Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 15, 2016
Valid: July 16-20, 2016 (Saturday-Wednesday)

Summary:

There is more substantial model consensus today regarding the placement of the cold front arriving today across the northern Mid-Atlantic. The front now appears that it will stall in the vicinity of I-95 on Saturday into Sunday morning before lifting northward as a warm front later Sunday into Monday. There is less support that a wave of low pressure will form along the stalled boundary on Saturday, however. Overall, there are still competing factors for ozone formation over the weekend. It will be hot with light westerly surface winds and potentially strong sea/bay breeze fronts both days, along with westerly transport aloft from the Ohio River Valley. However, most locations – at least west of I-95 – should be post-frontal, with a presumably clean air mass in place. In addition, the front should trigger afternoon clouds and scattered precipitation in its vicinity both days, but the weather models are still struggling to identify exactly where. Given all of these factors, the chances for an ozone exceedance on Saturday and Sunday will remain Appreciable, mainly south and east of I-95. The stalled front from the weekend will lift into PA/NJ as a warm front on Monday, which combined with the approach of a back door cold front from the north, will trigger clouds and precipitation across portions of the northern Mid-Atlantic. It appears that the I-95 Corridor will remain clear enough through the afternoon, however, that it will be at risk for an ozone exceedance. Therefore, the chance for an ozone exceedance on Monday will remain Appreciable for the I-95 Corridor. The back door cold front will move through the northern Mid-Atlantic on Tuesday. Although there is some uncertainty regarding the extent and location of its associated precipitation, we expect it to be widespread enough, with a strong enough front, for the chance of an ozone exceedance to drop to Slight. Another area of surface high pressure will build into the Ohio River Valley on Wednesday, returning the I-95 Corridor to full sunshine and light surface winds, although it will be post-frontal, with presumably a clean air mass in place. The chance for an ozone exceedance will rise to Marginal on Wednesday as a result.

Discussion:

The weather forecast models are in closer agreement today on the general synoptic pattern for the medium range, but they have trouble with the location and strength of afternoon convection through Monday, but especially on Saturday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An embedded shortwave within the broad upper level trough currently across southeastern Canada/Great Lakes will pass to the north of the forecast region on Saturday. The weak cold front moving into the region today will only reach to roughly I-95 and stall on Saturday as a result. There is less support in the model guidance today that a wave of low pressure will traverse this cold front on Saturday; the NAM is the only model that still has this feature. On Sunday, the upper level trough will lift slightly northward, returning the northern and central Mid-Atlantic to mostly zonal flow aloft. A strengthening mid-level ridge/Bermuda High across the southeastern U.S. will push the stalled front slightly northward as a warm front. A potent upper level closed low centered near northern ON will reinforce the trough, with its southern edge reaching the northern Great Lakes area by 00Z Tuesday. This strengthening upper level trough will push a back door cold front toward the Mid-Atlantic beginning late Monday, with it reaching the vicinity of the NY/PA border around 12Z Tuesday. The front will steadily advance southward to reach the MDL by Tuesday afternoon. In addition to showing similar tracks for this cold front, the GFS and EC are in closer agreement regarding its strength and associated precipitation in today’s model runs, as discussed below. The Mid-Atlantic will remain in an area of upper level troughiness on Wednesday, with the rest of the CONUS engulfed by a broadening upper level ridge centered over the Plains.

There is less uncertainty in Saturday’s forecast given today’s forecast model guidance, though some questions remain regarding the northward extent of afternoon precipitation. The models are in agreement that the cold front moving into the northern Mid-Atlantic later today will stall along I-95 on Saturday and into Sunday morning. As mentioned above, there is less support for a wave of low pressure to move along the front, but the air mass in place will be humid enough to support development of scattered afternoon convection. The models are still struggling to identify exactly where clouds and precipitation will develop, however. The GFS and EC keep the majority of the precipitation south of the MDL through 00Z Sunday. This precipitation pattern is also supported by the 00Z NMM/ARW, 03Z SREF and 06Z 13 km GFS. The 06Z 4 km NAM and the 12km NAM both develop a line of convection/thunderstorms along the I-95 Corridor by 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor are relatively fast but westerly from the Ohio River Valley. Although it doesn’t look like there will be much ozone upwind today, the ORV is historically a source region for ozone precursors. In addition, there will be light westerly surface winds, with a strong indication in the Hi-Res model guidance for sea breeze formation. All of the 06Z air quality models resolve Moderate ozone along and east of the I-95 Corridor for Saturday, with the BAMS models also showing isolated patches of USG near PHL/TTN and central NJ. Overall, there are still competing factors for ozone formation on Saturday. Areas along and west/north of I-95 should be post-frontal, and the location of the stalled front should trigger clouds and afternoon precipitation in the vicinity of I-95. However, it will still be hot, with light surface winds, a potentially strong sea breeze, transport aloft from the ORV, and at least partly sunny skies. Therefore, given these factors, and the potential for isolated exceedances today, the chance for an ozone exceedance on Saturday will remain Appreciable for locations along and east of the I-95 Corridor.

Sunday has most of the same competing factors as Saturday. It will still be hot and at least partly sunny, with even lighter surface winds and another likely strong sea and even bay breeze. Transport aloft will continue to be westerly, but shorter than on Saturday. In addition, surface high pressure will arrive across the northern Mid-Atlantic, centered over upstate NY by 12Z. This will promote clear skies and subsiding winds across the northern half of the forecast region. 06Z GFS back trajectories for the I-95 Corridor will remain generally westerly, but will shorten noticeably, originating from northern WV. The main forecast question will be the exact north-south placement of the cold front, which should move northward in the second half of the day as the Bermuda High/mid-level ridge strengthens northward. Wherever the front ends up, clouds and scattered showers could limit ozone formation. The deterministic models and the 09Z SREF again struggle with afternoon convection on Sunday, with the bulk of precipitation across the southern Mid-Atlantic, but chances for late afternoon scattered convection along I-95. It appears that the current air quality model runs keep the front far enough south that locations along and east of the I-95 Corridor rise into the mid-to-upper Moderate range for ozone. Furthermore, the NCDENR model resolves patches of USG ozone across southern NJ, DC, and eastern VA. As a result, the chance for an ozone exceedance on Sunday will remain Appreciable, mainly for locations along and east of the I-95 Corridor.

Monday could be a potential day of interest for the I-95 Corridor, as it will be pre-frontal, but there is still uncertainty in the precipitation forecast. The warm front (formerly stalled/cold front from Fri-Sat-Sun) will keep pushing northward into PA/NJ in response to the strengthening mid-level ridge/Bermuda High. 06Z GFS back trajectories for the I-95 Corridor are southwesterly, which suggest that the front will lift far enough north for much of the I-95 Corridor to be behind the front. Additionally, a back door cold front will be approaching from the north, potentially enhancing convergence along the warm front, and triggering clouds and precipitation. The main question for Monday is how far eastward these clouds will extend by the late afternoon. The deterministic models keep this precipitation to the north and west of the I-95 Corridor through 00Z Tuesday. This solution is also supported by the 03Z SREF and the 06Z 12 km NAM (though this only goes out to 18Z Monday) and 13 km GFS. If the I-95 Corridor remains clear through the afternoon, a day of intense sunshine and hot conditions with light southwesterly surface winds will promote ozone production. The 06Z BAMS models show the entire I-95 Corridor reaching into the upper Moderate for ozone, though there are no distinct areas of USG ozone resolved. Assuming that precipitation does not reach the I-95 Corridor until Monday evening, the chance for an ozone exceedance will remain Appreciable on Monday.

The arrival of the back door cold front from the north on Tuesday should limit ozone formation across much of the Mid-Atlantic due to associated precipitation and an air mass change. There remain some questions regarding the extent of precipitation associated with this front. The EC resolves widespread precipitation all along the I-95 Corridor by 18Z Tuesday, whereas the GFS has this precipitation shifted slightly southward. 06Z GFS back trajectories for the I-95 Corridor are northwesterly, suggesting that the front will clear at least the northern extent of the I-95 Corridor by 12Z. The BAMS models seem to favor a more GFS-like solution, developing upper Moderate ozone east of the I-95 Corridor, with USG ozone across central and eastern NJ. At this point, we are confident enough in the arrival of the cold front – it has stronger upper level support than the current front – that the chance for an ozone exceedance will drop to Slight on Tuesday.

A broad area of surface high pressure will arrive across the Ohio River Valley by 12Z Wednesday, promoting clear skies and light winds across most of the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are shorter and more localized, with combined with ample afternoon sunshine and subsiding surface winds, will allow ozone concentrations to rise along the I-95 Corridor, although it will be post-frontal, with a presumably clean-ish air mass. The chance for an ozone exceedance on Wednesday will rise to Marginal as a result.

-Brown/Huff