Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 24, 2015
Valid: July 25 – 29, 2015 (Saturday – Wednesday)

Summary:

Widespread clear skies, light winds, and increasing heat and humidity will keep chances for Moderate ozone and PM2.5 in the forecast throughout the medium range period. With a center of surface high pressure moving over the region on Saturday, clear skies and very light winds will allow for an Appreciable chance for isolated USG ozone, mainly in the Washington, DC metro area. The main forecast question is the degree of modification of the air mass in place over the region. Unsettled weather will impact the Mid-Atlantic on Sunday and Monday, allowing afternoon convection to hinder the continued increase of pollutants. An upper level ridge will reach the Ohio River Valley by Tuesday and the Mid-Atlantic on Wednesday, promoting sunny, hot, and humid conditions. As a result, Tuesday and Wednesday are possible days of interest, with model discrepancies as to possible precipitation and onshore back trajectories being the main limiting factors for rising ozone at this time.

Discussion:

The weather models are in generally good consensus throughout most the medium range period, but the NAM begins to diverge on Monday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, an upper level ridge axis aligned with the western Great Lakes is continuing eastward, pushing a trough off of the east coast. With a closed high aloft covering the entire state of TX, the majority of the CONUS is engulfed by this upper level ridge, except for the Pacific Northwest and New England. As the ridge axis continues moving towards the Northeast, it will flatten, arriving over the Mid-Atlantic on Saturday and allowing the widespread clear and calm weather to continue. To the northwest of this ridge axis, the weather models all show an upper level trough digging over the Hudson Bay. As the ridge moves out to sea on Sunday, the southernmost impacts of the trough will be felt in the Mid-Atlantic in the form of a surface trough stretching from eastern NY to central VA. The global weather models are picking up on afternoon and evening precipitation across the northern Mid-Atlantic (NMA) associated with a weak cold front moving southeastward through PA. This weak front will dissipate on Sunday but remain over the region through Monday, acting as a focus for isolated convection triggered by weak shortwaves aloft. Also on Sunday, the upper level ridge begins to peak over the Great Plains; this axis will move eastward steadily throughout the remainder of the medium range period. By Monday, the NAM is alone in creating a strong shortwave over the Midwest, and will be treated as an outlier from here on out. Dilute shortwave energy over the central Mid-Atlantic (CMA) will keep the chance for afternoon clouds and showers in the forecast for Monday, with help from the expected hot and humid conditions and the remnants of Sunday’s weak cold front. By Tuesday, the ridge axis will be over the Great Lakes, keeping sufficient subsidence over the NMA but allowing the threat of afternoon convection to continue in VA and NC. On Wednesday, there is some model divergence regarding the details of afternoon precipitation in the Mid-Atlantic as the ridge reaches the Ohio River Valley (ORV). However, both the GFS and the EC both show scattered convection popping up in the CMA and western Mid-Atlantic to some extent on Wednesday; these are likely diurnally driven storms due to the hot and humid conditions.

Saturday will be a pleasant day across the northeastern US with temperatures in the high 80s ͦF and dew points in the 50s ͦF. A center of surface high pressure over WV will keep skies clear and winds very light to calm throughout the region. Yesterday, ozone reached the mid-Moderate range in the Washington, DC metro region. This morning, hourly ozone mixing ratios are rising very quickly south and east of I-95, which suggests scattered locations may reach the upper Moderate range today for 8-hour average ozone. With little change in meteorological conditions and air mass characteristics on Saturday, isolated upper Moderate to low USG ozone is not out of the question. The 06Z runs of the air quality models today are picking up on this, with all of the models showing scattered upper Moderate ozone along the I-95 Corridor from DC/BAL to ILG/PHL/TTN, with the BAMS and NCDENR models indicating USG over DC and southern BAL. The main forecast question is how fast the air mass in place modifies, and we should have an idea based on how fast ozone continues to rise today in the DC metro region. Another contributing factor to rising ozone along the I-95 Corridor will be the possibility for bay and sea breezes to develop in the afternoon, as suggested by this morning’s hi-res mesoscale models. A limiting factor for rising ozone on Saturday may be northerly back trajectories from western NY, where ozone and PM2.5 concentrations are low this morning, suggesting the air arriving aloft tomorrow will be relatively clean. PM2.5 concentrations are slowly rising this morning along and west of the Appalachians. On Saturday, rising humidity will push PM2.5 into the low Moderate range from west to east, with Good conditions likely continuing for another day along the Atlantic coast.

Sunday will see rising temperatures and dew points as surface high pressure persists over the CMA and SMA. In the NMA, however, a weak cold front moving southeastward through PA as well as a surface trough stretching through the eastern Mid-Atlantic will bring unsettled weather to the forecast. These two features will mostly act as lines of weak convergence, promoting isolated to scattered afternoon and evening convection in PA, NJ, and MD. The resulting cloud cover will hinder afternoon ozone formation, limiting maximum concentrations along the I-95 Corridor to the bottom half of the Moderate range. In areas protected by the surface high pressure, southeasterly onshore surface winds will help limit ozone to the Good/Moderate threshold. Most areas will see PM2.5 reach into the Moderate range due to the increasing humidity and relatively light surface winds. Just along the coastline the 06Z runs of the 45km BAMS CMAQ air quality models are resolving little particle buildup, likely due to onshore surface winds, which will limit PM2.5 to the Good range.

Monday will be another day of unsettled conditions as temperatures hover at or above 90 ͦF, even in the NMA, with continuing high humidity. Again, mostly clear skies are expected to reign over the Mid-Atlantic as the heat index rises throughout the day. A lack of a substantial source of subsidence will allow the heat and humidity to cause scatted pop-up convection along the remnants of Sunday’s cold front, but little organization will keep the impacts localized. This will allow another day of ozone rising into the Moderate range along the I-95 Corridor, but onshore trajectories – albeit short – will be a limiting factor to widespread Moderate conditions. The entire Mid-Atlantic will see another day of widespread Moderate PM2.5 as humidity and light and variable surface winds persist.

Tuesday and Wednesday are days of interest as the axis of the upper level ridge will reach the ORV and then the Mid-Atlantic. Despite global weather model precipitation discrepancies, both days are expected to be mostly sunny, calm, and hot and humid across the region. The exact location of afternoon and evening precipitation cannot yet be pinned down, but there is agreement that it will most likely occur throughout the CMA and SMA where the effects of the incoming upper level ridge are less prominent. USG conditions are possible along isolated areas surrounding the I-95 Corridor on Tuesday. The 45km BAMS CMAQ and MAQSIP-RT air quality models both resolve an area of USG ozone just west of the interstate along the central MD/PA border. Wednesday is very similar to Tuesday in both weather and air quality forecasts as the ridge aloft continues to subdue any kind of clouds or showers and promote rising temperatures and humidity. The approaching ridge axis will spread the clear skies southward into the CMA, allowing ozone to rise into the Moderate range throughout VA and northern NC. The main possible limiting factor at this point are onshore back trajectories both days. On Tuesday, GFS back trajectories are fully onshore (southeasterly), while on Wednesday, they shift more southerly, along the Atlantic coast, but are still very clean. This maritime air mass transport may be sufficient to limit rising ozone. High dew points will perpetuate the threat of Moderate PM2.5 throughout the region.

– Eherts/Huff