Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Friday, June 8, 2018
Valid: June 9-13 (Saturday-Wednesday)
Summary
A series of low pressure waves will bring unsettled conditions to the Mid-Atlantic throughout the weekend and into next week. Today’s approaching cold front will push southward into the CMA resulting in unsettled conditions along the boundary. Weak surface winds and southwesterly back trajectories could become an issue with particles and ozone if thunderstorms and associated cloud cover are not widespread throughout PA and MD. A few air quality models are highlighting locations in NC for increased ozone and particle concentrations, possibly in response to numerous small fires burning east of I-95 in NC. A sea breeze front and weak onshore flow aloft will transport these pollutants inland where scattered shower and thunderstorms are expected. These conditions will result in a Marginal risk of an ozone exceedance with a focus on the SMA. Unsettled conditions will impact most of the Mid-Atlantic on Sunday as the frontal boundary slowly advances southward into the CMA. Mostly cloudy skies and widespread locally heavy precipitation in the NMA and CMA will limit temperatures to below average values. In the SMA, an increasingly humid air mass and periods of sun will promote scattered thunderstorms in the afternoon. The risk for an ozone exceedance will remain Marginal due to above average temperatures and the scattered nature of thunderstorms in the SMA. High pressure moving southward into southern ON/NY will push the cold front southward on Monday, resulting in another soggy day across the Mid-Atlantic. Mostly cloudy skies and widespread showers and thunderstorms across the region will keep a Slight risk for an ozone exceedance. There is considerable uncertainty in the forecast for Tuesday and Wednesday as the GFS and ECMWF solutions vary. Although it appears conditions could remain unsettled for most of the region, there remains a possibility that conditions in the CMA and SMA could be favorable for ozone formation. The lack of confidence in the forecast will result in a Marginal risk for an ozone exceedance.
NWP Model Discussion
The weather models have come into closer agreement with synoptic scale features until the end of the medium range period. A broad upper level ridge will continue to build over the central US and southern Canada by 12Z Saturday. The presence of a longwave trough over eastern QC and the arrival of a longwave trough in the Pacific Northwest/western US will amplify the central US ridge over the upper Mississippi River Valley/western ON by 00Z Monday. The western trough will continue to push eastward early next week, moving over the upper Great Planes and southern Canadian prairies by 12Z Tuesday as it depresses the central US ridge. The GFS and ECMWF solutions begin to diverge after 12Z Tuesday as they both handle the progression of the western trough and how it interacts with a reinvigorating longwave trough over eastern Canada. The ECMWF is slower to phase the two features over southern Canada as it develops an elongated trough that parallels the US/Canadian border by 12Z Wednesday, depressing the ridge across most of the northern half of the CONUS through the rest of the work week. The GFS is slightly quicker and cleaner with this phasing, merging the two systems by 18Z Wednesday, and allowing upper level ridging to build over the west central US by 00Z Thursday. Returning to Saturday, shortwaves will crest the central US ridge before propagating across the Midwest over the weekend and into the Mid-Atlantic by 00Z Tuesday. The ECMWF has trended southwards toward the GFS solution, bringing the shortwaves over IL/IN/OH by 12Z Monday and impacting the Mid-Atlantic as early as 18Z Monday. The arrival of this perturbation will impact the Mid-Atlantic throughout the day on Tuesday, before advancing eastward, off the NY/NJ coast by 18Z Wednesday. Differences between the two solutions will impact the precipitation forecast early next weeks as the GFS and similar NAM solutions are slower with the southward push of a cold front into the NMA and CMA on Monday.
The Dailies
Day 1 (Saturday): Today’s approaching cold front will push southward into the CMA resulting in unsettled conditions along and ahead of the boundary. Periods of sun and clouds across the region will allow temperatures to reach near average values. Convection-allowing models are suggesting that an increasingly humid air mass ahead of the front will promote afternoon showers and thunderstorms in the CMA and southern portions of the NMA. Weak surface winds and southwesterly back trajectories could become an issue with particles and ozone if thunderstorms and associated cloud cover are not widespread throughout PA and MD. This is highlighted in the air quality models as they develop Moderate ozone and particles across the Mid-Atlantic. The BAMS-MAQSIP and several of the NC versions are more aggressive with USG ozone along I-95 and the western edge of the NMA and CMA. It is important to note that the BAMS-CMAQ and a few versions of the NC model develop elevated particles across south central PA and MD. The BAMS-CMAQ develops USG particle concentrations in this region and the NC models develop upper Moderate particles across PA and MD. The NC models also highlight elevated particles in NC with some locations reaching the upper Moderate range. This feature in the SMA could be a result of numerous small fires burning east of I-95 in NC. A sea breeze front and weak onshore flow aloft will transport these pollutants inland. These conditions will result in a Marginal risk of an ozone exceedance.
Day 2 (Sunday): Unsettled conditions will impact most of the Mid-Atlantic on Sunday as the frontal boundary slowly advances southward into the CMA. Shortwaves moving over the Ohio River Valley will develop waves of low pressure along the frontal boundary that will move into the CMA in the afternoon hours. Mostly cloudy skies and widespread, locally heavy precipitation in the NMA and CMA will limit temperatures to below average values. In the SMA, influence from mid-level high pressure over the Southeast and mostly sunny skies in the morning hours will allow temperatures to rise above average in a very humid air mass. These conditions will promote scattered thunderstorms and showers across the region in the afternoon hours. The air quality models are responding to the more widespread unsettled conditions as they drop regional ozone into the low/mid Moderate range. Surprisingly a few versions of the NC and the BAMS-MAQSIP keep elevated ozone along I-76 in PA and in locations in the CMA despite anticipated precipitation. This could be in response to localized back trajectories, but cloud cover, below average temperatures, and a shift to breezy easterly surface winds and precipitation will be enough to knock ozone into the low Moderate range. The return of unsettled conditions, below average temperatures and low Sunday emissions will diminish the risk of an ozone exceedance in the NMA and CMA. In the SMA, a shift to southerly flow will limit ozone production for coastal regions but above average temperatures and periods of sunshine for inland locations will result in a Marginal risk of an exceedance.
Day 3 (Monday): High pressure moving southward into southern ON/NY will push the cold front southward on Monday, resulting in another soggy day across the Mid-Atlantic. Persistent clouds and a cooler air mass filtering in behind the front will keep temperatures below average in the NMA and CMA. A very moist air mass and above average temperatures in the SMA will result in a very uncomfortable start to the week. Mostly cloudy skies in the morning will give way to scattered showers and thunderstorms in the afternoon. Unsettled conditions across the region are resulting in widespread Good and scattered Moderate ozone in the air quality models for Monday. The risk of an ozone exceedance will return to Slight.
Day 4-5 (Tuesday-Wednesday): There is uncertainty in the forecast for Tuesday and Wednesday as the GFS and ECMWF solutions vary. The ECMWF brings a stronger wave of low pressure through the Ohio River Valley and into the CMA on Tuesday, resulting in near average temperatures and widespread unsettled conditions that will last through Wednesday. The GFS is slightly drier and has temperatures across the entire Mid-Atlantic reaching well above average values by Wednesday. Due to these uncertainties, the risk of an ozone exceedance will increase to Marginal for now, as ozone-friendly conditions are possible in the SMA.
-Enlow