Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: May 31, 2017
Valid: June 1-5, 2017 (Thursday-Monday)

Summary:
Unsettled weather will persist as a quasi-permanent upper level trough over the eastern U.S. dominates the forecast, keeping the chances for an exceedance Slight through the medium range period. Thursday may be the driest and most pleasant day, as seasonable and less humid air filters into the region behind a weak cold front. Unsettled conditions will return to the NMA on Friday but the extension of Bermuda high pressure across the SMA will bring sunny skies for most of the day. The forecast for Saturday and beyond is fairly uncertain, given the differences in the weather models regarding the recharging of the quasi-permanent upper level eastern low and the track of its corresponding surface low. A lingering frontal boundary will remain across the CMA on Saturday, keeping chances for scattered showers in the forecast, but a weak ridge should promote mostly sunny skies across much of the region. Sunday and Monday look wet as the next surface low moves into the region.

Discussion:
There is agreement between the weather models on placement of major atmospheric features until 00Z Sunday but they diverge after that. Both the GFS and ECMWF have the same general trend in synoptic features through the end of the medium range period, however. The models consulted for this analysis were the 06Z NAM, 06Z GFS and 00Z ECMWF. Thursday, an upper level trough with a closed circulation will continue to progress slowly eastward, with the closed low at its center moving across southern ON. This feature will spin off shortwaves over the northern Mid-Atlantic (NMA) between 06Z Thursday and 00Z Friday. As this upper level feature moves eastward, an extension of the Bermuda High will begin to build into the southern Mid-Atlantic (SMA) and will be aided by mid-level trough moving over the Midwest, starting 06Z on Friday. The upper level trough, which will continue to progress eastward, with its center moving over eastern QC and northern New England, will bring another round of shortwaves to the NMA starting around 18Z Friday and will continue until 18Z Saturday.

The models diverge beginning on Saturday regarding the redeveloped of the semi-permanent eastern upper level trough. The GFS and EC both reinvigorate the trough, but they differ regarding the timing and strength. On Saturday, all three models show a strong shortwave diving down from SK/MB and beginning to dig out a new longwave trough. The EC phases this disturbance with the remnants of the previous upper level trough, strengthening to form a new longwave trough over the eastern US with a closed circulation over eastern ON by 00Z Monday. The GFS is slower to phase the two systems, keeping the western shortwave separate and forming a small closed low over MB by 06Z Sunday. Similar to the EC, the GFS will phase this closed low with remaining energy from the previous trough, however, the trough formed from the merger will be much weaker and not form a closed low during the medium range period. At this time, WPC is favoring a blend of the model, but with a bit more preference to the EC. Regardless of the exact details, the reinvigorated upper level low will move across the Midwest/Ohio River Valley late in the period, pushing the Bermuda High extension back out to sea, bringing shortwaves and unsettled weather to the majority of the Mid-Atlantic.

Despite the Bermuda High building over the SMA through Saturday, today’s air quality models are not developing much in the way of Moderate ozone. They have isolated Moderate at different locations across the region through the period, with little to no consensus, which does not add confidence to their guidance. They are likely reacting to the unsettled nature of the synoptic pattern.

Thursday may be the clearest day of the period, with mostly sunny skies across the region. The exception may be far eastern NC and VA, where a weak frontal boundary will linger. Previous model runs have suggested potential unsettled weather for the NMA in the morning through afternoon hours, but today’s model runs are trending toward dry and clear skies throughout the day. It will be post-frontal, with a less humid and presumably clean air mass filtering into the region. Remnant dilute smoke continues to linger today across the Gulf Coast and especially Florida, due to numerous local fires, Saharan dust, and transported smoke from seasonal burning in Mexico and Central America. This smoke is not expected to impact the Mid-Atlantic through the end of the medium range period, but forecasters in NC should continue to monitor conditions, since they are closest to the sources of smoke and dust. The ample early June sunshine should allow some locations to reach the Moderate range for ozone, but there is only a Slight chance for an exceedance, given the post-frontal seasonably cool and dry condition, with breezy surface winds.

Friday will be similar to Thursday in terms of sensible weather, with seasonable temperatures and less humidity across most of the region. Shortwaves will be moving over the NMA on Friday resulting in scattered cloudy conditions and rain showers, with the potential for isolated thunderstorms in late afternoon and evening. These unsettled conditions will be focused in the NMA since the Bermuda High extension will be promoting clear skies and lighter winds in the SMA. The exception again may be along the coastal SMA, where the semi-permanent lingering frontal boundary will remain. A Slight chance for an exceedance will continue, with more widespread Moderate ozone possible across the SMA, which will be under the extension of the Bermuda High.

The forecast for Saturday and beyond is fairly uncertain, given the differences in the weather models regarding the recharging of the quasi-permanent upper level eastern low and the track of its corresponding surface low. Previous days’ model runs had more clouds and showers across the region on Saturday, but beginning yesterday and continuing today, the trend is toward drier weather. WPC, following a blend of the EC and GFS, keeps the lingering frontal boundary across the region on Saturday, but brings it farther north, across the central Mid-Atlantic. A weak mid-level ridge will move over the NMA, north of the cold front and ahead of developing surface low pressure in the Midwest. The weather models are split on precipitation associated with this front, which may move slowly northward as a warm front on Sunday, associated with the low pressure system that will begin to move into the SMA later Saturday night. The chance for an exceedance will remain Slight on Saturday, but it may be the best day for more scattered Moderate ozone, depending on how sunny it is.

The surface low pressure will begin to impact much of the Mid-Atlantic on Sunday as it strengthens over the Great Lakes. The lingering frontal boundary will begin to push northeastward as warm front in the morning hours, resulting in abundant clouds and precipitation for much of the northern and central parts of the region. Clouds and precipitation will progress eastward in the afternoon to evening hours, impacting the entire NMA but 00Z Monday. Mostly sunny skies are expected in the SMA on Sunday, however, this clearing will allow convective rain showers and thunderstorms to possibly take place between 18Z Sunday and 00Z Monday. Widespread unsettled will put the Mid-Atlantic at Slight risk of an exceedance.

The surface low will be move slowly into the NMA on Monday morning, bringing widespread clouds and rain to the NMA. This system will continue to move eastward into the afternoon and evening hours, bringing a cold front down into the SMA by 00Z Tuesday. The timing of this cold front will determine how much rain and clouds the SMA will get on Monday. As mentioned above, there is uncertainty in the timing and strength of the low pressure system between the models. If the system progresses slower than expected, Monday could be a mostly sunny day for the SMA resulting in possible Moderate ozone levels. Despite the uncertainty there remains a Slight risk of exceedance for the Mid-Atlantic region on Monday.

-Enlow/Huff