Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 31, 2014
Valid: August 1-5, 2014 (Friday-Tuesday)

Summary:

A coastal front will bring cloudy skies, periods of heavy rain, and Good air quality to most locations on Friday-Sunday, with the cleanest conditions expected on Saturday. The exception will be PM2.5, which will remain in the Moderate range in the northern Mid-Atlantic on Friday, since precipitation will move from south to north. The forecast for Monday and Tuesday is uncertain and hinges on how long the coastal front lingers, but Good to Moderate conditions are expected.

Discussion:

The weather forecast models are in close alignment on the main features of the medium range period through Saturday, but they diverge beginning on Sunday. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The closed low aloft over ON/QC will retrograde northward on Friday, but shortwave energy dropping down over the Great Lakes/northern Mississippi River Valley on Saturday will reinforce the longwave trough over the eastern U.S. This larger area of shortwave energy will move eastward into the Ohio River Valley (ORV) on Sunday. The forecast models disagree on how fast this larger shortwave will progress through the ORV and into the Mid-Atlantic. The ECMWF is about 12 hours faster with the progression of the shortwave than the NAM, while the GFS is in the middle. The ECMWF brings the shortwave into the northern Mid-Atlantic (NMA) by 00Z Monday. For both the ECMWF and GFS, this shortwave promotes a transition to more zonal flow aloft. Consequently, the ECMWF is faster with the transition to more zonal flow over the Mid-Atlantic on Monday and Tuesday, although the persistent trough over eastern Canada remains. As the upper level trough over the eastern U.S. finally transitions to more of a zonal flow, the GFS is faster than the ECMWF to build in mid-level ridging centered over the Great Lakes on Tuesday. This difference, coupled with the variations in the speed of the larger shortwave over the Mid-Atlantic, results in uncertainty regarding the precipitation forecast beginning Sunday, as discussed below.

The persistent frontal boundary that has been lingering out in the Atlantic, parallel to the coastline, will move back towards the Mid-Atlantic shore as a warm front on Friday. The forcing will come from the gradual expansion of the western Atlantic ridge as the strong Canadian closed low retreats northward. The coastal front will approach the southern Mid-Atlantic (SMA) first, hugging the NC/VA coastline by 12Z Friday. Small shortwaves moving through the eastern side of the upper level trough will trigger waves of low pressure that will ride along the coastal front from south to north. As a result, rain and cloud cover will spread northward on Friday. Overcast skies and widespread rain will keep air quality in the Good range in the SMA. Scattered cloud cover will become mostly cloudy by the afternoon in the NMA, with scattered showers all day, which should keep ozone in the Good range for most locations. Light onshore surface winds and a shift toward onshore flow aloft will promote falling PM2.5 concentrations in the NMA, but continued Moderate conditions are likely, particularly at inland locations west of I-95.

The coastal front will be located along the entire Mid-Atlantic coast on Saturday and Sunday before pulling away from the NMA coast on Monday. Waves of low pressure will ride along the front through the weekend, particularly on Saturday, promoting periods of heavy rain. The forecast models are not in agreement regarding the exact location and timing of the waves, and thus of the heaviest rainfall, especially on Sunday, when the ECMWF dries out the fastest due to its more progressive flow aloft. Regardless of the timing of precipitation, overcast to mostly cloudy skies and onshore flow on both Saturday and Sunday will keep air quality in the Good range, with the cleanest conditions likely on Saturday.

The forecast for Monday and Tuesday is uncertain and hinges on how long the coastal front lingers before surface high pressure builds back in from the west. On Monday, the GFS has the wetter solution, keeping the coastal front intact along the coast and continuing to move waves of low pressure along it. The ECMWF is relatively dry, expect for the SMA, where the front remains. By Tuesday, both models are predicting drier conditions, although a weak cold front will be approaching from the Great Lakes later in the day. The ECMWF develops a line of precipitation along the far NMA, associated with the front, while the GFS does not, likely due to its more enhanced mid-level ridging to the west. Thus, Good to Moderate air quality is expected at the end of the period, depending on the track of the coastal front, with better chances for Moderate conditions on Tuesday as skies clear and temperature reach the upper 80s °F.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 30, 2014
Valid: July 31 – August 4, 2014 (Thursday-Monday)

Summary:

Surface stagnation will promote scattered Moderate air quality on Thursday and Friday, with PM2.5 as the leading pollutant. Clouds and rain associated with a stationary front along the Atlantic coast over the weekend will keep air quality in the Good range. Monday’s forecast is uncertain, but Good to low Moderate conditions are expected.

Discussion:

The weather forecast models extend their trend of close agreement today on the main features of most of the medium range period, although they do diverge on Monday. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The deep upper level longwave trough will remain over the eastern U.S. on Thursday, but the closed low over ON/QC will open up on Friday and retrograde northward. Shortwave energy over the Great Lakes will reinforce the trough on Saturday. This larger area of shortwave energy will move eastward into the Ohio River Valley (ORV) on Sunday. At this point, the models diverge. The ECMWF keeps the large shortwave moving eastward to the Mid-Atlantic on Monday, and its exit promotes a transition to more zonal flow aloft. In contrast, the GFS is about 24 hours slower than the ECMWF, keeping the large shortwave over the ORV on Monday before shearing its energy across the Mid-Atlantic on Tuesday. The main result is that the GFS is much wetter than the ECMWF on Monday across the Mid-Atlantic. At the surface, the coastal front in the Atlantic, parallel to the East Coast, will slowly move westward back to the coast beginning Friday, mainly along the southern Mid-Atlantic (SMA), and then along the entire Mid-Atlantic coast on Saturday, where it will remain through the end of the period. A series of shortwaves aloft will trigger waves of low pressure along the front Friday-Sunday.

Thursday has the best chance for Moderate air quality before the coastal front transitions the region to a period of mostly Good air quality. The center of surface high pressure will be offshore but near enough to keep surface wind speeds light. Stagnation overnight will give way to light southwesterly winds in the afternoon. Back trajectories are slow and localized, which will promote accumulation of pollutants. The 06Z mesoscale models indicate partly cloudy skies and scattered shower activity across the region, with the chance for isolated thunderstorms, but they do not show an afternoon sea breeze. This afternoon’s air quality observations will give us an idea of how quickly the air mass is modifying, which will strongly impact Thursday’s forecasts, especially for ozone. Most of the air quality models indicate Moderate ozone along/north/west of I-95, with Moderate PM2.5 ranging from regionwide (BAMS CMAQ) to localized along I-95 in the DC-BAL-PHL corridor (NOAA, NC). Thus, scattered Moderate air quality seems likely on Thursday, with PM2.5 as the leading pollutant.

Temperatures and humidity increase slightly on Friday as the flow shifts southerly. Clouds and showers will move in from south to north in the afternoon, following the track of the coastal front. There will be stagnation overnight before winds pick up and begin turning onshore (southerly to southeasterly) in the afternoon. Back trajectories indicate a transition to onshore flow as well. Mostly cloudy skies should keep ozone in check at most locations, while PM2.5 will remain Moderate.

With the coastal front draped along the East Coast over the weekend, it will be mostly cloudy to overcast with more widespread showers as waves of low pressure form and ride along the frontal boundary. Light onshore flow, aloft and at the surface, will help to advect a clean air mass into the region. Good air quality is expected for both Saturday and Sunday.

Monday’s air quality forecast is uncertain, given the divergence in the GFS and ECMWF precipitation predictions. The wetter GFS solution would extend the period of Good air quality, while the drier ECMWF solution would promote the chance for scattered Moderate conditions, given light surface winds and mostly sunny skies.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 29, 2014
Valid: July 30 – August 3, 2014 (Wednesday-Sunday)

Summary:

Mostly Good air quality will bookend the medium range period, promoted by a clean air mass on Wednesday and clouds and showers associated with a coastal front over the weekend. Surface stagnation will push ozone and PM2.5 into the Moderate range Thursday-Friday, especially along I-95.

Discussion:

The weather forecast models continue to be in close alignment on the main features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. A deep upper level longwave trough will remain over the eastern U.S. through Thursday, anchored by a closed low over eastern ON/western QC. The closed low will begin to retrograde northward on Thursday afternoon, pulling the pool of coolest air aloft along with it. The longwave trough will remain over the eastern U.S., reinforced by shortwave energy dropping down over the Great Lakes on Saturday. This larger shortwave will continue to move eastward over the Mid-Atlantic on Sunday, while a series of small shortwaves will move through the flow on the east side of the longwave trough over the Mid-Atlantic on Friday and Saturday. The GFS and ECMWF show the standard minor discrepancies with the placement of the shortwaves. The main result is that the ECMWF has a wetter forecast for Friday and Saturday in the Mid-Atlantic. At the surface, the center of high pressure will move overhead on Wednesday and remain just offshore on Thursday. The remnants of yesterday’s cold front will linger out in the Atlantic, parallel to the coast, and gradually move back westward and reach the coastline along the entire region by Saturday-Sunday.

Given the presence of the upper level trough at the beginning of the medium range period and the approach of the coastal front by the end of the period, mostly Good to low Moderate air quality is expected. The day of most interest will likely be Thursday, when ozone may reach the mid-Moderate range along I-95.

The center of surface high pressure will move over the region on Wednesday. Stagnation overnight and into the morning will give way to very light westerly winds in the afternoon, with the formation of a sea breeze. These conditions will promote accumulation of ozone and its precursors along I-95, but with temperatures in the low-mid 80s °F and a relatively clean air mass in place, ozone should remain in the Good range, with a chance for Moderate ozone at isolated locations. This forecast is supported by the NOAA, BAMS, and NC (CMAQ D) models. The NOAA and NC models develop Moderate ozone in coastal NJ/DE, likely in response to the sea breeze. A weak cold front will approach the region from the northwest in the afternoon and should dissipate before moving very far into the northern Mid-Atlantic (NMA). The 09Z SREF indicates a high probability of precipitation on Wednesday afternoon along western PA and WV, likely due to the approach of the cold front, which will keep ozone in the Good range at these locations. PM2.5 concentrations will increase in response to the stagnation, but the relatively dry air mass should keep the daily average in the Good range, with a chance for isolated Moderate conditions. The impact of smoke from wildfires burning in the western U.S. and Canada is still an outstanding question. The NAAPS model continues to show a large plume of smoke arriving from the west today and remaining over the region through Friday. Upwind PM2.5 concentrations in the Great Lakes and Plains this morning do not seem to be affected by the smoke, so for now, we will assume that the smoke is staying aloft and not impacting surface air quality.

The surface high pressure center will be just offshore on Thursday, keeping skies mostly sunny and winds very light to calm. It will be slightly warmer as the cool pool of air aloft retreats to the north. Short and localized westerly back trajectories, with some recirculation at lower levels, will facilitate mid-Moderate ozone along the I-95 Corridor. PM2.5 concentrations will continue to gradually rise as well, with scattered locations reaching the Moderate range.

Surface winds will begin to turn onshore on Friday, both aloft and at the surface, as the center of surface high pressure slides farther offshore and the coastal front approaches slowly from the Atlantic. Surface winds will be very light yet again, but increasing clouds in the afternoon associated with the approach of the coastal front and shortwaves moving through aloft will likely take the edge off of any rising ozone levels along I-95. The SREF indicates a high probability of precipitation across most of the region west of I-95 in the afternoon, likely in response to weak surface convergence inland. Thus, Good to low-mid Moderate ozone is expected, with the highest levels once again along I-95. PM2.5 will likely respond to rising atmospheric humidity and reach the Moderate range at scattered locations, particularly inland.

The forecast for Saturday and Sunday will depend on the inland progression of the coastal front and any waves of low pressure that form along it. Back trajectories show onshore (easterly) flow on Saturday and more south/southeasterly flow on Sunday, along the coast. Scattered afternoon showers seem likely both days, with higher chances on Sunday due to the large shorwave moving overhead. It will be mostly cloudy along the coast, in the vicinity of the stationary (or slowly moving) front. Good to low-mid Moderate air quality seems probable, with PM2.5 as the leading pollutant, and higher concentrations of both ozone and PM2.5 inland.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 28, 2014
Valid: July 29 – August 2, 2014 (Tuesday-Saturday)

Summary:

A deep and persistent upper level trough over the eastern U.S. will keep air quality in the Good range for most locations through the medium range period. Surface stagnation Wednesday-Friday may allow ozone and PM2.5 to reach the Moderate range along the I-95 Corridor, depending on how quickly the air mass modifies and the potential impact of smoke from wildfires in the western U.S. and Canada.

Discussion:

The weather forecast models continue to be in excellent agreement this morning on the main features of the medium range forecast. The 00Z ECMWF, the 06Z NAM, and the 06Z GFS were consulted for this analysis. There is little change from yesterday’s discussion. A deep upper level longwave trough will sit over the eastern U.S. through the end of the period, anchored by a closed low over eastern ON/western QC. An omega block over North America will reinforce the persistence of the trough. The trough will keep a cool pool of air over the northern Mid-Atlantic (NMA) though Thursday morning, at which point the closed low will begin to retrograde to the north. A new area of shorwave energy over the western Great Lakes will help to fortify the longwave trough on Saturday. As the cool pool retreats on Thursday and Friday, mid-level ridging will try to build in from the east, but it won’t make much progress. At the surface, Monday’s cold front will stall out in the Atlantic, parallel to the coast, before beginning to slowly move back west as a warm front at the end of the period.

With the persistent upper level trough over the eastern U.S., high temperatures will be below average throughout the medium range period, reaching only the low-mid 80s °F in the NMA. The warmest days will be Thursday and Friday, as the cool pool aloft retreats northward. Humidity will creep up beginning on Thursday as well, but dew points will remain relatively low, reaching only the low 60s °F in the NMA by Saturday.

Good air quality still looks to be the rule across the region for most locations through the period, but there is a chance for Moderate conditions to develop along the I-95 Corridor beginning Wednesday and continuing through Friday.

Good air quality is expected regionwide on Tuesday as an unseasonably cool and dry air mass builds in from the west behind Monday’s cold front. The high pressure system of Canadian origin will remain over the Mid-Atlantic through the end of the period. Surface high pressure will promote mostly sunny skies and little to no precipitation through Friday, although there is a chance for isolated showers daily given the cool air aloft.

The dominant factor for air quality Wednesday-Friday will be surface winds. As the center of surface high pressure moves overhead on Wednesday, surface winds will stagnate and reverse direction (northwest to south/southwest). Light recirculating flow at the surface, along with an afternoon sea breeze, will pool ozone and its precursors along I-95. Given the relatively clean air mass in place and below average temperatures on Wednesday, ozone may stay in the Good range, but there is a chance for isolated Moderate conditions, especially in the Philadelphia metropolitan area. The center of high pressure will move just offshore on Thursday and then slightly farther offshore on Friday. Surface winds will remain light, with stagnation overnight and through the morning. Sea/bay breezes are likely both days. Aloft, back trajectories are short and from the Ohio River Valley on Thursday, shifting southerly along the Atlantic coast for Friday. As the initially clean air mass modifies, the chances for Moderate ozone will increase. The air quality models keep ozone in the Good range through Thursday but develop scattered Moderate conditions along I-95/northeastern Mid-Atlantic on Friday. How quickly ozone rises into the Moderate range will depend on how quickly the air mass modifies. For now, it seems likely that ozone will reach the Moderate range at isolated to scattered locations along I-95 possibly on Thursday and more likely on Friday.

PM2.5 may also increase into the Moderate range beginning on Wednesday and continuing through Saturday for isolated to scattered locations, in response to the surface stagnation. The most likely areas for Moderate PM2.5 will be those with strong local precursor emissions, such as the Susquehanna Valley. The BAMS CMAQ brings PM2.5 into the Moderate range across the region beginning Thursday, while the NC CMAQ C and D keeps conditions Good, with only the Susquehanna Valley reaching Moderate on Friday. The impact of smoke from the western U.S. and Canadian wildfires may also be a factor. The NAAPS model runs wouldn’t load this morning, but we will check them again this afternoon and also monitor upwind areas to determine the impact, if any, of the persistent smoke on PM2.5 concentrations.

Air quality on Saturday looks to be Good across the region, particularly along coastal locations. The center of high pressure will move well to the east, allowing onshore flow to develop. The coastal front out in the Atlantic will slowly move westward as well. Onshore flow and mostly cloudy skies will keep ozone in the Good range, while PM2.5 may be in the Moderate range at inland locations.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, July 27, 2014
Valid: July 28 – August 1, 2014 (Monday-Friday)

Summary:

Air quality will be in the Good range across the region at the start of the period as a cold front moves through on Monday and ushers in an unseasonably cool and dry air mass. An omega blocking pattern will keep a longwave trough aloft over the region through the end of the period, which will promote persistent Good air quality for most locations. However, PM2.5 and ozone will rise into the Moderate range Wednesday to Friday at isolated to scattered locations, primarily along the I-95 Corridor, as surface high pressure moves overhead.

Discussion:

The weather forecast models continue to be in excellent agreement on the large scale features of the medium range period. An intense shortwave will dig out an upper level longwave trough over the eastern U.S. on Monday and pull a cold front through the Mid-Atlantic region. The movement of the cold front has slowed slightly compared to yesterday’s analysis; it is now expected to clear the region by 12Z Tuesday morning. The potent shortwave will round the eastern side of the trough and merge with an existing closed low/shortwave over eastern ON/western QC early Tuesday. This will reinforce the deep longwave trough over the eastern U.S. and set up an omega block over North America that will persist through the medium range period. The main closed low/shortwave energy will begin to retrograde northward on Friday, but the longwave trough will persist over the eastern U.S. A series of small shortwaves will rotate through the trough over the Mid-Atlantic during the medium range period, keeping the chance for daily isolated showers in the forecast.

With the amplified upper level trough locked in over the eastern U.S. and a cool pool aloft over the Mid-Atlantic, Good air quality will be the rule for most locations, with isolated to scattered Moderate conditions later in the period.

The cold front will move through the region on Monday, bringing an unseasonably cool and dry air mass in its wake. Showers and thunderstorms overnight into the morning will clean out the atmosphere, and gusty winds will provide ample ventilation. Good air quality is expected across the region.

Tuesday-Friday will be dominated by the upper level trough and high pressure at the surface, moving down from Canada and gradually progressing eastward to the Mid-Atlantic by Thursday-Friday. Temperatures will be below average through the period, and dew points will remain low for the end of July (e.g., mid-upper 50s °F in the northern Mid-Atlantic). Forecast guidance suggests a gradual trend toward warmer and more humid conditions late in the period, as surface high pressure moves overhead, but it will still be relatively cool and dry (e.g., high temperatures reaching the low-mid 80s °F and dew points in the upper 50s/low 60s °F by Friday in the northern Mid-Atlantic).

With the arrival of the Canadian high pressure system on Tuesday, air quality will be in the Good range across the region. As surface high pressure moves eastward on Wednesday and surface winds begin to stagnate, PM2.5 may creep up into the Moderate range at scattered locations, continuing through Friday. It will depend on how quickly the air mass modifies and if any of the smoke from the persistent wildfires in the western U.S. and Canada is pulled down to the surface over the Mid-Atlantic by subsiding air associated with the high pressure system. For the second day in a row, the NAAPS model shows smoke moving into the region from the west beginning on Wednesday and continuing into Thursday. The possible impact of the smoke will need to be monitored to determine its effect on Mid-Atlantic PM2.5 air quality later in the medium range period.

There is also the chance for ozone to climb into the Moderate range at isolated to scattered locations beginning on Wednesday and continuing to Friday, mainly along the I-95 Corridor. The 12Z 4 km NAM shows a sea breeze on Wednesday afternoon, which seems likely to develop, considering the light synoptic surface flow. Sea/bay breezes also seem a good bet for Thursday and Friday. In addition, the upper level transport becomes slow and localized on Thursday and Friday, which will help to promote rising ozone mixing ratios, despite the relatively cool temperatures. As with PM2.5, it will depend on how quickly the air mass modifies and how much vertical mixing is promoted by the cool air aloft. The air quality models are not in agreement regarding the later part of the period. The BAMS ozone models keep conditions in the Good range through Thursday, while the NC ozone models (CMAQ C and D) develop Moderate ozone along I-95 on Wednesday (isolated) and Thursday (widespread). With full sun and light to calm surface winds, possibly aided by sea/bay breezes, Moderate ozone seems likely somewhere along I-95 late in the period.

-Huff