Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 26, 2017
Valid: July 27-31, 2017 (Thursday-Monday)

Summary:

A combination of unseasonably cool air and unsettled conditions will continue to keep the risk of an exceedance in the Mid-Atlantic minimal throughout the medium range period. The Mid-Atlantic will see a wide range of conditions on Thursday as a cold front and associated wave of low pressure approach the region from west in the late afternoon/evening hours. Cloud cover and the chance for precipitation will increase throughout the afternoon hours with showers likely in the late afternoon in the NMA and CMA, although there is still some question as to whether precipitation will reach the I-95 Corridor by late afternoon. The NC, NOAA and BAMS models are consistently highlighting upper Moderate to USG ozone along the I-95 Corridor from eastern PA through MD, likely in response to a southward model trend in precipitation. Given uncertainty in the precipitation forecast, there is a Marginal risk of an exceedance for Thursday, with the focus on areas along and just west of I-95. Friday still looks like a washout for the NMA and CMA as Thursday’s approaching cold front and associated wave push through the region and into the SMA, but uncertainty is creeping into the forecast given a southward trend of the weather models, especially the NAM. Following the recommended GFS/EC solutions, mostly cloudy skies in the morning hours will give way to widespread showers in the NMA and CMA by 18Z and more widespread heavy showers across the entire Mid-Atlantic between 21Z Friday and 00Z Saturday. Unsettled conditions in the NMA and CMA and onshore flow in the SMA will lower the risk of an exceedance to Slight on Friday. Saturday will be another mixed bag of conditions as the wave of low pressure from Friday slowly moves off the Atlantic coast; the trend in the models is for the wave to depart the region more slowly. Showers will linger throughout the morning and afternoon hours in the vicinity of the MDL. The rest of the Mid-Atlantic can expect mostly sunny skies, slightly below average temperatures, and strong northerly flow aloft, resulting in a Slight risk of an exceedance on Saturday. Conditions will be drier and more pleasant to end the weekend and start the work week as both Sunday and Monday appear to be mostly sunny with near average temperatures. High pressure building over the Midwest on Sunday will shift flow both aloft and at the surface to northerly. Again, the wild card for the end of the period will be the chance for transported smoke from Canadian fires. Strong subsidence and northerly flow associated with approaching high pressure may pull dilute smoke, located across most of Canada, into the Mid-Atlantic. The NAAPS model does not currently show any smoke making its way into our region, but forecasters should keep a close eye on the transport pattern for the end of the medium range period. Risk of an exceedance will remain Slight on Sunday, given the presumably clean air in place, but will increase to Marginal on Monday as high pressure moves into the NMA, increasing subsidence and pushing temperatures up to around normal.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models have come into closer consensus in today’s runs in regards to the upper level synoptic setup, particularly at the end of the periods. The GFS and EC are in fairly close agreement throughout the medium range period but the NAM begins to diverge around 18Z Friday as it slows the progression of an upper level low over the Ohio River Valley east- and southward relative to the GFS/EC. By 12Z Thursday, an upper level closed low over central QC will pull a cold front into the Ohio River Valley. A series of shortwaves associated with this cold front will move over the NMA and CMA between 12Z and 18Z Thursday, flattening weak mid-level ridging over the region. As this set of shortwaves passes over to the Atlantic coast by 00Z Friday, they will cause weak upper level troughing to occur over the region. As this is happening, another strong shortwave will drop down across the Midwest, digging out and reinforcing the upper level longwave trough over the Great Lakes by 12Z Friday. This shortwave will form a wave of low pressure along the cold front, driven by the parent trough still in central QC. This wave will move through the Ohio River and into the Mid-Atlantic at the surface by 15Z Friday. By this time the NAM diverges from the GFS and EC, keeping the center of shortwave energy from entering the Mid-Atlantic through the end of its run at 18Z Saturday. Instead the NAM drops the energy southward, stretching the upper level trough across the Mid-Atlantic with potent shortwaves overhead. This solution is very different from the GFS and EC therefore the NAM will be considered an outlier when dealing with upper level features between 12Z Friday and 18Z Saturday. The WPC agrees, suggesting a blend of the GFS/EC/UKMET for this system. They note a steady southward trend in recent models runs, however, which is increasing the spread of solutions for the wave/cold front and increasing uncertainty. The GFS and EC, in comparison, move the shortwave energy to the doorstep of the western Mid-Atlantic by 00Z Saturday before dropping slightly southward into the CMA by 12Z Saturday, spreading out the shortwave energy into the central portions of the region. The shortwave energy/trough axis is expected to continue to progress eastward, reaching the Atlantic shore by 00Z Sunday, before pushing out to sea by 12Z Sunday. The GFS and EC slightly diverge with a feature over the Mississippi River Valley and Mid-Atlantic starting 18Z Monday, resulting in disagreement with what happens behind the departing trough. The EC continues to deepen a weak trough over the eastern US, pushing southward into FL as early as 12Z Sunday, whereas the GFS is slower to develop this trough as it reaches into the Southeast by 00Z Tuesday. Despite this small difference at upper levels between the models, conditions are expected to be similar with either outcome.
The Dailies:

The Mid-Atlantic will see a wide range of conditions on Thursday as a cold front and associated wave approach the NMA in the late afternoon/evening hours. Mountainous portions of the NMA and CMA will have a foggy start in the morning hours but mostly sunny skies should prevail by 12Z. Increasing cloud cover in the afternoon hours will limit temperatures to near average values. The probability for precipitation in the NMA and CMA will increase as cloud cover does, with precipitation likely between 21Z Thursday and 00Z Friday in locations between I-95 and I-99 in the NMA and along I-81 in the CMA. Recirculating back trajectories in the NMA could enhance ozone accumulation during periods of sunshine or in the event that precipitation does not reach the I-95 Corridor, but transport in the residual layer should be limited by mostly Good ozone conditions today. Back trajectories in the SMA are also slow but have an onshore component to assist in limiting ozone across the region. The 06Z air quality models appear to respond to these conditions as onshore flow in the SMA results in widespread Good ozone across the SMA, while conditions favorable for ozone accumulation cause the models to develop Moderate ozone in the NMA and CMA. The BAMS-MAQSIP develops scattered USG across PA and northern NJ, but it has been consistently over-predicting ozone in recent weeks, so USG ozone is likely overdone. One area that is consistently highlighted between the NC, NOAA and BAMS models is the I-95 Corridor through MD, which suggests the highest regional ozone may be located here. The main forecast question will be the timing and east- and southward extent of precipitation. A combination of these conditions, and given uncertainty in the precipitation forecast, result in a Marginal risk of an exceedance for Thursday, with the focus on areas along and just west of I-95.

Friday still looks like a washout for the NMA and CMA as Thursday’s approaching cold front and associated wave push through the region and into the SMA, but uncertainty is creeping into the forecast given the southward trend of the weather models, especially the NAM. Mostly cloudy skies in the morning hours will give way to widespread showers in the NMA and CMA by 18Z, with more widespread heavy showers expected across the entire Mid-Atlantic between 21Z Friday and 00Z Saturday. Precipitation is most likely to begin just before 15Z Friday lasting into Saturday morning. Unsettled conditions will limit temperatures to the upper 70s-low 80s °F in the NMA and low-to-mid 80s °F in the CMA, while periods of sunshine and a later arrival of rain will allow locations in the SMA to mid-to-upper 80s °F. Despite widespread and at times heavy rain expected across the NMA and CMA, the 06Z BAMS and NC CMAQ-B models keep low Moderate ozone along the I-95 Corridor, with an isolated patch of USG ozone in northern NJ in the BAMS models. This area of elevated ozone is possibly due to the models picking up on the NAM’s delay in precipitation, which keeps the I-95 Corridor dry through Friday afternoon. Despite this feature in the air quality models, unsettled conditions in the NMA and CMA, following the GFS/EC solution, and onshore flow in the SMA will lower the risk of an exceedance to Slight on Friday.

Saturday will be another mixed bag of conditions as the wave of low pressure from Friday only very slowly moves off the Atlantic coast. Showers are expected to linger in the vicinity of the Mason-Dixon Line on Saturday, tapering off sometime between 18Z Saturday and 00Z Sunday. The rest of the Mid-Atlantic can expect mostly sunny skies, slightly below normal temperatures and strong northerly flow aloft, with a presumably clean and much less humid air mass building in behind Friday’s cold front. This trend is appearing in the 06 BAMS models as they blanket the Mid-Atlantic with Good ozone on Saturday. Risk of an exceedance will be Slight.

Conditions will be drier and more pleasant to end the weekend and start the work week as both Sunday and Monday appear to be mostly sunny with near average temperatures. High pressure building over the Midwest on Sunday will shift flow both aloft and at the surface to northerly. This surface high pressure will then move into the Ohio River Valley by 12Z Sunday and into the western NMA by 00Z Monday, resulting in diminishing surface winds on Sunday. Despite mostly sunny skies and stagnating surface flow, strong northerly flow and a seasonably cool Canadian air mass moving into the region will limit any ozone formation that occurs on Sunday and Monday. Again, the wild card will be the chance for transported smoke from Canadian fires. The start point for back trajectories for Sunday and Monday have shifted a bit to the east, now originating in QC instead of ON. This puts them farther away from the source of fires along Hudson Bay in ON, but there is still a lot of dilute smoke aloft over most of Canada. Strong subsidence and northerly flow associated with the approaching high pressure may pull smoke into the region. The NAAPS model does not currently show any smoke making its way into the Mid-Atlantic, but forecasters should keep a close eye on the transport pattern for the end of the medium range period. The atmospheric conditions are influencing the 06Z BAMS models as they keep widespread Good ozone across the Mid-Atlantic on Sunday with a few isolated areas of Moderate ozone in the CMA. Risk of an exceedance will remain Slight on Sunday but will increase to Marginal on Monday as high pressure moves into the NMA, allowing temperatures to rise and subsidence to increase.
-Enlow/Huff