Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 14, 2015
Valid: July 15 – 19, 2015 (Wednesday – Sunday)

Summary:

Friday will be the main day of interest during this period as it will be the second day of full sun and calm winds in the Mid-Atlantic. Pre-frontal precipitation and brisk, northerly, post-frontal surface winds will keep air quality Good across the Mid-Atlantic tomorrow. While an area of high pressure will extend southward into the region on Thursday and keep skies clear and winds light, clean air filtering in behind Wednesday’s front will limit ozone and particles to the upper Good range. By Friday, however, light southeast surface winds and clear skies will allow the air mass over the Mid-Atlantic to modify sufficiently, allowing for the formation and buildup of ozone along and northwest of the I-95 corridor. Less humid conditions behind the front will likely limit any particle buildup to isolated locations just northwest of the interstate, with concentrations reaching into the low Moderate range. Saturday will be unsettled as a surface wave moving across NY allows clouds and showers to form over much of the region, limiting ozone to the Good range. Sunday’s forecast is questionable, as a more humid air mass is expected to take over and increase particle concentrations and ozone precursors in the atmosphere. Model discrepancies regarding precipitation chances will decide whether Sunday will be another day of inhibited production or rebounding ozone concentrations.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a shortwave over the Great Lakes is digging out a trough that will reach the Mid-Atlantic by 12Z Wednesday. The weather in the rest of the country is being dominated by a large upper level ridge with its axis through the Great Plains. The arrival of the trough over the region on Wednesday will be evidenced at the surface by the passage of a cold front from the northwest, preceded by a period of clouds and showers on Wednesday. By 00z Thursday the front will reach the Delmarva, but unlike past Mid-Atlantic fronts this summer, will continue to dip southward. Thursday morning will see the exit of the shortwave and trough aloft off to sea, allowing a period of zonal flow to take hold over the Northeast as the cold front reaches the Carolinas. At the surface, the center of an area of high pressure will build near the QC/ON border, keeping abundantly clear skies throughout the Mid-Atlantic. This high pressure center will move eastward to ME by 12Z Friday with its influence keeping skies clear in the Northeast for one more day. Shortwaves embedded in the zonal flow over the Great Lakes on Friday morning provide the first source of disagreement between the global weather models, which translates to forecast differences for the remainder of the period. While the EC solution shows a single, relatively disorganized shortwave over Lake Huron, the GFS has three shortwaves along a NW-SE axis over the Lakes. These short waves will likely be accompanied by organized mesoscale convective systems so that, by Saturday morning, these forecast differences translate to differing precipitation forecasts for the Mid-Atlantic. While both models show unsettled periods of clouds and rain in the Mid-Atlantic on Sunday, the weather models have yet to come into consensus on the exact timing and location of the precipitation.
Wednesday will see the arrival of a large shortwave aloft, reflected at the surface via a cold front that will sweep southeastward through the NMA. Clouds and scattered rain showers will move through the area ahead of the front, limiting ozone production along the front’s path. The front will reach the Delmarva by 00Z Thursday, allowing the frontal impacts to reach the majority of the Mid-Atlantic and inhibit ozone production on Wednesday. Particle concentrations will also stay in the Good range, mostly due to the brisk northerly winds and cleaner air behind the front, with back trajectories on Wednesday originating from the currently clean state of IA. Areas that remain south of the frontal boundary throughout the day – namely along the coast south of the Bay – may see daily PM2.5 concentrations reach just into the Moderate range as calm winds and humid conditions ahead of the front promote particle buildup.

Thursday will be a much more settled day weather-wise as the front continues through the Carolinas, leaving abundantly clear skies in its wake throughout the Mid-Atlantic. An area of surface high pressure centered in western QC will sprawl southward through the central Mid-Atlantic (CMA), continuing to promote clear skies and decreasing surface winds. These winds will slowly shift northerly to easterly throughout the day, containing any ozone buildup to southern portions of the I-95 Corridor. The air quality models are reflecting this logic, showing isolated pockets of low Moderate ozone at locations along and south of the DC/BAL axis. The rest of the region is expected to remain in the Good range on Thursday when it comes to both particles and ozone due to the cleaner, drier state of the new air mass filtering in behind Wednesday’s frontal passage.

Friday is the first day of interest for the period as the center of the area of high pressure reaches ME, continuing to inhibit clouds and surface winds in the Mid-Atlantic. These conditions will allow the air mass to begin to modify in favor of ozone formation, with light southeast winds confining the area of Moderate ozone to along and northwest of the I-95 Corridor. The 45-km BAMS-CMAQ air quality model resolves a pocket of USG ozone just west of the BAL metro area on Friday. Although relatively little credibility can be given to a nation-wide air quality model for Day 4 forecasts, consensus among weather models for weekday clear skies and calm winds keep the possibility for isolated pockets of USG ozone alive. Particles will build up in a very similar pattern as ozone, with the 45-km BAMS models showing concentrations reaching just into the Moderate range to the west of the interstate.

By Saturday the pattern in the vicinity of the Mid-Atlantic once again becomes unsettled, as shortwaves embedded in the zonal flow aloft promote rising air and scattered showers throughout the region. The WPC surface analysis attributes these showers to a weak surface wave that moves from Lake Ontario through southern New England as it continues to dissipate. Although there is model disagreement regarding the exact surface impacts of the waves aloft and at the surface, both the EC and the GFS resolve clouds and showers across the NMA and CMA on Saturday and Sunday. This spells Good air quality across the region on Saturday. The 850mb solutions suggest the very weak frontal boundary will move back northward into the region on Sunday, perhaps bringing a more humid and dirty southern air mass to the Mid-Atlantic. These conditions, along with any possible periods of sun, would allow ozone to rebound into the Moderate range on Sunday. Particle concentrations would also rise as well, but only into the upper Good range due to a lack of additional pollutants (such as smoke or Saharan dust).

– Eherts/Ryan