Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 26, 2016
Valid: July 27-31, 2016 (Wednesday-Sunday)

Summary:

The medium range is mired in uncertainty due to a split in the weather forecast models regarding the track and impact of a stalled front positioned across the central Mid-Atlantic and several waves of low pressure that will ride along this frontal boundary, particularly Thursday-Friday. There is only a Marginal chance for an ozone exceedance on Wednesday, with a presumably clean air mass situated over the northern Mid-Atlantic, which should keep ozone in check despite sunny skies, and afternoon clouds/precipitation across the central/southern Mid-Atlantic associated with the stalled frontal boundary. Confidence in the forecast plummets beginning Thursday, as the NCEP (NAM and GFS) models completely differ from the UK models (EC) in regard to a weak and shallow upper level trough developing over the Plains and moving eastward. This translates into a roughly 18-hour time difference in the arrival of a wave of low pressure that will ride along the stalled front through the Mid-Atlantic on either Friday morning (NAM and GFS) or Saturday (EC). This means that Thursday is a day to watch, as if the EC solution verifies, calm winds in the morning and full sun in the afternoon may allow ozone to rise rapidly along the I-95 Corridor. If the NAM/GFS solution verifies, the stalled front will move northward and bring afternoon precipitation to the entire Mid-Atlantic region. This uncertainty continues into Friday, when the NAM and GFS bring a strong wave of low pressure along the stalled front, leading to periods of heavy rain in the central and northern Mid-Atlantic. In contrast, the EC has another mostly dry day, for a continued chance of high ozone, mainly along the I-95 Corridor. Saturday looks wet, albeit for different synoptic reasons, and Sunday will feature afternoon pre-frontal convection ahead of the next cold front, for only a Slight chance of an ozone exceedance through the weekend.

Discussion:

The weather forecast models are really struggling to come into consensus regarding the placement and impact of the cold front stalling in the vicinity of the Mason-Dixon Line this afternoon, as well as several upper level shortwaves that will ride this frontal boundary into the Mid-Atlantic. The EC is noticeable slower (by 12-18 hours) and weaker than the NAM and GFS is its development of larger shortwave aloft on Friday, which translates to stark differences its precipitation forecast. At this point, there is no clear indication as to which set of models (NCEP vs. UK) are trending toward the correct solution. The WPC is recommending a blend of the various ensemble forecasts, but with below average confidence. Therefore, we will consider both general options, and present the possibilities related to the air qualify forecast for both.

The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level shortwave embedded within a broad Canadian trough will pivot across northern ON/QC on Wednesday, but remain far enough north for the entire Mid-Atlantic to experience zonal flow aloft. This zonal pattern will allow a stalled front to remain in place across the central Mid-Atlantic through Friday. Weak shortwave energy will stream into the northern and central Mid-Atlantic along this front throughout Wednesday. A mid-level ridge/Bermuda High will also be in place on Wednesday, and will remain in place across the southeastern U.S. through the medium range, despite being suppressed by a mid-level trough beginning Thursday. A broad, weakly organized shortwave trough will begin to strengthen and progress eastward across the Midwest/northern Plains on Thursday, with its axis reaching the Great Lakes region by 00Z Friday. The EC solution begins to diverge with those of the NAM and GFS beginning roughly 18Z Thursday, with less substantially shortwave energy within its trough and its subsequently slower track. Smaller shortwaves getting absorbed into the base of this trough will continue to track into the northern and central Mid-Atlantic during Thursday along the stalled front. On Friday, the axis of the weak upper level trough will reach the western Mid-Atlantic by 18Z, allowing the stalled front to lift slightly northward, potentially as a warm front. The upper level trough will lift into the Northeast U.S. early Saturday, but another embedded shortwave behind it will develop across the Ohio River Valley, reinvigorating the trough across the Mid-Atlantic. This shortwave will bring another cold front towards the western Mid-Atlantic late Saturday. On Sunday, the shortwave will flatten out across the northern and central Mid-Atlantic, allowing this next cold front to progress eastward into the forecast region.

A stalled front will be positioned across the central Mid-Atlantic on Wednesday. This morning’s Hi-Res guidance appears to place the front just south of the Mason-Dixon line, which is slightly further south in comparison to yesterday’s guidance. This frontal boundary is expected to trigger clouds and scattered precipitation across portions of the central and southern Mid-Atlantic, particularly during the afternoon in coincidence with diurnal heating and convection. This precipitation pattern is supported by the deterministic models, as well as the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM. 06Z back trajectories for the I-95 Corridor are northwesterly, with drier and presumably clean air building into the northern Mid-Atlantic behind the front. This air mass change should limit ozone formation to the Moderate range along the northern extent of the I-95 Corridor, despite sunny conditions. The main area of interest, therefore, will be locations along and ahead of the front. There will ample heat and moisture regionally to support diurnal convection in these locations, especially given the presence of shortwave energy aloft. However, in the event that this convection is not as widespread as expected, convergence along the front, which may be aided by sea/bay breeze convergence, could cause locally enhance ozone formation. The 06Z air quality models show higher ozone concentrations near where the front is expected to settle, with both the NOAA and NCDENR models developing patches of USG ozone near DC as well as eastern VA. The BAMS models show this USG ozone slightly more inland, across portions of northern and central VA, as well as small area near BAL. Given the support for afternoon clouds and precipitation, however, these solutions seem overdone, which is supported by the fact that the air quality models as of lately have been over-forecasting ozone. Therefore, the chance for an ozone exceedance will remain Marginal on Wednesday, mainly for locations along and south of the front.

Uncertainty begins on Thursday in regard to the northern extent of afternoon precipitation, which leads to less confidence in the air quality forecast, making Thursday a day to watch. Following the NAM/GFS soultions, the approach of developing shortwave trough over the Ohio River Valley may allow the stalled front to advance slightly northward, though its exact placement remains a question. This is expected to bring its associated clouds and precipitation northward into the northern and central Mid-Atlantic during Thursday afternoon, though the deterministic only bring scattered showers into these locations by 00Z Friday. The Hi-Res guidance is a little more convincing, with the 03Z SREF and 06Z 4 km NAM showing clouds and scattered convection reaching northward across the I-95 Corridor beginning 18Z Thursday. 06Z GFS back trajectories for the I-95 Corridor are short and southerly at 500m AGL, suggesting that the front could reach PHL by 12Z. In addition to the clouds and rain triggered by the front, southeasterly surface winds will pick up in the afternoon, which may help to limit any rising ozone under sunny skies, especially along coastal locations. Once again, the question will be, if there will be enough diurnal convection along the front to limit ozone formation. If the EC solution verifies, it will be sunny across the northern Mid-Atlantic, with precipitation remaining to the south for another day. Calm winds in the morning and a full afternoon of sun would allow ozone to rise along the I-95 Corridor. The 06Z air quality models still appear to doubt the limiting effects of this convection, with the BAMS and NCDENR models showing USG ozone near PHL. Additionally, the BAMS models develop USG ozone in western PA near PIT, south central PA, and northern MD. Given the uncertainty in the weather model guidance, the chance of an ozone exceedance will rise to Appreciable on Thursday.

The divergence in the NCEP vs. the UK models continues on Friday. Following the NAM/GFS solutions, a shortwave trough aloft will allow the front to continue its northward progression on Friday, with a wave of low pressure moving northeastward along the front, across the central and northern Mid-Atlantic. These factors would promote even more substantial clouds and precipitation across the northern and central Mid-Atlantic, with the NAM and GFS both resolving period of heavy precipitation across the region beginning 12Z. Furthermore, 06Z back trajectories for the I-95 Corridor turn southerly, which will should usher clean maritime air into the forecast region. The BAMS air quality models are clearly responding to this, with no USG ozone models anywhere across the Mid-Atlantic on Friday. If the EC solution verifies, however, surface wave and its associated precipitation wouldn’t arrive until Saturday. This would leave at least partly sunny skies across parts of the northern Mid-Atlantic, which, depending on air mass characteristics, could allow for rising ozone, mainly along the I-95 Corridor. The trend is for increasing clouds and precipitation on Friday, even in the EC solution, so the chance for an ozone exceedance on Friday will remain Marginal.

There is considerable uncertainty throughout the weekend, due to persistent questions about the precipitation forecast. The GFS shows a surface trough developing out ahead of the next cold front on Saturday, with its associated precipitation staying west of the I-95 Corridor through 00Z Sunday. That said, 06Z back trajectories for the I-95 Corridor shift northerly, which should be sufficient to temper rising ozone along I-95, especially after a day of heavy rain on Friday. The BAMS models bring the entire I-95 Corridor into the Moderate range on Saturday, but do not develop any USG ozone. The EC solution brings the wave of low pressure along the stalled front, through the northern Mid-Atlantic on Saturday, similar to what the NAM/GFS did on Friday. This will result in a rainy forecast as well, although the cause will be different than the GFS solution. The GFS and EC come into closer consensus on Sunday regarding the track of the next cold front. The front will arrive across the northern and central Mid-Atlantic on Sunday, with widespread clouds and precipitation developing along and ahead of the front by 00Z, once again limiting ozone production along the I-95 Corridor. Therefore, the chance of an ozone exceedance will remain Slight for both Saturday and Sunday.

-Brown/Huff