Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: June 30, 2017
Valid: July 1-5, 2017 (Saturday-Tuesday)
Summary:
Widespread unsettled conditions will move into the Mid-Atlantic on Saturday as a cold front approaches from the west. Scattered showers/thunderstorms can be expected across the region, resulting in a Sight chance for an exceedance. Mid-level ridging will battle with an upper level trough beginning on Sunday. Mostly cloudy skies will linger in the western NMA, but clouds will clear for the rest of the NMA and CMA by midday, allowing ozone levels to rise. Sunny and warm weather in early July, the climatological peak of the ozone season, always pose a risk for an exceedance and so overall risk will be Appreciable. A mixture of sunny skies, above average temperatures and slowing westerly flow on Monday will be favorable for ozone accumulation across the Mid-Atlantic. The primary focus for poor air quality will be along the I-95 Corridor and eastern NMA/CMA as a frontal boundary stalls along the MDL. Risk of an exceedance will increase to High. The key forecast uncertainty for Tuesday, July 4th, is the position of a stalled or slow moving frontal boundary. As this time, we expect the boundary to be near the MDL on Tuesday. The area of concern will be in the eastern CMA as locations south of the frontal boundary could experience favorable conditions for ozone formation. As a result, risk for an exceedance will lower to Marginal. Despite uncertainty in the weather models, afternoon/evening thunderstorms are likely on Wednesday, bringing the risk for an exceedance down to Slight.
Weather Model Discussion:
The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be strong consensus between the weather models until the end of the medium range period. By 12Z Saturday, an upper level trough will be in place over the Midwest, with the axis extending southward over the upper Mississippi River Valley and a center of circulation over MB/ON. This trough will slowly move eastward, entering the Mid-Atlantic by 18Z Saturday, and weaken by 12Z Sunday as shortwaves spinning through the trough pull it to the northeast. The trough will be depressed over the NMA at this time, as the westward extension of the Bermuda high build into the southeast U.S., bringing zonal flow over the SMA at upper levels. The trough will continue to slowly progress eastward as shortwaves spin along the axis over the Great Lakes/southern ON through 00Z Monday. The westward extent of the trough will linger in the NMA throughout Monday, as mid-level ridging continues to build over the Mid-Atlantic through 00Z Tuesday. By 12Z Tuesday, the upper level trough will move over northeastern Canada, resulting in zonal flow over the Mid-Atlantic that will continue through 12Z Wednesday. At the same time, a strong shortwave will develop over the central Midwest, digging out a very weak trough over KS/MO by 12Z Tuesday. There is disagreement on the eastward progression of this feature through the end of the medium range period as the EC and GFS diverge by 18Z Tuesday. The EC moves the shortwaves into the Mississippi River Valley/Ohio River Valley by 12Z Wednesday, phasing the weak trough with a long trough over southern Canada, forming a weak longwave trough over the western NMA by 00Z Thursday. The GFS on the other hand, oddly keeps this disturbance in place over eastern KS through the end of the medium range period.
The Dailies:
Mostly cloudy skies in the morning will give way to a few periods of partly cloudy skies around lunch before precipitation moves into the region. Precipitation associated with the front will begin in the western NMA by 12Z and push eastward to the I-95 Corridor in the NMA, central CMA, and western SMA, by 00Z Sunday. The 03Z SREF suggests that widespread pop-up thunderstorms can be expected ahead of the precipitation associated with the front, as high probability of precipitation is shown across the Mid-Atlantic between 18Z Saturday and 00Z Sunday. This is supported by breaks in the clouds during peak heating hours (especially in the CMA and SMA) and a southerly flow advecting humid air into the region. The air quality models are in agreement as ozone levels across the Mid-Atlantic return to the mid/upper Good range in the 06Z model runs. Risk for an exceedance will be Slight due to clouds and precipitation.
Drier conditions will return to most of the region on Sunday with the exception of the SMA. Saturday’s cold front will be hung up across the SMA by 12Z Sunday, resulting in scattered partly sunny skies and scattered showers throughout the day. Mostly cloudy skies will linger in the western NMA, but clouds will clear for the rest of the NMA and CMA by midday. Mostly clear skies in the CMA and eastern NMA will allow temperatures to reach near normal. Strong subsidence aloft will result in mostly sunny skies and near average temperatures. Back trajectories will be westerly but fast with source regions far to our west. The 06Z model runs predict widespread Moderate ozone across the entire NMA and scattered USG ozone developing along the eastern NMA/CMA. Scattered Moderate with widespread Good ozone is expected across the SMA as showers, clouds, and fast westerly flow will limit ozone formation. Sunny and warm weather in early July, the climatological peak of the ozone season, always pose a risk for an exceedance and so we will set the overall risk at Appreciable.
Clear skies will be present over most of the Mid-Atlantic on Monday as high pressure begins to build into the region overnight. A weak frontal boundary will push into the NMA between 12Z and 18Z Monday, resulting in a slight chance for scattered precipitation in the late afternoon/evening hours. Mostly sunny skies across the rest of the region will allow temperatures to climb above average. A mixture of sunny skies, above average temperatures and slowing westerly flow will be favorable for ozone accumulation across the Mid-Atlantic. The 06Z air quality models are responding to this as more widespread Moderate ozone develops across the region. The primary focus for poor air quality will remain along the I-95 Corridor and eastern NMA/CMA. If surface flow is weaker than the air quality models are anticipating a swath of USG/High ozone could develop along the I-95 Corridor. Risk for an exceedance will rise to High on Monday with focus along the eastern NMA/CMA.
The key forecast uncertainty for Tuesday, July 4th, is the position of a stalled or slow moving frontal boundary. As this time, we expect the boundary to be near the MDL on Tuesday. There is disagreement in the weather models regarding conditions for Tuesday, as EC lifts a warm front across the NMA but the GFS keeps clear skies and dry conditions in the NMA. Despite the uncertainty, a shift to northwesterly flow behind the frontal boundary will allow ozone to fall into the Good range across the NMA. In the SMA and CMA, mostly sunny skies, above average temperatures and westerly back trajectories will allow ozone to continue to rise, developing more widespread Moderate ozone. The area of concern on Tuesday will be in the eastern CMA as locations south of the frontal boundary could experience favorable conditions for ozone formation. As a result, risk for an exceedance will lower to Marginal. For fine particles, July 4th is always problematic due to widespread fireworks displays. Light winds overnight could result in very local high concentrations of particles.
Continued uncertainty on Wednesday as the GFS and EC continue to diverge with the track of a low pressure system. Despite this uncertainty, unsettled conditions can be expected across the Mid-Atlantic. Afternoon/evening thunderstorms are likely, bringing the risk for an exceedance down to Slight for Wednesday.
-Enlow/Ryan