Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 10, 2015
Valid: July 11 – 15, 2015 (Saturday – Wednesday)

Summary:

A “mini ridge” aloft will build over the northern Mid-Atlantic through Sunday, with an area of surface high pressure moving over PA, priming the atmosphere for a period of peak ozone concentrations by Sunday. A frontal boundary currently in central VA will dip southward tomorrow and bring clouds and showers to VA, southern MD, southern DE, and eastern NC, while a small area of surface high pressure just north of the front will keep skies clear in the northern Mid-Atlantic. This day of sun will allow ozone precursors to build up in the atmosphere, making Sunday – a warm and sunny day throughout the region – the main day of interest during the medium range period. By Monday, a developing upper level trough over the eastern US will return unsettled conditions to the Mid-Atlantic. On Monday a surface wave will approach from the west, lifting a frontal boundary back north into the northern Mid-Atlantic and bringing a period of clouds and rain through the region starting on Monday evening, with unsettled conditions continuing through the end of the period . The particle forecasts through the period will depend on the lingering effects of wildfire smoke and Saharan dust, with possible additional contributions from sulfate haze on Sunday. Periods of Moderate conditions are likely, especially for the southern part of the region.

Discussion:

The weather models are in general consensus on the synoptic features for most of the medium range period, but diverge somewhat on Tuesday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a zonal flow aloft over the Mid-Atlantic and a building upper- and midlevel ridge over the southeastern US are slowing the southward progress of a cold front in northern VA. The axis of this ridge at 500 mb extends from AB southeastward through the northern Plains. A small area of shortwave energy currently over the western Ohio River Valley (ORV) will make its way eastward in the flow along the surface boundary, triggering a surface wave and bringing an area of uplift and resulting precipitation to the central Mid-Atlantic (CMA) tomorrow. Aloft, this feature is followed immediately by a “mini-ridge” in the otherwise zonal flow, allowing surface high pressure to move across PA on Sunday. The axis of the large upper level ridge will stretch from SK to the Tennessee River Valley by 12Z Saturday as the now closed ridge over the Southeast retrogrades. The westward movement of the center of this ridge into the southern Plains will allow for a trough to start digging out down the east coast, as evidenced by the frontal boundary now being able to dive as far south as the SC/GA border by 12Z Sunday. At this time a bundle of small shortwaves over the Great Lakes will become apparent, amplifying the growing trough, with some model discrepancy as to the exact characteristics of the shortwaves. The bottom line for air quality is that this the third day of model solutions supporting a trend toward developing a longwave trough over the eastern US beginning on Monday. Although the global forecast models are having trouble resolving the specific pattern of the evolving trough on Monday and Tuesday, they end up in the same place by Wednesday. Even though there is considerable uncertainty regarding the precipitation forecast for the end of the period, as described below, the beginning of the week looks unsettled, with few chances for ozone production.

The differences in the shortwave bundle are reflected at the surface via differing precipitation forecasts overnight Sunday and throughout the day on Monday. For example, by 12Z Monday, the NAM analyzes a large area of scattered centers of shortwave energy over the eastern Great Lakes making for scattered precipitation in the northwestern Mid-Atlantic; the GFS has only a few weak areas of shortwaves over the Great Lakes, keeping most rain to the west of the area; and the EC shows a tight shortwave circulation over the eastern Great Lakes bringing widespread precipitation to the entire northern and central Mid-Atlantic (NMA, CMA). The WPC model discussion explains a favoring towards a blend of the NAM and EC solutions, which both bring precipitation to the NMA by Monday afternoon. At 500 mb, these models show the bundle of shortwaves continuing to dig out the trough over the Mid-Atlantic. By 00Z Tuesday, the trough axis reaching from the QC/ON border southward to the Carolinas, with scattered shortwaves throughout its base promoting widespread clouds and precipitation throughout the eastern seaboard. In the west, although no longer closed, the ridge is still prominent and stretching from southern CA through the Gulf Coast with its axis along the eastern edge of the Rockies. By 12Z Tuesday, a large shortwave becomes apparent over the southern Hudson Bay and will continue to move eastward until the end of the medium range period, when it reaches the eastern border of QC. 00Z Wednesday analyses show substantial precipitation differences between models along the east coast. This is mainly due to a discrepancy in shortwave placement in the base of the east coast trough, with the EC solution placing the largest shortwave along with the heaviest precipitation in the southern Mid-Atlantic (SMA). The GFS keeps the bulk of the shortwave energy farther north, promoting precipitation in the NMA but keeping the SMA under clear skies. This causes the 18Z EC solutions to have a deeper trough along the east coast, but by this point the energy aloft in both models is diluted enough to keep surface impacts minimal. By the end of the day on Wednesday, although the models show discrepancies regarding the placement of any lingering precipitation, overall they are in very close agreement regarding large scale synoptic features. A wide ridge centered over the central Plains and the southern end of a trough with its axis mainly stretching southward through QC will share the atmosphere aloft over the CONUS by Wednesday night.

A cold front currently in northern VA will move very little over the next 48 hours as an area of surface high pressure building into the NMA and the continuing ridge in the southeastern US will sandwich the front in the CMA. Shortwave energy aloft will move eastward along the frontal boundary and bring an area of precipitation through the CMA tomorrow, from WV/western MD through VA to the eastern VA/NC coast. Convection is likely, initiating in the Richmond area before the disturbance moves off the coast of the Outer Banks overnight. This surface is a new feature in today’s model analyses, but all of the mesoscale and global models show it with fairly close consensus. The rain and cloud cover will keep ozone concentrations in the Good range throughout the CMA tomorrow. The rest of the Mid-Atlantic – areas north of the Mason-Dixon Line (MDL) and west/southwest of Cape Hatteras – will remain mostly clear throughout the day. Seeing as the disturbance will remain relatively localized to VA and the immediate surroundings, surface high pressure will be allowed to build over PA/NJ throughout the day on Saturday. The disturbance passing to the south will promote light onshore surface winds in the NMA and the synoptic pattern will keep back trajectories northerly – these factors, along with the heavy rain that moved through much of the NMA last night, will help to limit rising ozone, but scattered Moderate conditions are likely along the northern portion of the I-95 Corridor, from roughly ILG to TTN. The PM2.5 forecast is a bit trickier. Saharan dust is keeping hourly concentrations this morning in the 20s ug/m3 in NC, with areas just north of the frontal boundary in VA in the single digits. The NRL NAAPS model analyzed a plume of smoke dipping into the NMA behind this front this morning, and it is evident on satellite imagery. The smoke is prominent in visible satellite imagery throughout the northern Plains and over the Great Lakes, but has been slow to reach the surface and make its impact on surface monitors in the NMA. Hourly concentrations are rising in northern, western, and central PA this morning, which suggests that the smoke is gradually moving toward the eastern and central portions of the Mid-Atlantic. The 15km BAMS-CMAQ air quality model shows scattered areas of Moderate PM2.5 in the vicinity of the western edge of the PA/MD border tomorrow, without the consideration of possible smoke impacts. Depending on the vertical motion of this smoke plume, the NMA could see particle concentrations reach into the Moderate range on Saturday. Saharan dust should continue to impact NC on Saturday, for continued mid-Moderate PM2.5 conditions.

Sunday is still the main day of interest during the period. As the southeastern ridge retrogrades westward on Sunday, the frontal boundary in the CMA will dive southward through SC and allow a surface high pressure centered in southern MI to extend into the Mid-Atlantic and move over PA and then off shore during the day. Aloft, a “mini ridge” will build directly over the NMA, promoting abundantly clear skies and light and variable winds throughout the region on Sunday. This optimal synoptic set up should allow ozone to rebound well into the Moderate range, with the 45km BAMS CMAQ and MAQSIP-RT models showing widespread Moderate in the NMA and CMA, along with isolated areas of USG concentrations along the southern I-95 Corridor. A possible limiting factor will be the potential for strong onshore surface winds along the Atlantic coast, due to a weak coastal low off the northern NC coast. This low will begin as Saturday’s surface wave and will strengthen off of the coast, near HAT, on Sunday. All of the models show this low, but the GFS is the strongest. If the development of the low verifies, its circulation would limit rising ozone in DE, NJ, and PHL. Particle concentrations will again depend heavily on the possible intrusion of Canadian wildfire smoke. However, Moderate concentrations are resolved along the Appalachians and throughout the CMA in the air quality models, suggesting that the hot, increasingly humid, sunny, and calm weather conditions will be sufficient to bring PM2.5 into the Moderate range regardless. Any additional particle sources – such as wildfire smoke – would bring concentrations higher into the Moderate range. In the SMA, the Saharan dust is expected to dissipate by Sunday morning, helping particle concentrations drop closer to the seasonal average.

Model discrepancies on Monday make the forecast uncertain, but since the GFS is the model in least consensus, we will rely on a blend favoring the NAM and EC. An area of shortwave energy aloft over the eastern Great Lakes will help drag the frontal boundary northward as a warm front early on Monday, but its weak nature will keep the surface impacts minimal for the first half of the day, except for an increase in heat and humidity. The 06Z run of the 12km NAM resolves widespread cloud cover in the NMA which dissipates into the CMA. This gradient will intensify throughout Monday afternoon as a surface wave approaches from the west, increasing the strength of the frontal boundary and bringing an area of precipitation eastward through the NMA during the evening hours. In the western NMA and CMA, these daylong widespread clouds and showers will limit ozone to the Good range. However, in areas in the eastern Mid-Atlantic (EMA) where cloud cover will be sparser ahead of the evening rain, lingering ozone precursors could help daily concentrations to reach into the Moderate range on Monday, from PHL to NYC. Particles will also likely linger in the Moderate range ahead of the rain where light southerly surface winds and humid conditions will continue to promote the buildup of PM2.5.

Tuesday and Wednesday look unsettled, with the warm front stalling over southern PA and a back door cold front stalling in NY State. Tuesday will see the arrival of the surface wave along with its resultant cloud cover and rain showers mainly in the NMA and CMA. The WPC surface analysis resolves a lee trough extending southward through central VA and NC, which will likely promote clouds and scattered showers in its vicinity as well. Widespread cloud cover and precipitation, as well will keep ozone and particles in the Good range throughout the Mid-Atlantic on Tuesday. By Wednesday morning the center of the surface wave will reach Boston, allowing the Mid-Atlantic to clear briefly before the impacts of its trailing cold front are felt. By 18Z Wednesday, extensive cloud cover and scattered showers are expected to spread over the entire Mid-Atlantic and limit ozone and particles to the Good range for the end of the period.

– Eherts/Huff