Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 17, 2015
Valid: June 18-22, 2015 (Thursday – Monday)

Summary:

Frontal boundaries, along with their clouds and precipitation, will continue to oscillate in a general north/south pattern across the Mid-Atlantic through the medium range period, pausing only for a clearing that will precede the Sunday arrival of the remnants of Tropical Storm Bill. This will keep the chances for USG ozone slight for most of the period, with an increase to marginal on Friday and Saturday. A warm front will move northeastward through the western Mid-Atlantic on Thursday, promoting clouds and showers ahead of the boundary as it stalls through central PA down to southeastern VA. An area of high pressure will build in behind the next cold front on Friday, allowing clear skies and calm winds to take over in the northern Mid-Atlantic for Friday afternoon and into Saturday. This break from the clouds and showers along with subsidence ahead of the approaching post-tropical depression make Friday and Saturday the most likely days for isolated to scattered Moderate ozone, mainly along and east of I-95. Heavy rain associated with the remnants of Bill will move through overnight Saturday into Sunday, returning air quality to the Good range. Model differences regarding the speed of clearing and the passage of the next cold front on Monday make the forecast uncertain, but generally Good to low/mid Moderate air quality is expected.

Discussion:

Most of the weather models are in generally good agreement regarding the upper level pattern and the fate of Tropical Storm (TS) Bill through Saturday. However, the 00Z and 06Z runs of the NAM diverge noticeably from the other model solutions on the strength of TS Bill by Thursday night, making the NAM an outlier through the end of the period. For this reason, the 06Z GFS, 00Z ECMWF, and 00Z CMC were the focus for this analysis. By 12Z Thursday, the forecast models show the center of TS Bill over eastern OK. A strong ridge persisting over the southeastern US will maintain a zonal flow over the Mid-Atlantic. This will allow a weak warm front – currently a cold front in southern VA as of 15Z Wednesday – to move northeastward through the northern Mid-Atlantic (NMA) on Thursday. By 12Z Friday, the weather models agree in moving the center of TS Bill eastward to the MO/AK border, as well as shearing a ribbon of short wave energy off the center of rotation and into the northern flow towards the PA/MD border. The base of a shallow trough in the northern stream flow will move through the northeastern US on Friday, dragging a cold front northwest to southeast through the NMA. This front will act as a focus for clouds and scattered showers as it encounters the sheared shortwave energy from Bill on Friday. This front will stall in the vicinity of the Mason-Dixon Line on Friday afternoon into Saturday. A shallow upper level ridge behind the trough will allow surface high pressure to build in to the NMA behind this front on Friday and into Saturday. The presence of high pressure, back trajectories from the ORV (Friday), and possible subsidence ahead of the remnants of TS Bill (Saturday) make Friday and Saturday the most likely days for Moderate ozone and PM2.5 concentrations, especially in the northeastern Mid-Atlantic/ I-95 Corridor. The southeastern ridge blocks yet another air mass boundary from continuing southward through VA, pushing Friday’s front back northeastward through the Ohio River Valley (ORV) and southwestern PA as a warm front beginning Saturday afternoon. The continued presence of sheared shortwave energy in the vicinity of the Mason-Dixon Line (MDL) from approaching post-tropical depression Bill will promote showers along and south of this front throughout the day on Saturday.

By 00Z Sunday, discrepancies regarding the speed of the remnants of TS Bill become apparent between the forecast models. While there is agreement that a heavy period of rain will impact all areas in the Mid-Atlantic north of the VA/NC border overnight Saturday into Sunday morning/afternoon, the models have yet to come into agreement on the exact timing and intensity of the rain. Most notably, the CMC shows the bulk of the shortwave energy from TS Bill phasing with the energy in the base of a weak trough over the eastern Great Lakes overnight Saturday. This merger increases the breadth and intensity of the precipitation on Sunday morning over the NMA greatly. The WPC discounts this possibility, however, making the GFS and EC solutions – which keep the heaviest rain moving through PA from southwest to northeast – more likely. All of the models show Monday as a transition from clouds to sun as the tropical system moves out to sea. However, there is disagreement between the GFS and EC as to the speed of this clearing and the fate of yet another approaching cold front on Monday. Both of the models bring the next front into NMA on Monday, but the EC is about 12 hours faster than the GFS.

After a mostly sunny day for most of the region on Wednesday, clouds and convection will be back in the forecast for Thursday as Tuesday’s cold front moves back northeastward as a warm front. The mesoscale models continue the trend of moving a MCS-like area of organized convection and rain showers through MD/PA/DE/NJ Wednesday night into Thursday morning. If this feature verifies, which seems likely given its multi-model support and general run-to-run consistency, heavy rainfall will help to clean out the atmosphere overnight along impacted areas. The cold front currently along the VA/NC border will begin to move northeastward as a warm front and will stall roughly northwest to southeast over PA and the Delmarva. This front will act as focus for clouds and precipitation, enhanced by upper level shortwaves from Tropical Storm Bill. After the area of organized convection exits in the morning, the mesoscale models are showing scattered thunderstorms in the afternoon, mainly across the NMA. These widespread clouds and periods of convection on Thursday will limit ozone to the Good range, which is supported by the air quality models. PM2.5 will be the leading pollutant with isolated locations along the frontal boundary, where surface convergence will be strongest, possibly reaching the Moderate range. PM2.5 conditions in NC this morning have dropped into the Good range, due to the passing of the cold front. However, just to the south in SC, PM2.5 concentrations are still well in the Moderate range, due to the location of the ridge of high pressure overhead and the influence of smoke from local agricultural fires and prescribed burning. The NRL NAAPS model continues to show smoke impacting surface PM2.5 concentrations in NC through Saturday. Taken together, this suggests that PM2.5 concentrations will likely rebound to the Moderate range in NC on Thursday and possibly continue in the Moderate range through Saturday.

Friday is a day of interest, with PM2.5 as the leading pollutant. A back door cold front will move through the region, stalling in the southern Delmarva, and gradually moving south to the VA/NC border. This front will weaken as it moves south, decreasing the chances of widespread clouds and precipitation in the afternoon. Surface high pressure centered over southern ON will help promote clearing skies in the NMA, but cleaner air behind the front should limit ozone to the generally Good range, with the chance for isolated Moderate conditions. Scattered clouds and convection in the SMA should mostly limit ozone to the Good range. The primary impact of the front will be as a line of convergence, probably allowing PM2.5 concentrations to rise along the boundary, which is supported by the air quality models. The air quality models are split on the chance for Moderate ozone conditions on Friday, however. The 06Z BAMS-RT and BAMS-CMAQ show isolated Moderate ozone along the Delmarva. In contrast, the NCDENR model keeps ozone in the Good range across the board.

Saturday is a possible day of interest as well, due to chances for strong subsidence ahead of the remnants of TS Bill. On Saturday, the weak cold front in the southern Delmarva will begin to move northward as a warm front. The BAMS air quality models again show isolated Moderate ozone in the Delmarva, possibly in response to the convergence along the frontal boundary. The exact location of the warm front along with associated clouds and precipitation is still uncertain, but areas with more clearing and less precipitation may see ozone reach the Moderate range, particularly along the I-95 Corridor. Back trajectories on Saturday show strong subsidence, suggesting that Bill may still be strong enough to promote widespread sinking air in the Mid-Atlantic on Saturday. If this verifies, it will most certainly promote ozone formation. The air quality models are not really picking up on this, however, making the impacts of Bill uncertain.

On Sunday, Good air quality is probable as the remnant tropical moisture from Tropical Storm Bill will likely produce widespread heavy rainfall throughout the region overnight Saturday into the early afternoon. The warm front stalled in the NMA will also help to produce heavy rainfall.

The next cold front will arrive on Monday. The timing is uncertain, given the differences between the EC and GFS solutions, but the expected arrival of a new air mass, as well as pre-frontal clouds and convection, should keep air quality in the Good to low/mid Moderate range.

-Huff/ DeBoe/Eherts