Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 10, 2016
Valid: August 11-15, 2016 (Thursday-Monday)

Summary:

Oppressive heat and humidity will reign across the Mid-Atlantic beginning on Thursday and lasting through the weekend, with temperatures climbing above average and dewpoints in the 70s °F. Furthermore, heat indices are expected to reach up above 100 °F beginning on Friday. Ample regional moisture and instability, paired with the slow approach of a cold front, will support diurnal clouds and convection essentially every day during the forecast period. This will be further aided by the development of a prefrontal surface trough on Saturday and Sunday, which will act as a focus for robust afternoon convection. There is a small chance for some rising ozone in locations that experience periods without precipitation during the weekend, mainly in the vicinity of the I-95 Corridor. However, widespread cloud cover and the presence of a thoroughly cleaned out air mass should suppress most local ozone production. Once the front moves into the northern Mid-Atlantic and progresses southward towards the central Mid-Atlantic on Monday, the clean southerly/southwesterly transport pattern from the weekend will be swapped out for an equally clean northwesterly transport pattern behind the front. The sum total of these limiting factors will keep the chances for an ozone exceedance Slight each day of the medium range.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The Mid-Atlantic will remain under mostly zonal flow aloft on Thursday, allowing some shortwave energy to stream across northern and central portions of the forecast region. Concurrently, the semi-permanent mid-level ridge centered over the southeastern U.S. will persist over much of the Mid-Atlantic and remain in place for the entirety of the forecast period, though it will be slightly suppressed by an upper level trough arriving over the weekend. On Friday, this upper level trough will shift eastward over the northern Plains, with its center traversing southern MB/ON through 00Z Saturday. The leading edge of the trough will remain to the north of the Mid-Atlantic region, across New England, for the entire day on Friday. The axis of the upper level trough will shift slightly eastward on Saturday towards the Great Lakes, pulling its associated cold front into the Ohio River Valley during Saturday afternoon. Both the NAM and EC show the shortwave disturbance that has brought torrential rain to the southern U.S. in the past week getting absorbed into the periphery of the nearby trough by 00Z Sunday. The GFS holds onto this disturbance until Sunday, before also bringing it up into the flow around the base of the upper level trough. On Sunday, the upper level trough will flatten slightly and pivot directly over southern ON/Great Lakes, with its associated front remaining just to the north and west of the Mid-Atlantic through most of the day. The GFS and EC differ slightly on Monday, with the GFS completely flattening the upper level trough across southern QC/New England, while the EC maintains the structure of the trough with its axis pivoting and lifting northeastward across southern QC around 12Z. Both solutions, however, bring the trough’s cold front into the northern Mid-Atlantic during the morning, progressing it southward to roughly the Mason-Dixon Line by 00Z Tuesday.

The effects of an impending heat wave will be felt beginning on Thursday, and are expected to last through at least the weekend. The NWS has issued an Excessive Heat Watch for northern DE, PHL, and western NJ, effective today through Saturday evening. On Thursday, southwesterly surface winds will funnel heat and moisture into the Mid-Atlantic, allowing temperatures to climb above average, with dewpoints in the 70s °F. The GFS and EC develop widespread precipitation across the northern Mid-Atlantic by 18Z, while the NAM continues to keep the heaviest precipitation further south across the central Mid-Atlantic at this time. The 00Z ARW model and 03Z SREF support the GFS/EC solution. Despite this difference between the deterministic models, the NAM still does show precipitation along the I-95 Corridor by 18Z. 06Z GFS back trajectories for the I-95 Corridor are southerly and relatively fast, which will continue to transport clean air into the forecast region. If a NAM solution verifies, there may be some periods without rain across the northern Mid-Atlantic which, combined with intense heat and slight lighter surface winds (in comparison to those that are expected today), may support some rising ozone. The 06Z air quality models show some hints of this, with the BAMS and NCDENR models resolving some patches of low Moderate ozone along the I-95 Corridor across DC, northern DE, PHL, and west central NJ. The NOAA model, on the other hand, keeps the entire Mid-Atlantic well into the Good range on Thursday. Though either of these two solutions are possible, there will be enough moderating factors in place to suppress any substantial ozone production across the forecast region. As a result, the chance for an ozone exceedance will be Slight on Thursday.

Friday will be another scorching day for much of the Mid-Atlantic, with heat indices expected to climb up above 100 °F during the afternoon. As on Thursday, the GFS and EC develop widespread precipitation across the northern and central Mid-Atlantic beginning 18Z. The NAM shows a similar placement with its precipitation, but this precipitation is more scattered than the GFS/EC, with the heaviest rain focused across the central Mid-Atlantic. The 03Z SREF is again in agreement with the GFS/EC solution, with high probabilities of precipitation across the entire northern Mid-Atlantic and portions of the central Mid-Atlantic during Friday afternoon. 06Z GFS back trajectories for the I-95 Corridor remain fast and southerly, originating just off the coast of the Carolinas. Some ozone production is possible in the vicinity of I-95, given the intense heat, sustained southwesterly surface winds, and the possibility of few breaks of sunshine following the NAM solution. The BAMS and NCDENR models show an area of low Moderate ozone developing just to the north and west of the I-95 Corridor. However, the presence of relatively clean regional air mass and the likelihood of afternoon clouds and thunderstorms will keep the chance for an ozone exceedance Slight on Friday.

Saturday will be similar to the two preceding days, with oppressive heat and humidity across most of the Mid-Atlantic. Again, there is support for widespread diurnal clouds and precipitation due to the ample regional moisture and instability. Additionally, the development of a pre-frontal surface trough ahead of the cold front entering the Ohio River Valley will act as an additional focus for afternoon convection across the northern and central Mid-Atlantic. Surface winds remain southwesterly ahead of this front. 06Z GFS back trajectories for the I-95 Corridor also shift southwesterly to follow suit. All of the deterministic models bring precipitation eastward to I-95 by 18Z, as does the 03Z SREF. Even more substantial cloud cover and diurnal precipitation in comparison to Friday will limit ozone formation across the forecast region. The BAMS models reflect this, with the entire Mid-Atlantic falling back into the Good range for ozone. The only complicating factor could be if this cold front slows down, as they often can during this time of the year, especially as they encroach upon extremely warm and moist air masses like the one expected to be in place across the Mid-Atlantic on Saturday. If the front were to slow, breaks of sunshine could allow for isolated areas of rising ozone along and near the I-95 Corridor. However, given the continued support for clouds and precipitation, the chance for an ozone exceedance will remain Slight on Saturday.

The cold front will remain just to the north and west of the Mid-Atlantic for the majority of Sunday, supporting yet another round of clouds and precipitation across the forecast region during the afternoon. This will enhanced by the continued presence of a pre-frontal surface trough roughly aligned with the I-95 Corridor, which will promote robust diurnal convection. Both the GFS and EC show the northern and central Mid-Atlantic engulfed in precipitation by 18Z. 06Z back trajectories for the I-95 Corridor are more westerly, but this should have a limited impact on the air quality forecast given the presence of widespread clouds and rain. Accordingly, the BAMS models keep the entire Mid-Atlantic in the Good range for ozone on Sunday. As on Saturday, there could be some concern for local ozone production if the front slows down, but this seems unlikely at this point in time. Yet another day of persistent cloud cover, precipitation, and a relatively clean transport pattern will keep the chances for an ozone exceedance Slight for another day.

On Monday, the sluggishly progressing cold front will finally move into the northern Mid-Atlantic during the morning, before pushing southward to roughly the MDL by 00Z Tuesday. The EC develops an organized line of precipitation along the front, reaching the I-95 Corridor by 18Z. The GFS, however, shows only scattered precipitation across the northern and central Mid-Atlantic at this time. Even in locations that are spared of this intense frontal precipitation, there should be sufficient cloud cover to limit any rising ozone. Additionally, presumably clean, northwesterly winds both aloft and at the surface behind the front will also suppress ozone formation along I-95. 06Z GFS back trajectories for the I-95 Corridor are northwesterly, suggesting that front may actually pass to the south of I-95 by 12Z. As a result of these limiting factors, the chance for an ozone exceedance will be Slight once again to finish out the medium range.

-Brown/Huff