Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 29, 2018
Valid: June 30- July 4 (Saturday-Wednesday)

Summary

A strong upper-level ridge will settle over the Mid-Atlantic on Saturday and persist, with some oscillation, through the end of the medium range period. The weather models are still in relatively close consensus on the main synoptic features, but there is inconsistency in recent days as to the exact position of the center of high pressure aloft. These variations have introduced much more uncertainty in the air quality forecasts for the period, compared to yesterday. The exact placement of the center of high pressure could have a large impact on the air quality forecast across the eastern NMA/CMA beginning Monday, as flow aloft and at the surface have to potential to shift southerly across the Delmarva and entire I-95 Corridor. As a result, we still have confidence in the arrival and persistence of the mid/upper level ridges; these features will set the stage for a multi-day poor air quality event that has the potential to last through the medium range period. We have less confidence today, however, in the magnitude and geographic distribution of the exceedances – Monday, in particular, does not look as dirty as it did yesterday, and southern/western parts of the region may clean out on Tuesday and Wednesday. The best chances for ozone exceedances throughout the period will be the historically high ozone locations along the I-95 Corridor from BAL to NYC.

The main forecast question for Saturday is the potential for transport of dilute smoke from the northern Great Lakes into the I-95 Corridor. Large wildfires burning across eastern SK and western ON are producing a large plume of medium density smoke that is stretching across southern ON and into the source region of back trajectories (36 hr, ending at KPHL at 12 UTC) for the I-95 Corridor on Saturday. Given the many risk factors in place, the chance of an ozone exceedance will remain High on Saturday, with the highest ozone most likely along I-95 from RIC to NYC, as well as in western PA (PIT). On Sunday, temperatures will rise into the upper 90s°F across the NMA, with locations along the I-95 heat island possibly exceeding 100°F. Although light (< 10 mph) southerly surface winds are possible for coastal regions, the inward push of precursors/pollutants will probably promote rapid ozone formation along the entire I-95 Corridor. The risk of an ozone exceedance will remain High with a focus along I-95 again from RIC to NYC and locations in western PA (PIT). Uncertainty is much greater than it was yesterday for Monday’s forecast due to a variety of factors: the approach of a very weak cool front from the northwest, slightly stronger southerly (onshore) flow, and the amount of local NOx emissions (which may make Monday more like a typical Sunday). At this time, we think any southerly flow will be weak enough to keep chances for an exceedance High for the I-95 Corridor from BAL to NYC. Given questions about the weak front advancing from the northwest, the influence of onshore flow across the southern and eastern parts of the region, and mobile emissions, the risk of an exceedance will drop to Appreciable for Tuesday and Wednesday, with the best chances for an exceedance along the I-95 Corridor from BAL to NYC.

NWP Model Discussion

Aside from a few minor details, there is continued agreement between the weather models in regard to the synoptic pattern throughout the medium range period, which gives us confidence in the overall synoptic weather features, but we have much higher uncertainty today in the mesoscale details. A strong upper level ridge will build into the Mid-Atlantic on Saturday, with its axis moving over the region by 18Z, and the center of the ridge settling over the region as well. This ridge will encompass and influence most of the eastern CONUS throughout the medium range period. The arrival and persistence of this feature will set the stage for a multi-day poor air quality event that will probably last through the medium range period; however, we have less confidence today in the magnitude and geographic distribution of the exceedances. The center of high pressure aloft will remain over the NMA, unchallenged until Sunday, when a shallow longwave trough moves across the upper Great Plains and into the upper Midwest by 12Z. This trough will attempt to continue eastward, but the strength of the ridge will weaken and deflect it to the north, over ON/QC by 00Z Monday. The arrival of the trough will briefly dampen the upper-level ridge over the Great Lakes and send a weak shortwave trough southeastward towards the Mid-Atlantic. This trough will also quickly weaken as it pushes into the northwestern Mid-Atlantic by 00Z Tuesday, due to the lingering center of high pressure over the northeast U.S. In a change from yesterday’s guidance, the mid-level ridge will briefly weaken on Monday as it is temporarily dampened by the mid-level low as it moves across southern Canada. In the wake of the shallow longwave upper-level trough, the upper level ridge will back-build across the NMA and into the Midwest by 06Z Wednesday, placing the entire eastern two-thirds of the CONUS under a broad 594dm ridge through 00Z Thursday. In a change from yesterday’s forecast, the mentioned possible tropical disturbance has weakened in the weather models and its impacts on the Mid-Atlantic air quality forecast appear less clear.

The Dailies

Day 1 (Saturday): Multiple ozone exceedances are possible in the Mid-Atlantic on Saturday. Surface high pressure centered in the CMA will promote sunny skies and light surface winds across the region. Strong subsidence will inhibit clouds and convection. Temperatures will continue to rise, with most of the region reaching into the 90s °F. Light and variable winds and localized transport across most of the Mid-Atlantic are combining with temperatures well above average and pushing regional ozone higher in the air quality models. Similar to yesterday’s model runs, the air quality models develop widespread Moderate ozone and scattered USG across the region. The pockets of USG ozone are focused along the western bank of the Chesapeake Bay, the I-95 Corridor from RIC to NYC, and across western PA. In the SMA, it seems like the air quality models are picking up on the possibility of weak onshore component to the mid/upper level flow, as NC is under a mix of Good and Moderate ozone. This onshore component in the SMA flow appears to be the only limiting factor across the region on Saturday. Large wildfires burning across eastern SK and western ON are producing a large plume of medium density smoke that is stretching across southern ON and into the source region of back trajectories (36 hr, ending at KPHL at 12 UTC) for the I-95 Corridor on Saturday. The potential for transport of dilute smoke into the I-95 Corridor is a wildcard for Saturday’s ozone forecast. Given the many risk factors in place, the chance of an ozone exceedance will remain High on Saturday, with the highest ozone most likely along I-95 from RIC to NYC, as well as western PA (PIT).

Day 2 (Sunday): Conditions favorable for high ozone will continue into the Sunday for most of the Mid-Atlantic. Aloft, the center of high pressure will move over the eastern NMA/CMA. The exact placement of the center of surface/mid-level high pressure could have a large impact on the air quality forecast across the eastern NMA/CMA as surface winds have to potential to shift southerly across the Delmarva and entire I-95 Corridor. This is a major source of uncertainty for the ozone forecast beginning on Sunday, due to day-to-day inconsistency in the model runs. For example, in yesterday’s guidance, the threat of southerly flow was much reduced from Wednesday’s guidance. At this time, the close proximity of the center of high pressure aloft suggests that any southerly flow across the NMA and CMA, both aloft and at the surface, will be extremely light, which would not have much – if any – limiting effect on ozone. But it is something that we will have to monitor as the weekend progresses since southerly flow has the potential to quickly clean out affected areas. Once again in the SMA, the gradual eastward push of the center of high pressure and the possible development of an upper level shortwave trough over the Gulf will increase southerly/onshore transport. This will have a domino effect across the SMA and into the CMA on Sunday as flow across the region slowly shifts easterly. This is evident in the air quality models as both the BAMS models and several of the NC models keep NC and VA under Good and Moderate ozone. Throughout the NMA, temperatures will rise into the upper 90s°F with locations along the I-95 heat island possibly exceeding 100°F. Although light (< 10 mph) southerly surface winds are possible for coastal regions, the inward push of precursors/pollutants will probably promote rapid ozone formation along the entire I-95 Corridor. This can be seen in the air quality models as USG ozone blankets the eastern NMA/CMA, reaching westward into western PA. The risk of an ozone exceedance will remain High with a focus along I-95 again from RIC to NYC and locations in western PA (PIT).

Day 3 (Monday): Uncertainty is much greater than it was yesterday for Monday’s forecast, as a weak cold front could approach the northwestern Mid-Atlantic in the afternoon. The return of a few clouds as well as a few afternoon showers are possible in the western NMA and CMA as the front edges towards the region. The models are not in agreement with the strength of this frontal boundary and associated precipitation, as the GFS and ECMWF show slightly different solutions. The GFS is a little stronger with the boundary, developing scattered showers across western PA throughout the afternoon hours, whereas the ECMWF has a weaker front, keeping conditions dry into the night. This boundary is impossible to see at 850 mb, and looks more like a surface trough in the surface wind guidance. At this time, we do not recommend counting on this front to end the threat of an ozone exceedance on Monday. Despite the uncertainty in this frontal boundary, transport aloft will continue to shift southerly across the region, possibly relieving most of the region from poor air quality. This is the one day that the mid-level ridge seems to weaken, allowing for a better chance of onshore flow, albeit still very light in the NMA. This shift in transport is resulting in Good range ozone across the SMA and western half of the NMA and CMA. Transport aloft will also shift southerly across the eastern NMA and CMA but it remains uncertain if it will be strong enough to diminish the risk of an ozone exceedance. The air quality models seem to buy into this southerly flow as the BAMS models develop Moderate range ozone across the Delmarva and most of the I-95 Corridor. Both BAMS models keep extreme eastern PA and northern NJ under USG ozone as pollutants are pushed northward. Another key forecast question for Monday, and continuing on Tuesday and Wednesday, is the amount of local NOx emissions. With the Independence Day holiday on Wednesday this year, there is not a clear holiday weekend. This means that Monday may be more like a “normal” Monday in terms of mobile emissions, or it may be more like a Sunday. With the near record-breaking temperatures on Sunday and Monday, a/c demand will be very high, and shore traffic is also probably going to be heavy, which may compensate for any potential reductions in work/school commutes. Given the uncertainty in the forecast, we are less confident in the chances for an ozone exceedance on Monday. But at this time, we think any southerly flow will be weak enough to keep chances for an exceedance High for the I-95 Corridor from BAL to NYC.

Day 4-5 (Tuesday-Wednesday):  Monday’s approaching weak cold front will keep uncertainty in the forecast for Tuesday. The GFS and ECMWF continue to disagree with precipitation associated with the frontal boundary as the GFS has a few showers across PA on Tuesday, west of I-81, but the EC keeps the region totally dry. Although showers and associated cloud cover could have an impact on the air quality forecast, the main forecast question will be the eastward push of the front as it is forecast to dissipate in the NMA sometime in the afternoon. If the front is able to push far enough east with limited cloud cover, it could act as a converging factor to combine with strong July Sun and weak surface winds to allow rapid ozone formation to take place along I-81 in PA. Although it will be very humid, the back-building ridge aloft could suppress any developing convection. Across the rest of the region, mostly sunny skies and above average temperatures will be favorable for ozone formation, but continued onshore transport will diminish the risk of an exceedance. The air quality models have most of the Mid-Atlantic under Good/Moderate range ozone, with eastern PA and the I-95 Corridor under USG ozone. Light onshore flow, aloft and at the surface, is possible again on Tuesday, adding to uncertainty regarding the air mass characteristics. Given questions about the weak front advancing from the northwest, the influence of onshore flow across the southern and eastern parts of the region, and mobile emissions, the risk of an exceedance will drop to Appreciable, with the best chances for an exceedance along the I-95 Corridor from BAL to NYC.

The potential lingering frontal boundary across the NMA will keep uncertainty in the forecast. The arrival of a very moist air mass will combine with the lingering boundary and above average temperatures to possibly spark a few afternoon showers and thunderstorms across the northern half of PA. Throughout the rest of the region, skies will remain mostly sunny as the upper level ridge continues to build overhead. Temperatures across most of the Mid-Atlantic will remain in the 90s°F with weak surface winds. The flow aloft still has a southerly component, but become much weaker/shorter, with the center of the upper level ridge overhead. The primary forecast question will be how quickly the air mass is able to modify once the light onshore (southeasterly) flow is cut off across the region, especially given it is a holiday. In the SMA, back trajectories will also weaken but will remain onshore, keeping ozone levels in check. The risk of an ozone exceedance will remain Appreciable, with the highest ozone again most likely along the I-95 Corridor.

-Enlow/Huff