Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 14, 2016
Valid: July 15-19, 2016 (Friday-Tuesday)

Summary:

There is considerable uncertainty through the medium range forecast, due to persistent questions about the exact placement of a weak cold front entering the Mid-Atlantic on Friday and remaining in the area through Monday, as well as the approach of a back door cold front on Monday/Tuesday. There are many competing factors through the medium range that make the forecast challenging. On Friday, the weak cold front will approach northwestern PA by 06Z, and will stall across the state for the remainder of the day. Minimal convection and precipitation is expected ahead of the front, allowing for a full afternoon of sun across the I-95 Corridor. Hot conditions and light westerly surface winds will compete with clean regional and upwind conditions; there is a High chance for an isolated ozone exceedance along or east of I-95. Ozone levels should drop on Saturday. The only real area of interest is once again along and east of I-95, due to questions about the placement of the front and an associated wave of low pressure. At this point, either locations in the region will be post frontal or under the influence of clouds/precipitation, for a Marginal chance of an ozone exceedance. The risk for an ozone exceedance jumps back up to Appreciable for Sunday and Monday for the northern Mid-Atlantic, due the building mid-level ridge/Bermuda High and an area of surface high pressure that settles just to the north of the forecast region. There are questions about lingering precipitation associated with the surface wave on Sunday, as well as developing on Monday ahead of the approaching back door cold front which make us hesitant to assign a High risk for either of these days, especially Monday. Tuesday looks post-frontal for the northern Mid-Atlantic, with prefrontal precipitation affecting the central and southern Mid-Atlantic, for only a Marginal chance of an ozone exceedance.

Discussion:

The weather forecast models are in fair agreement on the general synoptic pattern for the medium range, but diverge in regards to a wave of low pressure traversing the Mid-Atlantic late Saturday, as well as with an approaching back door cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. Shortwave activity embedded within the broad and shallow upper level trough, currently across southern ON/Great Lakes, will shift eastward toward western NY and pull a weak cold front towards the northwestern edge of the Mid-Atlantic by 06Z Friday. The EC has slowed down in today’s runs to match the NAM, bringing this cold front very slowly into PA, remaining stalled across the northwestern portion of the state through Friday. The GFS shows a slightly faster track, with the front reaching central PA by 00Z Saturday, but its trend is towards consensus with the NAM and EC. These slight differences amplify on Saturday, however, as the models develop a wave of low pressure along the front. They differ on the track and strength of the wave, due to the slight differences in the placement of the stalled front. The NAM and EC keep the front roughly across central/southern PA through 18Z Saturday, whereas the GFS drops the front further southeastward, essentially aligned along the I-95 Corridor by 18Z. This morning, the WPC is leaning towards the GFS solution, with the front stalled and aligned along I-95 on Saturday. On Sunday, the cold front will reorient itself more east/west, which is reflected in both the NAM and EC, though the GFS brings it slightly further southward. At the same time, the mid-level ridge/Bermuda High will build back northward into the northern and central Mid-Atlantic as the upper level trough regresses slightly northward, returning much of the forecast region to zonal flow aloft. This will be short-lived, however, as another embedded shortwave trough aloft drops southeastward toward the Great Lakes by 00Z Tuesday. On Tuesday, the axis of this shortwave will swing down towards the western Mid-Atlantic, pulling a back door cold front along with it. The GFS has a slightly faster track with this front, its usual tendency during this time of the year, with the cold front reaching the MDL by 00Z Wednesday. The EC has a similar track but a slightly weaker front.

Friday is still a day of interest for the I-95 Corridor. The approaching cold front continues to look weak enough in today’s guidance that it should not trigger clouds and precipitation. None of the deterministic models develop any substantial precipitation, which is further supported by essentially all of this morning’s Hi-Res guidance. As discussed above, the NAM and EC keep the front across northern PA/NJ through most of Friday, while the GFS brings it further southward in the vicinity of central PA/NJ. Either way, the front does not look like it will clear I-95 on Friday. 06Z GFS back trajectories for the I-95 Corridor are southwesterly, but fast, and it is clean upwind. The air mass in place this morning across the northern Mid-Atlantic is much cleaner than anticipated due to widespread clouds and precipitation late Wednesday and into this morning, so how quickly this air mass modifies today will be crucial to Friday’s air quality. It will be very hot, with light but sustained westerly surface winds, which should blow I-95 emissions to locations along and east of the I-95 Corridor. The 06Z air quality models are struggling to reach a consensus with the impacts of this front, and possibly as a result, they are not providing widespread support for USG ozone, despite what appear to be favorable weather conditions. All of the air quality models place the highest ozone along and east of I-95, with only the NOAA model identifying a small area of USG ozone in east/central NJ. The rest of the models keep ozone in the Moderate range along and east of I-95, with Good ozone elsewhere. Although there is higher than usual uncertainty in the forecast, there appear to be enough factors in place conducive to rising ozone on Friday, most likely at isolated locations east of I-95. As a result, the chance for isolated ozone exceedances will be High for locations along and east of the I-95 Corridor on Friday.

There is considerable uncertainty on Saturday as well, given questions about the progression of the weak cold front, although the risk of an ozone exceedance is substantially less than on Friday. The uncertainty centers around a wave of low pressure that all of the weather models develop along the semi-stationary frontal boundary. The NAM keeps the front further north across northern PA, and it also develops a slower and more potent wave of low pressure, keeping its associated precipitation to the west of the I-95 Corridor through 00Z Sunday. The GFS and EC have slightly drier solutions, with the GFS pushing the front further south, but they still bring precipitation to I-95 beginning 18Z Saturday. If the GFS/EC solution verifies, afternoon convection and thunderstorms will temper rising ozone along the I-95 Corridor. A NAM-like solution, however, would provide another day of intense sunshine and clear skies for the I-95 Corridor. Given the consensus between the GFS and EC, and the much stronger wave developed by the NAM, we consider the NAM and outlier for this feature and side with the GFS/EC solution. 06Z GFS back trajectories for the I-95 Corridor on Sunday are fast and northwesterly, suggesting that presumably slightly drier and likely cleaner air will arrive behind the frontal passage. The current air quality model runs all have the same pattern for ozone, although they differ on the magnitude, they keep ozone mostly in the Good range for much of the region for the second day in a row. Once again, the highest ozone is located in a relatively small area along and east of I-95. The BAMS MAQSIP-RT model develops mid-to-upper Moderate ozone in these locations, while the BAMS CMAQ model only shows upper Good and low Moderate ozone. The NC model is a little more bullish, developing areas of mid-to-upper Moderate ozone across southern NJ, DE, and MD. Given the likely arrival of slightly cleaner air behind the front and chances for precipitation along I-95, the chance for an ozone exceedance on Saturday will drop to Marginal.

On Sunday, surface high pressure will build into the northern Mid-Atlantic, with the center settling across central NY by 12Z. This will promote clear skies and subsiding winds along the I-95 Corridor. Furthermore, back trajectories for the I-95 Corridor are short and westerly, which will also promote air mass modification. The cold front is expected to stall somewhere in the vicinity of the MDL, but its exact placement remains a question, as well as lingering precipitation on Sunday morning along I-95 associated with the departing wave. The BAMS air quality models are responding to strong afternoon sunshine and local stagnation, with areas of USG ozone along I-95. However, it will be Sunday, which is typically a lower day for ozone emissions. Taking all of these factors into consideration, the chance for an ozone exceedance on Sunday will rise to Appreciable.

Monday is another day of interest, but with higher than usual uncertainty persisting. It’s not clear what will happen to the stalled front in the central Mid-Atlantic. WPC keeps it near the MDL, but a building mid-level ridge/Bermuda High may shift it northward as a warm front. 06Z GFS back trajectories for the I-95 Corridor are recirculating, suggesting that the front will lift slightly northward across southern PA and central NJ. In addition, a back door cold front will approach from the north. This is a weak-ish front, but the EC and GFS develop precipitation ahead of it across northern and central PA and NJ on Monday afternoon. If this precipitation reaches far enough southward, it may be enough to temper any rising ozone. The BAMS and NC air quality models bring ozone up into the USG range along the I-95 Corridor, especially across northern and central NJ, PHL, and near BAL. Overall, Monday is a day to watch, but we will keep the chances for an ozone exceedance Appreciable for now, due to uncertainty regarding the precipitation forecast.

Tuesday remains uncertain, but the approach of cold front from the north should limit any substantial ozone production. The GFS shows a faster track for this front, with its associated precipitation reaching the I-95 Corridor as early as 06Z. The EC solution is slightly slower, but still has widespread precipitation across the region. This cold front is weak, but stronger than the one from Friday/Saturday, suggesting that it will track as predicted. As a result, the chance for an ozone exceedance on Tuesday will drop to Marginal on Tuesday.

-Brown/Huff