Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 15, 2014
Valid: August 16-20, 2014 (Saturday-Wednesday)

Summary:

Weak surface high pressure over the Mid-Atlantic on Saturday will promote scattered Moderate air quality. A weak cold front will bring mostly Good air quality to the northern Mid-Atlantic on Sunday, while Moderate conditions will continue to the south, mainly in NC. The forecast for Monday-Wednesday is uncertain, but Good to low Moderate air quality is expected across the northern Mid-Atlantic, which will be impacted by one or more surface waves moving along a stalled frontal boundary. Moderate conditions are expected to continue to the south, especially in NC, which will feel the effects of a mid-level ridge of high pressure.

The 2014 medium range air quality discussions will end today. They will resume in May 2015.

Discussion:

The weather forecast models are in general agreement today on the overall synoptic features of the medium range period, but they still differ on the details late in the period. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Saturday, the closed low over southern QC will begin to retrograde to the north as a vorticity lobe/shortwave spins down in the flow from ON. The closed low will open up on Sunday as the shortwave rotates through the base of the longwave trough over the Mid-Atlantic and pulls a cold front through the northern Mid-Atlantic (NMA). This front will stall roughly across the central part of the region and act as a focus for one or more waves of low pressure, which will travel along it from west to east early next week. The wave(s) will be triggered by shortwave(s) aloft moving through the Plains and into the Ohio River Valley (ORV) on Monday and across the Mid-Atlantic on Tuesday. The models are still having trouble with the track and location of the shortwave(s), but they are coming into consensus regarding a main, larger shortwave that will approach the Mid-Atlantic on Monday afternoon. The NAM has the strongest shortwave, while the GFS and CMC split the shortwave energy into a series of smaller shortwaves. Weak ridging will build in over the eastern U.S. as the upper level trough/low departs toward eastern Canada. The bottom line for air quality is continued uncertainty for the forecast Monday-Wednesday, but with unsettled conditions likely, there is little chance for air quality to rise beyond the low-mid Moderate range.

Saturday will be warmer but still very dry. Surface winds will become southwesterly in the afternoon as surface high pressure moves over the central part of the region. PM2.5 should remain in the Good range for most locations due to the dry air mass in place. The exception will be NC, where PM2.5 is in the Moderate range today, due to the slow progression of a weak cold front, which will stall across NC on Saturday and continue to promote Moderate PM2.5. With full sun in the afternoon and westerly transport aloft, ozone may reach the Moderate range at isolated locations, particularly along the I-95 Corridor, depending on how quickly the air mass modifies.

The weak cold front will move through the NMA on Sunday, reaching approximately to the Delmarva/N. VA by 00Z Monday. The front will trigger clouds and scattered showers across the NMA on Sunday morning and afternoon. Given the dry air mass in place, widespread rain and lines of thunderstorms are not expected. Depending on cloud cover, ozone may reach the Moderate range at locations east of I-95, where the front will arrive the latest in the day. The slight rise in humidity on Sunday may be sufficient to push PM2.5 into the Moderate range at scattered locations in the NMA, while PM2.5 will likely remain in the Moderate range across the SMA, particularly NC.

The forecast for Monday-Wednesday is uncertain, since it depends upon the track of waves of low pressure that will move along the stalled front across the central Mid-Atlantic. The models seem to be coming into consensus for the main low to approach the region Monday afternoon and evening, and move across the region mainly on Tuesday. Thus, clouds would begin to build in from the west on Monday, and then Tuesday would feature overcast skies and periods of potentially heavy rain, with widespread showers. The main low looks to be exiting the region to the east on Wednesday, allowing for a break in the precipitation. The best chance for continuing Moderate conditions will be in NC, which should be far enough south to feel limited or no impacts from the line of disturbances moving through to the north. A building mid-level ridge in the Southeast will promote rising pollutant concentrations as well. In the areas that will be impacted by the stalled front and surface waves, generally Good ozone seems likely for Monday-Wednesday, with Moderate PM2.5 possible, depending on the predominant winds and intensity of precipitation. If the main surface low is strong enough, flow may turn onshore ahead of it on Tuesday, which would further promote Good air quality.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 14, 2014
Valid: August 15-19, 2014 (Friday-Tuesday)

Summary:

Air quality will oscillate between Good and Moderate through the period, with uncertainty continuing regarding a stalled frontal boundary across the central part of the region on Monday-Tuesday. The best chances for rising Moderate conditions appear to be in the southern Mid-Atlantic early next week.

Discussion:

The forecast models are in better agreement today regarding the overall synoptic pattern for the medium range period, but considerable uncertainty remains in the details of the key features. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. The synoptic pattern consists of a closed low over southern QC on Friday that will begin to retreat northward on Saturday. At the same time, a shortwave lobe will drop down in the flow from ON and re-amplify a longwave trough over the Mid-Atlantic/East Coast on Sunday, pushing a weak cold front into the northern Mid-Atlantic (NMA), where it will stall in the vicinity of the Mason-Dixon line. The upper level trough will exit to the northeast on Monday as weak ridging builds in from the west. A large shortwave originating in the Rockies today will gradually move eastward during the medium range period, across the Plains and Ohio River Valley (ORV) and into the Mid-Atlantic on Monday/Tuesday/Wednesday. This feature will develop a surface low along the stalled frontal boundary across the central part of the Mid-Atlantic and ORV. The models still disagree on the speed and position of the re-developing upper level trough on Sunday, as well as the track and speed of the large shortwave. The bottom line for air quality is that the GFS and NAM are a bit slower with the cold front on Sunday and keep it farther north than the ECMWF and CMC. At the same time, the GFS is by far the fastest and farthest east with the large shortwave, bringing it into the NMA fully a day ahead of the ECMWF and CMC (e.g., Monday versus Tuesday). The WPC appears to be leaning more toward the ECMWF/CMC solution, keeping the surface low associated with the shortwave over the ORV on Tuesday.

Friday still looks quiet, with Good air quality for most locations from the cool and dry air mass over the region. It will be mostly sunny, but northwesterly transport from interior ON should keep pollutant levels relatively low.

Surface winds will decrease and shift southwesterly on Saturday as surface high pressure settles over the southern Mid-Atlantic (SMA). Temperatures will rebound into the low-mid 80s °F, but humidity will remain relatively low. Clouds may begin increasing from northwest to southeast across the NMA in the afternoon as the weak cold front approaches from the north/northwest. The best chance for Moderate air quality will be in the SMA, which will be farthest from the influence of the retreating closed low over southern QC. Isolated Moderate ozone may occur along the I-95 Corridor, depending on surface stagnation and the degree of air mass modification.

The weak cold front will move through the NMA on Sunday, reaching roughly the Mason-Dixon line. The impact of the front will depend on how far south it moves; the slower GFS and NAM solutions will keep its impacts slightly farther north. Surface winds will shift from southwest to northwest behind the frontal passage in the NMA. Clouds and scattered rain showers will impact the NMA though the morning and afternoon, triggered by the approach of the front. Rising humidity and westerly transport aloft will allow PM2.5 to reach the Moderate range at scattered locations in the NMA, depending on the intensity of precipitation associated with the cold front, while ozone will likely remain in the Good range. Both ozone and PM2.5 will rise higher into the Moderate range in the SMA, south of the front.

The forecast for Monday and Tuesday will depend on the location, speed, and intensity of the large wave of low pressure that is expected to form along the stalled frontal boundary across the central part of the region. The GFS solution would bring the low into the NMA on Tuesday, while the ECMWF delays its progress until Wednesday. The CMC is also slower, but brings the low on a track that keeps it farther to the north. Although it is too early to know the specific impacts of this large wave, clouds and precipitation seem likely for at least some parts of the NMA on Monday-Tuesday, given the proximity of the stalled boundary and the eventual arrival of the large wave of low pressure. Good ozone seems mostly likely for the NMA, with PM2.5 reaching the Moderate range in response to humid conditions, depending on the intensity of precipitation. Continued and rising Moderate air quality is expected across the SMA, particularly NC, which looks to be far enough away from the front to feel the impact of building Bermuda High, particularly on Tuesday.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 13, 2014
Valid: August 14-18, 2014 (Thursday-Monday)

Summary:

Air quality will be in the Good range for most locations across the region through Saturday, due to a closed low centered over southern QC. Divergence in the weather forecast models continues to make the air quality forecast highly uncertain for the end of the period, with the best chance for rising Moderate conditions in the southern Mid-Atlantic, since a weak cold front will impact the northern Mid-Atlantic on Sunday.

Discussion:

The uncertainty in the forecast for the end of the medium range period continues again today. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. The models are in close agreement on the evolution and placement of the closed low aloft, which will rotate over southern QC through Saturday. At that point, the models diverge. They all agree that shortwave energy will crest the western U.S. ridge and drop down into the Plains on Saturday, but they have different solutions regarding the fate of the shortwave. The ECMWF is by far the slowest, keeping the shortwave very compact and moving it slowly into the Ohio River Valley (ORV), reaching IN by 00Z Tue. The GFS is the fastest, scooting the shortwave along the ORV and into the Mid-Atlantic by 00Z Mon. The CMC splits the difference by shearing off a portion of the Plains shortwave to the east, similar to the GFS solution, and leaving a portion over the Plains, similar to the ECMWF, but displaced farther south (e.g., over AK at 00Z Tue). At the same time, the models are coming into consensus regarding a re-developing longwave trough over the Mid-Atlantic on Sunday, which will push a weak cold front into the northern Mid-Atlantic; the front will reach approximately the Mason-Dixon line by Monday and stall. A lobe of shortwave energy will drop down from ON on Sunday to dig the longwave trough back over the Mid-Atlantic as the closed low over QC retreats to the northeast. The ECMWF has the weakest trough and moves it through the fastest, allowing the eastern edge of ridging over the Great Lakes to approach on Monday. However, the ECMWF’s ridge is disrupted by the large shortwave located over the ORV, which will form a surface wave along the frontal boundary draped across the central Mid-Atlantic. The GFS and CMC, in contrast, re-establish the longwave trough over the Mid-Atlantic more slowly on Sunday, and extend it farther south. The GFS’s longwave trough absorbs its Plains shortwave late Sunday, giving it the wettest solution. The differences in how the models handle the re-developing eastern longwave trough and Plains shortwave have various implications for air quality late in the period, discussed below.

With a cool and dry air mass building in over the Mid-Atlantic on Thursday and Friday, and a reinforcing cold front moving through on Thursday, Good air quality will be the rule across the region. Surface high pressure will settle over WV on Saturday, allowing flow aloft and at the surface to turn southwesterly, which will begin a trend toward warmer and moister conditions. However, Saturday still looks to be below average for temperatures and dew points, and back trajectories from the northern Great Lakes will help to promote another day of Good air quality for most locations.

Things will get interesting on Sunday and Monday as the western upper level ridge tries to build eastward and the Bermuda High extends over the southern Mid-Atlantic (SMA). The model guidance is clearly trending toward the passage of a weak cold front from the north on Sunday, tied to the re-developing longwave trough aloft. All of the weather forecast models have rain and clouds across the northern Mid-Atlantic (NMA) on Sunday afternoon, although the GFS is the wettest. Temperatures will be close to average, with increasing humidity. Ozone and PM2.5 will likely rebound into the Moderate range across the SMA. The approach of the cold front will keep ozone in the Good range in the NMA, and PM2.5 may reach the Moderate range at scattered locations, depending on air mass modification and intensity of precipitation.

All of the models clear things out on Monday, at this point, although the stationary front across the central portion of the region will most likely act as a focus for clouds and showers. The ECMWF solution offers the greatest chance for a brief period of Moderate air quality in the NMA on Monday. The GFS and CMC, with their slightly less progressive and deeper tough aloft, keep cold air advection and northwesterly flow aloft over the NMA. Rising Moderate conditions seem probable for the SMA, which will be south of the stalled boundary and experience southwesterly flow and warm air advection.

With the striking divergence in the forecast model guidance beyond Saturday, it is pointless to speculate on the continued potential for upper level ridging over the Mid-Atlantic next week and the associated chance for an extended period of deteriorating air quality.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 12, 2014
Valid: August 13-17, 2014 (Wednesday-Sunday)

Summary:

The passage of a cold front on Wednesday and the arrival of a cool, dry, and clean air mass in its wake Thursday and Friday will keep air quality in the Good range across the region. Air quality for Saturday and Sunday is uncertain due to divergence in the upper level forecast, with either continued Good air quality (ECMWF) or rising Moderate conditions (GFS/CMC).

Discussion:

The weather forecast models are in close agreement on the synoptic features for the beginning of the medium range period, but they diverge beginning Saturday. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Wednesday, a strong upper level longwave trough over the eastern U.S. will cut off over southwestern ON late in the day. The closed low will spin over southern QC through early Saturday, keeping the longwave trough over the eastern U.S. On Saturday-Sunday, the models completely diverge. The ECMWF drops shortwave energy down from ON as the closed low drifts slowly northeastward. This shortwave reinforces the longwave trough over the eastern U.S. on Sunday. At the mid-levels, this feature keeps the cool pool of air over the northern Mid-Atlantic (NMA) through the end of the weekend. At the surface, the shortwave/reinforced trough pulls a cold front through the NMA on Sunday and triggers clouds and showers across the NMA. In contrast, the GFS exits the closed low to the northeast faster than the ECMWF on Saturday, and its shortwave dropping down in the flow from ON is weaker than the ECMWF’s, and it does not reinforce the longwave trough over the eastern U.S. on Sunday. Instead, the GFS has more of a zonal flow that keeps the NMA dry on Sunday. The CMC’s solution is similar to the GFS’s, which lends support to the warmer, drier forecast for the weekend. The PHL NWS suspects that the ECMWF’s reinforcing shortwave is too robust and has low confidence in the cold front on Sunday, but it is a possibility that we will monitor. These two varying solutions also impact the chances for a possible transition to a synoptic pattern conducive for deteriorating air quality beyond the end of the medium range period, as discussed below.

The days of most interest during the period are the ones with most uncertainty – Saturday and Sunday. Until then, air quality should be uneventful. The strong upper level ridge will push a cold front through the Mid-Atlantic on Wednesday. Periods of heavy rain will move through the region today and continue overnight associated this system. The main area of low pressure over the Great Lakes today will keep moving to the northeast and reach western QC tomorrow, while another low will move up the coast and into New England. The rain will end on Wednesday from southwest to northeast across the region, with gradual clearing, but there will be a chance for scattered showers most of the day. A cool and dry Canadian air mass will build in for Thursday and Friday behind the frontal passage. Below average temperatures, low humidity, and northwesterly flow will promote Good air quality.

Pollutant concentrations will begin to rebound on Saturday as the center of surface high pressure settles near WV. Temperatures will rise to near normal levels, although humidity will remain relatively low (mid-upper 50s °F dew points in the NMA). Both ozone and PM2.5 may rise into the low Moderate range at the usual locations along the I-95 Corridor, depending on the degree of surface stagnation and air mass modification, but continued northwesterly back trajectories from the Lake Superior region should limit most locations to the Good range.

The ECMWF solution will mean Good air quality for Sunday, as a cold front either approaches or passes through the NMA, with clouds, rain, and possibly thunderstorms during the day. The drier and warmer GFS/CMC solution will offer the chance for air quality to reach the Moderate range at more widespread locations across the region, as transport aloft turns westerly and temperatures and humidity gradually increase.

Looking ahead beyond the medium range period to early next week, the ECMWF offers the best chance for deteriorating air quality in the Monday-Tuesday time frame. The ECMWF’s weekend trough quickly gives way to upper level ridging over the eastern U.S., while the GFS is faster to develop a weak trough over the Great Lakes on Tuesday that will displace the ridge. We will continue to monitor these developments.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 11, 2014
Valid: August 12-16, 2014 (Tuesday-Saturday)

Summary:

Generally Good air quality is expected this week, with conditions more reminiscent of early Fall than late Spring, as a cold front impacts the region Tuesday-Wednesday and then a closed low lingers over southern QC/New England through Friday. There is a chance for synoptic conditions conducive to deteriorating air quality beginning just beyond the end of medium range period.

Discussion:

The weather forecast models are in excellent agreement this morning on the synoptic features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. A rapidly amplifying longwave trough centered over the Great Lakes on Tuesday will move over the Mid-Atlantic on Wednesday and cut off over southeastern ON late in the day. The closed low will rotate slowly eastward though the end of the period, lingering over southern QC and New England. It will begin to pull away toward eastern QC and NB on Saturday as an upper level ridge tries to build in from the west.

It still looks to be a very quiet week for air quality. Tuesday will be cloudy and rainy all day, with breezy onshore winds, as the strong longwave trough aloft pushes a cold front toward the Mid-Atlantic from the Great Lakes and a weak coastal low/wave moves up from the Carolinas. The areas of heaviest rain will target primarily the northern Mid-Atlantic (NMA) late Tuesday and overnight into Wednesday. Air quality will be in the Good range regionwide, with the chance for lingering Moderate PM2.5 at locations just west of I-95 (e.g., Susquehanna Valley), where PM2.5 concentrations are well into the mid-Moderate range this morning and won’t be impacted by the heaviest rain until Tuesday evening.

The cold front will clear the region on Wednesday morning. Rain will taper off in the morning from southwest to northeast as the main surface low moves into southern ON. Scattered showers will linger through the afternoon in the NMA. Breezy west/northwesterly winds behind the front and falling dew points as a new air mass builds in will promote Good air quality.

Generally Good air quality should be the rule across the region for Thursday-Saturday. The possible exception could be Moderate PM2.5 if the persistent smoke plume over eastern Canada from fires in western Canada and the U.S. mixes down toward the surface again in the wake of Wednesday’s cold front, similar to what occurred this past week. The NAAPS model does not show any of the Canadian smoke extending down in the Mid-Atlantic, but with the low aloft over New England/southern QC, it will need to be monitored. Otherwise, the closed low just to the north will keep a cool pool of air over the Mid-Atlantic until Saturday, at which point it pulls away to the northeast. Temperatures will be below average, and humidity will be quite low for mid-August (e.g., mid-50s °F in the NMA). Northwesterly transport, breezy surface winds, and cool air aloft will promote mixing. Pollutants may begin to rebound on Saturday, but with the center of high pressure to the west (e.g., OH), continued northwesterly back trajectories, below average temperatures and dew points, there is no reason to expect anything beyond low Moderate air quality at this point.

There is a hint of a more conducive pattern for deteriorating air quality setting up for early next week, but it is too soon to draw any conclusions. The GFS and ECMWF bring an upper level ridge over the eastern U.S. and build the Bermuda High westward Sunday-Monday. Both models have a strong shortwave moving into the Great Lakes on Monday, however, that may limit the impact of the ridge.

-Huff