Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 28, 2015
Valid: July 29 – August 2, 2015 (Wednesday – Sunday)

Summary:

Heat, humidity, light winds, and clear afternoon skies will continue in the northern Mid-Atlantic on Wednesday, keeping the chances for USG ozone High. The focus for these conditions is expected to be metropolitan areas north of the Mason-Dixon Line, but there is considerable uncertainty regarding the impact of possible southerly low level flow. The mesoscale models are showing more sustained southerly surface winds along the east coast, which, combined with weak southerly transport at 500 m AGL, should push the highest chances for USG ozone to locations north of I-95. The air quality models are all indicating a strong north-south gradient in ozone for Wednesday, but they disagree on the location of the Good/Moderate boundary. Persistence is likely the key factor for PM2.5 concentrations on Wednesday, with continued Moderate levels at inland locations, and persistent Good to low Moderate conditions along the coast. The arrival of a cold front on Thursday will generate widespread pre-frontal clouds which will end the threat of USG ozone, with Moderate conditions possible east of I-95. The front will slow and stall along the Atlantic coast by Friday morning, allowing lower dew points, a cleaner air mass, and sunny skies to prevail over the Mid-Atlantic. This will lower PM2.5 back to the Good to low Moderate range for Friday and Saturday, but allow ozone to rebound downwind of the I-95 Corridor by Saturday. Sunday will be the third post-frontal day of widespread surface high pressure, sunny skies, and light westerly surface winds, allowing ozone and PM2.5 to reach higher into the Moderate range.

Discussion:

The weather models are in generally good consensus throughout most the medium range period. The 06Z GFS and 00Z ECMWF were consulted for this analysis. The forecast for the Mid-Atlantic will be driven by two large, upper level features – a ridge, whose axis is currently over the Great Lakes; and a trough, which is currently a closed low over the SK/MT/ND border. By 12Z Wednesday, the closed low aloft will reach ON’s western border while the ridge will be pushed eastward out of ON and into QC, with the ridge axis moving over the Mid-Atlantic. By Thursday morning, the ridge will reach eastern New England, with the closed low over the Hudson Bay. The trough associated with this low will pull a cold front through the Mid-Atlantic on Thursday, with the main impacts starting between 12Z and 18Z Thursday. Although the front will reach the Atlantic coast by 12Z Friday, it will slow substantially by then and will stall along the eastern seaboard through Saturday. This will keep widespread clouds and showers along the coast of NC. Once the low aloft reaches QC and its associated trough dips down into the central Mid-Atlantic (CMA) on Saturday and Sunday, the synoptic pattern will stall temporarily and keep a zonal flow over most of the Mid-Atlantic. In the coastal southern Mid-Atlantic (SMA), however, the remnants of the front will linger as a surface trough and act as a focus for clouds and showers through Sunday.

Wednesday continues to be a day of extreme interest as it will be a hot and humid day across the region, with temperatures reaching into the low 90s F. With a surface high pressure center settled over western VA and the upper level ridge axis moving overhead, surface winds will be very light across most of the region. This morning’s runs of the mesoscale models are showing more sustained southerly surface winds along the east coast, from NC to NJ, however. The air mass transport pattern continues to be a question mark, with the 06Z GFS 36-hr back trajectories ending at PHL for tomorrow morning showing southerly transport at 500 m AGL, but localized transport at 1000 m AGL. If the southerly flow is more dominant, chances for USG ozone will focused to the north of I-95. The air quality model runs this morning are still calling for areas of USG ozone tomorrow, but the models are still not in agreement on the location of the highest ozone, but they are closer than yesterday. The 06Z 15km BAMS MAQSIP-RT and CMAQ air quality models both resolve pockets of USG ozone in the greater Pittsburgh, Harrisburg, and Philadelphia metropolitan regions for Wednesday, but keep areas south of the Mason-Dixon Line mainly in the Good range. In contrast, the 06Z NOAA NAM-CMAQ air quality model shows small pockets of USG ozone popping up in Baltimore and Washington, DC, as well as the PHL/TTN and central NJ areas. The 06Z 12 km NCDENR CMAQ model is similar to the NOAA model, with pockets of USG ozone in BAL and the PHL/TTN metro area, but Good ozone for most of the rest of the region, particularly south and west of BAL. The models seem to be having as much trouble as we are in determining the impact of the southerly low-level flow. The BAMS models seem to be responding the strongest to the possibility of increased onshore surface flow by pushing the highest ozone concentrations north of I-95. Another potential limiting factor for ozone across the CMA tomorrow involves continued chances for scattered convection associated with the weak but stubborn surface trough that continues to linger from the remnants of Sunday’s weak cold front. This wind shift surface trough will continue to slide southward tomorrow, acting as a trigger for pop-up storms and showers across VA, western MD, and parts of NC for much of the day. So overall, chances for USG continue to be High for Wednesday, but there is considerable uncertainty in regard to location and geographic extent. At this point, the highest chances will be just north of the northern part of the I-95 Corridor, from approximately BAL to PHL/TTN to NYC. If the southerly flow verifies and is strong enough, it may be sufficient to cause a strong gradient in ozone concentrations, from as low as the Good range in locations south of the Mason-Dixon Line. The speed at which hourly ozone rises across the region today should give us a better idea of the chance for USG ozone tomorrow.

The particle forecast continues to be somewhat tricky as well, especially for coastal locations. Hourly PM2.5 concentrations in PHL and DE this morning have remained in the Good range despite calm winds and fog. In contrast, inland locations are well into the Moderate range. For Wednesday, persistence will be a major factor for the PM2.5 forecast. Locations that are currently observing Moderate particle concentrations will likely continue to experience Moderate PM2.5 on Thursday, while locations that are currently in the Good range along the coast may remain in the Good range on Thursday if the onshore flow verifies.

A weak cold front will move into the region from the northwest on Thursday, bringing pre-frontal clouds and showers through the Mid-Atlantic. Reaching Pittsburgh around 12Z, the front will slow as it moves into the hot and humid air mass established over the region, reaching only to southern PA by 00Z Friday. The high-res mesoscale weather models are not resolving the precipitation as a strong, unbroken line of rain, but an area of optically thick clouds is expected ahead of the frontal passage. The mesoscale models are consistent in showing the clouds reaching the I-95 Corridor by 21Z and the east coast by 00Z Friday. As a result, locations west of I-95 will probably see Good ozone, while areas along and east of I-95 may have another day of Moderate ozone conditions. Without a substantial washout from the prefrontal precipitation, PM2.5 will reach higher into the Moderate range throughout the Mid-Atlantic again as the humidity increases ahead of the cold front.

The main impacts of the frontal passage will be lower dew points and a shift in surface winds to the northwest, but temperatures will persist in the 90s ͦF on Friday. The decrease in humidity will limit PM2.5 to the mostly Good range across the region. The cleaner air mass filtering in behind the front will keep ozone mainly in the Good range on Friday, with the exception being near where the front will stall along the coast. A surface wave will develop along the frontal boundary in eastern NC, keeping clouds and the chance for scattered showers east of the Appalachians in the SMA. Extending northeastward from the low will be the remnants of Thursday’s cold front, providing a line of convergence along coastal DE and the Delmarva. The northwesterly surface winds will allow emissions from I-95 to build surrounding the coastal front, allowing ozone to rebound into the Moderate range across sunny DE and the Eastern Shore of MD.

Saturday will be another warm, pleasant, and sunny day across the region as a post-frontal surface high pressure center strengthens over WV. The remnants of the cold front along the coast will be too weak to promote any clouds or showers, but will act as an area of light surface convergence. Light westerly surface winds will push shore traffic emissions to areas east of the I-95 Corridor, placing the area of most likely Moderate ozone from NYC to DC to Richmond and eastward to the coast. As the post-frontal air mass slowly modifies, particles will reach to the Good/Moderate threshold at widespread locations.

Sunday will see the continued modification of the air mass over the Mid-Atlantic as the stationary surface high pressure center over WV continues to strengthen, allowing the continued buildup of particles and ozone precursors. Back trajectories will be from IN and OH at higher levels, and will be very short and more southwesterly at low levels. These factors, combined with the sunny skies and light westerly surface winds, will allow another day of ozone buildup east of the I-95 Corridor. Seeing as it will be a Sunday, ozone concentrations will be limited to the Moderate range. Particles will likely see a small increase into the lower Moderate range, simply due to the stagnant air mass.

– Eherts/Huff