Daily Archives: July 27, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday July 27, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 27, 2017
Valid: July 28-August 1, 2017 (Friday-Tuesday)

Summary:

A shift to a more winter-like pattern for much of the medium range period, with a lingering low aloft bringing unsettled and unseasonably cool weather to the Mid-Atlantic through Monday, will keep the risk of an exceedance minimal for most of the medium range period. A wave of low pressure forming along a cold front draped across the Mason-Dixon Line (MDL) will bring unsettled conditions to the region on Friday. Although the latter half of Friday still appears to be wet, the weather models are trending towards precipitation starting later in the day with a larger focus on the CMA and southern NMA. The later arrival of widespread showers and thunderstorms will allow temperatures in many locations to rise into the mid-to-upper 80s °F. Hi-res weather models are suggesting that showers and thunderstorms will increase across the CMA around 18Z Friday, before developing/moving into the NMA and SMA around 21Z Friday. This may leave locations along I-95 in the NMA with enough afternoon sun to support some ozone production, which is being highlighted by the air quality models. This uncertainty will result in a Marginal risk of an exceedance with a focus on the I-95 Corridor through the NMA. The upper level low over the Mid-Atlantic will strengthen and slowly move toward the Delmarva coast on Saturday, and then linger there through Monday, gradually weakening. There is some discrepancy among the models on the exact path of the low, but they are in consensus with a winter-like pattern for the weekend, keeping the low over the region, near the Atlantic coast. This pattern will promote cool, rainy, and windy conditions, with brisk northeasterly flow aloft, which will keep the risk for an exceedance Slight through Monday. Mid-level ridging will build into the Mid-Atlantic from the west on Tuesday as the persistent low finally exits to the northeast in the afternoon. Temperatures will return to around average under sunny skies. Influence from high pressure centered over the western Ohio River Valley and near normal temperatures could allow some scattered Moderate ozone to form. Risk of an exceedance will be Marginal on Tuesday.
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 consensus in regards to the synoptic setup over the medium range period, but have trended toward a very different fate for the strong shortwave dropping down into the Midwest this afternoon, which will reach the Mid-Atlantic overnight Saturday and looks like will linger through Tuesday morning. The only significant disagreement between the weather models is with the NAM, which is slightly south and west than the GFS and EC with the eastward progression of the shortwave, which will cut off into an upper level trough/closed low over the Mid-Atlantic on Saturday. At 12Z Friday, the strong shortwave will be dropping across the Midwest, reinforcing a longwave trough over the Ohio River Valley by 00Z Saturday. This shortwave energy will enter PA/WV/western MD/northern VA from the west between 00Z and 06Z Saturday. All three models develop a closed center of circulation over the northern half of the Mid-Atlantic by 12Z Saturday. The closed upper level low will spin roughly in place and strengthen on Saturday and before weakening on Sunday as the center drops slightly southward over the CMA by 12Z. By 18Z Sunday, the EC and GFS move the shortwave to the coast and spread the energy out, while the NAM keeps the circulation closed. The GFS has the center of circulation over NJ while the NAM is a little slower with the eastward push, keeping the center over MD/VA. Both models show the shortwave energy lingering in the vicinity of the coastal Mid-Atlantic on Monday; the EC has a slightly stronger circulation centered over NJ and the Delmarva, while the GFS has a weaker trough centered a bit farther northeast, reaching into New England. Regardless of how quick the system pulls to the northeast, the shortwave energy appears to impact the Mid-Atlantic until 18Z Tuesday, at which point both models take it New England/Gulf of Maine/Canadian Maritimes. Any form of this outcome appears to keep ridging at both upper and mid-levels out of the Mid-Atlantic until Tuesday afternoon.
The Dailies:

Although the latter half of Friday still appears to be wet, the weather models are trending towards precipitation starting later in the day with a larger focus on the CMA and southern NMA. The Mid-Atlantic will start the day with periods of sun and clouds as a wave of low pressure approaches the region along a cold front situated near the Mason-Dixon Line (MDL). Cloud cover will increase around lunchtime as scattered showers become more likely. Hi-res weather models are suggesting that showers and thunderstorms will increase across the CMA around 18Z Friday, before developing/moving into the NMA and SMA around 21Z Friday. Widespread heavy rain will cover much of the Mid-Atlantic to the south of I-80 beginning 00Z Saturday and lasting into the night. The later arrival of widespread showers and thunderstorms will allow temperatures in many locations to rise into the mid-to-upper 80s °F on south/southwesterly flow aloft. The 06Z BAMS and NOAA air quality models are picking up on later arrival of precipitation as both highlight the same locations along the I-95 Corridor by developing USG ozone in southeast PA and western NJ, including PHL/TTN and ILG. The models also highlight the Washington D.C. area with mid-Moderate to USG ozone. The air quality models have really gone crazy this week, with very high bias for ozone predictions. We do not think that USG ozone is likely, given the clean conditions this week and the expected cloud cover on Friday. The models do seem to be identifying the areas with the highest observed ozone, however, which means that the locations along I-95 highlighted may see Moderate ozone if precipitation holds off until the late afternoon or early evening. This uncertainty will result in a Marginal risk of an exceedance with a focus on the I-95 Corridor through the NMA.

Saturday now looks to be the washout day. It will be unseasonably cool as the wave of low pressure from Friday strengthens and slowly moves toward the Delmarva coast. Widespread heavy rain is expected to last through the night and into Saturday across the NMA and northern CMA. The 03Z SREF shows high probability of precipitation across the NMA south of I-80 lasting through 00Z Sunday before tapering off around 06Z Sunday. Cloudy skies, persistent heavy rain, and gusty northeasterly winds throughout Saturday will limit temperatures to the mid-70s °F and keep ozone formation minimal across the NMA and northern CMA. The rest of the region can expected lingering showers in the morning hours but most of the SMA and southern CMA will clear out by 12Z. A cool Canadian air mass moving over the Mid-Atlantic will overcome partly sunny skies in the CMA and mostly sunny skies in the SMA to keep temperatures below average. The 06Z BAMS models respond to these conditions as Good ozone dominates the Mid-Atlantic with scattered Moderate ozone in the SMA due to mostly sunny skies. Strong northerly flow aloft in the SMA and a cloudy, soggy day in the NMA will bring the risk of an exceedance to Slight on Saturday.

Sunday will be another cool day as temperatures remain below average for most of the Mid-Atlantic. The lingering wave of low pressure will keep conditions along the eastern Mid-Atlantic unsettled throughout Sunday. Weather models are showing scattered precipitation along coastal locations in the CMA and NMA, possibly reaching as far inland as I-95. The 03Z SREF has a medium probability of precipitation in these locations. There is disagreement between the models regarding the inland push of cloud cover. The NAM has clouds reaching as far west as I-79 by 18Z Sunday, the GFS as far west as I-81 by 18Z, but the EC only has high level clouds reaching well into the NMA. The uncertainty in cloud cover is most likely caused by the position of the lingering low along the coastal Mid-Atlantic. Regardless, the presence of the low will keep a brisk northeasterly flow moving into the region, which will keep ozone and particles low. The threat for transported smoke is gone due to the lingering low, which has changed the direction of transport aloft from northerly to northeasterly. These conditions appear in the 06Z BAMS air quality models as they show Good ozone across the entire Mid-Atlantic. Risk of an exceedance will remain Slight.

Conditions will be more settled across the Mid-Atlantic on Monday and Tuesday as high pressure edges into the western Mid-Atlantic. Mostly sunny skies will allow temperatures to rise but strong northerly flow aloft, courtesy of the lingering low, will keep temperatures below normal on Monday. The wave of low pressure is expected to remain off the coast of NJ, possibly brining isolated showers to the eastern NMA on Monday. The EC keeps the low closest to the coast, promoting another day of rain east of I-95 in the NMA and CMA. The 06Z air quality models keep ozone in the Good range across the region once again on Monday in response to below average temperature and strong northerly flow, keeping risk of an exceedance Slight. Mostly sunny skies will continue on Tuesday as the high pressure moves into the CMA as the persistent low exits to the northeast. A dissipating frontal boundary along the NY/Canada border could increase clouds with a few isolated showers in the northern NMA but should not impact regional ozone. Temperatures will continue to rise on Tuesday, reaching near normal values across the region. Flow aloft and at the surface will begin to slow across the SMA on Tuesday allowing ozone to accumulate in some locations, but given several days of clean conditions, ozone should not rise too quickly. Influence from high pressure and near normal temperatures could allow some scattered Moderate ozone to form. Risk of an exceedance will be Marginal on Tuesday, with the focus on the western part of the region.

-Enlow/Huff