Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 2, 2017
Valid: June 3-7, 2017 (Saturday-Wednesday)

Summary:

Sunny skies and carry-over of rising ozone from today will pose the largest risk for the SMA and parts of the I-95 Corridor at the beginning of the medium range period, resulting in an Appreciable risk for exceedance on Saturday and Marginal on Sunday. On Saturday, weak high pressure moving overhead will allow clear skies to persist for much of the Mid-Atlantic. Stagnation across the SMA and possible air mass modification across the NMA will allow for rising ozone. This will continue into Sunday as weak high pressure remains in the SMA; however, a warm front will begin to creep its way into the NMA, bringing the possibility of partly cloudy skies and afternoon rain. The best chance for another day of possibly mid-to-upper Moderate ozone will be along the CMA, where clear skies are expected to last through the evening. There is still substantial model disagreement on the details of the weather forecast beginning on Monday. A cold front is expected to pass through the region, potentially bringing widespread showers and cloudy conditions. There is uncertainty on the timing of the frontal passage, but air quality is expected to improve. The weather on Tuesday and Wednesday is extremely uncertain due to major differences in the weather models, with two different outcomes possible (sunny vs. cloudy and wet). Despite the uncertainty, the arrival of an unseasonably cool air mass behind Monday’s cold front will limit ozone formation across the region and return the chances for an exceedance to Slight.
Discussion:

There is consensus between the weather models regarding the overall picture for the medium range forecast. However, there is disagreement in placement and timing of key features starting as early as 06Z Sunday. This uncertainty will carry through the end of the forecast period, with major discrepancies on Wednesday. The models consulted for this discussion were the 06Z GFS, 06Z NAM and the 00Z ECMWF. Saturday, an upper level trough will progress over the Northeast U.S. with its axis moving offshore by 06Z Sunday. At the same time, a mid-level ridge will build between the exiting upper level trough and a strong shortwave dropping down from the Canadian Prairies into the upper Midwest. The mid-level ridge will enter the Mid-Atlantic by 12Z Saturday and continue to move eastward into Sunday morning. As this ridge moves overhead, it will reinforce a Bermuda high extending into the Southeast U.S. This ridging will be short lived, however, as the strong shortwave over the upper Midwest phases with the remnants of the trough moving over New England and develops a new trough centered over western ON by 12Z Sunday. There is still substantial uncertainty in the strength and evolution of this trough on Monday. In yesterday’s model runs, the EC strengthened and progressed the trough much more quickly than the GFS and NAM. For today’s model runs, the 06Z GFS and 00Z EC are now both on the same page through 12Z Monday. Both the GFS and EC show a trough forming over ON/MN 12Z Sunday, with its axis sweeping over the western Great lakes, centered over WI extending southwest by 00Z Monday. The NAM is much slower and is considered an outlier, by us and by WPC. By 12Z Monday, both the EC and GFS have a closed upper low centered over eastern ON with a trough extending into Southeast U.S. The GFS and EC diverge through the end of the period regarding the track and strength of the upper level trough. The GFS has a weaker trough with no closed circulation by 18Z Monday, centered over PA by 12Z Tuesday. In contrast, the EC keeps a closed center of circulation over the eastern Great Lakes, moving southeastward with its center, reaching NY by 12Z Tuesday. A series of strong shortwaves spinning around the axis of the trough will impact both the NMA and SMA staring 18Z Monday and lasting through the end of the medium range period. The track and strength of the upper level trough/closed low continue to diverge between the GFS and EC on Tuesday. The EC continues to maintain a stronger trough with a closed upper level circulation progressing eastward over New England by 00Z Wednesday, whereas the GFS has a weaker trough and forms a closed circulation over the northern and central Mid-Atlantic by 18Z Tuesday. 18Z Tuesday through 00Z Thursday is the period of greatest uncertainty regarding the track and strength of the upper level trough. The EC continues the northeastward progress of the system, whereas the GFS continues its slower southeastward progression of the system. The WPC is favoring the EC solution with a surface low off the coast of NJ by 12Z Tuesday, progressing northeastward out to sea by 12Z Wednesday. This outcome will bring unsettled weather to much of the Mid-Atlantic for Monday but clear out for the central and southern region by 12Z Tuesday, with clearing continuing through the end of the period.

On Saturday, mostly sunny skies are expected to continue for much of the Mid-Atlantic region due to weak high pressure overhead. The only exception is in the NMA, where the hi-res models are showing an area of organized precipitation moving through PA/NJ in the morning hours. 06Z GEFS precipitation probabilities and 03Z SREF show a low chance of this precipitation occurring, whereas the hi-res NAM models show this area of organized precipitation across PA around 12Z. If this precipitation verifies, it will help to cleanse the atmosphere of ozone precursors. However, there is relatively low confidence in the magnitude of this disturbance. Once again, the main forecast question for the NMA will be air mass characteristics and potential carry-over of ozone from today. A weak cold front/surface trough is predicted to move through the NMA this afternoon, so it could bring a reinforcing shot of cleaner air. In addition, back trajectories are fast and northwesterly and thus, clean for tomorrow. But locations in the central Mid-Atlantic (SMA) saw mid-to-upper Moderate ozone yesterday, despite the arrival of cleaner air behind yesterday’s cold front. As a result, there is certainly the potential for rising ozone in portions of the NMA and CMA. In the SMA, back trajectories are showing recirculation and very stagnant air, which will allow for carry-over of mid-to-upper Moderate ozone (and possibly USG in Charlotte) from today. Once again, the air quality models are not showing much ozone development for Saturday, but given their known early ozone season low bias, we are not putting a lot faith in them. As a result, the chance for an exceedance will rise to Appreciable for Saturday, for locations across the SMA and possibly the southern NMA and the CMA.

There is uncertainty in the forecast for Sunday due to questions about the chances for precipitation across the region. The trend in the model guidance is for drier conditions in general. The models agree that the best chances for clouds and rain will be in NMA, essentially north of about I-76. A warm front will lift over the region, putting most of the Mid-Atlantic in the warm sector, allowing temperatures and humidity to rise. The models also have clouds and precipitation building across the SMA from west to east in the late afternoon, resulting in a clear patch across the CMA. The 06Z BAMS air quality models hint at this clearing, with a swath of Moderate ozone stretching across the NMA and CMA, following the southern stretch of the I-95 Corridor. Back trajectories for NC are once again slow, suggesting a relatively stagnant air mass. Given the uncertainty in location of clouds and rain, and the potential for another day of at least Moderate ozone the exceedance risk level will be Marginal on Sunday.

Favoring the EC solution, a surface center of low pressure will pass just to the north of the NMA around 12Z Monday morning. This will bring an associated cold front across the Mid-Atlantic throughout the day, resulting in widespread cloudy conditions and rain showers. Showers can expect to start around 12Z and will become the heaviest around 18Z across the region. There is uncertainty with the strength of this passing front, since the GFS has a weaker low and thus drier conditions for Monday. The 06Z MAQSIP and CMAQ air quality models respond to this, once again showing a swath of Moderate ozone across the NMA, suggesting the front will either be relatively weak or will slow down as it makes its way into the region, following the GFS solution. At this point, we are following the WPC and favoring the faster, stronger, and wetter EC solution. But if the front is weak or does not make it to the region on Monday, another day of isolated Moderate ozone is possible. However, we expect more widespread Good conditions, bringing exceedance risk down to Slight.

There is extreme uncertainty regarding the weather forecast for Tuesday and Wednesday, but either way, air quality should remain in the Good range. The track of the surface low is not clear beyond Monday. As explained earlier, the GFS brings the surface low down into the SMA on Tuesday, and keeps a weak closed low over the region, which would result in unsettled weather for much of the Mid-Atlantic between 12Z Tuesday and lasting through 00Z Thursday in the SMA. The EC shows the surface low progressing northeastward, leaving the forecast region by 06Z Tuesday, resulting in mostly sunny skies for the CMA and SMA for the rest of the medium range period. Even if the clearer EC solution verifies, however, Monday’s system will have ushered in an unseasonably cool air mass, which should limit any ozone formation at the end of the period. As a result, air quality is expected to improve with the passing of Monday’s system, and exceedance risk is Slight for Tuesday and Wednesday.

-Enlow/Huff