Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 10, 2016
Valid: June 11-15, 2016 (Saturday-Wednesday)

Summary:

There remains a higher than usual level of uncertainty with the air quality forecast for the weekend, due to continued questions about possible severe weather triggered by an advancing warm front on Saturday, as well as lack of confidence in air quality model guidance, which remains low-biased. Nevertheless, there is a High chance for an ozone exceedance on Saturday, given a quick rebound in temperatures into the upper 80s to 90s °F, subsiding winds through the morning, and the arrival of a modified air mass from the west. Factors that may limit rising ozone include breezy winds in the afternoon and chances for convection, although today’s weather models have even less support for clouds and storms across our region. A southward-moving cold front will track through the region on Sunday morning, clearing the I-95 Corridor by 15Z. Cooler and much drier air will arrive behind the front, along with very breezy northwesterly winds, but the cooler air will be slow to build in, allowing another day of temperatures into the upper 80s to 90s °F at locations roughly south of the Mason-Dixon Line. As a result, these locations have an Appreciable risk for an ozone exceedance on Sunday. On Monday, the cold front will move back northward as a warm front and stall in the vicinity of the Mason-Dixon Line. There are questions regarding the exact placement of the front, which will impact the air quality forecast. An Appreciable chance for an ozone exceedance will continue on Monday for those locations that remain south of the front, where the modified air mass and sunny skies will persist. The forecast is uncertain for Tuesday and Wednesday due to questions about the exact track of the front, which should remain over the region, but exactly where is not clear. In addition, a weak area of upper level vorticity over the Gulf Coast may trigger clouds and showers that extend into the southern Mid-Atlantic. Given this uncertainty, there is a Marginal chance for an ozone exceedance for the end of the period.

Discussion:

The weather forecast models are still 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. The broad upper level ridge over the majority of the country today will continue to strengthen and move eastward throughout Saturday, with its eastern edge extending over the Mid-Atlantic. The mid-level ridge in place over the southeastern U.S. for the past two days will also remain in place, building northward through the weekend. A potent shortwave moving southeastward across eastern ON and southern QC on Saturday will cut off by 12Z Sunday in the vicinity of ME. The NAM, which was considerably slower yesterday regarding the track of the feature, has caught up to the GFS and EC in today’s guidance. A cold front associated with this low will push southward across the Mid-Atlantic Sunday morning, but the low will be far enough away from the forecast region to limit the southward extent of the front’s track. Beginning Monday, the closed low will become quasi-stationary, churning slowly over northern New England and Canadian Maritimes. The quasi-stationary nature of the closed low will allow Sunday’s cold front to only progress as far as northern NC on Monday morning and then move back northward as a warm front, stalling in the vicinity of the Mason-Dixon Line on Tuesday and possibly sagging slightly farther south on Wednesday. The EC and GFS differ on the location of the front, with the GFS having a slightly stronger front that stays farther to the south. The WPC favors the EC solution, so we will as well. Aloft, the broad upper level ridge will amplify significantly over the central Plains on Monday and extend its axis all the way up into southern Canada. The southern edge of the ridge, as well as the southeastern U.S. mid-level ridge, will be deformed by two features developing starting on Monday. The weaker of the two is a broad area of troughiness forming Monday afternoon over the Gulf Coast, and progressing eastward into the southern Mid-Atlantic through Wednesday. The other feature is a more potent shortwave developing over the southwestern U.S. on Monday and riding the ridge’s flow into the central and northern Plains on Tuesday, where it closes off and intensifies through Wednesday.

Saturday remains a day of interest, with higher than usual uncertainty regarding the approaching warm front and any convection it may spawn. Winds will stagnate overnight Friday and into Saturday morning in response to surface high pressure moving directly over the northern Mid-Atlantic, but then turn southwesterly and strengthen later in the day. The northward encroachment of the southeastern U.S. mid-level ridge will turn transport westerly and cause substantial mid-level warming across the northern and central Mid-Atlantic, with high temperatures rebounding into upper 80s to low 90s °F. The general weakening trend (in the sense of being able to fire up convection) in the strength of Saturday’s warm front continues in today’s model guidance, bringing into the question the ability of this front to trigger severe weather. All of the deterministic models, as well as the 03Z SREF, show isolated precipitation developing across PA and NJ during the late afternoon; their diffuse precipitation pattern agrees with the notion of a weak front with limited lifting along the frontal boundary itself. The 06Z 4 km NAM is even less impressive with its convective development along the front, resolving only a few scattered showers and thunderstorms stretching southward across PA and MD that die off by the early afternoon, followed by a weak line of convergence forming over northern PA around 18Z. The model guidance is trending toward keeping the most organized convection to our north, across New York State. In spite of this, the SPC maintains a SLIGHT risk for severe convective activity across much of PA, NJ, and part of northern DE and MD. The presence of intense directional shear associated with the veering wind profile, as well as the enhanced regional instability caused by increased humidity and warm air advection at mid-levels certainly supports, at least dynamically, the potential for convection in the northern Mid-Atlantic, mostly north of the Mason-Dixon Line. The main question will be whether the weak warm front itself will be provide enough lift for this convection to actually initiate, which will in turn control how much sunshine there will be across the central and northern Mid-Atlantic. The air quality models are also struggling to resolve the impacts of this warm front. The 06Z models show more widespread Moderate ozone in comparison to yesterday’s runs, with Moderate ozone for areas near the I-95 Corridor, as well as southern and western PA, most of MD and DE, and parts of VA and NC. Interestingly, none of the models show any areas of USG ozone for the forecast region. However, the ozone models have been underforecasting this week, certainly in DE, southern MD, and parts of NC yesterday, as well as the isolated urban USG observations to our west yesterday. This makes us less trusting of the widespread Moderate predictions for tomorrow. Given the lessening support for afternoon convection and the presence of the mid-level ridge over the southeastern U.S., at a minimum, Moderate ozone conditions appear likely across the region. The chances for an ozone exceedance are High, but the question is, where? There is still a higher than usual amount of uncertainty. The mostly likely locations will be south of the Mason-Dixon Line, including DE, BAL, DC, southern VA, and across NC.

A cold front associated with the upper level closed low will advance southward across the northern Mid-Atlantic on Sunday morning, and continue this progression through the rest of the day. As with yesterday’s guidance, the track of this front has appeared to slow slightly in response to the eastward placement of the closed low, but the 06Z Hi-Res models still show the front clearing the entire I-95 Corridor by 15Z. Locations behind the front on Sunday will experience an air mass change very similar to the one seen on Tuesday of this week, with much drier air building in its wake for much of the northern Mid-Atlantic. The cooler air appears to lag somewhat behind the frontal passage, with locations roughly south of the Mason-Dixon Line reaching the upper 80s to low 90s °F for the second day in a row. 06Z GFS back trajectories are fast and northerly for the I-95 Corridor, originating from interior ON. As with Saturday’s warm front, there appears to be a weakening trend regarding the clouds and precipitation expected along this cold front, due to the very dry air mass behind the front. The deterministic and Hi-Res models show only light showers developing across the central Mid-Atlantic in the afternoon. This lack of intense frontal precipitation, combined with the slowing of its progression, suggest that parts of the central Mid-Atlantic and the entire southern Mid-Atlantic will still be under a modifying air mass on Sunday, and they will experience full afternoon sunshine ahead of the front. The 06Z air quality models support this, showing Moderate ozone conditions for essentially the entire state of NC and parts of southern VA, though none of the models show any USG ozone for these locations. How high ozone will be able to climb on Sunday will be partially dependent on how much ozone is produced on Saturday, as well as the southward progression of the cold front. That said, the chances for an ozone exceedance will remain Appreciable on Sunday for these areas.

On Monday, the cold front will lift back northward and become stationary somewhere in the vicinity of the Mason-Dixon Line. The EC and GFS are not exactly on the same page in regard to the fate of this front. Both have the front stalling over the region through the end of the period (and beyond), but they differ on the exact location, which is to be expected. As mentioned above, we are following the WPC and favoring the EC solution, which brings the front farther north on Monday. This frontal boundary should act as focus for clouds and thunderstorms, with the EC showing a band of heavy precipitation stretching across the northern and central Mid-Atlantic. Areas north of the frontal boundary should remain relatively clean due to Sunday’s air mass change, very fast northern back trajectories, and breezy northwesterly surface winds. Areas south of the front, especially the southern Mid-Atlantic, will remain under sunny skies. The exact placement and strength of the stationary front will determine how high ozone levels can climb, but mid-to-upper Moderate seems likely at a minimum. Due to this uncertainty, the chance for an ozone exceedance on Monday will be Appreciable.

The forecast for Tuesday and Wednesday is uncertain due to questions about the location of the stationary front and the impacts of the weak upper level troughiness developing across the southern portion of the nation. The southern trough may trigger clouds and thunderstorms across the southern Mid-Atlantic, with widespread showers showing up in both of the deterministic models. The northern and central portions of the Mid-Atlantic will still be mostly clean on Tuesday, with continued fast and northerly back trajectories taming rising ozone levels in response to afternoon sunshine. Winds aloft and at the surface subside on Wednesday, which may allow ozone to rise, but at this point, only a Marginal chance for an ozone exceedance is expected under the relatively progressive pattern aloft both days.

-Brown/Huff