Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 5, 2015
Valid: June 6-10, 2015 (Saturday – Wednesday)

Summary:

A consecutive series of cold fronts will sweep through the Mid-Atlantic during the medium range period, keeping the chances slight for USG ozone. However, there is a chance that next Wednesday may be a transition day to a possible period of deteriorating air quality at the end of next week. A cold front will continue its way southward through the Mid-Atlantic tomorrow, bringing scattered showers to areas south of the Mason-Dixon Line and clearer skies to the north. Sunday’s weather will be dominated by surface high pressure centered in New England; however, an approaching cold front will bring the next round of showers through the region starting in northwestern PA overnight. This cold front, along with an accompanying line of showers, will sweep southeastward through the rest of the Mid-Atlantic on Monday and early Tuesday, clearing the northern Mid-Atlantic by Tuesday afternoon but continuing to bring clouds and showers to areas south of VA on Wednesday. On days that it will be clear in parts of the region (Saturday/Sunday and Tuesday/Wednesday), it will be post-frontal, with presumably cleaner, drier Canadian air filtering in behind the cold fronts. The other days (Saturday afternoon, Monday, Tuesday morning) during the medium range period will be mostly cloudy and rainy.

Discussion:

The models are in close agreement for the beginning of the period, but begin to disagree somewhat on Sunday, and then diverge by the end of the period. The 06Z NAM, 00Z GFS, 00Z ECMWF were consulted for this analysis. Currently, the 500 mb pattern features two short waves, one over NC and the other over the QC/ON border, which is digging out a longwave trough over eastern Canada. A ridge of high pressure is located over the central US, with the axis amplifying over the northern Plains.

The medium range period is still a progressive one, with consecutive cold fronts impacting the region. All of the weather models agree that the short wave over QC/ON will rotate around the base of the Canadian upper level trough, pulling a surface cold front into the Mid-Atlantic Friday night into Saturday morning. The short wave over NC will be absorbed into the base of the Canadian longwave trough as it moves eastward to the Canadian Maritimes during the day on Saturday. This merging will help to push the cold front southward into NC by Saturday evening. By Sunday morning, surface high pressure will build into the northern Mid-Atlantic (NMA), resulting in some clearing. On Saturday afternoon, another upper level trough will start to form over the northern Plains. Several short waves will shear off from a closed low over the southwestern US and phase with the shortwave energy digging out the northern Plains trough, which will help to reinforce the developing trough Saturday night into Sunday morning. Differences in the models begin to become evident on Sunday night. All of the models strengthen the trough as it moves over the Great Lakes on Monday, bringing an associated surface warm front northward into the NMA and a cold front into the Ohio River Valley. However, the GFS is faster bringing the trough eastward, while the EC is slower (the typical bias). This translates into differences in the timing of precipitation on Monday. The faster GFS solution brings widespread precipitation across the region by Monday afternoon, while the EC confines precipitation to west of I-95. The GFS continues to be faster than the EC on Tuesday, placing the axis of the upper level trough over the Mid-Atlantic, while the EC has the trough axis along western edge of the Appalachians. As a result, the GFS essentially clears precipitation out of the region on Tuesday. The EC brings shortwave energy aloft across the Gulf Coast, which allows precipitation to continue across the southern Mid-Atlantic (SMA) on Tuesday. By 12Z Wednesday, a weak upper level ridge will build over the Mid-Atlantic. There are substantial differences between the GFS and EC regarding a possible closed low over the southern Plains, making the precipitation forecast for Wednesday across the Mid-Atlantic uncertain. At this point, the region looks mostly clear, except for possible showers across the SMA associated with the remnants of Tuesday’s cold front.

As a cold front continues to dive south through the SMA tomorrow, surface high pressure centered on the QC-ON border will begin to encroach into the NMA. This feature will keep any lingering showers and cloud cover associated with the cold front confined to eastern PA and south of the Mason-Dixon Line. Pop-up convection is possible among the rain showers in the afternoon over the central Mid-Atlantic (CMA) as the front passes through. At this time of year, any periods of afternoon sunshine will promote rapid ozone formation, given light winds and sufficient concentrations of ozone precursors. The NMA will experience morning stagnation, recirculating back trajectories aloft, and clear skies in the afternoon. However, the air mass currently over the region is quite clean, and the Canadian air mass arriving behind the front will be clean. The various air quality models are showing isolated pockets of low Moderate ozone. Based on the relatively clean air masses that will be impacting the area, generally Good ozone seems most likely, but isolated Moderate conditions are not out of the question. PM2.5 seems more likely to be the leading pollutant, with Moderate conditions possible across southwestern PA and the Susquehanna Valley.

On Sunday, the center of surface high pressure will continue to move eastward into New England and expand southward into the NMA and CMA, pushing the cold front to the southern border of SC. An upper level trough over the Great Lakes will enhance a surface low pressure center moving eastward through southern ON, bringing a warm front into the Mid-Atlantic from the southwest. The arrival of the warm front will increase cloud cover over the western Mid-Atlantic (WMA) throughout the day, as well as bring a southern continental air mass back into the region. A cold front will follow, sweeping a line of rain eastward into the Mid-Atlantic starting in northwestern PA overnight Sunday. The influence of the cooler and much drier Canadian air mass across most of the NMA and CMA should keep air quality generally in the Good range. The air quality models show isolated areas of Moderate ozone and PM2.5 across the region; they are likely responding to mostly sunny skies and light winds in the morning.

On Monday, the surface warm front will continue to travel northeastward and will be draped north to south across central PA/VA by 12Z. Although model discrepancies persist regarding the intensity of the upper level and surface lows impacting the region, there is general model consensus that there will be widespread cloud cover and west to east moving precipitation across the Mid-Atlantic region on Monday. The GFS, with its faster cold front, brings precipitation across the entire region by 00Z Tuesday. The EC has more intense precipitation across the NMA and keeps precipitation west of I-95 through the evening. Mostly cloudy to overcast skies will limit ozone formation across most of the region. Lower level back trajectories (1000 and 500 m AGL) show transport aloft from the northern ME/QC border, which will help to promote primarily Good air quality across the region. Depending upon the timing and intensity of the expected precipitation in addition to the potentially modified air mass behind the surface warm front, scattered locations could witness Moderate levels of PM2.5.

An approaching cold front from the northwest will reach the Delmarva coast/central VA around 12Z on Tuesday. Both the GFS and EC agree that there will be clearing of the NMA in a north to south fashion behind the cold front throughout the day. However, precipitation and cloud cover will continue in the SMA, which, combined with the arrival of a new, presumably clean air mass in the NMA, will once again limit ozone formation. Depending on the speed of the front and location/intensity of the precipitation, particles may continue to linger in the Moderate range at some locations; the 06Z BAMS-CMAQ show particles reaching into the Moderate range across most of the region south of PA/NJ. On Wednesday, the cold front will finally move out of the southern SMA region but may produce a few lingering showers. A weak upper level ridge over the region, and high pressure at the surface should keep the NMA cloud free. Although back trajectories show clean transport aloft from the Great Lakes region, the air mass appears to begin modifying by Wednesday. Sunny skies and light and variable surface winds on Wednesday may allow for ozone to reach the Moderate at scattered locations. Looking ahead, the EC shows westerly transport aloft with a persistent ridge over the Mid-Atlantic on Thursday and Friday, making Wednesday a possible transition day to a potential period of deteriorating air quality at the end of next week.

-Huff/Herdt/Eherts/DeBoe