Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 26, 2017
Valid: May 27-31, 2017 (Saturday-Wednesday)

Summary:

The weather pattern will remain generally unsettled through the medium range period, but today’s models are trending toward a less amplified upper level pattern, with a somewhat complex pattern of weak fronts and uncertain timing of shortwaves.  As a result, there is substantial uncertainty regarding the exact timing and location of clouds and precipitation, especially over the Memorial Day weekend.  On Saturday and Sunday, the weather will be dominated by waves of low pressure moving along a frontal boundary draped somewhere across the central Mid-Atlantic.  A stronger cold front, which was expected to move through the region on Tuesday, now has weakened substantially.  At the end of the period, a semi-stagnant pattern will develop, with the Bermuda High trying to build into the region from the south but running up against persistent troughing centered over ON/QC.  Overall, only a Slight chance for an exceedance is expected through the period, given chances for clouds and precipitation over the Memorial Day weekend, and then only a slow warm-up to around average temperatures on Tuesday and Wednesday.

Discussion:

There is a general agreement in the weather models regarding the key synoptic features of the medium range period.  Today’s model runs are trending toward weaker features at 500 mb, however, especially at the end of the period.  This translates into a synoptic pattern with weak fronts and uncertain timing of shortwaves.  As a result, the forecast remains unsettled, but there is substantial uncertainty regarding the timing and location of clouds and precipitation, especially over the Memorial Day weekend.  At the end of the period, a semi-stagnant pattern will develop, with the Bermuda High trying to build into the region from the south but running up against troughing centered over ON/QC.

The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF.  On Saturday, a series of upper level shortwaves will move across the northern Mid-Atlantic (NMA) throughout the day.  All of the models bring a more substantial shortwave through the region, developing a wave of low pressure along a frontal boundary that will move into the central Mid-Atlantic.  The placement and strength of the shortwaves all vary between the models, which adds uncertainty to the exact development and progression of clouds and precipitation associated with the front and surface waves.  A weak upper level ridge will build over the NMA ahead of an upper level trough with a closed circulation, centered over MB by 00Z Sunday.  The weak ridge will continue to progress eastward and will pass over the NMA between 06Z and 18Z Sunday.  Following this upper level high, a strong shortwave will move through the Mid-Atlantic on Sunday; like Saturday’s shortwave, the weather models are not in consensus on the strength and location of this feature either, which adds uncertainty to the precipitation forecast.  But the models are consistent in keeping the frontal boundary draped across the central Mid-Atlantic, with additional waves potentially moving along it.

The main story for the end of the period is the weakening upper level trough, which now appears will remain centered over southern Canada.  The models are in closer agreement today that the closed low will remain well to the north of the Mid-Atlantic, centered in ON on Monday.  The GFS and EC diverge slightly on Tuesday and Wednesday regarding the placement of the upper level low in eastern ON.  Both models have a weaker trough over the eastern US, but the GFS splits off a vorticity lobe and keeps the trough contracted farther north, while the EC extends the trough farther south.  Both models agree that shortwaves will continue to rotate through the trough axis and pass over the central and northern Mid-Atlantic, but once again, timing and strength is highly uncertain at this time.

Despite the uncertainty regarding the exact details of the precipitation forecast, ample clouds and scattered precipitation will be enough to keep an exceedance in the Slight range for Saturday.  A wave of low pressure, associated with upper level shortwaves, will ride along a frontal boundary, moving from the Ohio River valley into the central Mid-Atlantic.  This feature will bring unsettled weather to much of the Mid-Atlantic.  The deterministic and hi-res model guidance is not in consensus on the locations with the highest chance for precipitation, but it seems most likely at locations in the vicinity of the Mason-Dixon Line (MDL), where the front is likely to track, moving from west to east during the day.

More shortwaves will be moving overhead on Sunday, impacting the majority of the Mid-Atlantic.  Waves will move eastward along the stalled boundary across the central part of the Mid-Atlantic, causing scattered showers early in the day.  Widespread precipitation is expected for many parts of the northern and central Mid-Atlantic between 18Z Sunday and 06Z Monday.  Cloudy conditions and onshore flow should be enough to keep exceedance Slight for the forecast region.

On Monday, there will be another round of shortwaves, originating from the closed upper level Canadian low, moving over much of the Mid-Atlantic.  The uncertainties in timing of these shortwaves are evident in the models, which translates into questions about the precipitation forecast.  Nevertheless, clouds and precipitation should be enough to keep exceedance Slight for another day.

On Tuesday, the primary cold front associated with the Canadian upper level closed low is expected to move into the NMA.  This feature is much weaker in today’s model guidance.  The front appears likely to either dissipate or stall somewhere across the region, oriented more west/east than north/south.  Due to the weakening front, the models are currently showing clear skies for much of the NMA, but there is the possibility for showers and storms that will be convective in nature.   The 06Z air quality models developed isolated areas of low Moderate ozone along the I-95 Corridor, likely response to the potential for clear skies.

Wednesday looks similar to Tuesday, with a trend toward clear skies, especially for the SMA, where the influence of the Bermuda high will be strongest.  Temperatures are not expected to rise much beyond average, however, given the cool pool extending southward from the trough over eastern Canada.  The main forecast questions for the end of the period will be the northward extend of the southeastern mid-level ridge, as well as location of any frontal boundary that separates the two main air masses between the ridge to the south and the trough to the north.

-Enlow/Huff