Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 14, 2015
Valid: August 15-19, 2015 (Saturday – Wednesday)

Summary:

A mid-level ridge will build into the Mid-Atlantic for the medium range period, acting as a block for approaching frontal boundaries, and allowing above average temperatures and humidity to persist through the end of the period and beyond. A High chance of USG ozone exists for Saturday, dependent on how fast the current air mass modifies (i.e., today’s persistence for tomorrow) and the fate of weak frontal boundaries (e.g., bay/sea breeze fronts, weak back door surface trough). Short and recirculating back trajectories and consistent air quality model guidance lend confidence to a possible exceedance forecast in the usual area of concern along the I-95 Corridor. An Appreciable chance for USG ozone will continue on Sunday and Monday, with the main questions centering on lower Sunday emissions and a possible shift to sustained onshore flow aloft at 500 m. The forecast for Tuesday and Wednesday is uncertain, but the global models are showing widespread light precipitation across the region both days due to weak 500 mb troughing/shortwaves. In addition, the southerly 500 m AGL flow will continue. These factors may be enough to limit ozone to the Good to low Moderate range. Looking ahead to the end of next week, the Bermuda High will extend westward beginning next Thursday, centered over the Southeastern US. Although the influence of the Bermuda High will keep temperatures and humidity above average in the Mid-Atlantic, the southern location of the high should allow southerly transport aloft to dominate. Thus, although warm, stagnant, and sunny weather will continue through at least next weekend, the risk for USG ozone should remain Marginal due to southerly flow aloft. Particles will increase as humidity rises and the air mass over the Mid-Atlantic modifies over the next few days, peaking in the upper Moderate range on Tuesday in the Pittsburgh metropolitan area.

This is the last medium range discussion for the 2015 summer season. The discussions will return in 2016 and run from mid-May to mid-August.

Discussion:

The weather models are in generally close consensus throughout the medium range period, but exhibit differences regarding precipitation on Tuesday and Wednesday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. The large trough that had settled over the eastern seaboard for the past couple of days has almost completely lifted out of the region, and a mid-level ridge building into the Mid-Atlantic is promoting abundantly clear skies and calm conditions. A center of surface high pressure will persist over WV throughout the period to varying degrees as the mid-level ridge (850 mb) slowly moves eastward. Whereas in previous days’ guidance, a weak back door cold front was expected to impact the northern Mid-Atlantic to some degree on Saturday-Sunday, and a stronger cold front was expected to move into the region on Monday-Tuesday and stall near the Mason-Dixon Line (MDL), this morning’s GFS and EC runs now show these fronts staying to the north and west of the Mid-Atlantic. The mid-level ridge now appears to dominate the Mid-Atlantic through the end of the medium range period and beyond, into at least next weekend, as the Bermuda High extends westward, centered over the Southeastern US.

At 500 mb, however, the weak trough/upper level shear axis is still evident in the EC and GFS guidance, which complicates the forecast. The shortwave energy dropping down on Saturday from the northern stream flow is still there today, but not quite as strong as in yesterday’s analyses. This feature is associated with a weak back door cold front at the surface. This front is even weaker today than in previous model runs, and now looks to only bring a chance of clouds and scattered precipitation to areas west and north of roughly I-81 on Saturday. There is still a question as to whether this boundary will act as a line of converging winds in the NMA on Saturday and Sunday, as discussed below. On Monday, the weak shortwave energy aloft will remain along a shear axis running approximately along the ridge of the Appalachian Mountains from the Mid-Atlantic to the Texas Gulf Coast. The persistent western US upper level ridge will try to build eastward at the beginning of next week, but the weak trough/shear axis will inhibit it. Small shortwaves dropping down from the northern stream flow on Monday will phase, to certain degree, with the shear axis energy, and dig out a very shallow trough over the Mid-Atlantic on Tuesday-Wednesday. These shortwaves will bring scattered precipitation to the Mid-Atlantic on Tuesday-Wednesday, as discussed below. At the same time, a much more substantial shortwave will drop down over the Plains and pull a cold front as far east as the Great Lakes before pulling northward. Consequently, this next cold front will not make it to the Mid-Atlantic next week, but instead the mid-level ridge will continue to provide warm and humid weather to the region.

The weekend continues to be of interest for USG ozone conditions. Obviously, persistent sunny skies, very light to calm winds, and temperatures in the low 90s °F set the stage for rising ozone across the region. The main forecast questions continue to be 1) the degree to which the air mass currently in place modifies, 2) the impacts of weak frontal boundaries, including sea/bay breeze fronts and a weak back door surface trough, and 3) air mass transport aloft, which appears to turn more southerly at 500 m AGL beginning on Sunday. Saturday is the primary day of concern, given recirculating back trajectories and higher Saturday emissions. Historically, fewer emissions of ozone precursors on Sundays limit rising ozone to the upper Moderate range, all other things being equal. The air quality models have been resolving upper Moderate ozone concentrations along and surrounding the I-95 Corridor for the past several days, lending confidence to the expectation that this previously very clean air mass will modify substantially by Saturday. The CMAQ-based air quality models (e.g., NOAA, BAMS, NCDENR) do have a high bias by this point in the summer, however, which should be taken under consideration. The BAMS, NCDENR, and NOAA air quality models agree in resolving USG ozone in the Baltimore metropolitan area and along the southern NJ coast on Saturday. Upper Moderate ozone is resolved in a wide area surrounding this sector of the I-95 Corridor, but there is less consensus regarding the extent of these conditions. Due to the very light southerly/southwesterly surface winds, Moderate ozone will most likely be limited to areas along and just to the east of I-95, where emissions will be highest. Moderate ozone is also likely along the eastern coast of MD and VA, where the high-res mesoscale weather models are resolving areas of surface divergence, and therefore subsidence and limited atmospheric mixing. A complicating factor is the weak back door trough, which is no longer a cold front. The 09Z SREF shows a medium to high probability of precipitation on Saturday afternoon for areas west and north of I-81, similar to yesterday’s analyses. The hi-res weather models are showing only increased cloud cover in these areas, which adds uncertainty to the likelihood of clouds or scattered showers making their way to the I-95 Corridor. The 06Z 12 km NAM and 13 km GFS do show a line of convergence over southeastern PA and central NJ, however, associated with the trough. As we saw a couple of weeks ago in the Philadelphia metro area, a weak semi-stationary boundary can push ozone well into the USG range on local scale. Trying to forecast where this trough will settle is extremely difficult, however. We will have to make our best assessment based on the 12Z model runs. The same thing can be said about the bay and sea breezes, which may be an issue on both weekend days, given the weak synoptic winds. Particles are expected to reach the Moderate range at scattered locations due to a slight increase in the humidity compared to today and the continuing calm conditions.

Another day of above average temperatures, clear skies, calm surface winds, and short, looping, southwesterly back trajectories leave room for concern regarding building ozone on Sunday. The NCDENR air quality model shifts its USG concerns to the DC metropolitan area for Sunday, while the BAMS guidance looks almost identical to Saturday’s. Regardless, upper Moderate conditions are expected to extend along and east of the I-95 Corridor as very light southwesterly surface winds persist throughout the region due to the surface high pressure centered over WV. A question for Sunday is the fact that back trajectories at 500 m AGL turn due south, inland from the Atlantic coast. We have seen previously this summer that southerly flow aloft can completely suppress ozone to the Good range. Although this transport signal is somewhat weak, it may be sufficient to keep ozone out of the USG range. On the other hand, Saturday’s weak surface trough in the vicinity of PHL may linger, and sea/bay breezes are a threat as well. Particles are expected to continue to increase into the Moderate range at scattered locations as calm conditions and another uptick in humidity allow the buildup of PM2.5.

Monday will be another day of temperatures in the low to mid 90s °F, increasing dew points, light southwesterly surface winds, and surface high pressure throughout the Mid-Atlantic. The main difference is that lower level (500 m AGL) back trajectories are fully south along the Atlantic coast of NC. This is most likely due to the fact that the mid-level ridge will continue to build eastward, placing the Mid-Atlantic on its western edge by early next week. The NC air quality models appear to be responding to this change in upper level flow, as they begin a downward trend in ozone on Monday, continuing into Tuesday, with only isolated Moderate ozone at coastal locations. The BAMS models keep upper Moderate ozone all along the I-95 Corridor, however.

Tuesday and Wednesday have considerable uncertainty, with continued Moderate ozone very likely, but less chance for USG ozone. The next cold front that was expected to stall near the MDL on Tuesday-Wednesday will now stay well to the north and west of the region, allowing above average temperatures and summer-time humidity to continue. However, the weak shortwave energy/troughing aloft will keep the chances for scattered clouds and rain showers in the forecast. Both the EC and GFS show scattered light precipitation across the region on Tuesday and Wednesday, which may be enough to again limit ozone to the Moderate range. In addition, back trajectories at 500 m AGL will continue to be due south, from coastal NC.

Looking ahead, the Bermuda High will extend westward beginning next Thursday, centered over the Southeastern US. Although the influence of the Bermuda High will keep temperatures and humidity above average in the Mid-Atlantic, the southern location of the high should allow southerly transport aloft to dominate. Thus, although warm, stagnant, and sunny weather will continue next week, the risk for USG ozone should remain Marginal beyond the end of the medium range period, due to southerly flow aloft.

– Eherts/Huff