Medium Range Air Quality Outlook
Issued: July 31, 2017
Valid: August 1-5, 2017 (Tuesday-Saturday)
The influence of a mid-level ridge, with surface high pressure centered over the southern Mid-Atlantic, will keep ozone exceedances Appreciable through Thursday before a cold front brings widespread precipitation on Friday and a cooler and drier air mass into the Mid-Atlantic on Saturday. The center of high pressure will move into the Mid-Atlantic on Tuesday, promoting sunny skies and regional subsidence. Widespread Good air quality on Sunday suggests that the air mass in place over the region started clean, but observed PM2.5 concentrations this morning across the Great Lakes, Plains, and Ohio River Valley, as well as in parts of the NMA (PIT metro and the Susquehanna Valley) appear to be rising quickly, suggesting that dilute smoke from fires in the western US and Canada is mixing to the surface. With the addition of the possibly smoky air mass and the likelihood of bay/sea breezes forming, an isolated exceedance cannot be ruled out, with the main focus on locations susceptible to coastal mesoscale winds. Wednesday will be similar to Tuesday but with a slightly greater chance for afternoon showers and thunderstorms. High pressure will remain at the surface in western NC, resulting in another day of light/calm surface winds and localized flow aloft. Again, the main wildcard will be the presence of dilute smoke mixing into the local air mass. Thursday will be the warmest day of the medium range period for many locations in the Mid-Atlantic. Afternoon/evening thunderstorms are more likely than on Wednesday in the NMA and CMA. In addition, flow aloft will shift southerly, which may limit rising ozone. Given uncertainty about air mass characteristics, risk of an exceedance will remain Appreciable with a focus on the I-95 Corridor. A combination of a cold front pushing into the western Mid-Atlantic and a shortwave moving across the SMA will bring unsettled conditions the entire Mid-Atlantic on Friday. Mostly sunny skies are expected across the region in the morning hours before giving way to mostly cloudy skies by the afternoon. The main forecast question for Friday the extent of thunderstorms that popup in the afternoon hours across the central and eastern parts of the region. If precipitation is more isolated then conditions could be favorable for ozone formation as cloud cover would be less. As a result of these factors, risk of an exceedance will lower to Marginal with a focus on the eastern NMA and CMA. Saturday will a cooler and less humid day as the cold front pushes through the rest of the NMA and CMA before stalling along the eastern SMA. Risk of an exceedance will fall to Slight.
Weather Model Discussion:
The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are generally in consensus until 12Z Friday, when they diverge slightly, before coming back into consensus by 18Z Saturday. At 12Z Tuesday the entire eastern U.S. including the Mid-Atlantic, will be under weak upper level troughing and mid-level ridging, centered over the Ohio and Tennessee River Valleys. All three models show a shortwave developing across the Southeast U.S. between 18Z Tuesday and 00Z Wednesday. This shortwave will keep the influence of weak upper level troughing over much of the eastern U.S. through 00Z Thursday. By 00Z Thursday, a series of short waves in the northern stream flow that developed over AB will be dropping down across SK/MB and into the upper Midwest. As this shortwave energy drops southward, it will dig out an upper level trough over the upper Midwest by 18Z Thursday. Both the GFS and EC have this upper level lobe beginning to phase with a lobe of energy from a parent upper level closed low over Hudson Bay between 18Z Thursday and 12Z Friday; however, the models handle the timing of the phasing differently. Both models quickly strengthen the existing upper level longwave trough over the eastern U.S. and pull a cold front into the Mid-Atlantic on Friday-Saturday. The WPC seems to be siding more towards the GFS as it drops the shortwave energy (and associated surface low) more southward over northern IN/OH by 06Z Saturday. The GFS has a slightly stronger shortwave in the axis of its trough, centered farther south than the EC is showing, as well as a slower associated cold front and weaker, farther south surface low. The EC phases the shortwave energy into the lobe from the parent closed low almost immediately, sucking the energy into the center of circulation by 18Z Friday and then bringing it northeastward into ON near James Bay by 06Z Saturday. Despite the differences between the GFS and EC handling the strengthening of the long wave trough on Friday, both are back in general agreement by 18Z Saturday, with the upper level closed low over northern QC and the axis stretching southward along the Mississippi River Valley. By 00Z Sunday both models show this upper level trough consuming much of the eastern U.S. with weak shortwaves moving across the Ohio River Valley.
Tuesday will be a pleasant summer day for most of the Mid-Atlantic as the center of high pressure moving over the western part of the region will promote mostly sunny skies and increasing temperatures. Temperatures will rise into the mid-to-upper 80s °F with a few locations in the SMA reaching near 90 °F. The 03Z SREF shows the slight possibility of a few isolated pop up thunderstorms in the NMA and CMA but if any develop it will not be widespread enough to impact air quality. In addition to sky conditions, the nearby high pressure will result in diminishing surface winds and slow and localized flow aloft. Ozone and PM2.5 concentrations were generally quite low yesterday across the region, indicating that the air mass in place was clean. The most recent HMS smoke analysis shows dilute smoke from wildfires across Canada and the northwest U.S. is lingering over the NMA, however. Observed PM2.5 concentrations this morning across the Great Lakes, Plains, and Ohio River Valley, as well as in parts of the NMA (PIT metro and the Susquehanna Valley) appear to be rising quickly, suggesting that this dilute smoke is already mixing to the surface. The presence of this smoke aloft, combined with ample atmospheric mixing on Tuesday (>2km) bringing more smoke to the surface, could promote higher ozone and particle concentrations than the air quality models predict. The 06Z air quality models are showing a mix of Moderate and Good ozone across the Mid-Atlantic with a few isolated areas of upper Moderate ozone in locations impacted by bay/sea breezes, e.g., southern NJ, eastern DE, BAL metro and southern MD. The hi-res models are showing sea/bay breeze signals for Tuesday afternoon, suggesting that locations most at risk for quickly rising ozone will be those susceptible to coastal mesoscale winds. The main forecast question for Tuesday will be the composition of the air mass over the region, particularly in regard to smoke characteristics. With the addition of the possibly smoky air mass and the likelihood of bay/sea breezes forming, an isolated exceedance cannot be ruled out considering atmospheric conditions. A combination of these factors will bring the risk of an exceedance to Appreciable on Tuesday.
Weak shortwaves moving over the CMA and a weak frontal boundary approaching the NMA form the northeast will bring the possibility for afternoon/evening thunderstorms to the Mid-Atlantic on Wednesday. The models are showing more isolated cells popping up compared to Tuesday, but precipitation does not look widespread. Mostly sunny skies in the morning hours will slowly give way to partly sunny skies in the afternoon hours allowing temperatures to rise into the upper 80s/low90s °F. The 03Z SREF shows that precipitation is most likely between 21Z Wednesday and 00Z Thursday across the central portions of the Mid-Atlantic as cloud cover is modeled to increase. High pressure will remain centered at the surface and mid-levels in western NC, resulting in another day of light/calm surface winds and localized flow aloft, with bay/sea breezes likely. The 06Z BAMS models seem to be siding with drier conditions across the NMA and CMA as they develop more widespread Moderate ozone with a few isolated locations of USG ozone in MD, NJ, and southwestern PA. The NC model on the other hand, keeps regional ozone mostly Good with scattered Moderate ozone along the MDL and across MD. There will be two primary forecast questions for Wednesday: 1) how smoky will the air be and 2) how widespread will afternoon showers and thunderstorms be? If precipitation remains isolated and dilute smoke mixes to the surface, ozone exceedances are possible in the NMA and CMA. This increase the risk of an exceedance on Wednesday will remain Appreciable with a focus on the I-95 Corridor and the PIT metro area.
Thursday will be the warmest day of the medium range period for many locations in the Mid-Atlantic. High pressure lingering over western NC will keep mostly sunny skies throughout the day in the CMA and SMA and in the morning hours in the NMA. Temperatures will rise a few degrees across the Mid-Atlantic compared to Wednesday. A shift to southwesterly flow aloft and at the surface will combine with daytime heating and mostly sunny skies in the morning hours to allow afternoon/evening thunderstorms to develop across the NMA. Precipitation appears to be much more widespread in the models compared to Wednesday. The 03Z SREF show that the possibility of precipitation increases to likely between 15Z and 18Z in the central NMA. The 06Z air quality models pick up on the possible thunderstorms as low Moderate/upper Good ozone is shown in central PA and the western CMA. The BAMS models increase ozone along the I-95 Corridor in the NMA and CMA possible due to precipitation not pushing east of I-81. The models show the rest of the region under widespread Moderate ozone due to mostly sunny skies and light/calm surface winds in the SMA. The main questions for Thursday will focus on the impact of a shift to faster south/southwesterly flow aloft, which is a cleaner pattern compared to Tuesday/Wednesday, and the eastern extent of precipitation. Risk of an exceedance will remain Appreciable with a focus along and east of the I-95 Corridor.
A combination of a cold front pushing into the western Mid-Atlantic and a shortwave moving across the SMA will bring unsettled conditions the entire Mid-Atlantic on Friday. Mostly sunny skies are expected across the region in the morning hours before giving way to mostly cloudy skies by the afternoon. Precipitation is expected to start in the NMA between 12Z and 18Z Friday as the cold front moves across the Ohio River Valley. Popup thunderstorms are likely ahead of the precipitation associated with the front as southerly flow aloft and at the surface brings a more humid air mass into place. Precipitation associated with the front is only expected to push as far east as central PA/western CMA and SMA by 00Z Saturday. This makes the main forecast question for Friday the extent of thunderstorms that pop up in the afternoon hours across the central and eastern parts of the region. If precipitation is more isolated, then conditions could be favorable for ozone formation as cloud cover would be less. The 06Z air quality models are picking up on onshore flow aloft in the SMA as they show Good ozone across the entire region, where in the NMA they models seem unsure of the precipitation forecast. The BAMS models keep Moderate ozone across much of the CMA and all of the NMA with an area of USG ozone along the Susquehanna River in PA and northern NJ. The NC model appears more reasonable as I-81 serves as the dividing line between Good and Moderate ozone, with Good ozone to the west. The NC model also develops some low USG ozone long I-95 in the CMA and NMA possibly due to more isolated thunderstorms in the afternoon/evening. As a result of the relatively high chance for afternoon precipitation and southerly flow aloft, risk of an exceedance will drop to Marginal with a focus on the eastern NMA and CMA.
Saturday will be a cooler and less humid day across the Mid-Atlantic as the cold front pushes through the rest of the NMA and CMA before stalling along the eastern SMA. Temperatures will fall into the upper 70s/low 80s °F as another cool Canadian air mass arrives in the wake of the front. Mostly cloudy skies and showers in the morning hours will linger into the afternoon before giving way to sunny skies in the NMA and CMA, while in the SMA, the stalling front will keep scattered showers in the area into the evening hours. Southerly flow will shift to northerly behind the front ushering the presumably clean air mass into the region. Risk of an exceedance will fall to Slight on Saturday.