Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 10, 2015
Valid: August 11-15, 2015 (Tuesday – Saturday)

Summary:

Good air quality is expected for the beginning of the medium range period as a clean and dry air mass moves in from Canada behind tomorrow’s cold front. By the end of the period, however, a return to above average temperatures, mostly sunny skies, and light surface winds will promote rising ozone, although levels are not expected to exceed the Moderate range. On Tuesday, widespread clouds and showers will inhibit ozone production. For Wednesday and Thursday, the air mass change will promote Good ozone despite sunny skies and light surface winds. Friday and Saturday are days of Marginal interest due to rising temperatures and a slowly modifying air mass, although several factors will be present that should limit any ozone increases, including onshore back trajectories at lower levels and the approach of a backdoor cold front on Saturday. Particles will persist in generally the Good range through Friday as humidity falls behind the front.

Discussion:

The weather models are in very close consensus throughout the medium range period, having closed the timing discrepancy regarding this weekend’s frontal passage. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. An upper level closed low over the James Bay is currently digging a trough into the Great Lakes region, which will continue to strengthen, move eastward, and dip into the Carolinas by 12Z Tuesday. A cluster of strong shortwaves in the base of the trough will develop a surface low that will skirt just north of the Mid-Atlantic on Tuesday, pulling a strong cold front through the entire region. Clouds and showers will impact the Mid-Atlantic starting Monday night (associated with the leading warm front) and continuing through Tuesday morning, with another round of pre-frontal convection ahead of the cold front on Tuesday afternoon. Periods of heavy rain are expected in the central and northern Mid-Atlantic (CMA, NMA) on Tuesday morning. A bought of cleaner and drier air will filter into the CMA and NMA and persist through Thursday. The upper level trough will slowly lift north- and eastward Wednesday through Friday, allowing an area of surface high pressure to build eastward into the Mid-Atlantic. The close proximity of surface high pressure will dominate and keep skies clear until the next cold front approaches on Saturday. The weather models are in much closer agreement today regarding this next front. Due to the weak nature of the backdoor front, it will begin to dissipate and result in only cloud cover and scattered rain showers on Saturday for the NMA and northern parts of the CMA.

Tuesday will be a warm and humid day with breezy south/southwesterly surface winds (turning westerly behind the cold front), cloudy skies, and plenty of rain for the Mid-Atlantic. Southerly looping onshore back trajectories will combine with these factors to make for a day of mostly Good air quality. The BAMS and NOAA air quality models are resolving a swath of Moderate ozone in eastern DC/BAL metropolitan areas. They are likely responding to some pockets of clearing shown in the high-res mesoscale weather models during the afternoon hours, between the two rounds of precipitation. This guidance is likely over-predicting, seeing as the models have been doing so for the past few days and that periods of morning rain will rid the atmosphere of precursors. Since PM2.5 takes longer to respond to air mass changes, areas that receive only light rain in the western Mid-Atlantic (WMA) will see scattered locations reach the top of the Good range.

Temperatures will rebound quickly on Wednesday back into the mid-80s °F, although dew points will drop slowly as the Canadian air mass filters in behind the front. The arrival of the cooler and drier air mass is slightly slower in today’s analyses, with the full impact of the air mass change not occurring until Thursday. As a result, isolated Moderate ozone is possible, particularly along the coastline, where the cleaner air mass will reach last. Particles should remain in the Good range as well in response to the decreasing humidity.

Surface high pressure centered over IL will continue to expand eastward on Thursday, promoting clear skies and calmer conditions across the east coast. This is the day that the Canadian air mass fully makes itself felt over the region. Northwesterly back trajectories from interior ON and the coolest and driest weather of the week will promote Good air quality across the board despite sunny skies and light northwesterly winds.

Friday will be another day of calm and sunny conditions as the surface high pressure continues to build eastward, with the center settling over WV. Temperatures will quickly rebound into the upper 80s °F, but humidity will remain relatively low. Ozone will increase into the Moderate range in some locations as the air mass slowly modifies – specifically, downwind of the I-95 Corridor. On Friday, light southwesterly surface winds place these areas of interest in DC, BAL, southwest PA, NJ, DE, and along the eastern shore of MD. Although it is too far out to forecast with confidence, the lack of strong synoptic forcing on Friday will increase the chances for a possible sea breeze. This feature would greatly increase the chances of high ozone concentrations along and east of I-95.

Saturday will be a pre-frontal day for northern part of the region as a weakening backdoor cold front reaches PA in the morning. Temperatures will continue their upward trend from Friday, with highs approaching 90 °F. There are a few features that should limit any rising ozone to the Moderate range. Back trajectories at 500 m AGL hook onshore, which we have seen this summer tends to very effectively cap rising ozone. In addition, the weak backdoor cold front will move into the NMA. Scattered afternoon clouds and showers will decrease chances for Moderate ozone in the CMA and NMA. In NC and parts of VA, continuing high pressure and mostly clear skies may allow ozone to reach the Moderate range, depending on air mass transport aloft. Increased humidity ahead of the frontal boundary may allow for scattered Moderate PM2.5 concentrations in PA and NJ.

– Eherts/Huff