Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 14, 2016
Valid: June 15-19, 2016 (Wednesday-Sunday)

Summary:

The first half of the medium range period will be dominated by a 500mb omega block, which will keep a stationary front across WV and VA through Friday. A shortwave trough will cross the Great Lakes region on Thursday, but should remain to the west of the region, although the trend in today’s guidance is for a slower and weaker low. Widespread clouds and precipitation are expected along and south of the stalled front, but there is considerable uncertainty as to how far north they will extend. At this point, conditions along the northern I-95 Corridor are expected to remain warm and dry, with calm winds in the morning and light southerlies during day (which will allow sea and bay breeze convergence). The chance for an ozone exceedance is Slight, with an emphasis placed on the northern I-95 Corridor where full afternoon sunshine is most likely. The questions about the track of the low and associated precipitation carry over into Thursday. There may be enough afternoon clearing, in conjunction with light surface winds, for another day of ozone production along the northern I-95 Corridor. This is highly uncertain, but enough to keep the chances of an ozone exceedance Slight for another day. Friday will be wet for much of the Mid-Atlantic, again with some uncertainty in the extent of showers, with the approaching surface low triggering clouds and precipitation across the portions of the forecast region. Back trajectories will shift onshore for the I-95 Corridor, keeping rising ozone in check across the northern Mid-Atlantic on Friday where some clearing is potentially expected. The weekend forecast is now completely uncertain because the weather models diverge spectacularly regarding the fate of the upper level trough which traversed the region Thursday and Friday. At this point in time, the chance for an ozone will rise to Slight on Sunday for the western Mid-Atlantic, where the persistent upper level ridge will begin to encroach.

Discussion:

The weather forecast models are in relatively close agreement regarding the synoptic pattern through Friday, but they diverge completely at the end of the period. The models consulted for this analysis were the 06Z NAM and the 00Z GFS and ECMWF. The most dominant feature through Friday is still a large 500mb omega block formed by the broad and intense ridge over the central Plains, bookended by two closed lows, one over the Pacific Northwest and another in the vicinity of New England and the Canadian Maritimes. This blocking pattern will halt northward advancement of a front today across the southern Mid-Atlantic, with it becoming stationary across WV and VA as early as 06Z tomorrow. A weak shortwave trough will penetrate through the upper level trough across the northern Plains on Wednesday, and drop down across the Great Lakes region throughout Thursday. The coarse and Hi-Res models are all having trouble with the track of this low and the associated stationary front across the Mid-Atlantic. The trend today is for a weaker low, aloft and at the surface, which is slower to move into the region on Thursday. For example, the 06Z 4 km NAM still has the mid-level low over DET at 18Z Thursday. The WPC recognizes this uncertainty, and gives below average forecast confidence in the track of the shortwave and its associated surface low Wednesday into Thursday. The deterministic models are in better agreement today regarding the track and development of this trough on Friday, yet small differences still persist and have become magnified in today’s guidance. All of the deterministic models have the axis of this shortwave trough sagging down across the central and northern Mid-Atlantic on Friday, with the EC resolving it slightly further southward in comparison to its output yesterday. The key difference today, however, is the placement of the most intense shortwave energy embedded within the trough, which will have a distinct impact on the fate of the trough. The NAM and GFS are both further to the south across NC on Friday, whereas the EC keeps this energy across the central Mid-Atlantic. As a result, the EC’s trough interacts with the Canadian Maritimes trough from earlier in the period, siphoning energy off of this system throughout Saturday and forming an intense cutoff low just off the Mid-Atlantic coast by Sunday. This cutoff low will churn in place for the remainder of the period, circulating potent vorticity streaks along the eastern U.S. coast. The GFS, in contrast, keeps the trough away from the influence of the Canadian Maritimes system, with its shortwave energy detaching and drifting southward on Saturday and forming a weak cutoff low over the southeastern U.S. on Sunday.

Wednesday is still a potential day of interest. We still expect Sunday’s frontal boundary to move northward this afternoon and settle over WV and VA. This front will stall and will act as a guide for the surface low progressing across the Great Lakes on Wednesday. Today’s guidance, however, is trending slower with the track of this low, as mentioned above, which will have implications on the placement of clouds and precipitation, especially for the I-95 Corridor. The deterministic models have the heaviest precipitation developing late Wednesday afternoon and into the early evening along and behind the stalled front, impacting the majority of the central and southern Mid-Atlantic by 00Z Thursday. This is supported by both the 03Z SREF, with high probabilities of precipitation across these locations the latter half of Wednesday, and the 06Z 4 km NAM as well, showing scattered showers and thunderstorms across the central and southern Mid-Atlantic. However, the minor spread in the weather models in regard to the location and track of the shortwave/surface low translates into uncertainty in the clouds and precipitation forecast. All of the Hi-Res models in particular (4 km and 12/13 km) show clouds and precipitation, with some convection, associated with the approaching low, but not all in the same place or at the same time. The northern I-95 Corridor, however, looks to be spared from the majority of the clouds and precipitation associated with these surface features. Locations in this area are expected to experience intense afternoon sunshine, with temperatures rising into the 80s °F. Winds will stagnate overnight and turn southerly, remaining light throughout the afternoon, which will allow the formation and inland penetration of sea and bay breezes. The main forecast questions will be how close the clouds and precipitation associated with the stalled front make it to the I-95 Corridor, as well as how quickly the air mass in place ahead of the approaching front modifies today. The WPC’s analysis places the front further north in comparison to yesterday morning’s analysis, across northern VA, and therefore the chance of clouds and precipitation along the southern I-95 Corridor is slightly greater. The 06Z air quality models are still struggling to resolve the impacts of this front and approaching low, but both the NOAA and NCDENR models show upper Moderate ozone at some locations ahead of the front, while the BAMS-RT has the location of upper Moderate ozone displaced to central/western PA. At this point in time, given the uncertainty in the location of clouds and precipitation, there is a Marginal risk of an ozone exceedance along the I-95 Corridor, with emphasis being placed on its northern extent where full sunshine is more likely.

On Thursday, the surface low should slowly move southeastward across the central Mid-Atlantic, triggering scattered clouds and precipitation. The core of highest precipitation has shifted slightly southward in the deterministic models, in response to the more southward placement of the upper level trough (especially in the EC), with the heaviest rain over the central Mid-Atlantic. This precipitation pattern is also reflected in the 03Z SREF. 06Z GFS back trajectories shift onshore for the I-95 corridor, though it is worth noting that the transport does not appear particularly fast – so it could keep any ozone that forms on Wednesday close by. Given the trend in slowing the approaching low, as well as questions about the extent of precipitation, there is certainly a chance for a second day of upper Moderate ozone along the northern I-95 Corridor. Surface winds will stagnate again overnight and into the morning hours, with the NAM Hi-Res showing lighter east/southeasterly winds in the afternoon, while the GFS Hi-Res has breezier onshore winds. Abundant clouds and precipitation will promote generally Good air quality across most of the Mid-Atlantic, with the potential exception being the northern branch of the I-95 Corridor. Thus, the chance for an ozone exceedance will remain Marginal to account for uncertainty with the northeastward extent of clouds and precipitation.

The surface low from Thursday is expected to shift southward to the eastern NC coast by 12Z Friday, which is slightly further south compared to yesterday’s guidance. The models shift precipitation slightly southward, with the GFS and NAM keeping clouds and scattered showers across the central and southern Mid-Atlantic throughout the day on Friday, whereas the EC keeps the bulk of the precipitation over the northern Mid-Atlantic. Even if the northern Mid-Atlantic does clear, per the GFS and NAM solutions, strengthening onshore flow, both aloft and at the surface, will help keep the region clean and prevent ozone from rising dramatically. Therefore, the chance for an ozone exceedance will drop to Slight.

There is complete uncertainty in the forecast for the weekend, due to disagreement between the models regarding the fate of the shortwave trough that impacted the region on Thursday and Friday. The EC’s closed low aloft along the Mid-Atlantic coast keeps showers circulating inland across the eastern edge of the Mid-Atlantic. The GFS, however, has showers developing across the southern Mid-Atlantic in response to its closed low aloft, located over the southeastern U.S. The WPC is recommending a blend of EC and GFS guidance today, to account for the extremely divergent solutions for the weekend. So that means that most of the region will be clear, with around seasonable temperatures. High pressure moving down from ON on Friday into QC on Saturday and the Gulf of ME on Sunday will keep the flow aloft strongly onshore for the weekend, which should be sufficient to limit ozone to the Moderate range at most. With the eastern edge of the strong upper level ridge pushing up against the western Mid-Atlantic on Sunday, the highest ozone will likely be located there. At this time, given the questions about the upper level pattern, the chance for an ozone exceedance will rise to Marginal on Sunday, mainly for the western portions of the Mid-Atlantic.

-Brown/Huff