Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, August 9, 2015
Valid: August 10-14, 2015 (Monday – Friday)

Summary:

There is high confidence in overall Good air quality through Thursday as a strong cold front approaches on Monday and moves eastward through the region on Tuesday. Pre-frontal clouds and rain will encroach into the region west to east early enough on Monday to limit ozone production across the Mid-Atlantic. Widespread rain will reach the western flank of the Mid-Atlantic overnight Monday into Tuesday morning, with a Tuesday morning washout for eastern parts of the region. Behind the front on Wednesday, the first true Canadian air mass in recent memory this summer will build in on northwesterly flow. This cool, dry, and clean air will keep ozone and particles in the Good range on Wednesday and Thursday. Temperatures will quickly rebound to average values on Friday as the center of high pressure settles over the region. There is uncertainty in the cloud cover forecast for the northern Mid-Atlantic on Friday, associated with the impacts of a back-door cold front. In the event that the front remains to the north on Friday, a return to Moderate ozone is expected under sunny skies and westerly transport.

Discussion:

The weather models are in very close consensus throughout most of the medium range period, but show some discrepancies on Friday regarding the next cold front to potentially impact the region. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a more zonal upper level flow is temporarily established over the Mid-Atlantic. An area of shortwave energy over the Northern Plains will get caught up in a trough digging out in northern ON, which is expected to dip into the Mid-Atlantic starting Monday night. The main area of shortwave energy in this trough over southern ON will be reflected at the surface as an area of low pressure located over Lake Ontario at 12Z Tuesday. At this time, the quickly amplifying upper level trough will reach well into GA. The frontal boundaries associated with the surface low – a warm front closely followed by a cold front – will sweep eastward across the Mid-Atlantic on Tuesday, preceded by widespread clouds and showers. 500 mb analyses for 12Z Wednesday show the large area of shortwave energy in this trough sprawled over the western New England/Canada border; 850 mb analyses show cooler, drier Canadian air spilling into the Mid-Atlantic behind the front; and surface analyses show a large area of surface high pressure centered over IA extending into the Mid-Atlantic. As the energy in the upper level trough begins to lift north- and eastward on Thursday, surface high pressure will continue to dominate the weather in the Mid-Atlantic. The trough will continue to lift northeastward on Friday and further allow surface high pressure to build into the region, but model differences arise regarding an approaching back-door cold front and its pre-frontal impacts. Both the EC and GFS show this front, but there are the usual timing differences. Whereas the faster GFS brings increasing cloudiness and rain showers to the northern Mid-Atlantic (NMA) on Friday ahead of the approaching front, the EC slows this boundary enough to keep Friday clear across the region. Due to the question of cloud cover, this discrepancy has an impact on the air quality forecast – however, since the day in question is the fifth day of the period, there is time for the models to come into closer agreement.

Monday will be a seasonably warm day across the region, with increasing cloudiness from west to east beginning around 15Z as the warm front lifts into the region, with the cold front close behind. The cloud cover will limit ozone to the Good range for most locations, with the exception being coastal areas of the Mid-Atlantic, where the clouds will reach last. This will allow those locations to see another day of mostly to partly sunny skies before the front reaches the Atlantic coast. Despite periods of sun along the coast, breezy southeasterly surface winds should be sufficient to limit ozone to the Good range for most locations. The air quality models are split this morning, with the NOAA model bringing ozone to the upper Moderate range along the northern I-95 Corridor (PHL to NYC) and mid-Moderate range to the south and west (BAL, DC, and PIT). In contrast, the BAMS and NCDENR models keep ozone in the Good range for tomorrow. The NOAA model has been overforecasting for the past several days, so we tend to discount its guidance for tomorrow, and go with generally Good ozone for the region. In the event that cloud cover is not as widespread as expected, some pockets of low-to-mid Moderate ozone are possible. Particles will increase across the western and central Mid-Atlantic, especially in PA, WV, and western MD, where humidity will be highest ahead of the cold front. Since rain is not expected until the overnight hours, PM2.5 will have ample time to rise into the Moderate range in the calm morning and light afternoon inland surface winds.

Rain will move eastward through the region overnight Monday into Tuesday morning, with the heaviest precipitation expected between 06-15Z Tuesday. Clouds will linger over the entire region throughout the day on Tuesday, inhibiting ozone production and keeping concentrations in the Good range. Particles will drop into the Good range as well, providing the strength of the rain is consistent across the region – as model guidance currently suggests. Southerly, onshore back trajectories lend confidence to this clean forecast for Tuesday.

The cold front will quickly move through the region. Wednesday will be a dry and mostly sunny day, with dew points dropping more than 10 °F compared to Tuesday. With the cold front now well to the east and out to sea, cool, dry, and clean Canadian air will filter into the region on northwesterly winds (at the surface and aloft) throughout the day. Because of this, the abundantly sunny skies and building surface high pressure will have little impact on the ozone forecast, and it will stay Good for another day. Particles should also remain in the Good range in response to the arrival of the clean and very dry summer air mass.

Surface high pressure centered over IL will continue to extend eastward and keep skies mostly clear over the Mid-Atlantic on Thursday. The Canadian air mass will have fully moved into the region at this point. Below average temperatures and fast, northerly back trajectories from interior ON will keep ozone and particles in the Good range across the eastern seaboard.

Temperatures will quickly rebound to average values on Friday as the upper level trough retreats northeastward. Atmospheric humidity will remain relatively low, however. There is some question about the arrival of the next cold front from the north. The GFS shows a faster approach of this cold front compared to the EC, and the GFS has pre-frontal cloud cover and light showers impacting the NMA on Friday. If this cloudy forecast does not verify, Moderate ozone seems likely, given a shift to westerly transport at the surface and aloft. However, the air mass over the Mid-Atlantic will still be sufficiently clean and unmodified to limit rising ozone to the low-to-mid Moderate range. Particles may increase into the Moderate range at scattered western locations due to westerly transport from the Ohio River Valley, but persistent low humidity will keep most locations in the Good range.

– Eherts/Huff