Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 3, 2015
Valid: August 4 – August 8, 2015 (Tuesday – Saturday)

 

Summary:

Generally Good air quality is expected throughout the medium range period due to the passage of a cold front on Tuesday that will linger over the region, promoting unsettled weather. A large upper level trough reaching into NC will drop a cold front southeastward through the northern Mid-Atlantic early tomorrow morning, but it will stall along the I-95 Corridor through the evening hours. Clear skies will prevail behind the frontal passage, but a pool of clean air filtering in behind the front will limit the buildup of ozone and PM2.5 on Wednesday. Some areas of Moderate particles are possible on Thursday as humidity increases ahead of a substantial surface wave of low pressure arriving from the west. Resulting widespread clouds and showers are likely overnight Thursday through Friday afternoon as the wave moves through the Mid-Atlantic along the stalled front, inhibiting ozone production and scrubbing the atmosphere of precursors. A resurgence of cooler air behind the disturbance, as well as onshore surface winds and back trajectories, will keep air quality Good throughout the weekend despite a return to sunny skies.

Discussion:

The weather models are in generally good consensus throughout most the medium range period, but begin to show slight differences by Wednesday and larger ones on Thursday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large upper level closed low is centered over the James Bay, with the associated trough stretching from ND to NC and off of the East Coast. This low aloft will inch eastward over the coming days, with a general model consensus showing it moving off the Canadian Maritimes on Friday morning. This trough will drop a cold front southeastward through the Mid-Atlantic early Tuesday morning, which the WPC surface analysis shows reaching the I-95 Corridor around 12Z. It will promptly stall over eastern MD as a surface low develops over eastern VA and moves off the coast throughout the day, allowing the front to later continue southward into the Carolinas on Wednesday morning. This frontal boundary will vary in strength and latitude throughout the coming week, keeping cooler conditions to its north, with a substantial surface wave moving along it on Thursday into Friday. By Wednesday, an area of shortwave energy aloft over MO becomes apparent, with slight differences in its shape and exact position growing into major forecast discrepancies by Thursday, with the GFS as an outlier at this point. The EC solution appears to be a good consensus of the GFS and NAM at first, where the NAM is the stronger of the two weather models in terms of the resulting surface low pressure and precipitation and the GFS is more disorganized. This surface low will move along the stalled frontal boundary and impact the Mid-Atlantic on overnight Thursday into Friday. Despite upper level discrepancies, both the EC and GFS bring an area of widespread precipitation through the Mid-Atlantic from 00Z Friday to 00Z Saturday. However, from Thursday into Friday morning the EC begins to grow towards the NAM solution as it strengthens into a closed surface low, while the GFS remains the weaker and more diluted solution. The tighter EC surface low will allow skies to clear more quickly behind it as it moves out to sea overnight Friday. This makes Saturday the biggest question mark of the medium range forecast, as there is an even split regarding the cloud cover forecast. It is also worth mentioning that the EC 850 mb solution brings a pool of cooler air as far south as NC on Friday afternoon, while the GFS keeps this cleaner mid-level air in PA.

Tuesday will be a slightly above average day in terms of temperature and humidity behind an early-morning cold front passage. This front is expected to reach and stall near the I-95 Corridor by 12Z, placing an area of surface convergence along the interstate during the morning rush hour. The front has weakened since yesterday, as now there is little convection or other precipitation expected to form in the vicinity of the boundary until the overnight hours. Even then, only scattered storms are resolved in the high-res mesoscale models along the southern coast of MD around 00Z Wednesday. Surface winds are expected to be light and relatively sustained ahead of and along the front, with southwesterly winds promoting the highest ozone along the frontal boundary just east of I-95. Otherwise, the cleaner air mass filtering in behind the front will limit ozone to the Good range. Particles will follow the same general pattern, with moderate consensus among the air quality models that concentrations will reach the Good/Moderate threshold along the northern I-95 Corridor.

Wednesday will be only marginally cooler but noticeably less humid than Tuesday as the cold front dips southward into NC. Overall Good air quality is expected behind the front, where prevailing westerlies and fast back trajectories from the Great Lakes region will limit ozone production despite clear skies. Pockets of Moderate ozone are possible in southern NC below the front, however. Particles are expected to linger around the Good/Moderate threshold for another day due to light morning and overnight surface winds.

With continuing clean trajectories from the western Great Lakes and a frontal boundary draped through VA, most of the Mid-Atlantic is expected to see Good ozone on Thursday due to mostly cloudy skies. A rainy surface low will produce widespread precipitation in the Ohio River Valley and move eastward along the frontal boundary, increasing cloud cover in the Mid-Atlantic later in the day. Model discrepancies lend uncertainty to the forecast regarding precipitation on Thursday, as the ECMWF shows widespread rain throughout the northern and central Mid-Atlantic (NMA, CMA) overnight Thursday while the GFS brings only scattered showers to the CMA and southern Mid-Atlantic (SMA). Although there is uncertainty regarding the precipitation forecast, both models bring a pool of cooler air aloft over the NMA by 00Z Friday. Because of this, regardless of which rain forecast verifies, Good ozone is likely on Thursday. The WPC surface analysis appears to favor the EC solution of a closed surface low and widespread resulting rain, which would clean out the atmosphere. Given the close consensus between the NAM and EC with this system, we consider the GFS an outlier at this point and favor the stronger surface wave with the rainier forecast. Moderate particle concentrations are expected ahead of the system as humidity rises and converging winds increase, particularly along the frontal boundary in central and northern VA/the DC metropolitan area, except in locations that see heavy rainfall.

Friday and Saturday will see very similar conditions in one sense, as temperatures will hover around 80 °F and dew points around 60 °F with generally east/northeasterly winds on both days. However, Friday is likely to be a washout for the majority of the region if the EC/WPC forecast verifies – which seems most likely at this time. Skies will clear quickly behind the rain as the system moves out to sea overnight, allowing for a clear and sunny Saturday as strong high pressure centered over Hudson Bay tries to edge southward. Onshore trajectories aloft as well as light and sustained surface winds will prevail throughout the Mid-Atlantic, limiting ozone production in an already clean atmosphere.

– Eherts/Huff