Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 16, 2016
Valid: June 17-21, 2016 (Friday-Tuesday)

Summary:

The weather forecast models are in closer agreement today regarding the eastward advancement of an anomalously strong upper level ridge, which will build over the Mid-Atlantic beginning Saturday and persist through at least Monday. There is a growing consensus that a closed low will linger along the Mid-Atlantic coast through Monday, but it will be far enough off shore to allow the ridge to dominate the weather pattern. Friday will be cloudy and wet for most of the region, with onshore surface winds, as a potent shortwave aloft moves through the region. The upper level ridge will build over the western Mid-Atlantic on Saturday, with a mid-level ridge and surface high moving down from ON to the Ohio River Valley/northern Mid-Atlantic. The center of the mid-level ridge and surface high will progress southward to WV on Sunday. Strong subsidence associated with the upper level ridge will keep skies clear and dry across the region through Monday. The surface high will settle over western NC on Monday, its preferred position for high ozone across the Mid-Atlantic. Temperatures will be moderated to seasonable values over the weekend by fast northeasterly flow aloft, but with a return flow setting up on Monday, temperatures will jump into the upper 80s to 90s °F. Tuesday will be even hotter than Monday, but an approaching cold front may trigger showers and thunderstorms across the northern Mid-Atlantic that would limit rising ozone. Back trajectories for the I-95 Corridor are slow and recirculating on both Monday and Tuesday. Therefore, after only a Slight chance for an ozone exceedance on Friday, chances rise to Marginal on Saturday (mainly for the western parts of the region), and become Appreciable on Sunday, High on Monday, and drop back to Appreciable on Tuesday, due to uncertainty regarding the timing of the approaching cold front.

Discussion:

The weather forecast models are in relatively close agreement regarding the synoptic pattern through the medium range, with some slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The upper level shortwave currently over the Great Lakes area will dip down across Ohio and into the central Mid-Atlantic by 12Z Friday, with its associated surface low following closely in step. This shortwave will advance eastward throughout Friday, with its axis reaching HAT by 00Z Saturday. The NAM shows more potent shortwave energy associated with this feature, especially by late Friday afternoon, but the WPC considers the NAM to be too strong with this feature. The deterministic models are otherwise in agreement with placement and track of this shortwave. The anomalously strong and broad upper level ridge across the western and central Mid-Atlantic will build into the western portions of the Mid-Atlantic by 12Z Saturday, nudging the shortwave and its surface low eastward off the NC coast. There is much closer agreement in today’s guidance regarding the fate of this shortwave, with all of the models now showing the Canadian Maritimes cutoff low diving southward on Sunday and joining up with the departing shortwave to form a new cutoff low off the Mid-Atlantic coast, which was the EC’s solution all along. In this way, the NAM and GFS are now coming into consensus with the EC regarding a closed low lingering along the Mid-Atlantic coast through Monday, although there are differences in the strength and more importantly, how close the low is to the Eastern Seaboard. The timing and location of this cutoff low vary slightly from model to model. The NAM has the low developing slightly earlier than the GFS and EC around 06Z Sunday, whereas the GFS and EC develop this closed low by 12Z Sunday. The NAM and GFS keep this cutoff low hugging the southern Mid-Atlantic coast, with vorticity streaks circulating on land over HAT. The EC keeps this system far enough out to sea to limit its interaction with the Mid-Atlantic. At the same time, the upper level ridge will swallow up the rest of the Mid-Atlantic on Sunday, and remain the dominant feature across the entire CONUS through Monday. A broad upper level trough will begin to slide down from southern Canada beginning Monday afternoon, pulling a weak but extensive cold front into the northern Mid-Atlantic sometime Tuesday. Today’s guidance shows a much stronger trough than yesterday, which suggests more clouds and precipitation will be associated with this front, and a substantial air mass change in its wake. There are timing differences between the GFS and EC with the speed of the front, which is typical, with the EC being slightly (~6 hours) faster than the GFS.

At the surface, the approaching low will move southeastward across the central Mid-Atlantic on Friday morning and reach HAT by 00Z Saturday. Widespread showers and thunderstorms will remain across southern PA, DE, and most of the central Mid-Atlantic through 12Z, at which point precipitation will begin to shift southward, following the low. This is supported by both the 03Z SREF and 06Z 4 km NAM. The NAM shows more intense precipitation, due to its stronger upper level energy, but the models seem to be more on the same page with the regional precipitation pattern than they were yesterday. Skies over the northern Mid-Atlantic will clear by the afternoon as this low shifts southward, but the central Mid-Atlantic will remain socked under clouds and persistent rain throughout much of the afternoon. The low will shift southeastward towards the coast around 21Z, with scattered showers around its periphery impacting NC late Friday and into Saturday morning. Given the clouds and precipitation expected across the forecast region, generally Good air quality is expected throughout the Mid-Atlantic. The exception could be the northern Mid-Atlantic, where clearing is expected in the wake of the low, but onshore flow, especially upper level transport, should limit rising ozone. In addition, there are several areas of concentrated convection moving through the northern Mid-Atlantic today (Thursday). These areas of heavy precipitation and gusty winds (possibly severe) will help to clean out the atmosphere for Friday. Therefore, the chance for an ozone exceedance will remain Slight.

With increasing agreement regarding the encroachment of the upper level ridge and its associated surface high pressure, the weekend and Monday continue to be days of interest for the Mid-Atlantic. On Saturday, a surface and mid-level high will build southward from southern ON, settling across the Ohio River Valley and northern Mid-Atlantic. The approach of high pressure will cause winds to subside across the northern and central Mid-Atlantic, with a return to full June sunshine across the region. Rising ozone levels will be moderated, however, by north to south mid-level cooling, which will keep high temperatures in the lower 80s °F, and fast, northeasterly back trajectories, which should keep ozone in the Moderate range for most of the Mid-Atlantic. The only area of concern will be the western Mid-Atlantic, where the influence of the ridge will be felt sooner. As a result, the chance for an ozone exceedance will rise to Marginal on Saturday for those locations.

Sunday will be similar to Saturday, with the upper level ridge fully in place across the entire region by Sunday morning. The surface high will continue to shift southward, with its center over WV. Winds will essentially go calm in response to the high directly overhead, but temperatures will remain in the low-to-mid 80s °F. Winds will be stronger along the coast, where the influence of the meandering upper level closed low will be felt. Rising ozone is expected across inland portions of the Mid-Atlantic, except for locations east of I-95 and in eastern NC, which will be impacted by the closed low that the NAM and GFS place directly east of HAT by 18Z. 06Z GFS back trajectories for the I-95 Corridor remain fast and onshore, originating from off the NB coast, which could have a similar moderating effect as on Saturday. However, another day of intense sunshine will allow the chance for an ozone exceedance to rise to Appreciable, primarily for the central and western Mid-Atlantic.

Monday is the day of most interest, as the mid-level and surface highs will move to their preferred location for high ozone, over the southern Mid-Atlantic. This position will allow for a return flow, pushing very warm air northward, with high temperatures rising to the upper 80s to low 90s °F. No clouds or precipitation are expected, under the subsidence of the anomalously strong upper level ridge, so there will be another day of intense June sunshine and light winds across the Mid-Atlantic. The mid-level transport pattern will turn westerly, with flow from the Ohio River Valley, the historical source region for NOx. Back trajectories are slow and recirculating for the I-95 Corridor. The main forecast question will be how close to the coast the closed low will remain, which could potentially drive cleaner air inland and reduce ozone levels at coastal locations, but we don’t think this will be a factor. The only other question will be how quickly the air mass modifies over the weekend. The 06Z runs of the BAMS air quality models this morning are showing USG ozone along I-95, while the NC model keeps ozone in the upper Moderate range. Given the many factors in place that favor high ozone, the chance for an ozone exceedance will rise to High on Monday.

On Tuesday, the weather models are consistent in bringing a weak cold front into the northern Mid-Atlantic. Although there are timing differences with the front, both the EC and GFS develop widespread showers and convection across the northern Mid-Atlantic on Tuesday. If this verifies, it will limit any ozone to the Moderate range along the I-95 Corridor. However, Tuesday is day 5, and summer fronts historically slow down when moving into a warm and humid air mass. Therefore, given the possibility for the approaching weak front to stall or slow, there is an Appreciable chance for second consecutive ozone exceedance along I-95 on Tuesday.

-Brown/Huff