Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 5, 2015
Valid: August 6-10, 2015 (Thursday – Monday)

Summary:

Generally Good to low Moderate air quality is expected throughout the medium range period due to two substantial disturbances. The first will arrive in the form of a surface wave along a frontal boundary stalled over northern NC, which will bring widespread clouds and rain to the Mid-Atlantic south of the Mason-Dixon Line from Thursday through Saturday afternoon. North of these impacts, the clean post-frontal air mass in place will limit ozone production. Following this large system, we will see two days of at least partial sun, with Moderate ozone possible around the southern I-95 Corridor on Sunday. Then the next cold front will move through the region on Monday. Although there is some uncertainty in the precipitation forecast this far in advance, widespread pre-frontal clouds and convection are expected, which will drop particles and ozone into the Good range, while Moderate air quality may continue east of I-95.

Discussion:

The weather models are in generally good consensus throughout the beginning of the medium range period, but begin to differ on Friday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large closed low aloft just east of James Bay has an associated trough reaching into the central Mid-Atlantic (CMA). An area of shortwave energy aloft over NE/KS will move eastward with the flow and begin to dig out its own mini trough. The global and mesoscale models have come into much closer consensus today regarding the fate of this area of shortwave energy, but some discrepancies remain. In particular, the global weather models begin to show slight differences on Friday regarding the exact shape and speed of this shortwave. However, resulting model discrepancies do not impact the air quality forecast until Monday. This shortwave feature will begin to impact the Mid-Atlantic on Thursday afternoon as a wave of surface low pressure, bringing widespread clouds to areas south of the Mason-Dixon Line (MDL) and associated rain showers to I-95. The models now agree in containing the major impacts of this disturbance to areas south of the PA/MD border; as a result, PA and north/central NJ will see little to no precipitation. Saturday will be a transition day following this rainy system as it moves northeastward up the Atlantic coast. The persistent upper level ridge in the western US will flatten and spread towards the Mid-Atlantic on Sunday, promoting another day of mostly sunny skies. But the eastward extension of the upper level ridge will not spread any farther, as the semi-persistent eastern Canadian upper level trough re-develops for early next week. On Sunday, the models agree that an upper level low will reach the MB/ON border, reflected at the surface as an area of low pressure in southcentral ON. This low will have an associated southwestward stretching frontal boundary that will reach from the Great Lakes to northern KS on Sunday. The WPC surface analysis shows this front moving southeastward through the Mid-Atlantic throughout the day on Monday, which proves a favoring towards the GFS global model solution, which shows an extensive area of pre-frontal precipitation moving through the entire Mid-Atlantic starting on Monday night.

Extensive cloud cover will spread over the entire Mid-Atlantic from west to east tomorrow, due to both a stalled frontal boundary along the VA/NC border and an approaching surface low riding along the stalled front. Rain showers are expected to spread west to east across the CMA and southern Mid-Atlantic (SMA) throughout the day, reaching the coast by overnight Thursday into Friday. Model consensus has grown closer over the past couple of days, resulting in increased confidence that the rain will stay to the south of the MDL. Back trajectories from the Upper Peninsula of MI, along with the widespread clouds and afternoon rain, will limit ozone to the Good range for most areas. The exception will be where there is surface convergence resulting from the frontal boundary near coastal VA and NC, where the clouds may not reach until after nightfall. Good PM2.5 is anticipated across the region, due to either cleaner, drier air filtering in to the north of the front or the widespread evening washout.

Despite model discrepancies regarding the speed of the eastward motion of the surface low, widespread clouds and rain showers are expected to continue throughout the region on Friday – again, south of the MDL. Despite shorter back trajectories from the Lake Ontario region, the continuing cloud cover will keep ozone mainly in the Good range across the eastern seaboard. The BAMS air quality models are resolving an area of low Moderate ozone in northwestern VA, where an extended afternoon break in the clouds is resolved in the high-res mesoscale weather models. This most likely will not verify due to the fact that a substantial area of rain will have just cleared the region, and that the mesoscale models do not show the sun coming out until 21 to 00Z. PM2.5 is expected to stay in the Good range as well, except for a small area just north of the clouds in the Susquehanna River Valley of PA, which is being resolved by the BAMS/NCDENR air quality models. Both models are responding to the increased humidity just to the north of the rain and cloud cover, where a lack of heavy precipitation (such as there is to the south) may allow particle concentrations to build into the Moderate range.

There is general model consensus that the center of surface low pressure will move off the Mid-Atlantic coast overnight Friday into Saturday and make its way toward New England. It will take its time, however, and clear skies are not expected to overtake the region until about 18Z Saturday. Additionally, the air at 500 m AGL will originate from near Cape Cod. Ozone is therefore expected to stay mostly in the Good range. As the surface low shifts towards a more northeastward track and moves parallel to the Jersey coast, northeasterly surface winds will increase across the region, particularly along the coast, lending confidence to the Good air quality forecast. Particles will remain in the Good range as well, due to the sustained northeasterly winds and recent washout.

Sunday will be the first sunny day for the medium range period ahead of an approaching cold front. Clear skies, warm and seasonable conditions, and a slight increase in humidity will promote scattered afternoon cloud cover. However, this will not be sufficient to hinder scattered low Moderate ozone in the usual areas surrounding the southern I-95 Corridor. A stretch of Moderate PM2.5 is also resolved in the air quality models in DC, eastern VA, and much of NC. This is most likely due to the fact that winds will be calmest there for the longest behind Friday’s coastal low.

Monday will be pre-frontal, and following the GFS model solution, it will most likely be another washout throughout much of the region, dropping ozone and particles solidly back into the Good range. A cold front approaching from the northwest will sweep through the NMA, passing through northwest PA around 12Z. Widespread precipitation associated with this front will scrub the atmosphere of any precursors that managed to build since the last frontal passage. The cloud cover is not expected to reach the coast before nightfall, meaning that ozone will have an additional day of sun to build into the Moderate range in areas east of the I-95 Corridor. The same will go for PM2.5, as a calm morning and continuing humidity allow particles to build into the Moderate range ahead of the rain. Major model discrepancies do exist, however, lending uncertainty to this forecast. Whether or not the widespread showers verify, the atmosphere in place will be clean enough to limit max ozone to the Moderate range.

– Eherts/Huff