Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 13, 2016
Valid: July 14-18, 2016 (Thursday-Monday)

Summary:

There is growing consensus in today’s model guidance regarding the passage of a weak cold front on Friday and Saturday, though some questions remain regarding its north-south placement though the rest of the medium range period. Thursday will be an oppressively hot and humid day for much of the Mid-Atlantic. Most of the region will be impacted by convection driven by instability and a pre-frontal trough. There is still some model discrepancy regarding how far east the convection will extend, with the Hi-Res guidance in particular keeping thunderstorms west of I-95. Assuming the I-95 Corridor remains clear through the early evening, the chance for an ozone exceedance will rise to Appreciable on Thursday. On Friday, the weak cold front will move into the northern Mid-Atlantic in the morning and essentially stall. Minimal precipitation is expected along the weak front, meaning that the I-95 Corridor will experience a day of full sunshine, with hot conditions, westerly surface winds and westerly flow aloft. Therefore, the chance for an ozone exceedance will rise to High for locations along and east of the I-95 Corridor. On Saturday, the cold front will reach roughly to the Mason-Dixon Line by 12Z, triggering clouds and precipitation in the vicinity of the front. There is uncertainty, however, with how far south the front will actually progress, with it most likely stalling somewhere in the VA/MD/DE area. The chance for an ozone exceedance on Saturday will drop slightly to Marginal for the I-95 Corridor as a result. A strengthening mid- level ridge and surface high pressure building in along the NY/PA border on Sunday will promote clear skies and light winds across the northern Mid-Atlantic, with the chance on an ozone exceedance remaining Marginal for the I-95 Corridor. Monday’s forecast is uncertain and depends on the fate of the stationary front, which may be pushed northward as a warm front by the strengthening mid-level ridge. The chance for an ozone exceedance on Monday will remain Marginal for the I-95 Corridor as a result.

Discussion:

The weather forecast models are in fair agreement through most of the medium range, with more substantial agreement regarding the passage of a weak cold front and its associated precipitation Friday night and into Saturday, although there is still uncertainty at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad and weak upper level trough currently over southern MB/ON and the Northern Plains will push southeastward toward the Great Lakes region on Thursday, pulling a weak cold front into the Ohio River Valley by 00Z Friday. A mid-level ridge centered over the southeastern U.S. will be slightly suppressed by this approaching trough, but it will remain in place and continue to hook up with the Bermuda High through the entire medium range, strengthening at the end of the period. On Friday, a shortwave embedded within the broad and shallow upper level trough will swing down across the Great Lakes towards NY, bringing a weak cold front into northwestern PA around 12Z and stalls. As with yesterday’s guidance, the NAM appears to be too slow and weak, with its cold front only reaching the Mid-Atlantic by roughly 06Z Saturday. As a result, we will consider the NAM an outlier for this feature. The GFS and EC are in better agreement today with the track of the front, bringing it southward toward the central Mid-Atlantic by 12Z Saturday, where it will stall again, this time across VA and MD and southern DE. On Sunday, this stalled front will remain in place across the central Mid-Atlantic. The mid-level ridge/Bermuda High will begin to build northward into the northern and central Mid-Atlantic as the upper level trough slowly shifts northeastward, allowing the stalled front to lift slightly northward as a warm front. Upper level shortwave activity on the tail end of the departing trough may impact the northern and central Mid-Atlantic on Monday, ahead of a broad ridge forming over the rest of the CONUS.

Thursday will be an oppressively hot and humid day for much of the Mid-Atlantic, with temperatures rising into the 90s °F across much of the Mid-Atlantic and dew points in the 70s °F. Portions of the I-95 Corridor have already been placed under an Excessive Heat Advisory through Friday night as a result of this uptick in temperatures. The main forecast question is the possibility of diurnal convection due to increased regional instability and a possible pre-frontal trough. The deterministic models, especially EC, all develop precipitation across the region to the I-95 Corridor through 00Z Friday. However, this morning’s Hi-Res guidance does not support this, with the 00Z NMM/ARW, 03Z SREF, and 06Z 4km NAM keeping the majority of this precipitation to the west of the I-95 Corridor through the early evening. 06Z back trajectories for the I-95 Corridor are southerly at 500 m AGL (clean), although they shift due west at 1000 and 1500 m AGL, and there should be sufficient vertical mixing for the higher trajectories to impact surface air quality. Most of the 06Z air quality models are bullish for tomorrow, showing increased ozone concentrations essentially along the I-95 Corridor, especially its northern branch. The NOAA and BAMS model resolve USG ozone across northern and central NJ, and stretching down across PHL and towards BAL. Though the NCDENR shows a similar pattern, it only develops low to mid Moderate ozone in these locations. There will likely be lower regional ozone today, due to more widespread cloud cover than expected, so any ozone contributing to an exceedance on Thursday will be mainly locally produced. Given the hot conditions, a shift to southwesterly surface winds and a westerly transport component aloft on Thursday, there is an Appreciable chance for isolated ozone exceedances for locations along the I-95 Corridor, with the assumption that they will remain sunny through the afternoon.

Though there are still some questions regarding the exact placement of the cold front traversing the northern Mid-Atlantic on Friday, it appears weak enough, with essentially no pre-frontal convection, that it will not act to limit ozone production along the I-95 Corridor. Neither the GFS nor the EC develop any distinct precipitation along this front on Friday, save a few scattered showers. This lack of precipitation can also be seen in the Hi-Res guidance, including the 03Z SREF, the 12 km NAM, and the 13 km GFS. Furthermore, there will not be much of an air mass change behind this front, with temperatures and dew points dropping slightly but remaining above average. Back trajectories for the I-95 Corridor all shift westerly. With these factors in play, ample ozone formation is expected mainly along and east of the I-95 Corridor on Thursday, due to westerly surface winds. That said, the current air quality model runs only bring these locations into the upper Moderate range for ozone. Once again, there is the potential for diurnal thunderstorms to develop, though this will partially dependent on how much lift the weak cold front can provide to initiate convection. Given the clear trend for sunny skies and a second day of westerly transport and hot conditions, the chance for an ozone exceedance will rise to High for locations along and east of the I-95 Corridor, though any exceedances will likely be isolated.

On Saturday, the cold front should progress further south into the central Mid-Atlantic by the afternoon, promoting clouds and scattered precipitation, which will likely be enhanced by diurnal heating and convection. The northern Mid-Atlantic should be post-frontal, but it will remain quite warm and humid. Back trajectories for the I-95 remain westerly for Saturday but are a bit faster. The main forecast question will be how far south the front travels. If the front slows down and is not able to reach quite as far into the central Mid-Atlantic, its associated clouds and precipitation could limit ozone formation along the I-95 Corridor. The deterministic models keep the I-95 Corridor dry though the entire day, suggesting that front will indeed make it to the central Mid-Atlantic. This is also shown in the current BAMS models, with upper Moderate ozone developing along and east of the I-95 Corridor, and areas of USG across portions of central and southern NJ. The NC model, however, keeps essentially the entire Mid-Atlantic in the Good range for ozone, possibly due to more widespread clouds and showers. Given questions about the placement of the cold front, the chance for an ozone exceedance will drop to Marginal on Saturday for locations along and east of the I-95 Corridor.

The cold front is expected to stall across the central Mid-Atlantic on Sunday, possibly lifting slightly northward as a warm front during the afternoon, which will yet again trigger clouds and precipitation in the vicinity of the front. The northward progression of the mid-level ridge will allow surface high pressure to shift eastward from the Ohio River Valley and settle along the NY/PA border around 12Z Sunday. This will promote another day of abundant sunshine and subsiding winds across the northern Mid-Atlantic, allowing ozone levels to rise along the I-95 Corridor. Back trajectories for the I-95 Corridor are much shorter and localized, originating only from north/central PA, due to the close proximity of the front. The air quality models are not responding strongly to this, however, with the BAMS and NCDENR models only brining the I-95 Corridor up into the Moderate range on Sunday. Regardless, there are enough factors in place that will encourage ozone formation, unless the front is directly overhead. As a result, the chance for an ozone exceedance will remain Marginal on Sunday for the I-95 Corridor.

Monday’s forecast is uncertain and depends on the fate of the stationary front. This morning’s guidance points to the front moving northward as a warm front, out of the region. But there is a chance it will remain stalled over the central Mid-Atlantic for another day. If the front does lift north, and the Bermuda High builds northward, a day of intense afternoon sunshine and a shift to a short, southwesterly transport pattern should promote ozone production along the I-95 Corridor. Therefore, the chance for an ozone exceedance on Monday will remain Marginal for the I-95 Corridor.

-Brown/Huff