Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 21, 2016
Valid: June 22-26, 2016 (Wednesday-Sunday)

Summary:

This weekend continues to appear favorable for another ozone event along the I-95 Corridor, as another upper level ridge builds over the Mid-Atlantic. On Wednesday, the front that pushed into the northern Mid-Atlantic today will remain stalled in the vicinity of the Mason-Dixon Line. The front will not act as a trigger for clouds and precipitation, nor as a line of convergence, with the main effects being a slight decrease in temperatures and dew points behind the front. Although there will be abundant sunshine across most of the region, breezy surface winds should limit rising ozone, with a Marginal chance for an ozone exceedance. A wave of low pressure initiated by a small, yet potent shortwave aloft will traverse the stalled front on Thursday, bringing widespread clouds and precipitation to the northern and central parts of the region. There is still some question as to where the heaviest precipitation will fall, but the consensus among the forecast models is for the wave to move across PA and NJ. As a result, the chance for an ozone exceedance will drop to Slight on Thursday. On Friday, high pressure building southward from ON will push the front into NC, allowing the northern and central Mid-Atlantic to clear. The chance for an ozone exceedance will rise to Marginal for the I-95 Corridor, which will experience ample June sunshine and recirculating back trajectories but slightly cooler temperatures. Saturday and Sunday are potential days of interest, as an upper level ridge will build eastward across the region, with a mid-level ridge also forming and hooking up with the Bermuda High. Clear skies, rising temperatures, and subsiding winds across the Mid-Atlantic will promote ozone production, though a shift to onshore flow due to the circulation around nearby high pressure could temper rising ozone on Saturday. As a result, the chance for an ozone exceedance will rise to Appreciable for both weekend days.

Discussion:

The weather forecast models are in fair agreement regarding the synoptic pattern through the medium range, but they continue to differ with the placement of a stationary front and associated wave of low pressure which will impact the Mid-Atlantic on Thursday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The upper level trough that is pushing a cold front into the northern Mid-Atlantic today will spin over southern ON throughout Wednesday, with its associated cold front remain stationary in the vicinity of the Mason-Dixon Line by 12Z. A wave of low pressure, initiated by a small but potent shortwave developing across the northern Plains late Wednesday, will ride along this stalled front, possibly pulling it northward slightly as a warm front. The wave will traverse the Ohio River Valley overnight and arrive in the northern and central Mid-Atlantic as early as 06Z Thursday. There are still differences amongst the deterministic models with the intensity and track of this wave and its associated precipitation. The NAM develops a stronger and slower shortwave aloft than the GFS and EC, yet has a slightly faster progressing surface low, leading the WPC to suggest a GFS and EC blend with this feature. As a result, we consider the NAM an outlier beginning Thursday. The GFS and EC are in closer alignment today regarding the track of the wave, keeping it moving across PA and NJ and out to sea. The main difference is that the EC keeps the areas of heaviest precipitation over the northern Mid-Atlantic, while the GFS initially has the heaviest precipitation over NY. The exact north-south placement of the heaviest precipitation remains a question, but regardless, there is growing confidence that there will be widespread clouds and rain across the northern Mid-Atlantic on Thursday morning and afternoon, with the heavy precipitation moving into the central Mid-Atlantic in the later afternoon. High pressure developing over southern ON on Thursday will move southward early Friday, pushing Thursday’s front into the southern Mid-Atlantic, reaching NC by 12Z Friday. An upper level ridge amplifying across the Plains on Friday will shift eastward into the Mid-Atlantic beginning 06Z Saturday, building completely over the region by the late afternoon, and remaining in place on Sunday. Concurrently, a mid-level ridge develops and hooks up with the Bermuda High.

On Wednesday, the front stalled near the MDL will remain in place across the central Mid-Atlantic. It still does not appear that this front will be strong enough to generate clouds or convection. Furthermore, the 06Z Hi-Res guidance does not show any discernable convergence along this front, which often can be a matter of concern with weak frontal boundaries. The main impacts of this front will be reflected in temperature, with locations behind the front dropping a few degrees from Tuesday, while areas ahead of the front will remain warmer and under the influence of high pressure across the southeastern U.S. 06Z GFS back trajectories for the I-95 Corridor are northwesterly and shorter than yesterday, but breezy westerly surface winds should be sufficient to ventilate the atmosphere along I-95 and keep locations there out of the USG range. The 06Z air quality models only show areas of Moderate ozone across NC and southern DE. That said, we have seen ample ozone production recently on days with temperatures in only the low 80s °F, so the fact that the majority of the Mid-Atlantic is expected to remain mostly sunny and reach at least the mid-80s °F makes it difficult to rule out the possibility that there could be isolated areas of rapidly increasing ozone. Therefore, the chance for an ozone exceedance will remain Slight.

Although there is still some question about where the heaviest rain will fall associated with the wave moving along the stalled front on Thursday, but, as discussed above, it is becoming increasingly likely that the northern and most of the central Mid-Atlantic will be under the influence of clouds and, at the very least, scattered showers throughout much of Thursday, with the possibility of heavy rain and thunderstorms across PA and NJ. NC appears to be the only state that escapes the influence of this system on Thursday, with possibility of eastern VA also being spared. While the air quality models still keep these sunny areas in the lower Moderate range for ozone, the rest of Mid-Atlantic will likely be socked under persistent cloud cover and precipitation. Therefore, the chance for an ozone exceedance will drop to Slight.

High pressure building southward from Canada on Friday will push the stalled front southward into NC, allowing the northern and central Mid-Atlantic to clear in its wake. The movement of the front will trigger some diurnal showers and thunderstorms across the southern Mid-Atlantic. There will some mid-level cooling across the northern Mid-Atlantic in response to the departing front, with high temperatures only expected to reach the low 80s °F. Although it will be post-frontal in the northern Mid-Atlantic, 06Z GFS back trajectories for the I-95 Corridor are trending toward recirculation around New York City. Although temperatures will be cool relative to previous days, there should be ample sunshine to drive ozone production, and recirculation is always a red flag for the I-95 Corridor, as we saw for PHL last Wednesday. As a result, the chance for an ozone exceedance will rise to Marginal on Friday for areas along the I-95 Corridor.

Both Saturday and Saturday are potential days of interest, with an upper level ridge pushing eastward into the western Mid-Atlantic Saturday morning and completely enveloping the forecast region by 18Z Saturday and remaining in place through Sunday. At the same time, a mid-level ridge will strengthen over the eastern U.S. and connect with the Bermuda High. Surface high pressure will drift southeastward on Saturday morning and settle over eastern PA for the remainder of the weekend. These features will promote sunny skies, rising temperatures, and light surface winds across the Mid-Atlantic. Rising ozone could be tempered, however, by a shift to onshore flow due to circulation around the nearby center of surface high pressure, as it did last Friday and Saturday. Currently, the BAMS and NC models do not show any USG ozone developing throughout the Mid-Atlantic on Saturday. That said, two days of intense June sunshine, strong subsidence, and subsiding winds will allow the chance for an ozone exceedance to increase to Appreciable for both Saturday and Sunday.

-Brown/Huff