Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 13, 2016
Valid: June 14-18, 2016 (Tuesday-Saturday)

Summary:

There is High chance of an ozone exceedance on Tuesday across the southern Mid-Atlantic as yesterday’s cold front moves northward, allowing a hot, smoky, and modified air mass to surge into NC and southern VA in its wake. Conditions for the rest of the week will be dominated by a 500 mb omega blocking pattern, which will allow yesterday’s cold front to stall across VA through Friday. A strong shortwave moving through the central and northern Plains on Wednesday will barrel southeastward into the Ohio River Valley on Wednesday and Mid-Atlantic on Thursday and Friday. Wednesday remains a possible day of interest for the I-95 Corridor, where it will be warm and dry, with calm winds in the morning, ahead of a cold front moving through the region on Thursday. As a result, there is a Marginal chance for an ozone exceedance along the I-95 Corridor on Wednesday. As the cold front moves into the region on Thursday, followed closely by the surface low, clouds and showers will impact the region for both Thursday and Friday, lowering the chances for an ozone exceedance to Slight. There is some question about Saturday’s forecast, based on differences with the location of a closed low forming aloft off of the coast of the Mid-Atlantic. At this time, we are following the EC’s solution, which will keep the closed low along the coast, and promote another day with a Slight chance for an ozone exceedance.

Discussion:

The weather forecast models are in relatively close agreement regarding the synoptic pattern through the medium range, but they diverge somewhat at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF. Most of the medium range period, through Friday, will be dominated by an omega block at 500 mb, with closed lows over the northwestern U.S. and New England/Canadian Maritimes, and a strong ridge in between, centered over the Plains. This blocking pattern will keep yesterday’s cold front, currently located across NC and which will move back northward later today, stalled across VA through Friday. A shortwave trough will move through the flow under the upper level ridge on Thursday and Friday, forming a surface low that will track into the Mid-Atlantic along the stalled front on Thursday. The models diverge slightly on Friday with the speed and southern penetration of the cold front, with the GFS slightly faster than the EC (its usual bias). The WPC appears to favor the GFS solution, showing a cold front across southeastern VA and central NC on 12Z Friday. The shortwave trough will merge with some of the vorticity of the Canadian Maritimes upper level low throughout the latter half of Friday, allowing it to cut off somewhere along the coast of the Mid-Atlantic. The models disagree over the evolution and track of this strengthening trough. The EC has the trough becoming an intense cutoff low off the central Mid-Atlantic coast, with vorticity streaks circulating down across the majority of the forecast region. The GFS also closes the low off, but does so further eastward, far enough out to sea to spare the Mid-Atlantic most of its influence. Under the GFS solution, the Mid-Atlantic feels the impact of the upper level ridge, now positively tilted and spread across the entire U.S. At this point, based on historical performance, we favor the EC’s solution for the fate of the closed low along the Atlantic coast.

Tomorrow is a day of interest for the southern portions of the Mid-Atlantic. Yesterday’s cold front will move northward to the vicinity of central VA, allowing the modified and smoky air mass over the Southeast to surge northward. A weak upper level trough developing over the Gulf Coast late Monday will turn mid-level flow southerly, allowing the mid-levels to warm across the southern Mid-Atlantic and surface temperatures to increase. The mid-level trough in place over the southeastern U.S. will remain over place through Tuesday, promoting weak surface winds and clear skies. These factors will allow substantial ozone production across the southern Mid-Atlantic on Tuesday. The 06Z air quality models support this, with all of the models showing isolated pockets of USG ozone across western NC. It is worth noting that the air quality guidance has recently been underdoing ozone, so the fact that all of the models are resolving USG ozone conditions is a strong indication of tomorrow’s threat for poor air quality. As a result, the chance for an ozone exceedance is High for the southern Mid-Atlantic. The central and northern Mid-Atlantic will continue to feel the effects of the weekend’s air mass change, with persistent northwesterly surface winds and seasonable temperatures taming any rising ozone due to strong June afternoon sunshine. 06Z GFS back trajectories remain fast and northerly, originating from southern Hudson Bay in Canada. As a result, generally Good to low Moderate ozone is expected in the northern and central Mid-Atlantic, north of the stalled front.

Wednesday is also a day of potential interest. On Wednesday, the shortwave aloft and corresponding surface low will move into Ohio River Valley, while the stationary front stays in place over VA. These features will bring clouds and showers to much of the forecast region, except for the I-95 Corridor, which should remain dry. The NAM and EC are slightly more scattered with their development of showers, whereas the GFS shows much more widespread precipitation (as is expected for the GFS). The 03Z SREF shows high probability of precipitation across the central and southern Mid-Atlantic by Wednesday afternoon, and the 06Z 4 km NAM has overcast conditions. As a result, ozone production will be limited for much of the forecast region on Wednesday. The exception will be the I-95 Corridor, where it will be sunny, with calm winds in the morning turning southerly but light in the afternoon. It will be warmer, with temperatures rising into the mid to upper 80s °F. Light winds will allow bay and sea breeze fronts to move inland, acting as lines of convergence, seen in the 06Z hi-res model guidance. Presumably, the regional air mass will be relatively unmodified, as the northern Mid-Atlantic will remain north of the stalled front. So rising ozone will be driven mostly by local production. As a result, there is a Marginal chance for an ozone exceedance along the I-95 Corridor.

The surface low moves into the Mid-Atlantic on Thursday along the stalled front, bringing widespread clouds and showers to the region for most of day. The heaviest precipitation will likely be across the northern Mid-Atlantic, following the track of the surface low. The low will move to the VA coast on Friday morning and then possibly out to sea, depending on the location of the upper level closed low. At this point, Friday also looks wet and cloudy for the northern and central Mid-Atlantic for much of the day, while the south clears out. Back trajectories shift onshore for the I-95 Corridor on Thursday and Friday. Given the clouds, rain, and shift to onshore flow, the chance for an ozone exceedance will remain Slight for both Thursday and Friday.

On Saturday, the Mid-Atlantic will clear across the region as the surface low and persistent stationary front push south and east. There is some uncertainty regarding the upper level transport, due to the variations in the model solutions for the closed low aloft. Following the EC solution for now, there will be a cool pool over the northern and eastern parts of the region, with northerly flow aloft and cold air advection. Mid-level transport will cool the mid-levels as a result, which will promote Good air quality for the much Mid-Atlantic, and counteract the sunny conditions expected across the region. The chance for an ozone exceedance will remain Slight once again.

-Brown/Huff