Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Friday, July 6, 2018
Valid: July 7-11 (Saturday-Wednesday)
Summary
A fresh post frontal air mass filtering into the Mid-Atlantic on Saturday and Sunday will keep a low risk of an exceedance for the weekend before an arriving upper/mid-level ridge increases the risk to Appreciable though Tuesday. A cool and considerably less humid air mass will filter into most of the Mid-Atlantic on Saturday, in the wake of today’s cold front, setting the stage for a pleasant weekend. Strong northerly flow across the NMA and CMA (from Hudson Bay) will keep ozone formation minimal despite sunny skies under the strong June sun, resulting in a Slight risk for an ozone exceedance. The effect of overhead upper/mid-level high pressure will start to impact air quality on Sunday as ozone levels begin to gradually rise. A combination of mostly sunny skies and weakening surface winds will allow for afternoon ozone formation to take place across the NMA and CMA while onshore transport in the SMA should keep ozone formation minimal. The primary forecast questions on Sunday will be how quickly the air mass is able to modify given the recent frontal passage and lower Sunday emissions, and the impact of onshore surface winds in the morning along the east coast of the NMA and CMA. The risk of an ozone exceedance will increase to Marginal with a focus on the I-95 Corridor from DC to NYC and western PA. Nearby high pressure will weaken transport aloft on Monday, resulting in localized back trajectories, weak surface winds, as well as near average temperatures. Confidence is gradually increasing that conditions could be favorable for an exceedance along the I-95 Corridor from DC to NYC and in western PA, warranting an Appreciable risk on Monday. The main forecast question will be how quickly the air mass in place modifies. Uncertainty remains for both Tuesday and Wednesday as the weather models have yet to come into close agreement regarding a developing upper level trough and the track of a probable tropical cyclone off of the coast of the Carolinas. We have some confidence that unsettled conditions will move into the western part of the NMA, narrowing the chances for high ozone to the usual locations along I-95. Mostly sunny skies, slightly above average temperatures, westerly flow aloft, and southwesterly surface winds keep the risk of an ozone exceedance Appreciable on Tuesday, with a continued focus on the I-95 Corridor from DC to NYC. There is a great deal of uncertainty in the forecast for Wednesday stemming from questions in Tuesday’s forecast. Uncertainty surrounding the track of the potential tropical disturbance off the Carolina coast and differences in the weather models regarding the strength of the upper level trough forecast to move over the Northeast U.S. on Wednesday provide a range of possibilities across the Mid-Atlantic. At this time, given the opportunity for overnight/morning unsettled conditions and a weak frontal passage in the NMA, the risk of an exceedance will drop to Marginal.
NWP Model Discussion
The weather models are in general agreement with synoptic scale features until 00Z Monday, when solutions begin to diverge with a developing upper level trough in central Canada and the development of a probable tropical system off the coast of the Carolinas. The models have the same general features, but small details regarding their strength and timing translate into substantial uncertainty for Tuesday and Wednesday. The upper level longwave trough that will pass over the Northeastern CONUS today will continue to pull eastward on Saturday, moving out of the NMA around 00Z Sunday. The eastward pull of this trough will allow upper and mid-level ridging to quickly build eastward on Sunday, with a broad and shallow upper level ridge encompassing almost the entire CONUS. A shortwave passing over SK/ON on Monday will reinforce an upper level closed low over interior Canada that will move over Hudson Bay by 18Z Monday, suppressing the ridge over the eastern U.S. Embedded shortwaves will round the base of this closed low, fortifying troughing over the Northeastern U.S. by 12Z Tuesday and pushing the upper level ridge back westward. Differences between the GFS and ECMWF create uncertainty in how this upper level trough will interact with a tropical disturbance meandering off the Carolina coast. As of 12Z Friday, the NHC has given this disturbance a 70% chance of cyclone formation in the next 48 hours. The GFS has both the upper level shortwave trough and the tropical system stronger compared to the ECMWF, with a defined tropical system located along the NC/SC coast by 12Z Tuesday and a longwave trough over the Northeastern U.S./Great Lakes that is peppered with shortwave perturbations. These shortwave perturbations reinforce a longwave trough over the Northeast U.S. by 12Z Wednesday that picks up the tropical disturbance and moves it northeastward along the Atlantic coast, reaching east of the southern Delmarva by 00Z Thursday. Similar to yesterday’s model runs, the ECMWF is slightly slower with the development of the Canadian longwave trough, with general troughing over the Northeast by 12Z Wednesday that also picks up the (much weaker) tropical disturbance and moves it more easterly and out to sea. As the tropical disturbance is deflected, shortwave perturbations drop across the Great Lakes and finally reinforce the longwave trough over the Northeast U.S. by 06Z Thursday.
The Dailies
Day 1 (Saturday): A cool and considerably less humid air mass will filter into most of the Mid-Atlantic on Saturday, setting the stage for a pleasant weekend. Strong northerly flow (from Hudson Bay) will keep ozone formation minimal despite sunny skies under the strong June sun. In the SMA, lingering showers and thunderstorms seem likely throughout the day as the cold front is slow to push into the Southeast U.S. A clean post frontal air mass and below average temperatures in the northern half of the Mid-Atlantic and lingering unsettled conditions in the SMA are resulting in widespread Good range ozone in the air quality models. The BAMS-MAQSIP and NC models highlight a narrow strip along the I-95 Corridor with low Moderate ozone, possibly in response to mid-level high pressure moving overhead, resulting in weakening afternoon surface winds, but the clean post frontal air mass should keep ozone formation minimal. As a result, the risk of an ozone exceedance will remain Slight.
Day 2 (Sunday): The effect of upper/mid-level high pressure moving overhead will start to impact air quality on Sunday as ozone levels begin to gradually rise. A combination of mostly sunny skies and weakening surface winds will allow for afternoon ozone formation to take place across the NMA and CMA, while onshore transport in the SMA should keep ozone formation minimal. This is apparent in the air quality models as they develop widespread Moderate ozone and a few isolated spots of USG ozone across the northern half of the Mid-Atlantic and widespread Good air quality across the SMA. In addition to the I-95 Corridor, the air quality models also highlight the western NMA (PIT) with Moderate/low USG ozone, both in response to weakening flow and a full day of strong July sun despite northeasterly flow aloft. The primary forecast questions on Sunday will be how quickly the air mass is able to modify given the recent frontal passage and lower Sunday emissions, and the impact of onshore surface winds in the morning along the east coast of the NMA and CMA. The localized areas of USG ozone in the air quality models are taken as locations where ozone will be the highest; USG ozone is not anticipated given what will be a very clean air mass in place to start the day, along with lower Sunday emissions. Given the full sun and overhead high pressure, the risk of an ozone exceedance will increase to Marginal with a focus on the I-95 Corridor from DC to NYC and western PA.
Day 3 (Monday): Surface high pressure settling over western VA will continue to impact the Mid-Atlantic on Monday as temperatures return to near average values. High pressure will weaken transport aloft, resulting in localized back trajectories and weak surface winds. The air quality models continue to increase regional ozone in response to these conditions, with widespread Moderate and scattered USG ozone across the NMA and parts of the CMA. The models again highlight the I-95 Corridor and locations across western PA, but with more noticeable USG in response to a full day of June sun and localized transport aloft. In the SMA, the air quality models keep the region under Good air quality due to continued onshore flow aloft. Confidence is gradually increasing that conditions could be favorable for an exceedance along the I-95 Corridor from DC to NYC and in western PA, warranting an Appreciable risk on Monday. The main forecast question will be how quickly the air mass in place modifies.
Day 4-5 (Tuesday-Wednesday): Uncertainty remains for both Tuesday and Wednesday as the weather models have yet to come into close agreement. There is uncertainty in the specifics for Tuesday’s conditions across the SMA due to questions regarding the potential tropical disturbance approaching the Carolinas and the strength of a weak cold front that will move into the NMA in the late morning. If the GFS solution verifies, a combination of upper level high pressure to the northwest and an approaching tropical disturbance will result in onshore flow that will keep ozone formation limited in coastal regions despite uncertainties in the forecast. It’s hard to know at this time how much confidence to place in the GFS; most of this morning’s MREF members have the tropical system with varying strengths, but a few (3) quickly weaken it, similar to the EC’s solution. Regardless, locations further inland are more likely to feel the impacts of upper and mid-level high pressure, allowing ozone to increase. For the northern half of the Mid-Atlantic, the GFS and ECMWF both bring the weak cold frontal boundary into the NMA on Tuesday, but the GFS has a stronger front with fairly widespread precipitation and cloud cover, while the EC solution is dry and relatively cloud-free. Even in the GFS’s wetter solution, this frontal boundary will likely remain to the west of I-95 through the peak hours of ozone formation, allowing mostly sunny skies and slightly above average temperatures to persist throughout most of the day. These conditions combined with westerly flow aloft and southwesterly surface winds will be favorable for ozone formation along the I-95 Corridor on Tuesday. These conditions are resulting in a large strip of USG ozone in the air quality models along the I-95 Corridor from DC through NYC, and a mix of Moderate and Good air quality throughout the rest of the Mid-Atlantic. The risk of an ozone exceedance will remain Appreciable with a continued focus on the I-95 Corridor from DC to NYC.
There is a great deal of uncertainty in the forecast for Wednesday stemming from questions in Tuesday’s forecast. Uncertainty surrounding the track of the potential tropical disturbance off the Carolina coast and differences in the weather models regarding the strength of the upper level trough forecast to move over the Northeast U.S. on Wednesday provide a range of possibilities across the Mid-Atlantic. At this time, it appears unsettled conditions associated with Tuesday’s weak cold front will impact parts of the CMA and SMA. In the NMA, a shift to northerly flow behind the weak frontal passage could be enough to limit ozone formation even if mostly sunny skies occur. Given the opportunity for overnight/morning unsettled conditions and a frontal passage in the NMA, the risk of an exceedance will drop to Marginal.
-Enlow/Huff