Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 22, 2015
Valid: June 23-27, 2015 (Tuesday – Saturday)

Summary:

Chances for marginal USG on Tuesday will decrease to slight for the remainder of the week as a strong cold front ushers in a cooler and drier air mass on Wednesday and an amplified longwave trough builds over the eastern US by the end of the period. Tuesday has the highest chance for upper Moderate ozone, given sunny skies, well above average temperatures, an Appalachian lee trough, and transport aloft from the Ohio River Valley. However, a strong and widespread line of convection will sweep through the northern and central parts of the region on Tuesday afternoon and evening, reaching the I-95 Corridor by 18Z Tuesday to 00Z Wednesday. There is some uncertainty regarding the exact timing of this convection, but it should arrive early enough to limit ozone to the low-to-mid Moderate range for most of the I-95 Corridor. Weak surface high pressure will build over PA on Wednesday, promoting sunny skies and light winds; however, the arrival of a cooler, drier, and presumably cleaner air mass should limit ozone to the low Moderate range. Scattered precipitation associated with Tuesday’s stalled cold front along central VA/southern Delmarva, along with the developing longwave trough, will keep air quality in the generally Good range for the remainder of the medium range period.

Discussion:

The weather models are in generally good agreement for most of the synoptic features in the medium range period. The 06Z NAM, 06Z GFS, 00Z ECMWF, and 00Z CMC were consulted for this analysis. Strong shortwave energy currently over the central Plains will move eastward in the northern stream, reaching the ON/QC border by 18Z Tuesday. This feature will pull a cold front into the Mid-Atlantic from the northwest on Tuesday, reaching to central VA/southern Delmarva by 00Z Wednesday. A weak mid-level/surface ridge centered over PA will form behind the cold front and help advance it southward to the roughly the VA/NC border by 12Z Wednesday. The persistent, broad upper level ridge that has been in place over the southern US the past few days, and zonal flow at 500 mb will slow the southward progression of the front into the southern Mid-Atlantic (SMA). The forecast models are in agreement that a surface wave will form in the Ohio River Valley (ORV) on Thursday and will move eastward into the Mid-Atlantic along the stalled front, reaching coastal NJ/DE sometime on Friday morning. There is some disagreement on the speed and magnitude of this wave, with the NAM being the fastest and strongest. The GFS and EC are slower, and the GFS has a stronger wave than the EC but weaker than the NAM. These differences translate into varying precipitation forecasts for Thursday afternoon into Friday. Aloft, late Thursday into early Friday, the pattern will begin to change. Scattered upper level shortwaves will begin to dig out an upper level trough over the eastern US which will continue to amplify through the end of the period. By Saturday, the trough over the eastern US will be very amplified, reaching down to the Gulf Coast, while and equally amplified upper level ridge will be in place over the western US. This pattern will continue on into the weekend.

With sunny skies and temperatures well into the 90s ͦF on one of the longest days of the year, conditions on Tuesday will be prime for ozone formation throughout the morning and early afternoon. The main limiting factor will be the approach of a strong cold front in the afternoon. As this front approaches from the northwest, surface winds will strengthen and peak. The mesoscale models have been consistent in developing a strong line of pre-frontal convection from central PA through southern DE by nightfall. This line of storms is expected to reach the I-95 Corridor between 18Z Tuesday and 00Z Wednesday. The 06Z 4- and 12-km NAMs, along with the 06Z 13-km GFS, agree that this line of storms will be strong and widespread, lending support to this solution. However, the recent runs of the mesoscale models (Sunday to this morning) have been inconsistent in regard to the exact timing of the line of convection. In addition, the front may slow down overall as it will be moving into a very warm and humid air mass. The air quality model are in fairly close consensus on Good ozone for the I-95 Corridor, however, with only the eastern shore of MD/central DE/southern NJ possibly reaching the Moderate range. Thus, overall, it seems probable that the pre-frontal convection will reach the I-95 Corridor early enough in the day to limit ozone to the Moderate range, with locations to the south, such as southern MD/DE, having the highest chance for upper Moderate ozone, since the impacts of the front will arrive latest in the day. On the other hand, with dew points will be in the high 60s ͦF, sulfate PM2.5 formation will be favored, especially along an early morning lee trough extending along the I-81 axis through PA and southward through MD and VA. However, brisk surface winds picking up later in the morning will limit local particle build-up to the usual areas east of the I-95 Corridor.

Following Tuesday evening’s frontal passage, surface high pressure centered over the Ohio River Valley (ORV) will extend eastward into the Mid-Atlantic on Wednesday. This feature will bring abundant sunshine and light surface winds to the majority of the region, which will promote some ozone formation throughout the day. A slightly cooler and much drier air mass will build into most of the region on Wednesday. Temperatures and dew points will drop closer to average, with both forecasted to be 10 ͦF lower than on Tuesday. The main forecast question for Wednesday for locations north of the cold front will be the characteristics of the new air mass. Back trajectories are from the northern Great Lakes region, which will presumably mean the arrival of a clean Canadian air mass. Thus, this clean air should limit how far ozone will rise into the Moderate range. In the southern Mid-Atlantic (SMA), however, scattered clouds and some light showers are possible along the frontal boundary in the vicinity of northern NC, which will limit ozone formation.

Thursday will begin with surface high pressure centered over PA, making for clear skies and stagnant morning winds throughout the Mid-Atlantic. However, an approaching surface wave will promote late morning clouds and afternoon showers in the NMA, from west to east. There is some uncertainty regarding the timing and strength of the surface wave. WPC is siding more with the GFS solution this morning, bringing the wave across the ORV on Thursday and reaching the western Mid-Atlantic on Thursday evening, skirting just south of the PA/MD border. Thus, despite a calm and sunny start, the weather will deteriorate early enough in the day to limit ozone and PM2.5 concentrations to the generally Good range on Thursday.

On Friday morning the surface wave is expected to move off the NJ/DE coast, bringing the oscillating frontal boundary southward, well into NC. A series of waves of low pressure will ride along the frontal boundary, keeping a generally unsettled pattern across the Mid-Atlantic through the end of the period. This will make widespread cloud cover and scattered rain showers likely across the region throughout Friday and Saturday. Mostly cloudy skies will keep ozone formation at bay, while dew points in the 60s ͦF and local back trajectories make Moderate PM2.5 concentrations possible, especially on Saturday. However, model discrepancies on Saturday regarding the exact location and timing of precipitation make this forecast uncertain at this time.

-Huff/ DeBoe/Eherts