Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 13, 2015
Valid: August 14-18, 2015 (Friday – Tuesday)

Summary:

A somewhat complex and competing synoptic pattern during the medium range period, with a ridge at 850 mb and weak troughing at 500 mb, adds uncertainty and makes the air quality forecast challenging. The main forecast questions are 1) how quickly the current air mass in place modifies, 2) the fate of a weak back door cold front on Saturday-Sunday, and 3) the direction of air mass transport, particularly on Sunday and Monday. Friday may be a possible transition day, depending on how quickly the air mass in place modifies, with scattered Moderate ozone likely. The day of most interest appears to be Saturday, when light surface winds, clear skies, rising temperatures, and the possibility of an afternoon sea breeze will increase the chances for USG ozone to Appreciable. Similar weather conditions along with the uncertainty about the weak back door cold front moving through the northern Mid-Atlantic keep the chances at Appreciable on Sunday as well. The BAMS and NC air quality models are showing upper Moderate and pockets of USG ozone at locations along the I-95 Corridor on Saturday and Sunday; they have been consistent with this trend for several days, which gives more credence to the possibility of upper Moderate to USG ozone over the weekend. Potential complicating factors are afternoon sea breezes and the fact that the weak back door cold front could provide a line of surface convergence and push ozone concentrations higher than expected. Uncertainty in the precipitation forecast ahead of the next approaching cold front drop the chances for USG ozone to the Marginal range on Monday and Tuesday. The GFS solution is faster and would limit significant ozone production on Monday, while the EC solution would lead to another day of upper Moderate to USG ozone. Clouds and precipitation appear more widespread on Tuesday, which return ozone to the Good to low Moderate range.

Discussion:

The weather models are in generally close consensus throughout the medium range period, but show some discrepancies on Monday and Tuesday. The 00Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. A somewhat complex and competing synoptic pattern, with a ridge at 850 mb and weak troughing at 500 mb, adds uncertainty and makes the air quality forecast challenging for the medium range period. Currently, a large ridge over the western US and a large trough over the eastern US, with its base reaching to the Gulf coast, are the dominating upper level features. The upper level trough will begin to retreat northward this afternoon, allowing surface high pressure to extend into the Mid-Atlantic. The center of surface high pressure will settle over WV on Friday, where it will remain through Sunday. As the upper level trough retreats, a mid-level ridge (850 mb) will quickly build eastward over the Mid-Atlantic, allowing temperatures to rapidly rise into the low 90s °F Saturday through Tuesday. The center of the mid-level ridge will sit over the Mid-Atlantic on Saturday and Sunday. On Friday, a bundle of small shortwaves will crest the upper level ridge and move from ON into the Great Lakes. These shortwaves will pull a weak back door cold front into the northern Mid-Atlantic (NMA) on Saturday, with a weak trough at mid-levels (850 mb). The weather models are continuing to have trouble resolving this front. It certainly appears to be very weak. In this morning’s analysis, WPC has the front becoming a surface trough by the time it reaches PHL on Sunday, compared to previous analyses, which kept the front together before dissipating it on Sunday. As a result, considerable uncertainty persists regarding the impacts of this front on air quality. Beginning on Saturday evening, the upper level western US ridge tries to build eastward across the entire CONUS, but continues to inhibited by weak upper level troughing lingering over the eastern US. In particular, a stronger area of shortwave energy over the Ohio River Valley (ORV) will form an upper level closed low and prevent this upper level ridge from becoming very prominent east of the Mississippi River. By 18Z Monday, shortwave energy moving eastward in the northern flow will phase with the upper level closed low and establish a shallow trough over the eastern US. This shallow trough will reach as far south as TN and remain in place through Tuesday. This trough will bring the next, weak cold front into the Mid-Atlantic on Tuesday. The GFS is about 12 hours faster than the EC in moving this trough and accompanying cold front eastward, which leads to uncertainty in the precipitation forecast for the end of the medium range period.

Skies will be mostly clear and surface winds will be light as surface high pressure centered over WV dictates the weather for the Mid-Atlantic on Friday. Localized back trajectories indicate that the air mass will be fairly stagnant. The main forecast question for Friday is the degree of air mass modification. As the upper level trough retreats and the mid-level ridge builds eastward, flow at the surface and aloft will be very light. Moderate ozone is expected along the I-95 corridor in response to the clear skies and light winds. The NOAA and BAMS air quality models are supporting this idea by showing scattered low to mid Moderate ozone along the I-95 Corridor. The NCDENR model is by far the most aggressive for ozone tomorrow, with pockets of USG ozone in southwestern Washington, DC and southern NJ. At this point, the NC model seems overdone, considering how low ozone concentration were yesterday, and seem to be so far this morning. A potential complicating factor is an afternoon sea breeze, which is possible along the DE and NJ coasts due to the light synoptic wind field. This sea breeze would affect mainly locations in DE and NJ and push ozone to the mid to upper Moderate range, more in line with the NC model guidance. PM2.5 concentrations will remain in the Good range on Friday due to persistent relatively low humidity. There may be a few locations in western PA that reach into the Moderate range in response to the light surface winds.

Saturday is the main day of interest during this period as light surface winds, recirculating back trajectories, clear skies, and temperatures continue to increase. The locations with the best chances for upper Moderate ozone concentrations will be along the I-95 Corridor in MD, DE, PHL, and NJ, due to close proximity to emissions. Today is the third consecutive day that the air quality models are showing Moderate ozone along the I-95 Corridor on Saturday, and the models have steadily increased concentrations each day, which lends confidence to an upper Moderate ozone forecast for Saturday. Today, the air quality models (BAMS and NCDENR) are showing upper Moderate all along the I-95 Corridor. The NC model is also showing pockets of USG in the DC metropolitan area and PHL/ southern NJ. An afternoon sea breeze is also possible on Saturday due to the light synoptic winds. The sea breeze would affect mainly DE and NJ locations. With conditions already prime to see upper Moderate ozone, a sea breeze could be the tipping point to push ozone into the USG range along its line of convergence. The surface high pressure over WV may not provide enough subsidence to limit vertical mixing as it is not very prominent in the upper levels of the atmosphere. Based on today’s analysis, the chances for USG ozone on Saturday is Appreciable. If the air quality models continue the trend tomorrow of increasing ozone on Saturday, and the air mass in place shows signs of modification, then the chances will increase to High. PM2.5 concentrations will likely reach the Moderate range at scattered locations in response to the light surface winds and a slight uptick in humidity.

The main forecast question for Sunday is the fate of a back door cold front moving through the NMA. The weather models are showing scattered clouds and rain showers along the frontal boundary as it moves southward into PA Saturday evening into Sunday morning roughly west and north of I-81. This front should weaken as it reaches PHL by 12Z Sunday, limiting the chances for clouds and precipitation. The WPC forecast has the back door cold front weakening into a surface trough as it reaches PHL, which means the front may act as line of surface convergence. The weather conditions on Sunday will be similar to Saturday as light winds, clear skies, and rising temperatures continue. Back trajectories are still short, reflecting the location of the mid-level ridge aloft, but shift more southwesterly, from the vicinity of western VA. These conditions suggest that upper Moderate to USG ozone conditions are possible on Sunday. The BAMS air quality models are giving credence to this forecast by showing upper Moderate ozone in the same locations as Saturday (MD, DE, PHL, and NJ) as well as pockets of USG in DC and NJ. If the back door cold front weakens and dissipates by the time it reaches PHL, then ozone concentrations may stay in the upper Moderate range. Usually, ozone concentrations are limited to the upper Moderate range on Sundays, all other things being equal, due to decreased vehicle emissions. PM2.5 concentrations will likely continue in the Moderate range at scattered locations due to increasing humidity and light surface winds.

There is still considerable uncertainty in the forecast for Monday and Tuesday as the next cold front approaches the region from the northwest. Monday will see similar weather conditions to Sunday, which should lead to another day of Moderate ozone along the I-95 Corridor. The air quality models are responding by showing Moderate ozone along I-95 in MD, DE, and NJ, although the NC model backs off ozone compared to previous days, with only isolated Moderate. Back trajectories are still slow, but shift more southerly on Monday, which may be enough to limit rising ozone along the I-95 Corridor.

The main question for Tuesday’s forecast is the fate of the approaching cold front. The GFS is faster than the EC in moving this cold front eastward from the Plains into the Mid-Atlantic on Monday. The GFS is showing scattered clouds and rain showers in the NMA and central Mid-Atlantic around 18Z Monday while the EC keeps the clouds and precipitation out of the region until Tuesday. The GFS solution should keep help to limit significant ozone production while the EC solution would lead to another day of upper Moderate ozone. Ozone concentrations should be limited to the Good range on Tuesday as the cold front reaches the NMA and promotes widespread cloud cover and precipitation. Given the relatively weakness of the front, particles will likely remain in the Moderate range through the end of the period due to rising humidity and persistent light winds.

– DeBoe/Huff