Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 27, 2016
Valid: May 28 – June 1, 2016 (Saturday-Wednesday)

Summary:

An Appreciable chance for an ozone exceedance remains for Saturday across the northern Mid-Atlantic, mainly north and west of I-95. The main forecast questions for Saturday will be the influence of a shift to southerly onshore flow around the circulation of a weak tropical/sub-tropical low developing off of the Southeastern U.S. coast and the eastern extent of diurnal afternoon convection. The flow aloft turns completely onshore for Sunday and Monday, which should clean out the region from south to north and will drop the chances for an ozone exceedance to Slight. The highest ozone (Moderate) will be on Sunday, in the northern Mid-Atlantic, which will be the sunnier and drier of the two days. There is higher than usual uncertainty in the air quality forecast for Tuesday and Wednesday. A weak cold front will move into the northern Mid-Atlantic on Monday and the central Mid-Atlantic on Tuesday, before finally stalling along the Atlantic coast on Wednesday. The fate of the tropical/sub-tropical low is also uncertain; the low may possibly interact with the stalled front on Wednesday. At this point, Tuesday appears to have slightly higher chances (Marginal) for an ozone exceedance at locations ahead of the cold front. Chances for an ozone exceedance drop to Slight on Wednesday due to slightly lower temperatures and a somewhat higher chance of clouds and precipitation. PM2.5 will follow a gradual downward trend in concentrations, with mid-Moderate conditions continuing on Saturday along I-95.

Discussion:

The weather models are in close agreement on the main features of the medium range through Monday, but they diverge at the end of the period. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. The big story remains the tropical or sub-tropical low developing off the Southeastern U.S. coast on Saturday. The National Hurricane Center gives this system a 90% chance of developing over the next 48 hours. To that end, all of the deterministic models bring the low to the SC coast by the end of the day Saturday; the NAM is still slightly slower than the GFS and EC. The upper level ridge remains over the Mid-Atlantic on Saturday, but it will begin to be pinched on its southern edge by the tropical system. On Sunday, the tropical/sub-tropical low will spin over SC. There are some subtle differences in the models regarding the strength and exact track of the low. At that same time, a strong vort max over the Plains will move northeastward, into ON, and will carve out a shallow upper level trough, which will gradually move eastward across southern ON into southern QC on Monday and push the upper level ridge eastward to the Canadian Maritimes. The GFS/NAM and EC continue to differ slightly on the fate of the tropical low; the GFS and NAM keep it over the Carolinas through Tuesday, while the EC brings it northeastward along the NC coast. The Canadian upper level trough will slowly push a weak cold front into the northern Mid-Atlantic on Monday and Tuesday. Both models bring the front roughly parallel to the Eastern Seaboard on Wednesday, where it interacts with the tropical low to varying degrees. The EC is much drier on Tuesday and Wednesday, while the GFS is wetter. Both models build a relatively weak upper level ridge back over the Mid-Atlantic on Wednesday, but the EC is faster and farther north with the ridge than the GFS.

Saturday is the day of most interest during the period. With the ridge still over the region, it will be hot and humid. The main questions are afternoon cloud cover/convection and the impact of the shift to onshore flow aloft, which will begin tomorrow at lower levels (500 m AGL). Yesterday, there was a general eastward shift in the locations of highest ozone, and this trend is expected to continue today. The highest ozone seems to be following the eastward progression of the air mass, which very likely contains dilute smoke from the western Canadian wildfires. Tomorrow, 06Z GFS back trajectories for the I-95 Corridor swing due south at 500 m AGL, and surface winds will also be southerly, albeit light inland. Historically, onshore flow aloft cleans the region out quickly from south to north. The coarse resolution models and the 03Z SREF continue to show plenty of afternoon convection, triggered by topography, west of I-95. The 06Z 4 km NAM shows more isolated thunderstorms, however. The air quality models may finally be catching up observed conditions. The 06Z NOAA model shows isolated USG ozone in BAL, western PA, and northern NJ, with upper Moderate along the rest of the I-95 Corridor and western NC. The 06Z BAMS models show a similar pattern without the pockets of USG ozone. How high ozone rises today, especially in the southern parts of the region, should give us a better idea of how quickly the air mass is cleaning out (if at all). Certainly, there is much less NOx titration this morning along I-95, and PM2.5 is lower as well across the western and southern parts of the region, which suggests the air mass is not quite as dirty as the past two days. However, an Appreciable chance for an ozone exceedance continues for Saturday, mainly at locations along and just to the north and west of I-95, where there will presumably be sufficient afternoon sun, and where emissions will be pushed by southerly flow. PM2.5 seems to be slightly lower along I-95 this morning, at least in PHL, but it remains well inside the mid-Moderate range. Moderate conditions should continue across most of the central and northern Mid-Atlantic on Saturday, as PM2.5 will be slower to respond to the southerly flow compared to ozone.

On Sunday and Monday, 06Z GFS back trajectories aloft for the I-95 Corridor shift completely onshore, from the southeast. This should drop ozone down to the Good to Moderate range, with the highest ozone remaining in the northern part of the region. The tropical low does not look like it will be strong enough to generate enough subsidence along its periphery to cause a spike in ozone. The circulation will be coming from the southeast, where there will be presumably clean tropical/maritime air. There is a risk that the polluted air mass that was over the Mid-Atlantic on Wednesday and yesterday could be recirculated back into our region, but given the location of the tropical low, this risk seems small. Sunday will still be very warm and mostly sunny across the northern Mid-Atlantic, with the usual chances for diurnal convection. Monday looks much cloudier with greater chances for more widespread precipitation due to a cold front approaching from the northwest and an influx of humidity and possibly tropical moisture. It looks like a washout both days for the southern Mid-Atlantic. As a result, the chance for an ozone exceedance will drop to Slight both days, with the highest ozone expected on Sunday (Moderate) across the northern part of the region. The air quality models support this trend, with Good ozone in locations impacted by the tropical low. PM2.5 will follow a similar trend, with a gradual decrease in concentrations through the holiday weekend.

There is a great deal of uncertainty in the air quality forecast for Tuesday and Wednesday. The track and strength of the tropical low remains a question mark. A weak cold front will be moving into the central Mid-Atlantic on Tuesday and possibly settling along the Atlantic coast on Wednesday and interacting with what is left of the tropical low. In addition, a weak ridge may be building over the region by Wednesday. At this point, Tuesday looks more favorable for rebounding ozone, mainly at locations east and south of I-95, which will be ahead of the cold front. Wednesday may be a bit cooler with northwesterly (although slowish) transport behind the cold front. So for now, we assign a Marginal chance of an ozone exceedance for Tuesday, dropping to Slight for Wednesday in response to lower temperatures and possibly a greater chance for clouds and precipitation.

-Huff/Brown

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 26, 2016
Valid: May 27-31, 2016 (Friday-Tuesday)

Summary:

There is a High chance for an ozone exceedance in the Mid-Atlantic on Friday due to the continued effects of an upper level ridge and a very modified air mass in place. Chances for diurnal afternoon convection on Friday firing up over the mountains place the best chances for exceedances along and north of I-95. On Saturday, a weak tropical low will begin to affect the air quality forecast as the system’s circulation starts to turn back trajectories onshore. The effects of subsidence ahead of the system, which would promote continued ozone exceedances, will need to be weighed against the effects of onshore flow, which would promote a gradual clean-out. For now, there is an Appreciable chance for ozone exceedances on Saturday, primarily north of I-95 and possibly across western NC. Sunday and Monday look to be dominated by the impacts of the tropical system. If the current guidance verifies, it will be a washout along the southern Mid-Atlantic, and strong onshore transport will keep the chances for ozone exceedances Slight for the rest of the region. The weather model guidance diverges on Tuesday, making the forecast highly uncertain, with the tropical system either remaining over the southern Mid-Atlantic coast or moving off of the central Mid-Atlantic. In addition, a weak cold front moving into northern PA/NJ may act as a focus for clouds and rain. For now, we keep a Slight chance for an ozone exceedance on Tuesday.

PM2.5 has risen quickly today up into the mid-to-upper Moderate range at locations along I-95. Without an air mass change, concentrations are expected to remain at the same levels on Friday, and into Saturday for the northern Mid-Atlantic. A gradual downward trend should occur through the remainder of the period as onshore flow brings cleaner air into the region.

Discussion:

The weather forecast models continue to be in quite close agreement on the main aspects of the medium range period, except for the NAM, which is an outlier. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. There are some changes to the synoptic outlook in this morning’s model guidance, in particular with the fate of the back door cold front (BDCF) that, in previous days’ guidance, had threatened to move southward and impact the northern Mid-Atlantic. Now, the front stays well to our north and will not affect our region. The upper level ridge will build more northward over the Mid-Atlantic on Friday and remain in place through Sunday. The strength and placement of the ridge will prevent any shortwaves moving along its periphery from impacting the Mid-Atlantic.

The big story continues to be the potential tropical low that will approach the Southeastern U.S. on Friday. The National Hurricane Center is now monitoring this disturbance, and gives it a 50% chance of becoming a tropical system in the next 48 hours, with a 70% chance in the next 5 days. All three deterministic models, as well as the MREF ensemble members, continue to develop the system to varying degrees. The NAM is about 12 hours slower than the GFS and EC and is considered an outlier by Saturday. Today, the GFS and EC are in closer agreement than yesterday on the track of the system. They bring a vorticity maximum along the FL coast on Saturday and onshore in the vicinity of SC on Sunday. The EC continues to have a more organized system, while the GFS solution is more diffuse. The WPC recommends a GFS/EC consensus blend. The two models diverge by Tuesday, however, with the GFS weakening the system and keeping it over SC/NC, while the EC has a tighter circulation, and moves the low along the Atlantic coast to HAT and then offshore. The two models also handle the breakdown of the upper level ridge differently at the end of the period. The EC develops a stronger upper level trough over QC, which pushes a weak cold front to roughly the PA/NY border on Tuesday morning. The GFS has a much weaker upper trough, which in turn, slows the progression of the associated cold front. The WPC favors the EC solution.

The day of most interest in the period continues to be Friday. The forecast models are consistent in developing diurnal afternoon convection across most of the region, supported by the 03Z SREF. The 06Z 4 km NAM, however, shows only isolated thunderstorms developing over the mountains in the afternoon. With the strong ridge aloft sheltering the region from shortwave activity, it is hard to see what the trigger for widespread convection will be, aside from orographic lifting. One of the forecast questions will be identifying where and to what extent convection develops on Friday. The other question will be how high ozone rises today, which will give an idea of what the residual layer will hold. 06Z GFS back trajectories for the I-95 region swing more southwesterly for Friday, in the vicinity of the southern Mid-Atlantic, but USG ozone forecasts are posted there for today, which suggests that there will be plenty of ozone upwind. In addition, the air mass seems to be very modified, with widespread USG observed ozone yesterday across almost the entire Northeastern U.S. and a smattering of Unhealthy readings along I-95. And this morning, PM2.5 is well into the mid-to-upper Moderate range along I-95. I am convinced that the air mass contains dilute smoke that is pushing up PM2.5 and ozone higher than might otherwise be expected (certainly based on climatology for the past few years). Smoke would also explain why all of the air quality models have been and continue to be biased low for ozone and PM2.5. That trend continues tomorrow, although the 06Z NOAA model does have an isolated area of USG ozone stretching from PHL through TTN to northern NJ. Despite the lack of support from most of the air quality model guidance, there is a High chance for an ozone exceedance tomorrow in the Mid-Atlantic, most likely along and north of the I-95 Corridor, where skies are expected to remain sunny through the entire afternoon, and light southerly surface winds will push urban emissions northward. Without an air mass change, PM2.5 should persist in the mid-to-upper Moderate range, mainly along and north of I-95.

On Saturday, the Mid-Atlantic will begin to feel the effects of the weak tropical system developing off of the Southeastern U.S. coast. 06Z GFS back trajectories for the I-95 Corridor begin to shift southerly, with an onshore component at lower levels, due to the circulation around the low. Historically, a shift to southerly flow has very quickly cleaned out locations roughly to the south of the Mason-Dixon Line. Depending on the strength of the tropical low, subsidence on its periphery could be an issue. Often, subsidence ahead of tropical cyclones is sufficient to push ozone to an exceedance level. Saturday will be hot, with temperatures in the low to mid 90s °F. The coarse resolution weather models and the SREF fire up afternoon convection west of I-81, presumably trigged by orographic lifting and humid conditions, but it should remain mostly sunny along and east of the I-95 Corridor. The main forecast questions for Saturday will be the strength and track of the tropical system. A stronger system may result in higher ozone and additional exceedances due to subsidence, but the impact of the onshore circulation will have to be weighed as well. As a result, there is an Appreciable chance for an ozone exceedance on Saturday, mainly at locations north and west of I-95 and potentially western NC. We expect a slow downward trend with PM2.5 through the weekend as the effects of the shift to onshore flow gradually bring clean air into the region.

Sunday and Monday appear to be dominated by the tropical system. If the current guidance verifies, it will be a washout across the southern Mid-Atlantic due to the slow-moving system. 06Z GFS back trajectories for the I-95 Corridor for both days are completely onshore, and thus very clean. It will still be hot in most of the region on Sunday, while temperatures will drop slightly to the lower 80s °F on Monday due to more widespread clouds and precipitation associated with the tropical system itself and the moisture it pulls into the region. Given the relative weakness of the tropical system, there is uncertainty in the air quality forecast, but at this point, the chance for an ozone exceedance on Sunday and Monday appears Slight.

Tuesday’s forecast is uncertain as well. A weak cold front will likely be slowly moving into the northern Mid-Atlantic, which may be sufficient to act as a focus for clouds and rain. The fate of the tropical low is also a question – it could remain over the southern Mid-Atlantic (GFS) or move off of the central Mid-Atlantic coast (EC). 06Z back trajectories still show an onshore component for the I-95 Corridor. For now, we will assume a Slight chance for an ozone exceedance.

-Huff/Brown

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 25, 2016
Valid: May 26-30, 2016 (Thursday-Monday)

Summary:

There is a High chance for ozone exceedances in the Mid-Atlantic on Thursday and Friday due to the continued effects of an upper level ridge, which will keep skies mostly sunny, conditions very warm, and surface winds light. Transport aloft on both days is westerly, slow, and localized. There is strong evidence that ozone in the residual layer is high today and will remain high through at least Friday. There is a chance for diurnal convection both days, driven mainly by rising humidity, but any thunderstorms are expected to be isolated and limited to locations west of I-95. Considerable uncertainty remains for the end of the period due to the impacts of a back door cold front located to the north of the region and a weak tropical low that will settle off of the Southeastern US coast beginning on Saturday. Model guidance suggests a cleaning trend from south and east to north and west for the end of the period. There is still a Marginal chance of an ozone exceedance on Saturday, mainly for locations north and west of I-95. Back trajectories shift completely onshore for Sunday and Monday due to the circulation around the tropical low, which, if they verify, would essentially clean out the region.

Discussion:

The weather forecast models are in remarkably close agreement yet again today, and are showing relatively consistent run to run agreement, on the main synoptic features of the medium range period. The 00Z GFS and ECMWF and 06Z NAM were consulted for this analysis. The period begins on Thursday with the broad upper level ridge extending over the Mid-Atlantic region, with the northern edge of the ridge aligned along the northern Mid-Atlantic. The models are still showing many small shortwaves embedded under the ridge and riding along the periphery. On Friday, the upper level ridge axis will shift offshore to the east, and the ridge will build northward, allowing the Mid-Atlantic to be fully under the ridge. This shift will protect the Mid-Atlantic from the bulk of the shortwave activity to the west and north.

A back door cold front (BDCF) will be located to the north of the region on Thursday, along approximately southern NY into New England. There is still uncertainty regarding the day to day track of the front. The models seem to be trending toward moving the front more northward on Friday, away from the Mid-Atlantic, and then they begin to move it southward again beginning on Saturday. The front may be far enough south on Saturday and Sunday to affect the far northern Mid-Atlantic.

All of the deterministic models as well as the MREF ensemble members continue to bring a weak tropical system from the Bahamas up to the eastern coast of FL on Friday. The NAM and EC are in closer agreement on a stronger and more organized tropical low, while the GFS has a more diffuse circulation. This trend continues into Saturday and Sunday, with the NAM and EC showing a more organized low spinning off of the GA/SC coast. At this time, the tropical low begins to squeeze the upper level ridge from the south, but the ridge remains intact and over the Mid-Atlantic. By Monday, the GFS and EC diverge slightly regarding the fate of the tropical low and BDCF. The EC has a stronger shortwave moving through southern ON that pushes the BDCF further south, into the northern Mid-Atlantic, while the GFS has a weaker shortwave and in turn, keeps the front farther north, away from the Mid-Atlantic. In addition, the GFS moves the tropical low inland over GA/SC, while the EC keeps it spinning along the coastline. The WPC seems to favor the GFS solutions slightly more for the end of the period.

Thursday remains a day of interest for ozone exceedances across the region. It will be very warm, with high temperatures in the upper 80s to 90s °F, with light westerly winds shifting more southerly by the end of the day. Aloft, back trajectories are westerly, slow, and relatively localized. Upwind ozone is certainly expected to be high – there were widespread exceedances yesterday across most of the Great Lakes region, parts of the Ohio River Valley, and western NY and PA. In addition, preliminary analysis of ozonesonde measurements this morning (courtesy of Howard University) shows ozone in the 70s ppb in the residual layer. There is a lot of NOx titration across the Mid-Atlantic this morning, suggesting that there will be widespread ozone exceedances today. The coarse resolution weather models and the 03Z SREF still show diurnal convection firing up in the afternoon, but the 4 km NAM only has isolated thunderstorms. We continue to think that the strong ridge aloft will help to suppress most convective activity, and that coarse model predictions are somewhat overdone. The air quality models are still not showing any areas of USG ozone, but they appear to have a low bias based on yesterday’s guidance compared to observed conditions. Sean Nolan of PA DEP suggested that the reason the models are running low may be the influence of diffuse smoke from the Canadian wildfires – the air quality models do not include smoke in their boundary conditions. Thus, despite the persistent low biased air quality model guidance, there is a High chance for an ozone exceedance on Thursday in the region. The main forecast questions will be how high ozone climbs across the region today, and pinpointing the chances for afternoon convection on Thursday. PM2.5 also may be being impacted by the diffuse smoke, as well as rising humidity, with widespread Moderate concentrations expected on Thursday and probably Friday.

Friday now is also a day of interest, as the ridge will remain overhead, which will keep transport aloft localized and westerly. The back trajectory analysis is the main change from yesterday (from south/southwesterly and fast to slow and westerly). There is a question regarding the impact of surface winds, which will turn southerly, but remain light. Southerly winds may be sufficient to keep coastal locations and areas roughly south of I-95 in the Moderate range, while the highest risk for USG ozone will shift to eastern PA and NJ. The air quality models reflect this trend, with a hint of a clean out along the coast. There is also a chance for diurnal afternoon convection, but again, we expect any thunderstorms to be more isolated, not widespread. As a result, there is a High chance for an ozone exceedance on Friday as well.

Uncertainty remains high in the air quality forecast for Memorial Day weekend, due to questions regarding the strength and placement of the BDCF and tropical low. The GFS back trajectories begin to have an onshore component on Saturday and shift completely onshore on Sunday and Monday. This is due to circulation around the weak tropical system. The fact that all of the deterministic models and MREF members show the tropical system in roughly the same location lends credence to the likelihood of a shift to onshore transport for the Mid-Atlantic. Certainly, the southern Mid-Atlantic should feel the direct impacts of the system, with clouds, rain, and onshore surface winds. How far north and west these impacts extend is a question mark. In the northern part of the region, there is considerable uncertainty regarding the eventual southern progression of the BDCF. There is not consensus on the track of the front, nor has there been run to run consistency in the model guidance. The trend seems to be to bring the front southward late in the period, which would mean possibly an air mass change, or at least a shift to onshore surface winds and a concurrent decrease in high temperatures. All of these factors point to lower ozone, particularly on Sunday and Monday. Therefore, chances for an ozone exceedance drop to Marginal on Saturday, mainly for locations in the northern Mid-Atlantic north and west of I-95, while an ozone exceedance seems Slight for Sunday and Monday. PM2.5 will likely respond to the impacts of the tropical system and BDCF as well. The air quality models show a general clean out trend through the end of the period, starting with coastal and southern locations on Saturday and working inland and northward.

-Huff/Brown

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 24, 2016
Valid: May 25-29, 2016 (Wednesday-Sunday)

Summary:

There is a High chance for an ozone exceedance in the Mid-Atlantic on Wednesday and Thursday, given a stronger and farther northward extending upper level ridge compared to yesterday’s guidance. The weather forecast models now place the Mid-Atlantic completely under the ridge both days, which limits the chances for clouds and precipitation. Upwind ozone has risen quickly, with multiple exceedances on Monday from MI all the way to southern LA/AL. Back trajectory forecasts for Wednesday and Thursday bring this dirty air directly into the Mid-Atlantic. This pattern, combined with hot and sunny weather, makes exceedances likely for the next two days. Chances for an ozone exceedance drop to Marginal on Friday, as back trajectories shift south/southwestward and speed up, which will presumably transport cleaner air into the region. The weekend looks like a possible clean out, as the models bring a weak tropical system to the coast of the Carolinas, which will shift back trajectories onshore.

Discussion:

The weather forecast models continue to be in close agreement for most of the medium range period regarding the main synoptic features that will impact air quality conditions. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. There are some substantive changes to the synoptic pattern in today’s model runs. The first part of the period is still impacted by a broad upper level ridge that will build eastward today and move into the Mid-Atlantic tomorrow, as the pesky closed low exits to the northeast over the Canadian Maritimes. Similar to yesterday’s model runs, the deterministic models keep the ridge broad, without a sharp axis, on Wednesday. There are still several small shortwaves embedded under the ridge and at least three larger shortwaves in a broad upper level trough over the western U.S. The strength and placement of the eastern U.S. mid-level ridge mirrors that of the upper level ridge; at the surface, the Bermuda High begins to extend westward. On Thursday, the ridge continues to shift eastward, with the axis moving over the Mid-Atlantic. Today’s model runs build the ridge more northward compared to yesterday’s analysis. The main impact on air quality is that clouds and showers triggered by shortwaves moving through the flow on the edge of the ridge will be diverted northward on Wednesday and Thursday. The Mid-Atlantic remains completely under the ridge on Friday, which will shelter most locations from the impact of shortwaves moving through the flow aloft. There is a new wrinkle in the forecast for the weekend. Both the GFS and EC deterministic models and the MREF ensemble members bring a weak tropical system into the southeastern U.S. coastal waters on Saturday, and move it to the coast of the Carolinas on Sunday. The EC and GFS diverge somewhat, with the GFS bringing the weak system inland on Sunday, while the EC keeps it along the southern Mid-Atlantic coast. The impact of this system will be generate strong onshore flow aloft for the weekend. The other difference in today’s model analyses is that the weak back door cold front that will be located along the northern edge of the upper level ridge stays farther north on Wednesday through Friday – well into southern Canada. However, it begins to move southward on Saturday, and may travel as far south as the PHL-ILG metro region by Sunday morning.

Wednesday and Thursday continue to be the days of most interest for the period. As the ridge builds over the region, temperatures will quickly climb into the upper 80s to low 90s °F. It will be sunny across most of the region both days, with slow west/northwesterly back trajectories. Ozone air quality deteriorated quickly across the Great Lakes and Mississippi River Valley yesterday, with scattered exceedances from MI to southern LA/AL. Code Orange ozone forecasts are posted for most of those same locations today. Therefore, upwind ozone is expected to be high on Wednesday and Thursday. The air quality models are still not predicting any areas of USG ozone for either Wednesday or Thursday for the Mid-Atlantic. They are keeping ozone in the mid-to-upper Moderate range at locations across most of PA, NJ, MD, and DE, with scattered locations in VA and NC. The models could be responding to chances for diurnal afternoon thunderstorm activity, particularly on Thursday. The 03Z SREF still has a high probability of precipitation west of I-81 for Wednesday, and the 06Z 4 km NAM has a few scattered storms in western PA on Thursday afternoon. Overall, the chances for precipitation on Wednesday are much reduced compared to yesterday’s guidance, and there is still some uncertainty for Thursday, but with the ridge shifted farther north, it seems likely that the deterministic models’ precipitation forecast is overdone. As a result, there is a High probability for an ozone exceedance somewhere in the Mid-Atlantic on Wednesday-Thursday, most likely along the I-95 Corridor.

PM2.5 is also rising quickly to our west, with concentrations this morning well into the Moderate range, including areas of western PA. The air quality models bring PM2.5 into the Moderate range across the entire Mid-Atlantic region beginning tomorrow and continuing through Friday. This seems reasonable given rising humidity and increasing upwind concentrations.

On Friday, back trajectories shift south/southwestward and increase in speed. It will still be hot and sunny, with continued chances for diurnal afternoon convection. The air quality models begin back off on both ozone and PM2.5 levels on Friday – the locations of highest ozone and PM2.5 remain the same (e.g., I-95 Corridor), but the intensity decreases, possibly in response to the shift in upper air transport. The main forecast question for Friday will be how high ozone reaches on Thursday, particularly at upwind locations. Given the uncertainty in the transport pattern and ozone levels upwind, the chances for an exceedance drop to Marginal on Friday.

In contrast to yesterday’s analysis, Saturday and Sunday now look clean – back trajectories shift south/southeast with a strong onshore component in response to the weak tropical system that will be moving into the coastal southern Mid-Atlantic. If this onshore transport pattern verifies, it will clean out the region, with the possible exception of locations west of I-95, where Moderate ozone conditions may persist. The other wild card is the strength and progression of the back door cold front, particularly on Sunday. If the front reaches as far south as the Mason-Dixon Line, or even into central NJ, it will help to clean out the northern Mid-Atlantic. Given the shift to onshore flow and questions about the back door cold front, chances for an ozone exceedance drop to Slight for the weekend. PM2.5 also will likely see a downward trend for the weekend, which is supported by the air quality models.

-Huff/Brown

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 23, 2016
Valid: May 24-28, 2016 (Tuesday-Saturday)

Summary:

After a very quiet first three weeks in May, the Mid-Atlantic will transition to a synoptic pattern that is favorable for USG ozone beginning on Wednesday. The chances for USG ozone are Slight on Tuesday as a closed low aloft spins northward into New England and keeps the weather unsettled. A broad but persistent upper level ridge will move eastward on Wednesday, settle overhead on Thursday, and then build westward on Friday; the Mid-Atlantic will remain on the western periphery of the ridge through at least the beginning of next week. Temperatures will rise into the upper 80s to low 90s °F across the region Wednesday through Saturday. Wednesday may be a transition day, with Marginal chances for USG ozone, mainly along and east of the I-95 Corridor and across NC, where skies will remain sunny through the entire afternoon. The main forecast question for the end of the period is the extent and location of precipitation, triggered by a series of shortwaves aloft interacting with a stationary frontal boundary draped across NY and New England. While chances for USG ozone rise to Appreciable for Thursday-Saturday, questions about the precipitation forecast keep substantial uncertainty in the air quality forecast. PM2.5 is expected to remain in the Good to low Moderate range through Wednesday, with concentrations increasing to the widespread low Moderate range through the end of the period in response to rising humidity.

Discussion:

The weather forecast models are in close agreement on the synoptic scale features through Thursday, but they diverge on Friday and Saturday. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. The period begins with a closed low aloft spinning along the Eastern Seaboard, moving from over VA today to eastern New England on Tuesday; the low exits to the northeast by Wednesday morning. At the same time, a broad upper level ridge roughly centered over the Great Lakes on Monday will build eastward over the Mid-Atlantic on Tuesday. The models are in excellent agreement on the placement of these features, giving us high confidence for the first part of the medium range period. The upper level ridge is in place over the Mid-Atlantic on Wednesday; this is a broad feature, not particularly amplified, with many small embedded shortwaves. The upper level ridge continues to gradually move eastward through the end of the period, with the axis located along the East Coast on Thursday. By Friday, the Mid-Atlantic is on the western periphery of the ridge. The GFS and EC diverge regarding the amplification of the ridge at the end of the period, related to the placement of a strong shortwave spinning up over the Northern Plains. The GFS amplifies the Mid-Atlantic ridge and moves the shortwave farther north, into the southern Canadian Prairies, while the EC keeps the northern edge of the ridge suppressed compared to the GFS and also has the shortwave farther south, in the Dakotas. WPC sides with the EC solution at this time.

At the surface, high pressure moves over the region on Tuesday, and off shore on Wednesday, hooking up with the Bermuda High beginning on Thursday. A weak back door cold front over MI/ON on Tuesday stalls along northern NY and New England on Wednesday. The front remains north of the Mid-Atlantic and oscillates over NY and New England through the end of the period. The numerous shortwaves embedded in the upper level flow around and in the ridge interact with the stalled frontal boundary and trigger widespread showers and thunderstorms during the end of the medium range period. This activity is supported by the 09Z SREF for Thursday. Given the questions regarding the placement of the upper level ridge on Friday and Saturday, there is considerable uncertainty regarding the extent and location of any precipitation at the end of the period, but it is reflected in both the GFS and EC solutions.

On Tuesday, a warming trend will begin, with temperatures reaching near average for this time of year. The closed low will be moving northward along the East Coast, keeping clouds, scattered showers, and possibly isolated thunderstorms in the forecast. As a result, the chances for USG ozone are Slight. The air quality models have isolated Moderate ozone along western PA and NC, which seems reasonable. Good to low Moderate PM2.5 is expected to continue until around Thursday, when humidity begins to rise.

Wednesday is one of the two key days of interest during this period. As the upper level ridge builds eastward, skies will be sunny for most of the region, and temperatures will jump well into the 80s °F. Surface winds will be westerly, with slow-ish northwesterly transport aloft. Widespread Moderate ozone is likely, with a Marginal chance for USG ozone. The exception will be along the western Mid-Atlantic, particularly western PA and NC, where clouds and afternoon showers will develop. The main forecast question will be how quickly the air mass modifies, with a more quickly modifying air mass translating into higher chances of USG ozone.

Thursday is also a day of interest, given the rising temperatures as the upper level ridge builds overhead and the Bermuda High extends westward. The main source of uncertainty is the precipitation forecast. As noted above, the weather forecast models, including the SREF, develop widespread afternoon showers, mainly west of I-95. As a result, the best chances for upper Moderate to USG ozone will be along and east of the I-95 Corridor, as well as across NC, with an Appreciable chance for USG ozone. The air quality models keep ozone in the upper Moderate range, with no pockets of USG ozone at this time.

Conditions on Friday and Saturday look to be generally similar to Thursday. Uncertainty remains in the chances for USG ozone, given the questions regarding the precipitation forecast. As noted above, both the EC and GFS trigger widespread afternoon showers and thunderstorms across most of the region. Identifying the exact location and extent of any convection will be the main forecast question for the end of the period. Given temperatures in the mid to upper 80s °F, and at least partly sunny skies, Appreciable chances for USG ozone will continue, with considerable uncertainty.

-Huff/Brown