Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 6, 2015
Valid: August 7-11, 2015 (Friday – Tuesday)

Summary:

A strong surface wave moving through the Mid-Atlantic tonight into Friday morning will promote widespread cloud cover and heavy rainfall for most of the region, effectively cleaning out the atmosphere and leading to generally Good air quality. Clear skies throughout the weekend will promote localized ozone formation, especially along the I-95 Corridor, but temperatures in the low 80s °F, lower vehicle emissions and onshore flow will limit concentrations to the lower end of the Moderate range. The best chances for scatted Moderate ozone will be on Sunday. The weather models are not in consensus regarding the next cold front, which will impact the region sometime in the Monday-Tuesday period. As a result, the forecast is uncertain beyond Sunday, but generally Good to Moderate ozone and low Moderate PM2.5 seem most likely.

Discussion:

The weather models are in generally close consensus in regard to a strong surface wave moving through the Mid-Atlantic on Friday, but they begin to diverge on Saturday. The 00Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. The mesoscale models are continuing to confine the bulk of the rain associated with the surface wave arriving today to areas along and south of the Mason-Dixon Line (MDL). However, the model are not in complete consensus regarding the coverage, magnitude, and northern extent of the wave’s precipitation. Aloft, the shortwave energy associated with the surface wave is over IL this morning and will move eastward into the Mid-Atlantic this evening into Friday morning. The surface wave will ride along the stalled frontal boundary, which is currently draped across the VA/NC border. The surface wave will move off of the Atlantic coast on Saturday morning and will move northward along the eastern seaboard. At this time, the weather models agree to move a shortwave eastward from the northern Plains into the Great Lakes. The EC continues to move this shortwave eastward into New England on Sunday, while the GFS and NAM dissipate it. This leads to differences in the upper level flow. The EC’s solution develops a westerly flow at 850 mb while the GFS and NAM solutions form a mini-ridge at 850 mb, leading to a light southwesterly to southerly flow on Sunday. Overall, the main takeaway for Sunday is that the persistent western US ridge tries to build eastward over the Mid-Atlantic, but it won’t get very far. On Monday, the global weather models are re-developing a broad upper level trough over the northeastern US with its base reaching as far south as KY. The GFS is slightly faster in moving the axis of this trough eastward, placing the axis to the east of the Mississippi River on Monday. In contrast, the EC places the trough axis over the Mississippi River. The upper level trough will amplify on Tuesday as its base stretches southward into GA.

The strong surface wave currently over KY will move eastward into the Mid-Atlantic along a stalled frontal boundary tonight into Friday morning. The models are in general consensus on the fate of the surface wave, but still disagree on the details, which makes the precipitation forecast somewhat uncertain. Today the models are continuing the trend from yesterday to push the wave further southward along the VA/NC border. This surface wave will promote widespread clouds and moderate to heavy rainfall across most of the region, with the more southerly track keeping locations north of the MDL mostly dry. As a result, Good ozone is expected at locations along and south of the MDL. The air quality models are having trouble resolving ozone guidance for tomorrow. They all develop areas of Moderate ozone at various locations, from the I-95 Corridor (NOAA) to southern NJ/DE/eastern shore MD (NCDENR). This seems unlikely given the expected widespread cloud cover, which should reach northward to at least PHL/TTN, and breezy onshore surface winds. Thus, a few locations north and east of the MDL may see ozone reach into the low Moderate range on Friday, but they should be isolated.

Hourly PM2.5 surface concentrations this morning are hovering in the single digits to low teens ug/m3 across most of the region, with the exception of the Hickory, NC area, which is seeing hourly concentrations into the 50s ug/m3. We can’t tell from surface or satellite observations what the cause of this spike in particle concentrations is in Hickory, but there is an AQA issued for the area, so a fire seems the most likely culprit. Aside from this isolated location in NC, the heavy precipitation moving through the Mid-Atlantic tonight into Friday morning will clean out the atmosphere and keep PM2.5 concentrations in the Good range on Friday.

Mostly clear skies across the Mid-Atlantic will return on Saturday as the surface wave moves off the east coast and the stalled frontal boundary moves southward into SC. Friday’s washout coupled with continued onshore surface winds will suppress significant ozone production, keeping concentrations in the Good range on Saturday. A few locations along the I-95 Corridor may see ozone rise into the low Moderate range as surface winds will die down inland and skies will clear. PM2.5 concentrations will likely stay in the Good range for another day due to Friday’s washout along with low humidity.

Clear skies will continue on Sunday as winds die down across the region. These conditions are conducive to rising ozone, but lower Sunday emissions coupled with average temperatures will limit concentrations to the low Moderate range. Locations along the I-95 Corridor have the best chances to see ozone reach into the Moderate range. PM2.5 concentrations will likely continue in the Good range as well due to the low humidity. Some locations in the southern Mid-Atlantic (SMA) where the humidity is higher and the surface winds are light may see PM2.5 concentrations reach into the low Moderate range.

The forecast for Monday and Tuesday is uncertain due to model differences in the arrival of the next cold front sometime early next week. The passage of front has slowed by a day since yesterday’s analysis, with the front now arriving in the Mid-Atlantic on Tuesday afternoon. The GFS has a more traditional frontal passage, albeit very slow, with a line of pre-frontal convection moving across the region from west to east beginning Monday afternoon and continuing to Tuesday evening. In contrast, the EC develops a very strong surface low over the SMA and brings it up the Atlantic coast as a nor’easter-like system. Thus, the precipitation forecast is uncertain for Monday and Tuesday. The GFS solution brings widespread precipitation through the region Monday evening into Tuesday morning, while the EC brings the bulk of the precipitation through Tuesday morning. Surface winds shift southerly on Monday, increasing humidity and cloud cover. Cloud cover will depend on whether the GFS or EC solution verifies, with the GFS keeping skies cloudy in the NMA and the EC with more clouds in the SMA. PM2.5 concentrations may rise into the Moderate range in response to the rising humidity. On Tuesday, both models have a washout across the region, although for different reasons. We will monitor the model trends to determine which solution is more likely; for now, the WPC is siding with the GFS’s “traditional” cold frontal passage on Tuesday.

-DeBoe/Huff