Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 25, 2017
Valid: May 26-30, 2017 (Friday-Monday)

Summary:
After a brief break in the rain and clouds on Friday, unsettled weather will continue to be the norm across the Mid-Atlantic, ensuring only a Slight probability of an exceedance through the medium range period. The semi-persistent upper level trough that has been dominating the eastern CONUS will finally move into New England on Friday, allowing a mid-level ridge to build in the southern Mid-Atlantic. This brief pattern change will promote clearing skies, allowing for the best chance for some ozone formation during the medium range period. Waves of low pressure developing along a warm front on Saturday, however, will promote clouds and scattered precipitation across the central and southern Mid-Atlantic, which will clean out any potentially rising ozone that may build on Friday. This trend will continue on Sunday, as low pressure developing in the Ohio River Valley will bring scattered heavy precipitation and clouds to the Mid-Atlantic. On Monday and Tuesday, there is some uncertainty in the timing of the next cold front, but with another substantial upper level trough moving overhead, unsettled weather is expected.

Discussion:

There is close agreement among the weather models regarding the placement and timing of the large scale features until late in the medium range forecast period. Models consulted for this analysis were the 00Z ECMWF and the 06Z NAM and GFS. On Friday morning, the upper level trough that has dominated the eastern CONUS for the last few days will be centered over New England at 12Z and will quickly pass into the Canadian Maritimes by 00Z Saturday. At the same time, a mid-level ridge will build over the lower Mississippi Valley and progress northeastward, reaching southern Mid-Atlantic (SMA) by 00Z Saturday. Upstream, an upper level trough/closed low will be centered over SK/MB on Friday morning. This closed low will broaden slightly as it slowly moves eastward, reaching southeastern MB by 00Z Sunday. The mid-level ridge will continue to build northeastward into the central Mid-Atlantic on Saturday morning but will meet resistance from the mid-level troughs to the northeast and northwest, with a frontal boundary dividing the two major air masses (cool and moist to the north, dry and warm to the south). Aloft, several small shortwaves will move through the upper level flow over the Mid-Atlantic, helping to develop waves of low pressure along the weak frontal boundary draped across the central Mid-Atlantic. On Sunday, the closed low in Canada will begin to intensify and contract its center over southwestern ON. Several shortwaves rotating around the axis of the closed low will begin to dig out a longwave trough over the central CONUS. Behind this amplifying upper level trough, a ridge will be building over much of the western CONUS starting 00Z Sunday, which will help to reinforce the amplifying synoptic pattern over the CONUS. This morning’s EC and GFS runs have come into closer agreement on the track of the Canadian closed low, keeping it spinning slowly over southern ON through the end of the period. At the same time, additional shortwaves will continue to rotate around the axis of the closed low’s associated longwave trough, amplifying the trough over the central and eastern U.S. However, at mid-levels and the surface, the EC and GFS diverge regarding the timing of a weak cold front that will slowly move through the region in the Monday-Tuesday timeframe. The GFS is faster than the EC, bringing the front through the region on Monday, while the EC is about 24 hours slower. The main difference translates into the timing of precipitation associated with the frontal passage; the GFS is wetter on Monday, while the EC has more widespread precipitation on Tuesday. We are siding with the EC for now, following the WPC preference.

Friday, much of the Mid-Atlantic will see a break from the widespread heavy rain of the past few days as the semi-persistent upper level trough/closed low exits to the northeast. The mid-level ridge will building into the SMA will keep skies clear in the southern and central portions of the region. In contrast, clouds and scattered showers will linger in the northern Mid-Atlantic (NMA), mainly through the morning, but gradual clearing is expected by the late afternoon. Given the relatively clean air mass in place, there is only a Slight chance of an exceedance, with higher ozone levels across the SMA.

The recent trend in the models towards a cloudier Saturday has continued in today’s guidance. A warm front moving into the central part of the region will flatten the mid-level ridge and bring unsettled weather back to much of the Mid-Atlantic. Waves developing along the frontal boundary will bring rain showers to the central and southern Mid-Atlantic in the afternoon. The NMA should remain mostly dry but will be partly cloudy with light winds. With precipitation and cloud cover, there will only be a Slight chance of an exceedance, with higher ozone across the NMA.

A surface low located in the Ohio River Valley on Sunday will progress eastward, bringing clouds and scattered rain showers to the central and southern Mid-Atlantic during the morning to afternoon hours. The NMA will see scattered showers and cloudy conditions for much of the afternoon and into early Monday morning due to upper level shortwaves moving overhead. These conditions, along with onshore flow will keep the chances for an exceedance in the Slight category for Sunday.

There is uncertainty in the forecast for Monday and Tuesday, given the disagreement between the EC and GFS regarding the timing of the next major cold front. Rain showers and periods of heavy rain are expected for much of the region, but the timing of this is still uncertain. Another upper level trough will be building over the region for the end of the period, making unsettled conditions likely both days. Following the EC for now, Monday will be partly to mostly cloudy but generally dry, except for showers across NC. Then on Tuesday, following the slow progression of the cold front through the region, more widespread precipitation is expected. Despite the uncertainty regarding the precipitation forecast, a Slight chance of an exceedance will continue on Monday and Tuesday.

-Enlow/Huff