Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 20, 2016
Valid: June 21-25, 2016 (Tuesday-Saturday)

Summary:

Monday’s episode of high ozone along the I-95 Corridor will likely end on Tuesday as a weak cold front moves into the northern Mid-Atlantic overnight and reaches the Mason-Dixon Line by late afternoon. This front is weaker than it was in Friday’s guidance, but all of the models are consistent in its track and speed, which lends confidence in the forecast. The front will trigger a round of overnight and early morning precipitation, with the possibility of another round in the afternoon. There is some question about the extent of the afternoon clouds and rain, with locations along and east of I-95 possibly receiving full afternoon sun. Although we expect ozone along I-95 to drop to Moderate, we keep the chances for an ozone exceedance Appreciable, mainly along and east of I-95, to account for the lack of complete confidence in the precipitation forecast. The front will stall in the vicinity of the MDL through Thursday. Wednesday will be sunny and warm for most of the region, with any rising ozone potentially tempered by breezy northwesterly surface winds, for a Marginal chance of an ozone exceedance. A strong wave of low pressure will quickly move along the stalled front on Thursday, bringing widespread clouds and periods of heavy precipitation to parts of the northern and central Mid-Atlantic on Thursday, for only a Slight chance of an ozone exceedance. Then we reset for the weekend, with another, albeit less strong, upper level ridge building over the region. Surface high pressure will move into the Great Lakes on Friday and settle over eastern PA on Saturday. There is some uncertainty about the strength of upper level onshore transport on Saturday, so for now, we assign a Marginal chance for an ozone exceedance for both Friday and Saturday.

Discussion:

The weather forecast models are in relatively close agreement regarding the synoptic pattern through the medium range, but they differ slightly with the placement of a stationary front and associated wave of low pressure which will impact the Mid-Atlantic during mid-week. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The broad and anomalously strong upper level ridge which engulfed the majority of the CONUS through the weekend will surrender its control of the Mid-Atlantic on Tuesday as an upper level trough over southern Canada circulates southeastward into the U.S., pulling a weak cold front into the northern Mid-Atlantic early Tuesday morning. This upper level trough is much weaker than it was in Friday’s guidance, and it will only reach as far south as roughly the central Mid-Atlantic. This will prevent the cold front from moving farther south than approximately the Mason-Dixon Line on Tuesday, where it will remain through Thursday. A small but potent shortwave developing along the U.S./Canada border on Wednesday will form a wave of low pressure along the stalled front over the Ohio River Valley, which will move very quickly along the front, into and across the Mid-Atlantic on Thursday. The weather forecast models have some variations in regard to the track of the wave; the NAM is about 3 hours faster than the GFS and the EC, while the NAM and GFS are slightly farther north than the EC with the track of the wave. All of the models develop periods of heavy rain on Thursday in the vicinity of the MDL, along the northern and central Mid-Atlantic, but the exact placement of the heaviest precipitation is uncertain at this time. High pressure over central ON will help to push the cold front southward on Friday, into southern VA and NC, as an upper level ridge centered over the Plains builds eastward. The ridge will build into the Mid-Atlantic on Saturday, while the corresponding mid-level ridge hooks up with the Bermuda High.

On Tuesday, the cold front will move into the northern Mid-Atlantic overnight and reach I-95 by 12Z, at which point it will continue to move slowly toward the MDL by 00Z. The coarse resolution and Hi-Res models all develop a line of precipitation and possibly convection ahead of the front during the early morning hours. The eastern edge of this precipitation breaks up by the afternoon in all of the coarse resolution models, as well as the 03Z SREF and 06Z 4 km NAM, but the western edge remains intact across the western portion of the central Mid-Atlantic through 21Z. This front will likely be a focus for long-lived thunderstorms, with the SPC placing most of the central Mid-Atlantic in a SLIGHT risk for severe convective activity. The coarse resolution models develop a second round of precipitation and possibly convection in the afternoon ahead of the front, in line in with SPC’s thinking. The Hi-Res models do not show as impressive of an afternoon episode. Ozone levels across the Mid-Atlantic will be highly dependent on the extent of the clouds and precipitation associated with this front. If precipitation does dissipate over the eastern portion of the forecast region, and the expected afternoon round of showers does not occur, locations along and east of the I-95 Corridor could receive enough afternoon sunshine for ample ozone formation, which could be aided by high ozone concentrations in the residual layer due to widespread exceedances expected on Monday along the I-95 Corridor. Additionally, if the front stalls earlier than expected, as can often happen when interacting with such a warm air mass, the southern Mid-Atlantic may be spared of its influence and remain under full sunshine, though the forecast models do not suggest that this will happen. The 06Z air quality models show upper Moderate conditions along and east of the I-95 Corridor for Tuesday. Taking these factors into consideration, the chance for an ozone exceedance is Appreciable, mainly for locations along and east of the I-95 Corridor.

The stalled front in the vicinity of the MDL on Wednesday should be too weak by this point in time to trigger any substantial precipitation, save a few scattered diurnal showers across southern VA and NC. Usually this would be a concern, with a weak front possibly acting as a line of convergence, but this likely won’t be the case on Wednesday. Fast, northwesterly winds behind the front will help to ventilate the atmosphere across the northern and central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor originate front central MI at 500 m AGL, with even faster transport at the 1000 and 1500 m AGL. Only the southern Mid-Atlantic will remain under the influence of the southeastern surface high, but the air quality models only show scattered Moderate ozone across NC. Nevertheless, without a substantial air mass change, and with temperatures remaining in the mid to upper 80s °F with mostly sunny skies, there is a possibility for ample ozone production in the usual places along I-95. As a result, the chance for an ozone exceedance will drop but only to the Marginal category.

The strong wave of low pressure will quickly move along the stalled cold front on Thursday, from roughly IL to off of the Delmarva coast by the evening. This low will trigger widespread precipitation across much of the central and northern Mid-Atlantic, with periods of heavy rain possible. This precipitation will reach the western edge of the region by approximately 06Z Thursday, and push eastward through Thursday evening. There are some differences between the deterministic models with the strength and placement of the rain associated with the wave, as discussed above. The NAM and EC keep the heaviest rain across the northern Mid-Atlantic, especially PA and northern MD, while the GFS keeps this precipitation core to the north of the forecast region across NY. Additionally, the EC is almost 3-6 hours slower than the NAM and GFS with the track of this system. Regardless, it looks like Thursday will be a wet day for the majority of the Mid-Atlantic, except for southern NC, dropping the chance for an ozone exceedance to Slight.

On Friday, high pressure building in from the north in ON will push the cold front southward into the southern Mid-Atlantic, triggering clouds and scattered showers. The northern Mid-Atlantic is expected to clear as surface high pressure builds in from north. There is a pool of cooler air behind the departing front, which, along with relatively quick northwesterly back trajectories, should keep any rising ozone in the Moderate range. However, it is June, so there is a Marginal chance for an ozone exceedance, mainly along the I-95 Corridor, where anthropogenic emissions are highest.

Saturday is a potential day of interest, with an upper level ridge building in from the west and a mid-level ridge hooking up with the Bermuda High. Surface high pressure is expected to settle over eastern PA, which should promote subsidence, sunny skies, and calm winds across the northern Mid-Atlantic. Back trajectories for the I-95 Corridor are fast and from the Gulf of Maine, due to the position of the surface high, which may counter any rising ozone with clean maritime air. Though there is some inherent uncertainty given that this is five days out, the chance for an ozone exceedance will remain Marginal.

-Brown/Huff