Daily Archives: July 19, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Wednesday July 19, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 19, 2017
Valid: July 20-24, 2017 (Thursday-Monday)

Summary:

A strong ridge will remain in place to our southwest into the weekend resulting in favorable conditions for ozone formation across much of the Mid-Atlantic. A weak frontal boundary will push southward to the MDL on Thursday as a MSC moves across the Ohio River Valley. With an approaching MCS, cloud cover for the central and eastern part of the NMA is in question as a few high clouds may move in ahead of the system. Aside from this, skies in locations south of I-80 are expected to be mostly sunny allowing temperature to reach into the low-mid 90s F. Due to these conditions and a large plume of smoke upwind, risk of an exceedance will remain High for Thursday with a focus on PIT and the I-95 corridor. The weather models diverge on Friday with the position of the stalled frontal boundary. The GFS (and WPC) keep the front furthest north near the MDL. Skies are expected to remain mostly sunny on Friday, reflective of large scale subsidence, which will enhance photochemistry. The back trajectories are westerly in the transport layer and high particle concentrations, suggestive of smoke effects, are currently observed in western PA. Mostly sunny skies, westerly flow aloft and upwind smoke will keep Appreciable risk on Friday. Through the weekend, stronger MCS development is expected with more cloud cover possible for the NMA. Although the location and timing of these MCS are still uncertain, most of the weather models suggest enough cloud cover and local instability to lessen the regional ozone into the Moderate range. Risk of an exceedance will lower to Marginal on Sunday and Monday with a focus on the SMA due to weak southwesterly flow aloft and mostly sunny skies.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. Although there is still come small disagreement late in the forecast period in regards to the strength of an upper level low, the weather models have come into closer agreement with the upper level synoptic set up over the medium range period. A key forecast issue will be the location of a frontal boundary expected to reach the NMA late Thursday. The EC and NAM are much more aggressive with this boundary than the GFS although, given the season and the strength of the upper level ridge, this solution may be less likely. It does, however, introduce uncertainty in the daily forecasts as will be noted below.

The upper level ridge will remain over the southern Great Plains/Mississippi River Valley at 12Z Thursday, allowing a shortwave to crest the ridge, and drop over the Great Lakes and into the NMA by 18Z Thursday. A mesoscale convective system will be driven by this shortwave, moving across the northern NMA into Thursday evening. The shortwaves will slightly weaken the ridge over the Mid-Atlantic at mid-levels, amplifying the ridge axis into the upper Midwest by 12Z Friday. At upper levels the ridge will build into the Tennessee River Valley and the western half of the SMA by 18Z Friday, driving the mid-level ridge axis over the Ohio River Valley by 00Z Saturday. The upper level ridge will weaken by 06Z Saturday, as shortwaves continue to crest the ridge across the Midwest and into the Mid-Atlantic. At the same time, the previously mentioned upper level low will begin to form as shortwaves move over MB/SK, progressing eastward, digging out an upper level trough over southwestern ON/MN by 00Z Sunday. The GFS and EC begin to diverge between 00Z and 06Z Sunday as the GFS develops more of an elongated trough compared to the EC, over the Great Lakes. By 18Z Sunday, the trough will be over the Great Lakes with the axis extending to the southwest across the upper Midwest. The difference between the GFS and EC begin to stand out Sunday as the strength of mid-level low varies. By 12Z Monday, however, both models show similar strength and placement of this feature at upper and mid-levels as the positively tilted trough moves over the Ohio River Valley and the axis moving into the Mid-Atlantic between 00Z and 06Z Tuesday.
The Dailies:

While high pressure builds over the SMA, a weak back door cold front will push into the NMA around 06Z Thursday, before stalling between I-76 and the MDL by 12Z Thursday. A mesoscale convective system (MCS) is modeled to push into the NMA between 18Z Thursday and 00Z Friday. The 03Z SREF shows precipitation staying to the north of I-80 and most likely after 00Z Friday. With an approaching MCS, cloud cover for the central and eastern part of the NMA is in question as a few high clouds may move in ahead of the system. Aside from this, skies in locations south of I-80 are expected to be mostly sunny allowing temperature to reach into the low-mid 90s F. Light/calm winds in the morning hours will shift to southwesterly and breezy across the region by 21Z Thursday. The 06Z air quality models are responding to these conditions by developing a few isolated areas of USG ozone along the I-95 Corridor, PIT and along the Ohio River. The models develop widespread Moderate ozone with scattered Good in between. Slow westerly back trajectories in the NMA will be troublesome as the smoke plume across the Midwest continues to become thicker from wildfires in BC. Since the air quality models do not factor in regional smoke, risk of an exceedance will remain High for Thursday with a focus on PIT and the I-95 corridor.

The models are significantly different with respect to the location of the frontal boundary Friday with the EC and NAM pushing the boundary south of DCA by Friday morning. The WPC leans more toward the GFS solution and keeps the boundary north of the MDL. Climatology also supports the weaker GFS solution. In any case, there is likely to be clear skies on Friday, reflective of large scale subsidence, which will enhance photochemistry. The back trajectories are westerly in the transport layer and high particle concentrations, suggestive of smoke effects, are currently observed in western PA. A few of the models are hinting at possible scattered showers across the NMA between 18Z Friday and 00Z Sunday which may explain why the 06Z air quality models are backing off on regional ozone, developing widespread mid-Moderate ozone. Developments in western PA today will provide information on the nature of the upwind air mass and its effect on the mid-Atlantic Friday. In the meantime, given the nature of the smoke threat, we will keep the risk of an exceedance at Appreciable.

On Saturday, the key issue will be the timing and strength of an MCS developing to our west. Although the bulk of the convection will remain north, there is the threat of significant mid-level clouds on Saturday. The GFS is the slowest with the MCS, bringing it into the NMA between 18Z Saturday and 00Z Sunday, while the NAM and EC bring it into the region as early as 12Z Saturday morning. This creases uncertainty in the forecast for cloud cover, precipitation and possibly flow aloft and at the surface. Higher ozone is expected with the GFS solution. Aside from this uncertainty, temperatures are expected to remain in the mid-90s F. Back trajectories shift to northwesterly flow; although this is typically a cleaner back trajectory, if the smoke plume remains in place over the Midwest, this could bring in a more smoke filled air mass. The BAMS air quality models seem to be siding more with the GFS as Moderate ozone remains over most of the NMA, with a swath of USG ozone along the I-95 Corridor. Risk of an exceedance will rise to Appreciable due to uncertainty in the forecast.

Similar issues remain on Sunday as the GFS and EC once again disagree on timing of precipitation. The GFS moves showers and thunderstorms through the NMA early in the period while the EC keeps unsettled conditions over the region throughout the day. In the SMA mostly sunny skies, above average temperatures and southwesterly back trajectories will be favorable for ozone formation. The 06Z BAMS models respond to the precipitation across the NMA, dropping ozone into the low Moderate/Good range, while in the SMA mid-to-upper Moderate ozone develops along I-81. Risk of an exceedance will drop to Marginal with a focus on central NC.

Widespread unsettled conditions will move into the Mid-Atlantic on Monday as a weak cold front pushes to the MDL by 12Z. Widespread showers and thunderstorms are expected to begin around 18Z Monday and last through 00Z Tuesday. Temperatures will drop into the low 90s/upper 80s F across the NMA and CMA while more scattered showers in the SMA will allow temperatures to remain in the mid-90s F. Southwesterly flow in the SMA will combine with breezy southwesterly flow at the surface and mostly sunny skies. Risk of an exceedance will remain Marginal with a focus on the SMA.

-Enlow/Ryan