Daily Archives: July 2, 2015

Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday, July 2, 2015

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 2, 2015
Valid: July 3 – 7, 2015 (Friday – Tuesday)

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Summary:

Model consistency regarding a building ridge of high pressure at the end of the holiday weekend increases the chances for USG ozone to marginal on Sunday, Monday, and Tuesday. A frontal boundary oscillating around the VA/NC border through Monday will promote clouds and precipitation that will limit ozone to the Good range for most of the region on Friday and Saturday. By Sunday, the arrival of the upper level ridge and corresponding surface high pressure will allow ozone to climb into the Moderate range in the northern Mid-Atlantic, with the best chance for scatted USG ozone on Monday. There is some uncertainty about Tuesday, as questions regarding the speed and strength of an approaching cold front and a possible weak coastal low could provide sufficient clouds to limit rising ozone. Model guidance suggests that smoke from Canadian wildfires will linger across the region through Monday. This smoke, combined with local contributions from Independence Day celebrations, will keep PM2.5 concentrations in the Moderate range for most of the period.

Discussion:

The weather models are in general consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS. A cold front that moved through the northern Mid-Atlantic (NMA) this morning has stalled in northern VA. This front will oscillate northward and southward between approximately the Mason-Dixon Line (MDL) and the VA/NC border through Monday. Shortwaves in the Plains and Tennessee River Valley (TRV) will form multiple surface waves and move them eastward, riding along the stalled frontal boundary. The largest wave is expected to impact the Mid-Atlantic Friday evening into Saturday morning. Although the global and regional scale models have come into somewhat closer agreement today on the track and timing of the surface waves, there is still uncertainty regarding the location and timing of the heaviest cloud cover and precipitation through Saturday. On Saturday, a relatively narrow but amplified upper level ridge will develop over western ON and gradually move eastward through the end of the period, with the axis over the east coast on Monday and New England on Tuesday. The surface reflection of this ridge is a center of high pressure that will form over southern ON and move southeastward, reaching southeastern ON on Sunday and the eastern NY/PA border on Monday. On Monday and Tuesday, the EC and GFS both have substantial shortwaves over the Mid-Atlantic, which may act to form a weak coastal low along the remnants of this week’s frontal boundary. WPC has analyzed this low over BAL on Tuesday morning, but the EC and GFS do not have much precipitation associated with the feature, which makes its impact uncertain at this time. A developing trough over SK/MB will move eastward and form a closed low at 500 mb on Sunday. This feature will eventually pull the next major cold front toward the Mid-Atlantic beyond the end of the medium range period.

The quasi-stationary frontal boundary will be located over south/central VA on Friday. Waves of low pressure riding along the boundary will bring scattered showers and convection to locations mainly along and south of the MDL. The front will also promote mostly cloudy skies for much of the region, with the exception of eastern PA and north/central NJ. Consequently, ozone will be suppressed to the Good range for most of the region. Areas where there is sufficient afternoon sun, mainly along the northern branch of the I-95 Corridor, may see Moderate ozone, however. The NOAA model in particular has Moderate ozone all along the extent of the I-95 Corridor, as far south as Washington, DC and Baltimore. This seems slightly overdone, especially considering high temperatures will be below average, but conditions will need to be evaluated once the 12Z runs of the mesoscale models are available. The biggest story for air quality across the region will continue to be smoke from Canadian and Alaskan wildfires. The smoke still very evident on visible satellite imagery across the region this morning, despite scattered cloud cover. This smoke is still reaching the surface, with PM2.5 monitor hourly measurements in the 20s and 30s ug/m3 across the entire region, with the exception of northern NJ. The NRL NAAPS model updated this morning, and it shows smoke lingering over the region through Monday, with the thickest smoke over DC/BAL/PHL on Friday. Consequently, the smoke is expected to continue to keep particles well in the Moderate range tomorrow, with areas that experience heavier bouts of precipitation, mainly across VA and MD, will see some reduction in hourly PM2.5 concentrations.

On Saturday the CMA stationary front will weaken and move very little, keeping the chance for scattered cloud cover and showers in the Mid-Atlantic. Per a blend of the GFS and EC guidance, the most concentrated area of precipitation will shift from the CMA to the NMA throughout the day as a surface wave moves eastward along the frontal boundary, bringing a period of widespread precipitation to northern VA, MD, and southern PA/NJ. These cloudy skies will limit ozone to the Good range for most of the region. The showers will help to decrease PM2.5 concentrations as the smoke continues to dissipate slowly, but recirculating back trajectories and NAAPS model guidance suggest that some smoke will persist. In addition, Independence Day fireworks celebrations pose a local threat for PM2.5 buildup. As a result, particle concentrations will likely decrease but remain in the Moderate range.

Sunday and Monday are the days of most interest in the period, with the upper level ridge setting the stage for rising ozone levels, mainly in NMA. The air quality models are responding to these weather conditions by increasing ozone to the upper Moderate range on Sunday and USG on Monday along the I-95 Corridor. The main forecast questions will be the daily chances for clouds and convection associated with the quasi-stationary boundary, shortwaves aloft, and the possibility of the weak coastal low forming by the end of the period.

Overnight Saturday into Sunday, a cold front will dissipate as it quickly sweeps into the NMA. Although the air quality impacts of the air mass that will build in behind this front are uncertain, this frontal passage may help to mark the end of substantial smoke impacts in the region, although the NAAPS model brings another, albeit weaker, pulse of smoke into the region behind the front. Consequences of Independence Day celebrations may linger at more inland locations, where the light onshore surface winds will not reach. Most notably, surface high pressure centered over Lake Huron with its resultant clear skies, subsidence, and light winds will make Sunday a day of interest for ozone for the NMA. As a surface trough moves northeastward into the Mid-Atlantic around midday, clouds will build and precipitation will fall across the SMA starting in the late afternoon , limiting daily ozone concentrations to the Good range across the region. Clear skies will persist in the NMA throughout the day, however, making it the focus for continually building ozone into the evening hours.

By 12Z Monday, the center of surface high pressure is expected to be over northeastern PA, allowing clear skies and calm surface winds to dominate the weather north of the Carolinas. In the SMA, the persistent stationary front will continue to promote clouds and scattered rain showers, limiting ozone and PM2.5 concentrations. To the north of the frontal impacts, calm, warm and sunny conditions along with local and recirculating air aloft will allow for the buildup of both ozone and PM2.5 concentrations, with ozone being the most likely to reach USG levels. Lingering air quality impacts resulting from holiday weekend travel will continue to pose a threat to local concentrations.

The threat for rising ozone will continue into Tuesday as the daily high temperature persists in the 80s ͦF, clear skies remain over the region through the afternoon, and winds at the surface and aloft stay light. The forecast question towards the end of the period is the speed and strength of an approaching cold front from the west, which at this time appears to slow as it enters the Mid-Atlantic overnight Tuesday. If this front fails to bring a substantial period of rain, brisk surface winds, and/or a new air mass into the region, next week could see several days of deteriorating air quality.

– Eherts/DeBoe/Huff