Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 7, 2015
Valid: August 8-12, 2015 (Saturday – Wednesday)

Summary:

Overall Good air quality is expected to prevail throughout the medium range period in the Mid-Atlantic due to a relatively progressive pattern and only intermittent periods of calm, sunny conditions. The recent washout in the central and southern Mid-Atlantic, along with breezy northeasterly winds, will limit ozone mostly to the Good range on Saturday. Because of the anticipated sunny skies and lighter winds inland, the exception will be in areas of increased precursors directly downwind of the I-95 Corridor. Sunday will be another dry day, but possible cloud cover in the northern Mid-Atlantic and lower Sunday emissions will keep ozone mainly in the Good to low Moderate range once again. Throughout the entire weekend, particles are only expected to reach into the Moderate range at inland locations where winds will be lightest and humidity will be higher. Monday will see the arrival of the next disturbance, bringing afternoon clouds and showers eastward across the region, with Good ozone expected for most locations. Due to the light nature of the precipitation, as well as southerly shifting surface winds, PM2.5 will likely continue in the Moderate range in the western Mid-Atlantic. Immediately following Monday’s disturbance – a relatively weak surface low – an extensive cold front will sweep west to east across the eastern seaboard, bringing widespread clouds and showers to the region on Tuesday. Model discrepancies lend uncertainty to the exact precipitation forecast, but regardless, the passage of the cold front will continue the trend of Good ozone. PM2.5 will thrive on the increased humidity ahead of and along the front, depending on the strength of the precipitation. Skies will clear quickly behind the frontal passage on Wednesday morning, but a cooler, drier, and clean Canadian air mass is expected to arrive behind the front, which will return air quality to the Good range across the region.

Discussion:

The weather models are in generally close consensus throughout the beginning of the medium range period, but begin to differ on Monday. However, the discrepancies mainly occur aloft, and they do not have a large impact on the air quality forecast or its confidence. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, an upper level trough axis along the eastern seaboard with an associated area of shortwave energy over the Carolinas is the source of widespread showers and cloud cover. While the showers are limited to coastal areas south of the Mason-Dixon Line (MDL) this morning, the clouds spread from the Atlantic coast to the Appalachians. As the trough continues moving eastward today, the area of shortwave energy will organize enough to give some structure to its resulting surface low that will move northeastward and away from the coast by 12Z Saturday. This will allow an upper level ridge currently centered over the Plains to briefly build into the Mid-Atlantic for Sunday, which will be a clear and very calm day throughout the region. By Monday morning, a cluster of shortwave energy over the Great Lakes is resolved by all of the global weather models, and develops a surface low, but there are discrepancies regarding its strength and exact extent of associated precipitation across the Mid-Atlantic. Since the timing of this system is virtually identical for the GFS and the EC, the confidence of the air quality forecast is not impacted substantially by this difference in shortwave characteristics, with the bulk of precipitation likely remaining west of I-95 through the late afternoon. The models have come into closer consensus on the precipitation forecast for Monday and Tuesday, with the previously drier EC actually surpassing the GFS on the extent of the rain. The EC has completely backed off on yesterday’s solution of forming a nor’easter like coastal low and now has a traditional cold frontal passage on Tuesday, similar to the GFS. Both models analyze a large upper level trough digging southward into the northeastern US in a winter-like fashion, with the southernmost edge of the trough reaching as far south as southern GA on Tuesday. All in all, the models show a very similar solution for the end of the period, with a widespread area of pre-frontal rain moving eastward across the entire Mid-Atlantic from 18Z Monday to 06Z Wednesday. This looks to be a substantial cold front, with both the EC and GFS showing a pool of cooler Canadian air filtering into the Mid-Atlantic on Wednesday behind the frontal passage.

The clouds and showers currently impacting the central and southern Mid-Atlantic (CMA, SMA) will linger along the NC coastline through Saturday morning. Except for these showers and some scattered, topographically-driven showers in western MD and WV, Saturday will be a mostly sunny day throughout the region. Back trajectories from eastern New England, breezy northeasterly winds along the coast, and the recent washout will limit ozone to the Good range for most of the Mid-Atlantic. The air quality models haven’t been handling the impacts of the surface wave well, with substantial over-forecasting of ozone yesterday and (one hopes) today along I-95. This lends much less confidence to their guidance for tomorrow, which features scattered Moderate ozone downwind of I-95 and in the Pittsburgh metro area. Nevertheless, the I-95 Corridor will act as the usual area of increased precursor buildup on Saturday. Combined with the abundant sunshine and lighter surface winds inland, there is a chance for isolate Moderate ozone along and downwind of the interstate. Particles may see some buildup in these areas as well, but it will be limited to the low Moderate range due to the low humidity.

Sunday will be a calm and mostly sunny day as surface high pressure prevails over the Tennessee River Valley (TRV). However, some of the high-res mesoscale weather models are resolving some pop-up afternoon showers along the eastern edge of the Appalachians in the CMA and northern Mid-Atlantic (NMA), as well as widespread cloud cover. Depending on the optical thickness of these clouds and the characteristics of the northerly air building into the region on the western flank of the coastal low, ozone will likely be limited to the mostly Good range. The exception, according to the air quality forecast models, is a small pocket of possible low Moderate ozone in DC/along the southern coast of MD. We suspect that, given the full sun and very light to calm surface winds, area of Moderate ozone may be more scattered than suggested by the air quality model guidance. The calm winds will increase PM2.5 across the region, with the highest concentrations in the western Mid-Atlantic where humidity will be the highest.

Increasing clouds throughout the day on Monday ahead of an approaching surface low will limit ozone to the Good range for most of the region. Swaths of low Moderate ozone are possible east of I-95, where the clouds will reach last, but potentially breezy southerly surface flow may limit ozone to the Good range in these locations as well. The southerly will raise dew points and promote Moderate PM2.5 from the I-95 Corridor to the edge of the Appalachians, where only light rain will fall on Monday.

The surface low that sourced Monday’s rain will move across southern PA overnight Monday into Tuesday morning, reaching the PHL metropolitan area by 12Z Tuesday. At the same time, a low pressure center will pass well to our north – along the northern NY/Canada border – and drop an extensive cold front through the region immediately following the passage of the surface low. Although there are discrepancies between the global weather models as to the exact extent of the resulting precipitation, the combination of these two rain-promoting disturbances will keep clouds and at least scattered showers over the region through Tuesday night. The lack of sunlight will inhibit ozone production, keeping it in the Good range across the board. Particles may linger in the Moderate range in scattered locations until a cooler and drier Canadian air mass builds in on Wednesday. Areas that see heavier rain will observe PM2.5 concentrations dropping into the Good range.

Skies will be clear across the eastern seaboard by 12Z Wednesday behind the passage of Tuesday’s cold front. Dew points will decrease throughout the day as northwesterly surface winds filter in cleaner, drier air behind the front. Based on the 850 mb model guidance, Good air quality is expected across the entire Mid-Atlantic on Wednesday. However, if the air does hold some precursors, the full sun and light winds may be sufficient to allow Moderate ozone downwind – to the southeast – of the I-95 Corridor. Overall, however, Tuesday’s washout will be enough to limit concentrations to the bottom half of the Moderate range.

– Eherts/Huff