Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 28, 2016
Valid: July 29-August 2, 2016 (Friday-Tuesday)

Summary:

The weather models have finally converged on a similar solution regarding several waves of low pressure riding a frontal boundary into the northern and central Mid-Atlantic through the weekend. On Friday, the first of these waves will move directly overhead, triggering widespread cloud cover and periods of heavy precipitation across much of the region during the morning, with a second round of convection firing up in the afternoon in response to diurnal heating. The chance for an ozone exceedance on Friday will be Slight as a result. A second wave of low pressure will initiate across the Midwest on Saturday, and slowly progress eastward along a front that will stall in the vicinity of northern VA, potentially triggering another day of clouds and precipitation for much of the region. Though the eastward extent of this precipitation remains uncertain, onshore surface winds and the presence of a clean regional air mass will keep the chances for an ozone exceedance on Saturday Slight once again. This second wave will push toward the eastern U.S. coast on Sunday, which, combined with diurnal convection, with provide another sufficiently wet and cloudy day for the northern and central Mid-Atlantic. The chance for an ozone exceedance will remain Slight on Sunday as a result. For both Monday and Tuesday, the stalled front will push southward towards the southern Mid-Atlantic. A return to sunshine behind the front may allow for ozone to rise slowly along the I-95 Corridor both days. As a result, the chance for an ozone exceedance will rise to Marginal for both Monday and Tuesday.

Discussion:

The weather forecast models have come into relatively close agreement regarding the synoptic pattern for the medium range, with slight differences regarding the extent of precipitation developing in association with several waves of low pressure that will ride a stalled front into the Mid-Atlantic through Sunday. The timing differences with the waves and associated precipitation that were most striking in previous days’ guidance have resolved themselves. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. On Friday, shortwave energy will continue to stream out of the southern U.S and across the Mid-Atlantic along the base of a shallow upper level trough over the Ohio River Valley. A wave of low pressure initiating later today will progress eastward throughout Friday, departing toward the Northeast U.S. around 18Z Friday along a new frontal boundary created by the merging of Thursday’s stalled front and an approaching back door cold front reaching central PA by 12Z Friday. Concurrently, a mid-level subtropical ridge will remain suppressed across the southeastern U.S. through Monday. Another wave of low pressure will initiate over the Midwest on Saturday, slowly traversing the new front which will settle southward across the central Mid-Atlantic by 12Z. This wave will gradually work its way across the northern and central Mid-Atlantic through Sunday, only reaching the eastern U.S. coast by roughly 18Z. On Sunday, the axis of the broad upper level ridge will shift eastward over the Mid-Atlantic, ahead of an approaching broad and weak ridge setting up across the central U.S. This trough will be slow to depart, however, with the Mid-Atlantic remaining under its influence through most of Tuesday, with the leading of the approaching broad upper-level ridge reaching the Mid-Atlantic by 00Z Wednesday. The mid-level ridge will build back northward towards the northern Mid-Atlantic on Tuesday as well.

Friday appears to be a wet day for much of the Mid-Atlantic. The EC has come into agreement with the NAM and GFS, bringing an area of substantial precipitation across the northern and central Mid-Atlantic during Friday morning in association with a wave of low pressure traversing the new frontal boundary discussed above. The GFS shows the heaviest of this precipitation remaining across the northern Mid-Atlantic, while the NAM and EC develop more widespread (though still quite intense) precipitation covering the majority of the northern and central portions of the forecast region. Additionally, diurnal convection is expected to develop as the frontal boundary progresses southward into the central Mid-Atlantic during the afternoon. This precipitation pattern is supported by this morning’s Hi-Res guidance, including the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM. The NWS has placed much of the I-95 Corridor, as well as most of WV, under a Flash Flood Watch through Friday evening due to the high likelihood of period of intense rain throughout the day. 06Z GFS back trajectories for the I-95 Corridor are southwesterly, but this will have a limited impact on air quality given the widespread clouds and precipitation. The 06Z air quality models reflect the impacts of these factors, bringing most of the northern and central Mid-Atlantic down into the Good range, with some scattered low Moderate ozone across southern NJ and portions of DE and eastern MD. Regardless, the combined effects of drenching rain and persistent cloud cover will shut down ozone formation across most of the region. As a result, the chance for an ozone exceedance on Friday will drop to Slight.

Saturday could be another rainy day for the Mid-Atlantic, but it looks drier than Friday, as a second wave a low pressure slowly progresses eastward into the forecast region during the latter half of the day. The NAM is slightly slower than the GFS and EC with the feature, with only light precipitation reaching the I-95 Corridor by 00Z Sunday. The GFS and EC, however, developing moderate to heavy precipitation across the northern and central Mid-Atlantic by 00Z Sunday. The 03Z SREF agrees with the NAM solution, with the highest probabilities of precipitation remaining to the south and west of the I-95 Corridor through 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor are northerly, though somewhat short. If clouds and precipitation do not reach the I-95 Corridor by the late afternoon, ozone levels could possibly rise along I-95. The 06 air quality models show some evidence of this with the BAMS and NCDENR models showing Moderate ozone along the I-95 Corridor, with the BAMS MAQSIP-RT even resolving small patches of USG ozone over western MD and northeastern NJ. There should be a number of factors in place, however, to limit this ozone formation. Even if the I-95 Corridor stays dry, there may be sufficient cloud cover to suppress rising ozone. The 06Z 15-panel NAM shows widespread cloud cover reaching the I-95 Corridor by 21Z ahead of the approaching precipitation. Additionally, surface winds should be persistently onshore, given the circulation around the departing low from Thursday, which will push clean, maritime air into the forecast region. Lastly, and most importantly, the air mass in place is expected to be thoroughly cleaned out from Friday’s intense precipitation. Therefore, the chance for an ozone exceedance on Saturday will remain Slight.

On Sunday, the second wave of low pressure will slowly push eastward toward the eastern U.S. coast, providing yet another round of clouds and precipitation across most of the Mid-Atlantic. The deterministic models show the wave and its main batch of precipitation reaching the eastern Mid-Atlantic between 12Z and 18Z. Beginning 18Z, a second round of precipitation fires up across the forecast region, due to diurnal heating and convection. The EC shows more widespread and slightly more intense afternoon precipitation than the GFS, but both models show rain across the I-95 Corridor at this time. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m AGL, due southerly at 1000m, and southwesterly at 1500m. This clean transport pattern should further limit ozone formation along the I-95 Corridor. The BAMS models are do not appear to reflect the impacts of this precipitation, with the MAQSIP-RT model showing Moderate ozone across most of PA, MD, and DE. The CMAQ model shows Good ozone across most of PA, but still develops Moderate ozone across MD and DE. This could be a result of converging winds along the stalled front which will remain draped across the central Mid-Atlantic. However, given the likelihood of clouds and precipitation, the recent over-forecasting of ozone by the air quality models (especially the BAMS MAQSIP-RT in the past few days), and the decreased anthropogenic emissions expected on Sundays, ozone production should be mostly limited across the Mid-Atlantic. The chance for an ozone exceedance on Sunday will remain Slight as a result.

Monday will be a more clear and sunny day for the Mid-Atlantic, as the weekend’s wave of low pressure moves offshore by 12Z. The stalled front is expected to push further southward into the central Mid-Atlantic as a cold front, with high pressure centered over southern ON/QC building in behind the front. There is the possibility of diurnal clouds and thunderstorms due to ample regional moisture and the continued presence of shortwave energy aloft within the persistent, broad upper level trough. That said, only the GFS shows these afternoon showers developing, with the EC keeping the I-95 Corridor dry through 00Z Tuesday. 06Z GFS back trajectories for the I-95 Corridor shift westerly at 500m, and southwesterly at 1000m and 1500m. This, combined with the potential for afternoon sunshine, could promote rising ozone along the I-95 Corridor. Onshore surface winds and the continued presence of relatively clean air mass, however, should be sufficient to limit rising ozone to the Moderate range behind the front. If clouds and scattered precipitation do not develop in the afternoon, locations near the front could experience locally enhance ozone formation due to weak convergence along the front, which we have seen quite a bit in the past few days. The BAMS models appear to reflect this, developing a bullseye of USG ozone over DC, with Moderate ozone across the remainder of the central Mid-Atlantic. Given these factors, the chance for an ozone exceedance will rise to Marginal, mainly for locations near the front.

The Mid-Atlantic will remain under the influence of the upper level trough on Tuesday. High pressure will settle across Lake Ontario and upstate NY, as the cold front continues to push southward towards the southern Mid-Atlantic. As on Monday, the GFS suggests that diurnal precipitation could develop along the I-95 Corridor beginning 18Z, while the EC keeps the northern and central Mid-Atlantic dry through 00Z Wednesday. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m, but short and localized at 1000m and 1500m. Ample afternoon sunshine may be sufficient to support rising ozone along the I-95 Corridor. Given this possibility, and the fact that this is five days out, the chance for an ozone exceedance on Tuesday will remain Marginal.

-Brown/Huff