Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 31, 2016
Valid: June 1-5, 2016 (Wednesday-Sunday)

Summary:

The medium range period looks relatively quiet in terms of air quality, due to a progressive series of weak upper level troughs and associated cold fronts that will impact the Mid-Atlantic. Conditions for Wednesday will need to be monitored, however. A weak frontal boundary, currently draped along I-95, is expected to linger and possibly be joined by a second weak front arriving from the northwest. The current front has already pushed hourly ozone up this morning at locations in PHL, ILG, and BAL, due to the converging winds along the boundary. This pattern historically can produce isolated USG ozone at affected locations. The 06Z NOAA and BAMS air quality models show isolated USG ozone near PHL and Washington, DC for Wednesday, likely in response to the weak fronts. Consequently, there is a Slight chance for an ozone exceedance on Wednesday. Otherwise, high pressure over eastern Canada will promote sustained onshore winds, which will continue through Friday morning. Flow aloft will shift onshore as well for Thursday and Friday. As result, the chances for an ozone exceedance will drop to Slight for Thursday and Friday. On Saturday, a weak upper level ridge will promote sunny skies and a slight rebound in temperatures across the northern Mid-Atlantic, where chances for an ozone exceedance will jump back up to Marginal. A strong upper level trough will develop over the Plains/Mississippi River Valley on Sunday and move a stronger cold front into the Mid-Atlantic, for widespread clouds and showers, and a return to a Slight chance of an ozone exceedance.

Discussion:

The weather models are in excellent agreement with the main synoptic features of the medium range period, although the ECMWF and GFS diverge slightly by day 5. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. The weather pattern for the medium range is fairly progressive, with a series of weak upper level troughs and associated frontal boundaries impacting the Mid-Atlantic. A weak upper level ridge will first build over and then move through the region on Wednesday and Thursday. At the surface, a weak frontal boundary located along I-95 this morning will gradually dissipate as another weak cold front moves into the region from the northwest. The remnants of Tropical Storm Bonnie will linger along the SC/NC coast, slowly moving northeastward to HAT and dissipating by Friday. There will be mostly zonal flow aloft on Friday over the Mid-Atlantic as a strong northern stream shortwave lifts northeastward across ON and digs out a weak trough, with the Mid-Atlantic on the southern edge. This trough will pull a slow-moving, weak cold front into the region on Friday. Another weak upper level ridge will quickly move over the Mid-Atlantic on Saturday, with a strong upper level trough developing over the Plains/Mississippi River Valley on Sunday. The GFS has a more amplified and slightly slower trough, while the EC has a much more broad and slightly faster trough. This feature will pull a stronger cold front into the region late Sunday into Monday.

The medium range period looks generally quiet for air quality. On Wednesday, surface high pressure over NB/NS and the circulation around the remnants of Bonnie will shift surface winds onshore. The winds will not be particularly brisk, but they will penetrate completely inland across the entire region. This will allow temperatures to drop to around average for much of the region. The big question for Wednesday will be the fate of the weak frontal boundaries – the one currently along I-95 and the second one moving in from the northwest. Although temperatures don’t look to be warm enough for USG ozone, the 06Z NOAA and BAMS air quality models are all showing isolated USG ozone at various locations inland, in the vicinity of PHL and Washington, DC. My guess is that they are responding to the weak frontal boundaries, where converging winds can push ozone up higher than might otherwise be expected, assuming the fronts don’t act as focus areas for clouds and convection. This morning, for example, hourly ozone has jumped up very quickly at locations along the stalled front, near I-95. Forecasters whose regions will be potentially impacted by the weak fronts tomorrow should pay close attention to the 12Z hi-res model runs to determine the impact of converging winds. The 06Z 4 km NAM and 12 km NAM don’t show any widespread convection or cloud cover associated with the fronts. Thus, a Marginal chance for an ozone exceedance exists for Wednesday at locations west of I-95 in the northern and central Mid-Atlantic. The influence of Bonnie will keep the southern Mid-Atlantic generally wet and cloudy, with Good air quality.

For Thursday and Friday, flow aloft shifts onshore, in addition to persistent onshore surface winds. This flow will increase cloud cover across much of the region and drop temperatures a notch from Wednesday. On Thursday, the 09Z SREF has a high probability of precipitation west of I-81 in the afternoon, associated with the slowly approaching cold front. Then on Friday, this precipitation tracks eastward, with another round of afternoon pre-frontal convection for locations east of I-81. As a result, the chances for an ozone exceedance drops to Slight for Thursday and Friday.

Skies will clear across the northern Mid-Atlantic on Saturday as the weak upper level ridge moves overhead. The models keep things wet across the central and southern parts of the region, mainly due to the slow moving cold front. 06Z GFS back trajectories for locations along the I-95 Corridor are recirculating, likely due to the proximity of the weak cold front. It doesn’t look like temperatures will be high enough for USG ozone, but upper Moderate is possible, depending on how clear skies remain and any impact of recirculation. As a result, the chances for an ozone exceedance rise up to Marginal for Saturday, mainly along the northern extent of I-95.

The coarse resolution models develop a lot of precipitation across the region, due to the developing strong upper level trough to the west. A stronger cold front will be slowly approaching from the west. We expect mostly cloudy skies with afternoon precipitation, for a Slight chance of an ozone exceedance.

– -Huff/Brown