Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 17, 2016
Valid: June 18-22, 2016 (Saturday-Wednesday)

Summary:

The weekend and beginning of this coming week will be days of interest for the Mid-Atlantic, with all of the weather models showing the anomalously strong upper level ridge currently over the western and central U.S. dominating the regional weather pattern. However, the models are trending toward a stronger closed low off the Mid-Atlantic coast through Monday, which may prevent the ridge from fully building over the region. Mid-level and surface high pressure will build in across the Ohio River Valley and northern Mid-Atlantic on Saturday, dropping southward to western NC, its preferred location for high ozone across the Mid-Atlantic, on Sunday and remaining there on Monday. Subsiding surface winds and sunny skies under intense June sunshine, promoted by the strong ridge aloft, should allow for ample ozone production through Monday. Possible limits to rising ozone will be presence of the closed low, slowly rising temperatures, and fast northeasterly back trajectories for the I-95 Corridor on Saturday. Ozone should rise gradually, with a Moderate chance of an ozone exceedance on Saturday, an Appreciable chance on Sunday, and a High chance on Monday, when it will be warmer with weekday emissions. Tuesday remains uncertain, as a cold front associated with a broad and amplifying Canadian upper level trough is expected to move into the northern Mid-Atlantic. The timing of this frontal passage remains a question, but the trend is toward a stronger trough aloft, with more confidence in a Tuesday frontal passage, and substantial and organized precipitation and convection ahead of the front. As a result, the chances for an ozone exceedance on Tuesday drop to Appreciable. Wednesday should be post frontal, with only a Slight chance for an ozone exceedance.

Discussion:

The weather forecast models are in general agreement regarding the synoptic pattern through the medium range, but they differ on the location of a quasi-stationary low off of the Mid-Atlantic coast and the exact speed of a cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough currently sagging across the central Mid-Atlantic will move southeastward and reach HAT by 06Z Saturday, with its surface low following close by. The eastern edge of a peculiarly strong and broad upper level ridge will make its way over the western edge of the Mid-Atlantic around 12Z Saturday, and it will push the trough along the NC coast offshore. Shortwave energy from the Canadian Maritimes’ cutoff low will drop southward beginning Saturday afternoon and join up with this trough, causing it to strengthen once again, with all of the deterministic models showing a new cutoff low developing by 12Z Sunday. The GFS diverges at this point from both the NAM and EC with this feature, as is noted in the most recent WPC Model Diagnostic Discussion, showing weaker circulation with a much slower track than the other models. As a result, the GFS will be considered an outlier for this particular feature, but will be reconsidered later in the period in regard to a cold front arriving on Tuesday. The NAM and EC are in closer agreement today regarding the strength and placement of this closed low. Both show the system churning just off the Mid-Atlantic coast through Monday afternoon, with its western periphery brushing eastern VA and NC, especially in the EC. This trend in the models toward a stronger closed low, closer to the Atlantic coastline, continues in today’s guidance, with the effect of preventing the anomalously strong western upper level ridge from fully building over the Mid-Atlantic over the weekend and into Monday. In addition, the mid-level ridge and surface high drop southward faster through the period, which has impacts on the transport pattern aloft for the Mid-Atlantic. The EC shows the closed low shifting northward overnight Monday, still hugging the Eastern Seaboard until 12Z Tuesday, when it moves northeastward out to sea as it gets absorbed by a broad upper level trough reaching down from Canada into the Northeast U.S. This trough will pull a cold front into the northern Mid-Atlantic sometime Tuesday, but the weather models still disagree slightly on its exact speed. The EC brings this front to central PA by 18Z Tuesday, whereas the GFS is approximately 3 hours slower. At this point, the WPC is favoring a blend of the solutions, showing the cold front approaching the northern Mid-Atlantic at 12Z Tuesday. The trend in the forecast model guidance today is for a stronger trough on Tuesday-Wednesday, which in turn gives more confidence in the ability of the cold front to penetrate the northern Mid-Atlantic on Tuesday. This trough will deepen throughout Tuesday and into Wednesday, with the entirety of the Mid-Atlantic under its influence by the end of the period.

On Saturday, a mid-level ridge will move down from the north and become centered over the Ohio River Valley and northern Mid-Atlantic, with the corresponding surface high moving from NY to northern VA. Winds will subside across the northern and central Mid-Atlantic in response to the arriving center of high pressure. Full June sunshine, the strongest of the year, will prevail across the region, allowing ozone levels to rise. This potential increase in ozone will be tempered, however, by two factors, one of which will be the fast and clean transport pattern still in place for the I-95 Corridor. 06Z GFS back trajectories show transport originating from just south of the NB coast. Additionally, the close proximity of the trough off the Mid-Atlantic coast will drive clean air onshore into coastal locations across the central and southern Mid-Atlantic. In addition, temperatures will be relatively slow to rise, possibly in response the stronger closed low off of the Mid-Atlantic coast preventing the upper level ridge from fully building over the region. As a result, with temperatures only reaching the upper 70s to low 80s °F, it may not be hot enough for substantial ozone production. The western edge of the Mid-Atlantic may have the quickest rise in ozone, as this is where the influence on the approaching ridge will be felt first. The 06Z air quality models keep most of the western Mid-Atlantic within the Good range for ozone, with isolated patches of Moderate in western PA and along the I-95 Corridor. It is worth noting that the air quality models have been wildly inconsistent within the past week or so, failing to even pick up on the correct trend in the ozone levels across the Mid-Atlantic on most days. The main forecast questions will be how quickly the air mass in place modifies, along with the potential effects of the coastal closed low. We should have an idea of air mass modification this afternoon. Taking all of these factors into consideration, the chance for an ozone exceedance on Saturday will be Marginal, mainly for the western Mid-Atlantic.

On Sunday, surface high pressure will move southward during the morning, settling over western NC by 12Z, its preferred location for high ozone across the Mid-Atlantic. Return flow from this high will turn mid-level transport westerly. There is some question as to how fast the region will warm, with model guidance keeping temperatures in the mid-to-upper 80s °F. As on Saturday, there will be very light to calm winds across the northern and central parts of the region, which will allow bay and sea breezes to develop. The main exception will be coastal portions of VA and NC, which will be under the influence of the nearby cutoff low. In addition to its onshore flow, clouds and showers circulating around this low may affect these locations. Onshore back trajectories for the I-95 Corridor will slow and begin to recirculate. Strong subsidence associated with the upper level ridge will keep skies sunny all day. A possible limiting factor will be the fact that it is Sunday, when anthropogenic emissions are typically lower than Monday-Saturday. However, it is Father’s Day, and it will be such a beautiful weekend that there may be more shore/beach traffic than on a usual Sunday. Thus, the chance for an ozone exceedance will rise to Appreciable.

Monday continues to be the day of most interest, with mid-level and surface high pressure remaining in place over western NC. Another day of calm winds and cloudless skies, as well as a return southerly flow, will allow temperatures to climb into the upper 80s to 90s °F, so it will certainly be hot enough for ozone production. In addition, transport aloft will shift westerly, with recirculation in the morning for locations along I-95. The closed low off the Mid-Atlantic coast may still be close enough to induce onshore flow at coastal locations, but the influence of this system should weaken on Monday as it begins to move northward. How high ozone can reach on Monday will be largely determined by how much the air mass in place modifies over the weekend. But at this point, most factors point to widespread ozone exceedances along and west of the I-95 Corridor. The air quality models, although lately unreliable, are picking up on this potential event, with the BAMS and NC models all showing USG ozone along the I-95 Corridor, with the NC model even placing Unhealthy ozone along the northern extent of the I-95 Corridor. As a result, the chances for an ozone exceedance on Monday are High.

Tuesday remains a question for the air quality forecast, with both the GFS and EC showing a cold front moving into the northern Mid-Atlantic sometime during the day, with considerable and widespread precipitation and convection forming ahead of the front. The timing of this frontal passage will the key to determining how high ozone can rise before clouds and precipitation associated with the front take hold. As mentioned earlier, the EC and GFS both bring in the front by Tuesday afternoon, which would limit ozone levels across the northern Mid-Atlantic and the I-95 Corridor in particular. Cold fronts tend to slow and stall, however, during this time of the year as they push into much warmer and drier air masses, much like the one that will be in place over the region through Tuesday morning. But the fact that there has been consistency in the run-to-run guidance since yesterday regarding this frontal passage on Tuesday lends support to a lower ozone forecast. If this front does indeed slow, ozone concentrations could climb as high or higher than Monday across the Mid-Atlantic. The BAMS and NC air quality models keep ozone in the USG range along I-95, so they are buying into a slower frontal passage. At this time, given this uncertainty, the chances for an ozone exceedance on Tuesday will drop to Appreciable.

On Wednesday, the Mid-Atlantic will experience an air mass change as Tuesday’s cold front dives southwestward across the region. Fast, post-frontal back trajectories, combined with clouds and precipitation along the front, will help clean out the region and drop ozone out of the USG range. The chance for an ozone exceedance will drop to Slight as a result.

-Brown/Huff