Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 5, 2016
Valid: July 6-10, 2016 (Wednesday-Sunday)

Summary:

There is a strong likelihood for ozone exceedances along the I-95 Corridor through the first half of the medium range, due to the combined influences of a mid-level ridge and a developing lee trough along the eastern U.S. coast. The only caveat to this will be a series of shortwaves and associated weak fronts which will impact the Mid-Atlantic through the period, lending some uncertainty to the air quality forecast. On Wednesday, with an upper level shortwave arrive across the central Mid-Atlantic, but should remain far enough south to keep the I-95 Corridor under full afternoon sunshine. Temperatures will climb above 90°F, which combined with a slow, westerly transport pattern will allow for ample ozone production along the I-95 Corridor. The chance for an ozone exceedance will rise to High for locations along and east of the I-95 Corridor. On Thursday, a second shortwave aloft will impact the region, triggering widespread clouds and showers during the late afternoon. There are questions regarding the speed and intensity of this shortwave, but its associated precipitation looks scattered enough to keep the I-95 under sunny skies, allowing for another day of rapid ozone formation. Back trajectories remain very short and westerly for the I-95 Corridor as well. The chance for an ozone exceedance will remain High for locations along and east of the I-95 Corridor as a result. A backdoor cold front will drop down across the northern Mid-Atlantic during the morning, with a second cold front approaching in its wake. These frontal passages will promote diurnal clouds and precipitation during the late afternoon, but will likely be too weak to inhibit another day of air mass modification across the northern Mid-Atlantic, where ample sunshine will allow temperatures to rise above average once again. The chance for an ozone exceedance on Friday will drop slightly to Appreciable, due to uncertainty in the precipitation forecast. On Saturday, a second cold front will push southward across the northern Mid-Atlantic. The deformation of the mid-level ridge combined with a faster regional (albeit, westerly) transport pattern and the chance for diurnal precipitation will allow the chance for an ozone exceedance to drop to Marginal on Saturday. Mid-level cooling and a shift to northerly and presumably clean back trajectories is expected on Sunday across the Mid-Atlantic behind the cold front. However, post-frontal clearing will promote ample afternoon sunshine along the I-95 Corridor. The chance for an ozone exceedance will remain Marginal on Sunday as a result.

Discussion:

The weather forecast models are in fair agreement through most of the medium range, but are struggling to reach consensus with the development of several shortwaves aloft and their associated precipitation. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A zonal upper level flow pattern will be in place on Wednesday, and remain through the medium range, ushering a series of shortwaves across the Mid-Atlantic. The first of these shortwaves will arrive across the central and southern Mid-Atlantic by 12Z Wednesday. A second shortwave will traverse the Ohio River Valley during Thursday morning and reach the western edge of the Mid-Atlantic by 18Z. The deterministic models are showing varying degrees of intensity for this second shortwave, with the NAM being the strongest and slowest, and the EC being the weakest and fastest. This shortwave will shift offshore by 12Z Friday ahead of an upper level trough developing across the Northern Plains and progressing eastward into the Great Lakes region. The combined influence of the departing shortwave and approaching trough will push a backdoor cold front across PA around 12Z Friday, which will continue to progress southward across the northern Mid-Atlantic during the remainder of the day. The axis of the upper level trough will reach the western Mid-Atlantic by roughly 18Z Saturday, pushing Friday’s backdoor cold front further south into the central Mid-Atlantic. This system will also deform and suppress the mid-level ridge across the eastern U.S. The GFS shows the trough becoming a cutoff circulation by 06Z Sunday, remaining mostly north of the forecast region, whereas the EC has a broader trough through the end of the medium range which reaches down across the northern Mid-Atlantic. The cold front associated with this trough/cutoff will push southward across the northern Mid-Atlantic and into VA by 12Z Sunday. Beginning 18Z Sunday, the trough will begin to slowly lift into the Northeast/southern ON.

Wednesday is a day of interest for the Mid-Atlantic, with several factors aligning to support rapid ozone formation along the I-95 Corridor. The shortwave aloft will remain far enough south to limit most of its associated clouds and scattered precipitation to the central and southern Mid-Atlantic. This precipitation pattern is supported by all of the deterministic models, as well as the 03Z SREF and 06Z 4 km NAM. A lee side trough developing along the eastern coast and remaining in place through Friday will draw warm, moist air from the southeastern U.S. into the northern Mid-Atlantic. This, combined with a mid-level ridge in place over the eastern U.S., will allow temperatures to climb into the low 90s °F across the forecast region. 06Z GFS back trajectories for the I-95 Corridor are short and westerly. It goes without saying that all of these factors are highly conducive to ample local ozone production. The 06Z air quality models are responding in an alarming fashion to this setup, with USG ozone forecast all along the I-95 Corridor for Wednesday, with both the NOAA and NCDENR models resolving Unhealthy (Code Red) ozone across central and southern NJ, as well as near DC. As a result, the chance for an ozone exceedance will rise to High for locations along and east of the I-95 Corridor.

Thursday is also a day of interest for the northern Mid-Atlantic, though there is some uncertainty regarding late day precipitation associated with an approaching shortwave aloft. The NAM, which has a stronger and slower shortwave, keeps the majority of the precipitation to the west of the I-95 Corridor through 00Z Friday, which is also reflected in the 06Z 12 km NAM. The GFS and EC, however, show a broader shortwave, with scattered showers across the northern Mid-Atlantic beginning 18Z. This solution is supported by the 03Z SREF, showing a chance (albeit, low) of precipitation along the I-95 Corridor beginning 18Z Thursday. Despite these differences, it appears that the I-95 Corridor should receive enough afternoon sunshine for another day of above average temperatures and ample ozone production. The mid-level ridge and lee trough from Friday will remain in place as well. Back trajectories for the I-95 Corridor continue to be extremely short and westerly. The air quality models continue to show USG conditions near the I-95 Corridor, but there appears to be some discrepancy in how widespread this USG ozone will be. The BAMS models show more widespread exceedances, with essentially all of NJ, DE, and the western half of MD well into the USG range. NCDENR model, however, back things off in comparison to Wednesday, with only isolated exceedances in the vicinity of DC and portions of NJ. Given the numerous factors still in place that support local ozone formation, however, the chance for an ozone exceedance will remain High for locations along and east of the I-95 Corridor which remain clear through the afternoon.

Friday could potentially be a day of interest as well, but questions pervade the precipitation forecast. A backdoor cold front will push southward across the northern Mid-Atlantic during Friday morning, with a weak center of low pressure remaining just off the Mid-Atlantic coast. This may be sufficient to generate clouds and precipitation across the northern Mid-Atlantic, but the timing of this precipitation is questionable. The NAM keeps the low closer to the eastern U.S., with precipitation along the I-95 Corridor through 12Z, but clearing by the afternoon. The GFS and EC keep the I-95 Corridor clear most of Friday, but then develop more substantial precipitation around 00Z Saturday, ahead of a second cold front behind the backdoor front. Assuming that a GFS/EC solution verifies, the I-95 Corridor will receive a third consecutive day of intense sunshine. Furthermore, the regional air mass should be thoroughly modified, with two days of ozone exceedances driving up ozone levels in the residual layer. Back trajectories for the I-95 Corridor remain relatively short and westerly. The BAMS models continues to show widespread USG ozone all along and east of the I-95 Corridor. The chance for an ozone exceedance will drop to account for the possibility of clouds and precipitation, but only to Appreciable.

On Saturday, a second cold front will push into the northern Mid-Atlantic. The GFS keeps the majority of the precipitation associated with this front to the north of the forecast region, while the EC develops diurnal precipitation across the northern Mid-Atlantic around 18Z Saturday. As with Friday, the exact extent and timing of this precipitation will be crucial to determining how quickly ozone can rise in the vicinity of the I-95 Corridor. Back trajectories remain westerly, but speed out noticeably, originating in western IA. Additionally, the mid-level ridge will finally be suppressed by an approaching low pressure system. The air quality models appear to respond to these factors, returning most of the I-95 Corridor to the Moderate range, with only the far eastern edge of NJ reaching the USG range. That said, temperatures are expected to remain above average, which could support ozone formation if there is enough afternoon sunshine. Given the trend towards cleaner conditions, however, the chance for an ozone exceedance will drop to Marginal on Saturday.

On Sunday, a cold front will push southward into the central Mid-Atlantic. Mid-level cooling will occur across the northern Mid-Atlantic behind the front. Back trajectories for the I-95 Corridor turn northerly, in response to a low pressure system off the coast of ME, which presumably should usher cleaner air into the northern Mid-Atlantic. The only caveat will be widespread clearing and ample sunshine behind the front, which could promote local ozone production along the I-95 Corridor. At this point in time, however, the regional transport pattern should be clean enough to keep the chance for an ozone exceedance Marginal on Sunday.

-Brown/Ryan