Daily Archives: July 12, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Wednesday July 12, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 12, 2017
Valid: July 13-17, 2017 (Thursday-Monday)

Summary:

The risk for an exceedance remains High on Thursday and Appreciable on Friday before a pair of weak cold fronts lowers the risk of an exceedance to Marginal/Slight through the end of the medium range period. Thursday will be the hottest day of the period, and the available air quality models continue to highlight Thursday as the best chance for USG ozone along the I-95 Corridor. The main forecast question will be the southward and eastward push of precipitation associated with a frontal boundary draped across NY State; precipitation is expected to begin around 15Z and move from northwest to southeast, reaching the I-81 Corridor by 18Z Thursday and the I-95 Corridor after 00Z Friday. Aside from the clouds/precipitation forecast, almost all other factors are favorable for USG ozone (a third consecutive day of temperatures > 90 °F, light SW/W surface winds, the presence of a lee trough, W/SW flow aloft, and Bermuda High pressure centered in the vicinity of western NC). A weak cold front will push into the NMA on Friday, reaching I-95 by roughly 00Z Saturday. The front will bringing showers and cloud cover to most of the NMA and parts of the CMA. If the front slows down, or the precipitation is less widespread or later than expected, another day of mid-to-upper Moderate and isolated USG ozone is possible, especially at locations south and east of I-95. In the SMA, mostly sunny skies and temperatures nearing 100 °F will be favorable for ozone formation but till be countered by onshore back trajectories. Relief from the heat and humidity will arrive to the NMA and the northern CMA on Saturday as Friday’s cold front continues to push through the Mid-Atlantic to eastern NC, where it will stall through Sunday. A change in air mass across the NMA/CMA and unsettled conditions in the vicinity of the front in the SMA will bring risk of an exceedance down to Slight for Saturday. Sunday will be mostly sunny for much of the region. Mostly sunny skies with calm/light southwesterly winds will be favorable for ozone formation, but clean northerly transport from ON and lower Sunday emission should keep ozone from rising too quickly, with a Marginal chance of an exceedance. Unsettled conditions will impact most of the Mid-Atlantic on Monday as the frontal boundary in the SMA moves northward and another weak cold front moves into the NMA from the northwest. There is uncertainty in the precipitation forecast, with clear skies possible along and east of I-95, where rising ozone is possible, keeping the chance for an exceedance Marginal.

 

 

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 close agreement through 00Z Monday, at which time the GFS and EC diverge slightly with the strength of an upper level trough and strength of associated shortwaves moving over the eastern U.S. At 12Z Thursday, the Mid-Atlantic will be under zonal flow, with the westward extension of the Bermuda High building over the SMA and Southeast U.S. at mid-levels, and a developing upper level trough to the north over western ON/MN. Shortwaves moving over western ON/MN will fuel the upper level trough as it moves over the northern Great Lakes by 00Z Friday. The weak upper level trough will then phase with a much larger longwave trough that is over eastern Canada by 12Z Friday. As this trough phases and continues to drop southward, the shortwaves will interact with a frontal boundary draped across the Great Lakes and develop a surface low pressure in southern ON by 12Z Friday. This solution is slightly different from previous model runs this week; instead of a southward moving back door cold front, the developing low in the Great Lakes Thursday is strong enough to make for a more traditional northwest to southeast oriented cold frontal passage late Friday. The upper level trough will continue to progress eastward, with the axis moving over the Mid-Atlantic by 06Z Saturday, pulling the front into the Mid-Atlantic. By this time, the trough will be reaching into central VA with the help of an amplified ridge to the west over the Rocky Mountains. By 00Z Sunday, the amplified upper level trough will weaken and move to the northeast, while a shortwave developing over the SMA will keep some lingering troughiness over the Mid-Atlantic. At the same time, another series of shortwaves will develop over Lake Superior and reinforce the upper level longwave trough over the Great Lakes by 00Z Monday. This trough will slowly progress eastward throughout the day on Monday, moving into the western Mid-Atlantic by 00Z Tuesday. A combination of this primary upper level trough and the shortwaves still stretching across the Southeast and SMA will keep troughiness over the Mid-Atlantic through the end of Monday. The EC has a slightly stronger trough compared to the GFS, with more concentrated shortwave energy centered over MI/southeastern ON. In contrast, the GFS spreads out the shortwave energy across the Ohio River Valley, southeastern ON, and southern QC.

The Dailies:

Thursday will be the hottest day of the medium range period as temperatures rise into the upper 90s °F across the Mid-Atlantic. A frontal boundary draped west-east over NY State will bring periods of mostly cloudy skies and scattered showers across most of the NMA on Thursday. As shown in the 03Z SREF, precipitation is expected to begin around 15Z and move from northwest to southeast, reaching the I-81 Corridor by 18Z Thursday and the I-95 Corridor after 00Z Friday. The primary forecast question for Thursday will be the southward and eastward push of clouds and precipitation, with a focus on the I-95 Corridor. In the SMA, dew points in the low 70s °F and temperatures nearing 100 °F will make for an uncomfortable air mass under mostly sunny skies. A few clouds may form in the afternoon across the SMA, but clouds will not be thick enough to impact ozone formation. A combination of these factors with westerly transport aloft will make favorable conditions for ozone formation in locations south/east of where the precipitation sets up. The 06Z air quality models (BAMS and NC) are in consensus in developing USG ozone for Thursday. The NOAA air quality model is still unavailable at this time and was not consulted for this forecast. As expected, the air quality models highlight the I-95 Corridor from BAL to central NJ, with areas of USG ozone from BAL and the eastern shore of MD, across DE, and along and east of I-95 to central NJ. The BAMS models also have a strip of USG ozone in northern NJ. The air quality models are also in agreement that the I-81 Corridor in the SMA will see low-mid Moderate ozone, with upper Good ozone developing in the rest of the Mid-Atlantic. Factors favoring USG ozone along I-95 include a third consecutive day of temperatures > 90 °F, light SW/W surface winds, the presence of a lee trough, W/SW flow aloft, and Bermuda High pressure centered in the vicinity of western NC. Factors limiting USG ozone will be chances for late afternoon thunderstorms, relatively vigorous vertical mixing (e.g., 2 km at KPHL), and questions about how polluted the residual layer will be. As a result of these factors, with more weight to USG ozone favorable conditions, the risk of an exceedance will remain High for Thursday with a focus on the I-95 Corridor.

A cold front will push into the NMA on Friday afternoon, bringing showers and cloud cover to most of the NMA and parts of the CMA. The 03Z SREF shows that precipitation will begin just to the south of I-80 in PA and push south and east, reaching the CMA and I-95 Corridor by 21Z and dissipating sometime after 00Z Saturday. Despite periods of rain occurring throughout the day in the CMA and NMA, breaks in the clouds and westerly/southwesterly flow aloft will allow temperatures to crawl back into the mid-90s °F, ahead of the frontal passage. Light surface winds across the region will increase to breezy in the afternoon hours as daytime heating will allow for considerable vertical mixing (e.g., 2 km). The 06Z air quality models are developing lower regional ozone compared to Thursday in response to the increased chances for precipitation. Once again, however, the BAMS models are highlighting the I-95 corridor as the location for the highest ozone in the region, with mid-to-upper Moderate ozone in this area. The NC model is showing widespread Good ozone with a few areas of isolated Moderate ozone developing across the SMA and CMA. The hi-res models are mixed on precipitation along I-95 in the afternoon. The 06Z 13 km GFS has scattered precipitation across the NMA by 21Z, while the 06Z 3 km NAM keeps a line of thunderstorms west of I-95 until 00Z. If the front slows down, or the precipitation is less widespread or later than expected, another day of mid-to-upper Moderate ozone is possible, especially at locations south and east of I-95. Another area for possible Moderate ozone is the western NMA, which will be to the west of where precipitation is expected during the middle of the day. Both BAMS models develop low-to-mid Moderate ozone in this area, whereas the NC model has upper Good ozone. This is most likely in response to less cloud cover and precipitation occurring during the early/late morning hours. The air quality models are showing widespread Good ozone with scattered Moderate across the rest of the region. As a result of these factors, risk of an ozone exceedance will lower but remain Appreciable on Friday, with a focus on the I-95 Corridor.

Relief from the heat and humidity will arrive to the NMA and the northern CMA on Saturday as Friday’s cold front continues to push through the Mid-Atlantic to eastern NC. Despite some disagreement between the GFS and EC with the strength of this frontal passage, temperatures behind the front will drop to slightly below/near normal while dew points drop into the mid-60s °F. For locations in the SMA, the heat and humidity will stick around as the front is expected to slow and stall as it reaches the central SMA. Mostly cloudy skies, near average temperatures and northwesterly flow aloft will limit ozone formation across the NMA. As the front stalls in the SMA, the developing overhead shortwaves will interact with this front and result in cloud formation with the possibility for scattered showers. Although a recent frontal passage will bring in a new air mass, breaks in the clouds/rain, westerly flow and near average temperatures will allow for some ozone formation across the CMA and parts of the SMA. This is appearing in the 06Z air quality models as widespread upper Good ozone gives way to a few more isolated Moderate ozone areas across the SMA. The BAMS models do however, highlight a swath of low-to-mid Moderate ozone across the CMA. This could possibly be in response to weakening, converging winds between 21Z Saturday and 00Z Sunday. Given the expected effects of the frontal passage, risk for an exceedance will lower to Slight on Saturday.

Sunday will be unsettled across the SMA as overhead shortwaves interact with Friday’s stalled frontal boundary and develop a wave of low pressure. Temperatures across the SMA will remain in the mid-upper 90s °F with light/calm winds. In the NMA, Sunday will be dry with temperatures remaining near normal. Mostly sunny skies with calm/light southwesterly winds will be favorable for ozone formation, although lower Sunday emission should keep things from getting out of hand. The 06Z air quality models are picking up on these conditions as the BAMS models develop widespread Moderate ozone across the NMA, with a hint of isolated USG in the BAL metro area. The NC model is not as aggressive but still develops low-to-mid Moderate ozone in the eastern NMA. These factors will bring risk of an ozone exceedance up to Marginal for Sunday.

Unsettled conditions will impact most of the Mid-Atlantic on Monday as the frontal boundary in the SMA begins to move northward and another weak cold front moves into the NMA from the northwest. There are some differences between the GFS and EC in regard to the precipitation forecast, as the GFS brings more widespread precipitation earlier into the region than the EC. The GFS has precipitation developing in central NMA by 18Z Monday, expanding over the majority of the NMA by 00Z Tuesday. The weaker EC, only develops scattered showers across the NMA with precipitation developing in the eastern NMA by 12Z Tuesday. Despite the differences between the GFS and EC, scattered showers and partly cloudy skies across the central NMA will limit ozone in locations to the west of I-95, while mostly sunny skies and recirculating back trajectories will be favorable for ozone accumulation along and east of I-95. Risk of an ozone exceedance will remain Marginal for Monday because of the uncertainty in the precipitation forecast.

-Enlow/Huff