Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 6, 2017
Valid: July 7-11, 2017 (Friday-Tuesday)

Summary:

Friday will be a mixed bag of unsettled and clear conditions as an approaching cold front and lingering wave of low pressure impact the Mid-Atlantic. A round of morning/early afternoon showers across most of the region will give way to clear skies and near normal temperatures, allowing for some ozone formation to take place, mainly in the western part of the region, where clearing will occur first. As a result, the risk of an exceedance will remain Marginal on Friday with a focus on the western NMA. A weak cold front will push into the Mid-Atlantic on Saturday, resulting in scattered showers and some clouds in the NMA and CMA. At this time, it appears that the front will reach as east as the I-85 Corridor in the SMA, as air quality models develop a line of elevated ozone in this area. A mixture of clear skies, a slowing front and slow westerly flow in the SMA will be favorable for ozone formation, keeping the risk of an exceedance at Marginal for Saturday. The slowing cold front will stall along the southern Atlantic coast on Sunday as high pressure builds over the Mid-Atlantic. This high pressure will bring widespread clear skies across the region, but a cooler and relatively dry air mass will keep temperatures slightly below normal. Clear skies will allow for ozone formation to take place, but a swift northwesterly flow aloft will should keep ozone accumulation minimal, bringing the risk for an exceedance down to Slight. High pressure will continue to build over the Mid-Atlantic on Monday and move offshore in the afternoon, resulting in clear skies and above average temperatures. Despite uncertainty between the air quality models, risk of an exceedance will rise to Marginal on Monday, with a focus on the I-85 and I-95 Corridors. There is a lot of uncertainty for Tuesday’s forecast as the weather models continue to disagree on the southern extent of an upper level low and its associated cold front. Onshore flow across the SMA will be enough to keep ozone in the Moderate range but the main forecast question is if precipitation associated with the cold front will be able to reach locations along the I-95 corridor in the NMA and CMA. If precipitation is not able to push to the I-95 Corridor, conditions will be favorable for ozone formation, keep the risk of an exceedance at Marginal for Tuesday.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models continue to be in consensus with placement and progression of key atmospheric features until 00Z Monday. Similar to yesterday, the NAM outcome is similar to the GFS and EC but looks to be more aggressive with the strength of shortwaves moving across the Midwest/Great Lakes/Mid-Atlantic throughout its run period. By 12Z Friday, a series of shortwaves, which will have recently phased with a large upper level trough over the eastern U.S., will move over the Mid-Atlantic along the trough axis. These shortwaves will move offshore by 00Z Saturday as another series of shortwaves drop down across the upper Midwest/Great Lakes, rotating through the trough axis. The trough axis will remain over the Mid-Atlantic through at least 06Z Monday as shortwaves continue to develop over the Great Lakes and the trough center spins in place over eastern Canada. Similar to yesterday’s runs, the GFS and EC begin to diverge by 06Z Monday regarding the southward extent of the upper level trough over the eastern U.S. Unlike yesterday, the GFS and EC seem to have swapped solutions. In today’s runs, the GFS is farther south and east with the trough, while the EC keeps it farther west and north. The GFS drops the center of circulation to the southern tip of James Bay on Monday, while the EC keeps the center northward over Hudson Bay. The southward push of the GFS keeps the southern extent of the trough over the Mid-Atlantic through the end of the medium range period. The EC solution keeps the center of circulation over Hudson Bay, allowing more of a zonal flow to build over the Mid-Atlantic by 00Z Tuesday. Despite the differences in the placement of the upper level trough, a shortwave is modeled to sweep across the Mid-Atlantic between 12Z and 18Z Tuesday, keeping some lingering troughiness over the region into Wednesday. The main difference in the model solutions for air quality is the southward extent of the next weak cold front that may impact the NMA on Tuesday.

The Dailies:

Friday will be a mixed bag of unsettled and clear conditions as an approaching cold front and lingering wave of low pressure impact the Mid-Atlantic. The lingering wave of low pressure will bring a round of morning/early afternoon showers to much of the Mid-Atlantic, especially the eastern portions. Precipitation will begin in locations along the Appalachian Mountains around 12Z Friday, reaching the Atlantic coast by 00Z Saturday. As the precipitation progresses eastward, skies will clear from west to east ahead of an approaching cold front. Clear skies will allow temperatures to reach near normal across the region. The 06Z air quality models are picking up on this clearing as the models are in agreement with Moderate ozone developing in the western NMA, where skies will clear first. There continues to be disagreement between the models in the eastern NMA and CMA as the MAQSIP BAMS model continues to develop mid-to-high Moderate ozone whereas the NC and CMAQ BAMS models keep ozone in the upper Good/low Moderate range. The BAMS MAQSIP is most likely higher than the other models due to slow southwesterly back trajectories and periods of clear skies in the afternoon for locations along the I-95 Corridor. The air quality models are in agreement that the rest of the Mid-Atlantic will see ozone in the Good range as precipitation and cloud cover will move across the CMA and SMA during the peak heating hours of the day. Due to these factors, risk for an exceedance will remain Marginal with a focus on the western NMA, and to a lesser extent the I-95 Corridor, due to clear skies during the afternoon/evening hours.

Friday’s approaching cold front will push into the NMA and CMA by 12Z Saturday. Although not much precipitation is expected with this frontal passage during the day on Saturday, the 03Z SREF indicates that a few scattered showers cannot be ruled out, mainly across the NMA starting around 09Z and becoming less likely throughout the day. Mostly cloudy skies are expected along the western half of the Mid-Atlantic in the morning hours but are expected to clear quickly after 15Z. Clear skies will allow temperatures in locations east/south of the front to rise above normal, while a shift to northwesterly/westerly flow will bring temperatures behind the front to below normal. At this time, it appears that the front will push to the I-85 Corridor in the SMA by 00Z Sunday. The 06Z air quality models respond to the frontal passage by developing a band of Moderate ozone along the boundary, while locations behind the front drop into the Good range. A mixture of clear skies, a slowing front and slow westerly flow in the SMA will be favorable for ozone formation, keeping the risk of an exceedance at Marginal for Saturday.

As the cold front stalls along the Atlantic coast by 12Z Sunday, high pressure will build over the Mid-Atlantic. This high pressure will bring widespread clear skies across the Mid-Atlantic, but a cooler and relatively dry air mass will keep temperatures slightly below normal. Clear skies will allow for ozone formation to take place, but a swift northwesterly flow aloft should keep ozone accumulation minimal, despite diminishing surface winds. The 06Z air quality models show widespread Good ozone across the region, with a few isolated areas of Moderate ozone developing in populated locations in the CMA and SMA. As a result of below average temperatures and the arrival of a clean air mass on northwesterly flow aloft, risk for an exceedance will drop to Slight.

Much like Sunday, high pressure will continue to build over the Mid-Atlantic on Monday and move offshore in the afternoon. A mixture of clear skies, above average temperatures, and recirculating back trajectories are showing up in the 06Z air quality models as ozone level increase across the region but are focused on locations along the I-85/I-95 Corridors. The BAMS models develop upper Moderate ozone along the I-95 Corridor, while the NC air quality model keeps ozone in the low Moderate range. Due to the uncertainty between the air quality models and generally favorable ozone-producing conditions, risk for an exceedance will rise to Marginal on Monday.

There is uncertainty in the weather models for Tuesday’s forecast as the progression and strength of the next cold front is in question. As mentioned in the Model Discussion, the EC and GFS disagree on the southern push of an upper level trough. This disagreement impacts the timing of precipitation associated with the front that is expected to occur across the NMA on Tuesday. If the GFS solution verifies, precipitation could progress as far south as the MLD, whereas the EC solution would keep precipitation in northern and western PA. Clear skies and above average temperatures ahead of the precipitation will be favorable for ozone formation. Onshore flow across the SMA will be enough to keep ozone in the Moderate range. So the main forecast question is if precipitation will be able to reach locations along the I-95 corridor in the NMA and CMA by Tuesday afternoon. Given the uncertainty in the forecast, risk of an exceedance will remain Marginal with a focus on the I-95 Corridor.

-Enlow/Huff