Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 12, 2017
Valid: June 13-17, 2017 (Tuesday-Saturday)

Summary:

Tuesday is the main day of interest in the medium range period.  It will be the final day in the heat wave that began on Saturday/Sunday, with hot, stagnant, and mostly sunny conditions for much of the region.  The forecast question is how far southward a back door cold front and its associated clouds/precipitation will reach.  The weather models are in relatively close consensus that the front will move into northern PA/NJ around 18-21Z Tuesday and progress to around I-76 (ECMWF) or the Mason-Dixon Line (NAM, GFS) by 00Z Wednesday.  The NAM and GFS hi-res weather models trigger scattered convective thunderstorms across the NMA in the 21Z to 00Z Wednesday time period, which could limit potential USG ozone levels along the northern branch of the I-95 Corridor.  Summertime fronts have a tendency to slow down when moving into a very warm air mass, however, so extremely careful consideration of the 12Z model guidance today will be necessary to evaluate the expected southward extent of any clouds and precipitation associated with the cold front.  At this point, another USG ozone day seems likely for the more southern portions of the I-95 Corridor, including BAL and IGL.  The uncertainty is much greater for PHL and TTN, as well as the Susquehanna Valley and southern NJ.  Given uncertainty about the front, the risk for an exceedance will be High on Tuesday.  The models are in consensus that the front will reach to north/central VA on Wednesday.  A shift to easterly surface winds behind the front and widespread clouds and precipitation along and ahead of the front will drop the risk for an exceedance to Marginal.  The front will stall in the vicinity of central VA on Thursday and Friday before moving back northeastward as a warm front late Friday into Saturday.  Onshore flow aloft and at the surface north of the front and widespread unsettled conditions along and south of the front will drop the chance for an exceedance to Slight on Thursday and Friday.  The region will be in the warm sector on Saturday, but the chances for an exceedance will remain Slight due to onshore flow aloft and unsettled weather across the region.

Weather Model Discussion:

The models consulted for this analysis were the 06Z GFS, 06Z NAM and 00Z ECMWF.  There is close consensus between the weather models with placement of key atmospheric features through the period.  The westward extension of the Bermuda High that has been presiding over the Mid-Atlantic since Saturday will be flattened by an upper level trough/closed low moving over central/northern QC on Tuesday.  This upper level low will bring a back door cold front (BDCF) into the NMA on Tuesday afternoon.  The weather models are in general agreement on the track of this front, bringing it to somewhere between I-76 (EC) and the Mason-Dixon Line (MDL; GFS and NAM) by 00Z Wednesday.  For now, WPC is favoring the slightly slower EC.  The main impact on air quality will be clouds, showers, and thunderstorms forming along and slightly ahead of the frontal boundary on Tuesday afternoon, as discussed in the Dailies below.  As the upper level ridge flattens and weakens, shortwaves will move over the CMA and SMA starting 12Z Tuesday and continuing into Wednesday.  Farther west, the upper level ridge will be pinched between the QC upper level low and a second closed low moving northeastward from the northern Rockies on Tuesday to SK/MB by 00Z Thursday.  This will amplify the ridge axis over the northern Plains on Tuesday, with the axis gradually moving eastward to the Great Lakes on Wednesday, and then weakening slightly and moving over the Mid-Atlantic on Thursday and then off of the Atlantic coast on Friday.  At the same time, the closed low over the Canadian Prairies will spin in place and slowly migrate eastward, reaching ON by Saturday.  There are some minor differences in the way the EC and GFS handle this feature, but they will have little impact on air quality in the Mid-Atlantic.  By the end of the period, the flow over the Mid-Atlantic will be mostly zonal, with the region on the eastern edge of a weak upper level trough over the northern CONUS dug out by the central Canadian closed low.

The Dailies:

Tuesday is the main day of interest in the medium range period.  It will be the final day in the heat wave that began on Saturday/Sunday, with hot, stagnant, and mostly sunny conditions for most of the region.  The forecast question is how far south the approaching BDCF and its associated clouds/precipitation will reach.  The front is expected to enter the NMA around 18-21Z.  The 06Z 13 km GFS, 12 km NAM and 3km NAM all show the front pushing just south of the MDL by 00Z Wednesday, whereas the WPC, following the slightly slower EC, places the front only as far south as I-76.  The front is expected to trigger convective thunderstorms across the NMA which could limit potential USG ozone levels along the I-95 Corridor.  The 09Z SREF has a high probability of precipitation roughly along and west of I-81 by 21Z, shifting east to I-95 by 00Z Wednesday.  The 06Z 3 km NAM has a scattered line of thunderstorms reaching to roughly I-80 by 21Z, but these storms are not compelling in the guidance.  The 06Z 12 km NAM and 13 km GFS show a similar present but generally spotty line of storms in the late afternoon.  Summertime fronts have a tendency to slow down when moving into a very warm air mass, so extremely careful consideration of the 12Z model guidance today will be necessary to evaluate the expected southward extent of any clouds and precipitation associated with the BDCF.  At this point, another USG ozone day seems likely for the more southern portions of the I-95 Corridor, including BAL and IGL.  The uncertainty is much greater for PHL and TTN, as well as the Susquehanna Valley and southern NJ.   The air quality models have been showing their characteristic early season under-prediction of ozone bias, with a substantial miss of the scattering of exceedances along I-95 on Saturday.  For tomorrow, the 06Z air quality models are showing areas of Moderate ozone in southern PA with scattered USG ozone along and east of the I-95 corridor.  In areas that are not impacted by the approaching BDCF, very hot and sunny conditions will continue, with an Appalachian lee trough draped along the I-95 Corridor.  We look to be on the way to exceedances today (Monday) along I-95, so some carry-over of high ozone is likely into Tuesday.  In the SMA, passing shortwaves will bring an increase in cloud cover with the potential for scattered showers along the western edge of the region between 18Z Tuesday and 00Z Wednesday, suppressing ozone formation.  Risk of an exceedance for Tuesday will remain High for locations along I-95 due to widespread ozone-conducive conditions and the anticipated slower progression of the cold front across the NMA.

The BDCF is expected to push into northern VA by 12Z Wednesday, bringing temperatures back to around normal across the NMA and much of the CMA.  The front will stall over central VA and will cause widespread clouds and precipitation along and ahead of the front (south and west).  Precipitation will be the heaviest along the Mason-Dixon Line but will cover much of the CMA and SMA between 18Z Wednesday and 00Z Thursday.  Risk of an exceedance will lower to Marginal as temperatures return to near normal and flow at the surface turns to onshore behind the cold front.

The frontal boundary will stall across central/southern VA on Thursday and the first part of Friday before moving northeastward as a warm front late Friday.  Temperatures will drop dramatically on Thursday and Friday, with PHL only reaching the mid to upper 70s °F.  The flow aloft will shift northeasterly on Thursday and then due easterly on Friday as surface high pressure moves over eastern QC and then coastal NS.  The stalled frontal boundary across the CMA will continue to bring unsettled conditions across locations along and south/west of the stalled front on Thursday.  Mostly cloudy skies in the morning will give way to partly cloudy skies in the afternoon, with the chance of precipitation increasing between 18Z Thursday and 00Z Friday.  Continued onshore flow in the NMA and unsettled conditions in the SMA will allow ozone levels to decrease into the Good range with isolated Moderate in the SMA.  Risk of an exceedance will drop to Slight.

The warm front will lift northeast of the region on Saturday, putting the Mid-Atlantic in the warm sector.  Widespread unsettled conditions are likely.  Back trajectories are southerly onshore, suggesting continued mostly Good air quality despite a surge in temperatures.  The chance for an exceedance will remain Slight.

-Enlow/Huff