Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 15, 2017
Valid: June 16-20, 2017 (Friday-Tuesday)

Summary:

Risk for an exceedance will remain Slight through much of the medium range period as unsettled conditions will impact the Mid-Atlantic daily, with Marginal risk on Saturday and Tuesday due to possible clearing skies and light surface winds. Shortwaves interacting with surface boundaries on Friday will result in widespread rain showers across the NMA and CMA, while daytime heating and a humid air mass will trigger afternoon/evening thunderstorms in the SMA. Similar to yesterday, there is some concern regarding the 06Z BAMS air quality models as they continue to magnify ozone levels across the Mid-Atlantic compared to the NOAA and NC models, especially on Saturday. The BAMS models continue to show areas of scattered Moderate ozone along the path of the stalled front on Friday, which may be caused by the model not picking up on convection that seems likely to occur. Saturday and Sunday will be very similar as afternoon thunderstorms will be the theme of the weekend, although there is greater chance for clearing on Saturday. Again on Saturday, the 06Z BAMS models are showing a disturbing trend of USG/upper Moderate ozone across eastern PA/MD/NJ. This looks to be caused by a mixture of wind direction shift from easterly to southerly and light/calm winds across the area of interest. As we saw yesterday, the BAMS models seem to be picking up on the pattern of highest ozone across the region, but their magnitude is too high. As a result, the risk of an exceedance rises to Marginal on Saturday, but confidence is low. A cold front will be passing through the Mid-Atlantic on Monday and Tuesday. There is some disagreement in the eastward progression of the front on Monday as the GFS is a few hours slower than the EC, but the front is expected to reach I-81 by 00Z Tuesday. Widespread heavy pre-frontal precipitation will move across the region from west to east on Monday. The departure of the front is also in question as the GFS shows lingering precipitation along the Atlantic coast through 18Z where the EC clears out by 12Z. Despite any timing concerns, skies will clear west to east across the region. There is little air mass change behind the front, allowing for seasonably warm weather on Tuesday under sunny skies with westerly flow aloft. Questions about the fate of the front, which seems likely to stall, will push the risk of an exceedance up to Marginal on Tuesday.

 
Weather Model Discussion:

Strong agreement with placement, timing and strength of key atmospheric features continues today between the weather models. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The upper level trough/closed low that is currently over MB (12Z Thursday), will weaken and begin to open up as the center reaches western ON by 12Z Friday. This center of circulation will slowly move over north/central ON by 12Z Saturday, while upstream, potent shortwaves moving eastward in the northern stream flow, just south of the U.S./Canadian border, will reinvigorate the trough in the 18Z to 00Z Sunday time frame. As the trough moves into the upper Plains by 00Z Sunday, it will interact with a surface frontal boundary that is draped over northern MN, developing a surface low by 06Z Sunday in MN/WI. By 12Z Sunday, the upper level trough axis will be over the upper Mississippi River Valley, encompassing much of the upper Midwest, closing off a center of circulation over ON by 18Z. At this point the strongest shortwaves will spin around the center of circulation, pulling the surface low to the northeast, to the ON/QC border by 12Z Monday. At the surface low moves northeastward, an associated cold front will advance through the Ohio River Valley, reaching northwestern PA by 12Z Monday, I-81 by 00Z Tuesday, and pushing to the Atlantic coast of the NMA by 12Z Tuesday. By the end of the period, the GFS and EC diverge slightly. Both of the models stall the cold front over the Mid-Atlantic, as the upper level trough weakens and retreats northward on Tuesday. The EC keeps the front stalled farther south than the GFS, likely somewhere across the SMA or possibly in the vicinity of the Mason-Dixon Line. Both models have clearing across much of the region on Tuesday, with more clearing associated with the EC, which pushes the cold front farther south. The interesting thing about this cold front is that it will be strong in terms of convection, but weak from an air quality perspective, with the cooler and drier air staying to the north of the region. The WPC is using a blend of the EC and GFS for now.

The Dailies:

Unsettled conditions will impact the entre Mid-Atlantic on Friday as the frontal boundary remains in place over the western NMA and CMA. The front will remain roughly stationary from west/central PA, southeastward to the VA/NC border. Overhead shortwaves entering the region around 12Z, moving east to west, will interact with the boundary bringing scattered showers across the NMA and CMA. Precipitation will be most likely in the NMA and CMA between 12Z and 18Z, with clearing skies from west to east, keeping temperatures near normal in most locations. The 09Z SREF is showing a high probability of precipitation roughly along and east of I-99 from 18Z to 21Z, perhaps in response to the chances for afternoon convection. In the SMA, partly cloudy skies and above average temperatures will be assisted by a surface trough across central NC/VA, in triggering thunderstorms between 18Z Friday and 00Z Saturday. Back trajectories ending at the I-95 Corridor will be onshore, keeping clean maritime air flowing into the NMA, at locations north and east of the stalled front. The 06Z BAMS air quality models are showing scattered areas of mid-Moderate ozone along the stalled front, focusing on PIT, PHL, TTN, BAL-DC, and the Susquehanna and Lehigh Valleys of PA. In comparison, the NC and NOAA air quality models are relatively clean in the NMA with a few isolated areas of Moderate in the SMA. We continue to think the BAMS is not picking up on convection that is expected to occur, but is focusing instead on convergence along the stalled front. Scattered precipitation, cloud cover, and onshore flow should be enough to keep ozone out of the upper Moderate range. As a result of the expected meteorological conditions, risk of an exceedance will remain Slight for Friday.

Mostly cloudy skies across the region Saturday morning will give way to partly cloudy skies by afternoon, clearing from west to east. The frontal boundary draped across the NMA and CMA will lift northeastward and dissipate Friday night, resulting in drier conditions for Saturday. The Mid-Atlantic will be in the warm sector of the approaching storm system. A shift to southerly flow will bring a surge of warm and very humid air into the region, bringing dew points into the upper 60s °F to low 70s °F. Clearing skies and above average temperatures in a humid air mass will trigger widespread afternoon/evening thunderstorms across the SMA, with potentially a few isolated thunderstorms in the NMA. The 03Z SREF and 06Z GEFS show precipitation is most likely between 18Z Saturday and 00Z Sunday, east of the Appalachian Mountains in the SMA. The 06Z air quality models are acknowledging the drier conditions in the NMA and wet conditions in the SMA as both the NC and BAMS models increase ozone relative to Friday. The 06Z BAMS models, however, show a few areas of USG/upper Moderate in eastern PA/MD/NJ. This looks to be caused by a mixture of wind direction shift from easterly to southerly and light/calm winds across the area of interest, but confidence in this is very low. The NC model is showing areas of elevated ozone in similar locations but only reaching into the low Moderate range. As we saw yesterday, the BAMS models seem to be skillful at highlighting areas at risk for high ozone, but their magnitude is too high. Saturday seems likely to be the driest day in the period, and given light winds and plenty of warmth, ozone may surge in any areas that see sunny afternoon skies. Onshore back trajectories will continue for locations along the I-95 Corridor, so regions most at risk for any rising ozone will be inland, west of I-95. Given this uncertainty, the risk of an exceedance will rise to Marginal on Saturday.

Southerly flow will persist on Sunday as the Mid-Atlantic remains in the warm sector. The slow-moving cold front will approach the western part of the region in the afternoon. Strong pre-frontal convection, with periods of heavy rain, will move into western PA, WV, and western MD between 18Z and 00Z Monday. Across the eastern half of the region, a few clouds in the morning hours will give way to clear skies by noon. This clearing, along with above average temperatures and dew points in the low 70s °F, will result in another day of afternoon/evening convection, at locations roughly east of I-81. Both the NC and BAMS 06Z runs highlight an area of Moderate ozone along the I-95 corridor in the NMA on Sunday, likely in response to early afternoon clearing and very warm conditions, but strong southerly back trajectories and the possibility of afternoon convection argue against this outcome. Risk of an exceedance will return to Slight for Sunday.

The cold front will pass through the Mid-Atlantic on Monday, entering western PA just before 12Z Monday and reaching I-81 by 00Z Tuesday. There is some uncertainty in the timing of the frontal passage, however, widespread and heavy pre-frontal precipitation is expected across the NMA and CMA on Monday, possibly lingering into Tuesday along the Atlantic coast. The GFS is slightly slower than the EC, as usual, and keeps precipitation over the NMA coast through 18Z Tuesday. The risk for an exceedance will remain Slight on Monday

As mentioned in the model discussion, there is some uncertainty in the fate of the front on Tuesday. The upper and mid-level lows will weaken and retreat northward slightly, pushed by an encroaching Bermuda high ridge to the south. The front looks to be caught between these two features, possibly stalling along I-95 Corridor on Wednesday before pushing back north and stalling west to east anywhere from the MDL to NY State/New England on Wednesday. The GFS allows the front to return northward more quickly than the EC. There will be little in the way of an air mass change behind this front, as seasonably warm and somewhat humid conditions will continue on Tuesday. Westerly transport aloft and diminishing winds at the surface, under sunny June skies, will certainly allow for ozone formation. Given the uncertainty about the fate of the front, the risk for an exceedance will rise to Marginal on Tuesday.

-Enlow/Huff