Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Monday, July 9, 2018
Valid: July 10-14 (Tuesday-Saturday)
Summary
Mid-level high pressure will linger over the Mid-Atlantic throughout the medium range period keeping an elevated risk for ozone exceedances, with some day to day variations depending on mesoscale features and the risk of smoke transport. Tuesday is a day of interest, given above average temperatures, potential carry-over of locally high ozone from today, bay/sea breezes, slow westerly transport aloft, and convergence ahead of a weak cold front. The weak front will push into the NMA on Tuesday morning, reaching the vicinity of the MDL by 00Z Wednesday. Precipitation will not be abundant ahead and along the front, since the air mass will not be particularly moist. The front may act more as a line of convergence near I-95 instead of a focus for clouds and pre-frontal convection. Given the many ozone-conducive weather conditions that will be in place and anticipated poor air quality along the I-95 Corridor today, the risk of an ozone exceedance will be High on Tuesday with isolated exceedances possible throughout the Mid-Atlantic region, especially along and south of I-95. Wednesday will be post-frontal for the NMA and CMA, which would normally lower the chance for an ozone exceedance, but a huge, high density smoke plume is upwind in western Canada, along with other, smaller plumes in the central and western CONUS. High pressure building over the Upper Midwest and ON will result in northwesterly flow for the NMA and CMA on Wednesday, possibly tapping into the smoky air. If the smoke does not reach into the Mid-Atlantic, the shift to northwesterly flow should clean out the regional air mass, diminishing the risk of an ozone exceedance. Given the uncertainty about the smoke, the risk of an ozone exceedance will lower to Appreciable with a focus on the NMA and locations ahead of the slow moving frontal boundary in the SMA. High pressure will continue to influence conditions across the Mid-Atlantic on Thursday. Despite mostly sunny skies and near average temperatures, a shift to more northerly/northeasterly flow aloft will likely usher a clean air mass into the northern half of the region. This combined with unsettled conditions in the SMA will lower the risk of an ozone exceedance to Marginal on Thursday with a focus on the I-95 Corridor. The risk of an ozone exceedance will increase to Appreciable on Friday as weak surface high pressure moves through the NMA. Weakening flow aloft and a shift to light southerly/southwesterly surface winds will be favorable for ozone formation, but periods of clouds could prevent anything excessive. Ozone exceedances will be most likely along the I-95 Corridor and in western PA. Uncertainty enters the forecast for Saturday as guidance begins to diverge regarding precipitation associated with another weak cold front. Nevertheless, any unsettled conditions are expected to remain west of I-95, although partly cloudy skies and breezy surface winds from the south may be enough to prevent excessive ozone formation along the I-95 Corridor. As a result, the risk of an ozone exceedance will drop to Marginal on Saturday.
NWP Model Discussion
The weather models are in close agreement with the evolution of the synoptic pattern through most of the medium range period. An upper level longwave trough currently moving across Hudson Bay will drop across ON/QC today and tonight, depressing an upper/mid-level ridge over the Northeast U.S. by 12Z Tuesday and squeezing it back westward. A shortwave perturbation will drop southward across southern ON Tuesday night, reinforcing the longwave trough axis over the Mid-Atlantic by 12Z Wednesday. This weak trough axis will continue eastward to the Atlantic coast by 18Z Wednesday, picking up Hurricane Chris and pulling it out to sea by 00Z Thursday. As this occurs, another shortwave disturbance will crest the western upper level ridge and drop southward across the Great Lakes and over the NMA, reestablishing a longwave trough over the Northeastern U.S. by 06Z Thursday. This longwave trough will briefly separate from the northern flow by 18Z Thursday as its parent closed low continues to pull to the east, resulting in a lingering longwave trough over the northeastern U.S. into Friday. This meandering upper level trough will remain over the Northeast U.S. through Friday as it is picked up by another upper level low moving across Hudson Bay, pulling the northeastern trough east of the Mid-Atlantic by 00Z Saturday. The eastward pull of the upper level trough will allow the edge of the western upper level ridge to build eastward into the Mid-Atlantic on Saturday. The influence of this area of high pressure could be weakened by the arrival of a shortwave trough from the west on Saturday. The GFS and ECMWF disagree with the development of this shortwave disturbance over the Midwest. The ECMWF develops a rather weak shortwave over the Ohio River Valley around 00Z Sunday, whereas the GFS brings a well-defined shortwave trough into the NMA/CMA by 00Z Sunday. This disagreement results in two very different solutions at the surface with a much more unsettled GFS solution over the Mid-Atlantic on Saturday compared to the ECMWF.
The Dailies
Day 1 (Tuesday): Tuesday is a day of interest, given above average temperatures, potential carry-over of locally high ozone, and convergence ahead of a weak cold front. The weak front will push into the NMA on Tuesday, reaching the vicinity of the MDL by 00Z Wednesday. Precipitation will not be abundant ahead and along the front, since the air mass will not be particularly moist. The hi-res models suggest scattered showers and a few isolated thunderstorms are possible. Surface winds will be light and westerly, allowing bay and sea breezes to develop, which will enhance ozone build-up. Transport aloft will be westerly and relatively slow (36hr back trajectories ending at 12Z for KPHL originate from northeastern OH/western PA). In addition, the weather models show a pre-frontal surface trough in the vicinity of I-95. The primary forecast questions will surround the weak cold front as its southeastward push and extent of precipitation/cloud cover are in question. It may act more as a line of convergence near I-95 instead of a focus for clouds and pre-frontal convection. In the SMA, mostly sunny skies, light surface winds, and weak northwesterly flow aloft will be conducive for ozone formation. The air quality models respond to these conditions by developing widespread Moderate ozone with scattered USG ozone. The models are in agreement in highlighting the I-95 Corridor from RIC to NYC with USG ozone. The NC model is the most aggressive with a few patches of Unhealthy ozone along the I-95 Corridor between DC and NYC in addition to USG ozone across NC. Given the many ozone-conducive weather conditions that will be in place and anticipated poor air quality along the I-95 Corridor today, the risk of an ozone exceedance will be High on Tuesday with isolated exceedances possible throughout the Mid-Atlantic region, especially along and south of I-95.
Day 2 (Wednesday): Wednesday will be post-frontal for the NMA and CMA, which would normally lower the chance for an ozone exceedance, but a huge, high density smoke plume is upwind in western Canada, along with other, smaller plumes in the central and western CONUS. High pressure building over the Upper Midwest and ON will result in northwesterly flow for the NMA and CMA on Wednesday, possibly tapping into the smoky air. The NAAPS model (which has not been very reliable this season) brings smoke into the NMA on Wednesday in the wake of the frontal passage. If the source air mass is polluted with smoke, mostly sunny skies, light surface winds and near/slightly below average temperatures could be favorable for ozone formation in the NMA and CMA. If the smoke does not reach into the Mid-Atlantic, the shift to northwesterly flow would be able to clean out the regional air mass, diminishing the risk of an ozone exceedance. Tuesday’s weak cold front will linger in the SMA on Wednesday, promoting partly cloudy skies and possibly a few scattered showers. Despite the presence of this boundary, slightly above average temperatures, light surface winds and back trajectories sourcing from OH/IN will keep favorable conditions for ozone formation. The air quality models are picking up on ozone conducive conditions across the southern half of the region with widespread Moderate ozone. In the northern half of the region, the air quality models are responding to the shift to northwesterly flow aloft, dropping regional ozone into the Good/low Moderate range. The primary forecast question will be the possible transport of smoke into the region, which should be monitored closely by forecasters tomorrow. Given the uncertainty, the risk of an ozone exceedance will lower to Appreciable with a focus on the NMA and locations ahead of the slow moving frontal boundary in the SMA.
Day 3 (Thursday): High pressure will continue to influence conditions across the Mid-Atlantic on Thursday. Despite mostly sunny skies and near average temperatures, a shift to more northerly/northeasterly flow aloft will likely usher a clean air mass into the northern half of the region. The NAAPS model responds to this onshore flow by dissipating Wednesday’s smoke on Thursday. In the SMA, Tuesday’s front will linger along the NC/SC border, promoting a few scattered showers throughout southern NC. Partly cloudy skies and slightly below average temperatures will combine with northeasterly flow to prevent further rise of regional ozone. These conditions are evident in the air quality models as they develop Good ozone across most of the NMA with a mix of Good and Moderate across the CMA and SMA. The BAMS models highlight the DC metro area with a single patch of USG ozone and upper Moderate along the I-95 Corridor, likely in response to converging surface winds along the interstate, but northerly flow should be enough to limit ozone to the Moderate range. The risk of an ozone exceedance will lower to Marginal with a focus on the I-95 Corridor.
Day 4-5 (Friday-Saturday): The risk of an ozone exceedance will increase to Appreciable on Friday as weak surface high pressure moves through the NMA. Weakening flow aloft and a shift to light southerly/southwesterly surface winds will be favorable for ozone formation, but periods of clouds could prevent anything excessive. The air quality models increase regional ozone across the NMA by developing a mix of Moderate and USG ozone. The BAMS and NC-GFS2 handle the CMA and SMA differently as the BAMS models keep a mix of Good and Moderate ozone across the southern half of the region while the NC-GFS2 has a few pockets of USG across NC and in the DC metro area. Ozone exceedances will be most likely along the I-95 Corridor and in western PA.
Uncertainty enters the forecast for Saturday as guidance begins to diverge. A low pressure system moving through Canada will pull a very weak cold front into the NMA sometime on Saturday, possibly promoting unsettled conditions in conjunction with shortwaves moving overhead. The wetter GFS brings more widespread showers across most of the NMA and parts of the CMA throughout the afternoon while the ECMWF only has a few scattered showers in the western NMA throughout the day. Despite uncertainties in the precipitation forecast, any unsettled conditions are expected to remain west of I-95. Partly cloudy skies and breezy surface winds from the south may be enough to prevent excessive ozone formation along the I-95 Corridor. In the SMA, weak onshore flow should be enough to counter mostly sunny skies and near average temperature to keep ozone formation limited. The risk of an ozone exceedance will drop to Marginal on Saturday.
-Enlow/Huff