Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 24, 2017
Valid: May 25-29, 2017 (Thursday-Monday)

Summary:
A persistent progressive and unsettled weather pattern will continue for the Mid-Atlantic during the medium range period, allowing mostly Good air quality to persist. A strong upper level trough will continue to dominate the region on Thursday before it exits the region to the northeast early Friday. Friday afternoon there will be a mid-level ridge building northward into the region, bringing relief from rain and clouds across the southern and central Mid-Atlantic. This reprieve will be short lived due to increasing confidence in the weather models that the Memorial Day weekend will be unsettled. Waves of low pressure will develop along a warm front moving into the central Mid-Atlantic on Saturday, bringing clouds and scattered precipitation mainly in the afternoon and evening. Elsewhere, in the northern and southern Mid-Atlantic, Saturday may be the clearest day of the period. The next low pressure system will form in the western Ohio River Valley on Sunday and pull a cold front into the Mid-Atlantic on Monday, leading to cloudy and rainy weather for the end of the holiday weekend. There is uncertainty as to speed and strength of this system, but nonetheless, continued Good to low Moderate air quality is expected on Sunday and Monday.

Discussion:

There is general agreement among the weather models for the first part of the forecast period. The 00Z Wednesday ECMWF was unavailable, so the 06Z GFS and NAM were consulted for this analysis. The trough currently dominating the eastern CONUS will begin to recede to the north and east on Thursday morning, contracting over the northern Mid-Atlantic (NMA) by 00Z Friday. At the same time, the closed low over SK will essentially spin in place, while a weak upper level ridge builds between the two troughs. On Friday, the eastern trough will contract further and quickly move northeastward into New England and the Canadian Maritimes, as the upper level ridge progresses into the Great Lakes. Concurrently, a mid-level ridge will advance into the southern Mid-Atlantic (SMA), moving southwest to northeast tough Friday into early Saturday morning. The overall pattern for the end of the period from yesterday’s models continues in today’s runs. On Saturday, the weak upper level ridge moves over the Mid-Atlantic, while the mid-level ridge tries to build northward as a warm front lifts into the central Mid-Atlantic. The NAM and GFS both have small shortwaves moving through the upper level flow over the northern and central Mid-Atlantic, which translate into scattered precipitation along the warm front on Saturday afternoon and evening. On Sunday, the next major low pressure system will form in the western Ohio River Valley, which will impact the Mid-Atlantic on Sunday and Monday. There is some uncertainty regarding the track of the low pressure system, tied to the track of the Canadian upper level closed low. Today’s GFS run brings the closed low southward into the northern Plains Sunday afternoon, whereas yesterday’s 00Z EC run kept it farther north. As a result, the GFS develops a stronger and much deeper trough over the eastern U.S. on Sunday. This difference is important to watch since it may translate into a slightly different outcome for Monday. Since the GFS has the trough extending further south, it brings a stronger cold front through the Mid-Atlantic region on Monday compared to yesterday’s EC run. Regardless, Sunday and Monday look unsettled for the Mid-Atlantic.

Unsettled conditions will bring widespread Good air quality to the Mid-Atlantic on Thursday. A center of low pressure associated with the strong eastern upper level trough will pass through the Mid-Atlantic early Thursday morning, moving northeastward throughout the day. This system will bring heavy showers and cloudy skies to most of the region in the morning and afternoon hours. As the low pulls away, precipitation will become more scattered and skies will clear from south to north in the late evening.

The upper level trough will be exiting the Mid-Atlantic region to the northeast early on Friday, resulting in scattered precipitation and lingering clouds across the NMA. The building mid-level ridge will bring mostly clear skies to the SMA. Some scattered Moderate ozone may occur, mainly in NC, but Good air quality is expected elsewhere.

The trend we saw yesterday, with the weather models increasing unsettled weather for the central and southern parts of the Mid-Atlantic on Saturday, continues in today’s model runs. A warm front will lift across the central Mid-Atlantic, and waves of low pressure may develop along it, triggered by upper level shortwaves moving over the region. Precipitation is most likely in the afternoon through evening hours and will be scattered in nature. At this point, much of the NMA and SMA look clear, so scatted Moderate ozone is possible in those locations, with Good ozone in the areas affected by the clouds and precipitation.

Conditions for Sunday and Monday are somewhat uncertain due to the differences in the weather models regarding the upper level trough that will progress over MB/ON and over the Great Lakes/Upper Plains regions at the end of the period. WPC is putting more weight on today’s GFS solution, bringing the center of vorticity further south on Sunday, thus developing a stronger surface low and bringing more widespread unsettled weather to the Mid-Atlantic region on Monday. If the GFS solution verifies, the cold front will reach I-95 by 12Z Monday and then continue slowly through the eastern part of the region, reaching the far southeastern Mid-Atlantic by 12Z Tuesday.

-Enlow/Huff