. Both develop a wave of low pressure that rides along the front early Friday into Saturday, but the ECMWF keeps the front along the Mason-Dixon Line, while the GFS moves the front and track of the wave farther north. These differences impact the location of precipitation in the Mid-Atlantic on Friday.

On Monday, surface high pressure will slide east to the Gulf of Maine as a warm front approaches the western Mid-Atlantic. Light winds and periods of stagnation overnight and through the morning will give way to steady south/southeasterly winds in the afternoon. It will be mostly sunny with seasonable temperatures. Humidity will begin to increase but dew points will remain relatively low until Tuesday. Onshore winds will push pollutants inland, keeping Good air quality along the coast, with scattered Moderate ozone and PM2.5 inland, particularly along the I-95 Corridor.

As the cold front approaches the western Mid-Atlantic on Tuesday, it will generate rain and clouds in the afternoon. All of the operational models and the SREF show rain and clouds along the western Mid-Atlantic beginning 18Z Tuesday. The 12Z 4km NAM keeps a line of convection and clouds to the west of the Mid-Atlantic through 00Z Wednesday, however, which adds some uncertainty to the precipitation forecast. For now, we assume that precipitation will arrive in the afternoon and keep ozone in the Good range along the western part of the region. PM2.5 may also fall into the Good range, depending on the strength and extent of rain and thunderstorms. Along the eastern Mid-Atlantic, where it will be mostly sunny and humid, steady onshore winds will moderate rising ozone and PM2.5, keeping concentrations in the low-to-mid Moderate range.

The cold front will slowly move into the northern part of the Mid-Atlantic on Wednesday and reach roughly the Mason-Dixon Line on Thursday morning. The front will trigger rain and thunderstorms across the region, particularly the eastern Mid-Atlantic, on Wednesday afternoon. As a result, ozone will be in the Good range for most locations. Some of the storms may have be severe and include periods of heavy rainfall, which will return PM2.5 to the Good range for most locations as well.

With flow aloft turning mostly zonal late in the period, it is not clear how far south the cold front will travel. The ECMWF and GFS both back off on precipitation for Thursday, as surface high pressure builds into the region, centered over the northern Great Lakes. Given strong June sun and relatively light winds, ozone has the potential to reach the Moderate range at locations along the I-95 Corridor. Dew points are not expected to fall dramatically behind the front, so PM2.5 may rebound to the Moderate range as well at scattered locations.

Both the GFS and ECMWF show a wave of low pressure impacting the Mid-Atlantic on Friday-Sunday. The GFS has the wetter solution for Friday for the Mid-Atlantic. It is too far out to track this system, but we expect Good/Moderate air quality for the end of the period.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 21, 2014
Valid: June 22-26, 2014 (Sunday-Thursday)

Summary:

Stagnation and recirculating flow aloft on Sunday will allow ozone to reach the Moderate range west of the I-95 Corridor, with the chance for isolated Moderate PM2.5 as well. Moderate ozone conditions will continue until the arrival of the next cold front on Tuesday/Wednesday, while PM2.5 will increase gradually in response to rising humidity, with widespread Moderate conditions likely by Tuesday. There is still uncertainty regarding the track and timing of the next cold front, with precipitation likely arriving in the western Mid-Atlantic on Tuesday afternoon and widespread precipitation across the region on Wednesday. Thursday’s air quality will depend on the progress of the front, with Good air quality to the north of the boundary and the chance for lingering Moderate PM2.5 to the south.

Discussion:

The weather forecast models have come out of agreement today regarding the development of an area of low pressure and track of an associated cold front that will impact the Mid-Atlantic in the Tuesday-Thursday time frame. Weak upper level ridging will be in place over the eastern Great Lakes and Mid-Atlantic regions on Sunday and Monday. Shortwave energy over southern MB and the central U.S. Plains will begin to dig out a weak longwave trough on Monday. The weak troughing will transition to zonal flow aloft over the Mid-Atlantic on Wednesday and Thursday. At the surface, the 00Z ECMWF develops an area of low pressure over western MI by 12Z Tuesday. The 06Z GFS is weaker with the low and places it farther south, while the 12Z NAM is about 24 hours faster than the ECMWF with the low, placing it over southwestern QC. These timing and location differences translate into uncertainty regarding the precipitation forecast for the Mid-Atlantic for Tuesday-Thursday associated with the advancement with the surface low and its attendant cold front. In particular, the ECMWF keeps clouds and precipitation confined to the far western Mid-Atlantic on Tuesday, while the GFS and NAM bring precipitation all the way to the I-95 Corridor, leaving only the Atlantic coast dry. The WPC prefers the ECMWF’s solution, as do we since it confers better run-to-run consistency at this point.

Sunday will be sunny with low humidity as the persistent frontal boundary currently over the central Mid-Atlantic retreats to the south and Canadian surface high pressure builds into the region. The exception may be eastern VA/NC, which may be impacted by clouds and showers associated with the lingering frontal boundary along the VA/NC coastal waters. The most prominent feature the region on Sunday will be very light surface winds, with periods of stagnation all day, and recirculating flow aloft. With mostly sunny skies in the afternoon, Moderate ozone is likely along and west of the I-95 Corridor, as onshore surface winds will push pollutants inland. It seems probable that PM2.5 will remain in the Good range, due to the relatively dry air mass in place, but the BAMS and NC air quality models develop an area of Moderate PM2.5 across the northern Mid-Atlantic, most likely in response to the stagnant conditions and line of convergence inland.

The center of surface high pressure will move offshore on Monday, over the Gulf of Maine. It will be another mostly sunny and seasonably warm day. Surface winds will pick up relative to Sunday and turn south/southeasterly. Dew points will begin to increase as the flow becomes southerly, but humidity will still be reasonably low until Tuesday. Onshore transport will keep air quality in the Good range along the east coast, with continued Moderate ozone likely west of the I-95 Corridor. PM2.5 concentrations will rise into the Moderate range at scattered locations along and west of I-95 as well in response to the rising humidity.

The air quality forecast for Tuesday through Thursday will depend on the track of the surface low and its associated cold front. Since we prefer the ECMWF solution, clouds and precipitation arriving from the west on Tuesday afternoon/evening will keep ozone in the Good range in the western Mid-Atlantic and possibly PM2.5 as well, depending on the amount of rainfall. In the eastern part of the region, where it will be mostly sunny and humid, Moderate ozone is likely, along with Moderate PM2.5. Wednesday will be warmer and more humid, and most likely pre-frontal, with southwesterly flow. Again following the ECMWF’s solution, widespread clouds and afternoon showers/thunderstorms will keep ozone in the Good range for most of the region on Wednesday, with PM2.5 likely lingering in the Moderate range until the passage of the cold front. As the flow aloft becomes zonal, the cold front will slow and probably stall somewhere around the central Mid-Atlantic on Thursday. Locations to the north of the front will see northwesterly flow and Good air quality, while south of the front, clouds and rain will limit ozone to the Good range but PM2.5 will remain in the Moderate range.

-Huff