Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, August 10, 2014
Valid: August 11-15, 2014 (Monday-Friday)

Summary:

It will be a quiet week for air quality. Sustained onshore flow on Monday will push Moderate air quality inland, to scattered locations west and north of I-95. Unsettled weather Monday afternoon to Wednesday morning associated with the next cold front will give way to the arrival of a very cool and dry air mass on Thursday and Friday. Good air quality is expected regionwide, although PM2.5 may be impacted by smoke from Canada again late in the period.

Discussion:

There is little change from yesterday in terms of the synoptic pattern for the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The axis of a weak upper level ridge will be over the Mid-Atlantic and East Coast on Monday, compressed between the semi-permanent closed low over eastern Canada and a developing longwave trough over the Great Lakes. This longwave trough will amplify rapidly on Tuesday as it swings eastward, anchored by a strong vorticity maximum over the Great Lakes (i.e., MI). The trough will cut off late Wednesday over southeastern ON/southwestern QC/northern NY. Here, the models diverge. The GFS stalls the closed low and keeps it spinning over southern QC/New England through the end of the period. In contrast, the ECMWF slowly but steadily moves the closed low eastward toward NS. These differences have the most impact looking forward beyond the medium range period to next weekend, when the progressive ECMWF solution will allow for a faster departure from Good air quality.

It looks to be a relatively quiet week for air quality in the Mid-Atlantic. Although ozone mixing ratios are climbing rapidly this morning along I-95, sustained onshore flow will develop on Monday afternoon as the center of surface high pressure moves east of the region. Rain and clouds will move into the Mid-Atlantic from the west in the afternoon and evening, sparked by the developing longwave trough over the Great Lakes and interacting surface lows over IL/IN/MI. There are some model differences with the advance of the rain, as is typical. The operational guidance indicates most of the western edge of the Mid-Atlantic will be impacted, which is supported by the 12Z 4 km NAM (e.g., western PA, WV, MD, VA, and NC). The 12Z 12 km NAM keeps the northern part of the region clear however, limiting rain to western VA and NC. Given this rain in the west and onshore flow in the east of the region, the chances for Moderate air quality will be pushed to inland locations in the central/eastern Mid-Atlantic. The 06Z NOAA, BAMS, and NC ozone models are basically consistent in painting an area of Moderate ozone along northeastern PA and northern NJ. Smoke is still streaming down from Canada today and impacting surface PM2.5 concentrations. This will likely continue until late Monday into Tuesday, at which time the predominant northwesterly flow around the western edge of the semi-persistent upper low over eastern Canada will be displaced by the approaching longwave trough from the Great Lakes.

The longwave trough will move eastward on Tuesday. Its associated surface low will interact with a weak coastal low moving up from the Carolinas, bringing widespread rain and clouds to the region, as well as breezy onshore winds. The cold front will pass through the region on Wednesday morning. There are still minor timing differences with the speed of the front and the end of precipitation on Wednesday, the GFS being faster than the ECMWF with both. Rain likely will end in the late morning or early afternoon, with clearing from south to north as the surface low moves into ON/QC and the coastal low heads to New England. Good air quality is expected across the region both Tuesday and Wednesday, although PM2.5 may linger in the Moderate range at inland locations west of I-95 on Tuesday, depending on the strength of onshore flow.

Another presumably clean, cool, and dry Canadian air mass will build into the region on Thursday and Friday behind the frontal passage. We will have to watch for a recurrence of smoke transported from Canada as the closed low settles over southern ON/QC and the Northeast, so Moderate PM2.5 may be a concern. It will be mostly sunny and dry, but with high temperatures in the upper 70s °F, it will feel more like early Fall than late Summer. Ozone should remain in the Good range at most locations.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, August 9, 2014
Valid: August 10-14, 2014 (Sunday-Thursday)

Summary:

Sunny skies, light/calm winds, and continued transport of smoke from Canada will keep air quality in the Moderate range in the northern Mid-Atlantic on Sunday, while Good air quality will continue in the southern Mid-Atlantic due to widespread clouds and precipitation. Sustained onshore flow on Sunday will shift Moderate air quality to scattered locations inland in the NMA, west and north of I-95. The approach of the next cold front on Tuesday and its passage on Wednesday will promote generally Good air quality for the end of the period.

Discussion:

The weather forecast models are in general agreement on the overall synoptic features of the medium range, although there are typical discrepancies late in the period related to the passage of the next cold front. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The upper level western U.S. ridge will continue to move eastward and amplify over the Mid-Atlantic on Sunday and Monday, getting compressed between the departing closed low over eastern Canada and a developing longwave trough over the Great Lakes. This longwave trough will amplify rapidly and move into the Mid-Atlantic on Wednesday, pulling a cold front through the region, and then cut off early Thursday over the Northeast U.S./southern QC. The GFS is slightly faster with the progression of the trough and keeps the closed low farther north and west than the ECMWF on Wednesday and Thursday; these differences translate into timing differences with precipitation over the Mid-Atlantic on Tuesday-Thursday.

The primary day of interest for the medium range period is Sunday. Surface high pressure will settle just to the east of the region, keeping surface winds light and variable, with the possibility of a sea breeze in the afternoon in the northern Mid-Atlantic (NMA). Mostly sunny skies north of roughly northern VA will promote Moderate ozone, most likely in the vicinity of I-95. Chances for upper Moderate ozone will depend on if/where surface convergence occurs and how modified the air mass is. Regional ozone is rising relatively slowly this morning, which suggests that the air mass is still relatively clean. The 06Z air quality models all indicate scattered to isolated Moderate ozone, mainly along a band stretching across southern PA/MD/northern VA/DE/southern NJ. The BAMS models are the most aggressive, with pockets of upper Moderate ozone along western MD and PHL. Smoke is still streaming down from Canada this morning; it is quite evident in visible satellite imagery. The NAAPS model is capturing the smoke plume more accurately today, and keeps it over the NMA through Tuesday. Thus, continued low-mid Moderate PM2.5 is expected across the NMA on Sunday. Locations from central VA to NC will see Good air quality due to clouds and rain associated with disturbances traveling along the stalled frontal boundary across the Mississippi River Valley and Southeast.

Sustained onshore flow will develop on Monday as surface high pressure slides farther to the east. Precipitation will stay to the south, with scattered showers continuing across the SMA. Onshore winds will push Moderate air quality inland to locations west and north of I-95 in the NMA, with Good conditions expected elsewhere.

The remainder of the period will feature generally Good air quality. Tuesday and Wednesday will be unsettled as the cold front approaches from the west and a weak coastal low moves up from the south. The operational models and the 09Z SREF indicate widespread clouds and rain across the entire region on Tuesday afternoon/evening. The cold front will pass through the region on Wednesday. Guidance suggests that the rain will end by the afternoon, from south to north, as the main low tracks into QC. The ECMWF is slower to end the precipitation and keeps it farther south and east than the GFS. A new air mass will build into the region on Thursday, with slightly below average temperatures and low humidity. The proximity of the closed low aloft just to the north will keep a cool pool of air over the region, promoting deep mixing.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 8, 2014
Valid: August 9-13, 2014 (Saturday-Wednesday)

Summary:

Smoke transported from Canada and nearby surface high pressure will promote mid-upper Moderate air quality in the northern Mid-Atlantic over the weekend. Areas roughly south of northern VA will experience Good air quality from clouds and periods of heavy rain associated with a series of surface waves moving from west to east. Mostly Good air quality is expected across the region Monday-Wednesday as sustained onshore flow, a slow-moving coastal low, and an approaching cold front keep the weather unsettled.

Discussion:

The weather forecast models are in reasonable agreement on the overall synoptic pattern for the medium range period, although there are some minor discrepancies late in the period. The models are coming into consensus today on a more southerly solution for the series of surface waves that will impact mainly the southern Mid-Atlantic (SMA) today through Sunday. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. Aloft, a closed low over the Canadian Maritimes today will continue to retreat eastward, allowing the broad ridge over the Plains/Great Lakes to build over the Mid-Atlantic on Sunday. By Tuesday, a deepening upper level trough will displace the ridge and push a cold front through the Mid-Atlantic on Wednesday. Guidance suggests that this will be a strong trough that will cut off over ON/QC/NY on Thursday.

The main focus will be on the series of shortwaves that will trigger waves of low pressure along a stalled frontal boundary draped across the Mississippi River Valley (MRV) and Southeast and bring periods of heavy rain to the SMA through Sunday. There is still disagreement among the models regarding the location and progression of the waves, but they are trending toward the more southern solution of the ECMWF. The WPC once again prefers the ECMWF over the NAM and GFS this morning. The NAM is still the farthest north of the operational models with the track of the waves and extent of precipitation, but it is farther south than it was yesterday. The GFS places the heaviest precipitation north of the ECMWF on Saturday and is slower to move the wave toward the coast on Sunday. The WPC follows the ECMWF and places the axis of heaviest precipitation over southern-central VA and central NC on Saturday and coastal NC on Sunday, following the slow eastward progression of the wave. On Monday, the GFS and ECMWF develop a weak coastal low off of the NC coast. The models agree – although with some differences – that this low will slowly move up the Mid-Atlantic coast and interact with the approaching cold front on Tuesday-Wednesday.

The bottom line for air quality is some continued uncertainty regarding the northern extent of clouds and precipitation over the weekend, especially on Saturday. For now, it appears that areas roughly north of northern VA will remain clear, but this could change depending on the actual track of the surface waves. For central VA southward, especially NC, clouds and periods of heavy rain associated with the passage of the surface waves will keep air quality well inside the Good range over the weekend.

The northern Mid-Atlantic (NMA), where conditions will be dominated by surface high pressure over the weekend, is the area of most interest for air quality. The center of surface high pressure will be overhead or just to the north on Saturday, moving to New England on Sunday, and then continuing on to the Canadian Maritimes on Monday and Tuesday. Temperatures over the weekend will be seasonably warm, with mostly sunny skies and relatively low humidity. With high pressure nearby, surface winds will be very light and recirculating. Back trajectories on Saturday are still northwesterly from ON, but they turn very slow and localized on Sunday. Thus, rising ozone is a concern along the I-95 Corridor for both weekend days. The 06Z BAMS and NC ozone models all develop a bull’s-eye of USG ozone in the vicinity of Washington, DC on Saturday, likely in response to the stagnating winds. The 06Z NOAA ozone model is much less aggressive and keeps ozone in the mid-upper Moderate range in the DC/BAL area. Certainly, localized mid-high Moderate ozone is possible, given sufficient sunlight, a modified air mass, and calm or converging winds. The main forecast questions will be the extent of any afternoon cloud cover from the disturbances to the south and the degree of modification of the air mass.

PM2.5 concentrations remain in the Moderate range across the region this morning, despite the arrival of a relatively clean air mass behind the recent frontal passage. This is due to a smoke plume being pulled over the Mid-Atlantic and Great Lakes by the closed low over New England. The smoke plume was very striking in yesterday afternoon’s visible satellite imagery, and it is still evident this morning. None of the smoke models picked up on the surface transport (e.g., NOAA or NAAPS), although they do show the plume aloft. The influence of the smoke on PM2.5 in the NMA seems likely to continue through the weekend, keeping conditions in the Moderate range.

As the surface high retreats eastward on Monday, winds aloft and at the surface will turn onshore in the NMA, which will increase humidity and decrease temperatures slightly. Air quality along the coast should fall into the Good range, while Moderate conditions will continue at inland locations, particularly for PM2.5. The skies across most of the SMA should clear as the surface wave exits to the coast, but the with the recent periods of heavy rain, the air mass should be quite clean, allowing Good air quality to continue.

Tuesday and Wednesday will be unsettled, as the next cold front approaches from the northwest and the weak coastal low moves up the coast. Sustained onshore flow and widespread clouds and showers will keep air quality in the Good range on Tuesday. The front will move through the region on Wednesday, promoting another day of Good air quality.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 7, 2014
Valid: August 8-12, 2014 (Friday-Tuesday)

Summary:

There is major uncertainty in the air quality forecast beyond Friday given discrepancies in numerical model guidance regarding the development of waves of low pressure that will bring widespread clouds and periods of heavy precipitation to the WV/VA/NC/MD area Saturday-Monday. Depending on the modification of the air mass and the track of the surface lows, Moderate air quality is most likely on Friday and Saturday, mainly in the northern part of the region. Otherwise, the surface lows and a transition to onshore flow in the north on Sunday will promote mostly Good air quality.

Discussion:

The weather forecast models are in consensus on the overall synoptic pattern during the medium range period, but they do not agree on the mesoscale features. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The period will begin with an upper level closed low over New England that will retreat to the Canadian Maritimes while the western ridge builds eastward over the Mid-Atlantic. The axis of the upper level ridge will be over the Mid-Atlantic/East Coast by Monday, at which point a longwave trough will redevelop over the Great Lakes and begin pushing eastward on Tuesday. At the surface, high pressure centered over southern ON/QC and the northern Mid-Atlantic (NMA) on Friday and Saturday will slide to New England and the Gulf of Maine on Sunday and Monday. The frontal boundary making its way south of the region today will stall across the Southeast/Mississippi River Valley (MRV) through the end of the period.

The weather forecast models agree that a series of shortwaves moving from west to east in the upper level flow will trigger waves of low pressure somewhere along the stalled frontal boundary draped across the MRV/Southeast on Friday-Sunday, but there is no consensus yet as to exactly where the waves will track and where the areas of heaviest precipitation will be over the weekend. The operational NAM and its mesoscale models have consistently been the farthest north with the shortwaves and surface waves. For example, the NAM brings the surface wave across the central Mid-Atlantic on Saturday, through WV/VA/MD, to Washington DC by 00Z Sunday and the Delmarva by 12Z Sunday. The WPC considers the NAM’s solution to be too far north and thus an outlier, but it has had quite good run to run consistency so it needs to be monitored for the potential impact on air quality in the NMA. The WPC prefers the ECMWF, which is the farthest south with the wave, bringing it over southern VA/NC on Saturday and taking it to the NC/SC coast on Sunday morning. The GFS solution is located somewhat between the NAM and ECMWF, but the axis of heaviest precipitation is farther west and not as strong as the NAM or ECMWF, so the WPC considers it an outlier as well.

Cloud cover and heavy precipitation along the actual track of the surface waves will keep air quality well in the Good range for Saturday-Monday. Given the uncertainty with the model guidance, however, the demarcation line between areas that will experience clouds and precipitation is not clear. For now, we will follow the WPC and rely primarily on the ECMWF.

On Friday, the center of surface high pressure will be overhead, promoting mostly sunny skies and light surface winds. A relatively clean air mass will be in place, with transport aloft from interior ON. Light surface winds may allow isolated pockets of Moderate ozone along I-95 and western NC, depending on how clean the air mass is. The cleaner and drier Canadian air mass has been slow to filter into the region behind the cold front, so PM2.5 concentrations are still in the Moderate range this morning in many areas, especially western PA and VA/NC. There is dry air aloft, and presumably the transition of the air mass will continue to progress, and PM2.5 concentrations will continue to fall today and remain generally in the Good range tomorrow.

Per the ECMWF, clouds and rain will be restricted to the southern Mid-Atlantic (SMA) on Saturday, primarily western and central VA/NC, where air quality will be in the Good range. Areas roughly from northern VA northward will be mostly sunny, with very light and recirculating surface winds. The conditions, in conjunction with slow and localized back trajectories, will push ozone into the Moderate range along I-95, with Moderate PM2.5 possible as well, especially in the Susquehanna Valley. The main forecast questions will be the degree of modification of the air mass and the northern extent of clouds associated with the surface wave. If the farther north NAM solution for the surface wave verifies, there will be little chance of Moderate ozone in the NMA on Saturday.

Flow turns onshore – both aloft and at the surface – in the NMA on Sunday and continues through the end of the period. As a result, temperatures will drop slightly but atmospheric humidity will increase. Isolated areas of Moderate air quality are possible inland on Sunday and Monday, depending on the strength of the onshore winds. Good air quality will be the rule in the areas of the SMA that are impacted by the surface waves.

On Tuesday, a weak coastal low moving up from NC may keep precipitation across the eastern part of the region. The GFS favors the development of the low, while the ECMWF pushes the area of low pressure out into the Atlantic. Continued onshore flow in the NMA should restrict air quality to the Good to low Moderate range.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 6, 2014
Valid: August 7-11, 2014 (Thursday-Monday)

Summary:

Air quality is not expected to rise above the low-mid Moderate range during the period, especially in the southern Mid-Atlantic, where widespread rain over the weekend will promote very clean conditions. The best chances for Moderate air quality will be in the northern Mid-Atlantic Friday-Sunday, as surface high pressure promotes mostly sunny skies and light surface winds.

Discussion:

The weather forecast models agree on the main synoptic features of the medium range period but are having trouble with mesoscale features triggered by a series of shortwaves beginning on Friday, which will mainly impact the southern Mid-Atlantic (SMA). The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The medium range begins with an amplifying upper level trough over New England and the Canadian Maritimes that will cut off on Friday and then slowly progress eastward through the end of the period. As the upper level trough retreats, broad ridging will spread eastward over the Mid-Atlantic. The influence of the ridge will be blunted over the SMA in particular due to a series of shortwaves that will move through the upper level flow and develop MCSs/waves along a stalled frontal boundary, producing widespread rainfall over the weekend across VA/NC. There looks to be a significant difference in the sensible weather between the northern Mid-Atlantic (NMA), where surface high pressure will dominate and keeps skies mostly clear, and the SMA, which will experience the effects of the MCSs/waves, although the location of the specific dividing line is not clear at this point.

The cold front moving through the region today will clear NC on Friday morning. This boundary will stall along the Mississippi River Valley (MRV) and Southeast through the end of the period. Cooler and much less humid air will filter in behind the front, with northwesterly flow. Good air quality is expected across the region for Friday.

Surface high pressure located over southern ON on Thursday will gradually move eastward though the end of the period, reaching NY state on the weekend and sliding off the Cape Cod coast by Monday. The proximity of the high will keep skies mostly sunny in the NMA, with slightly below average temperatures (e.g., highs in low 80s °F). Humidity will begin to increase over the weekend as the flow turns southerly, but it should remain relatively low for early August (e.g., low 60s °F). Air quality is expected to remain in the Good to low-mid Moderate range, with the highest ozone and PM2.5 concentrations along the I-95 Corridor, as usual. Friday and Saturday are probably the days when stagnating surface winds will push pollutant levels slightly higher, before the flow shifts onshore on Sunday and Monday. Depending on how the onshore flow is on Sunday, inland locations may see PM2.5 rise into the Moderate range, particularly the Susquehanna Valley and western PA/MD.

From a meteorological standpoint, the forecast for the SMA is far more interesting, but the only question for air quality will likely be how low ozone and PM2.5 concentrations fall into the Good range, particularly on Saturday and Sunday. The forecast models do seem to agree that shortwaves moving from the Ohio River Valley (ORV) and MRV on Friday through Sunday will interact with the stalled frontal boundary and trigger MCSs and/or waves of low pressure. As is typical, the models are having trouble agreeing on the location, track, and strength of these mesoscale features. The WPC prefers a blend of the GFS and ECMWF guidance, as they consider the NAM’s wave/MCS to be too far north and too strong on Friday-Saturday. The blended guidance indicates there will be widespread rain across VA and NC on Saturday and Sunday, with periods of heavy rain and possibly some imbedded thunderstorms. Where exactly the transition line to the rain will be, moving south from MD/northern VA, is not clear at this point. Furthermore, clouds ahead of the approaching meso low on Friday may push into the western Mid-Atlantic on Friday afternoon, along western VA/WV. By Monday, the axis of the upper level ridge will be along the East Coast, helping to dry things out, although the models do develop a weak coastal low off of the NC/SC coast, particularly the ECMWF. So the development of these mesoscale features bear watching and seem likely to bring several days of Good air quality to locations roughly south of Washington, DC/northern VA on Friday-Sunday, possibly continuing into Monday.

-Huff