Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Wednesday, June 29, 2016
Valid: June 30-July 4, 2016 (Thursday-Monday)
Summary:
The broad upper level trough over southeastern Canada continues to dominate the medium range, with shortwaves embedded within its flow pulling a pair of cold fronts across the Mid-Atlantic. The first cold front will shift off the Mid-Atlantic coast on Thursday, except for its southern extent which will remain stalled across NC and influence the southern Mid-Atlantic through the forecast period. High pressure moving in from the Ohio River Valley will promote clear skies and light winds across the northern and central Mid-Atlantic, but a northwesterly and presumably clean transport pattern aloft will limit rising ozone. As a result, the chance for an ozone exceedance will remain Slight for Thursday. Friday is looking increasingly wet for the eastern half of the region, where the formation of afternoon convection and thunderstorms is expected ahead of a second cold front approaching from the west. There is still some uncertainty, however, regarding the extent of this precipitation, which is further complicated by increased anthropogenic emissions due to the arrival of the Fourth of July weekend. Despite growing consensus for the development of precipitation, the chance for an ozone exceedance will rise to Marginal to account for uncertainty in the forecast. Saturday will be another post-frontal day for the northern Mid-Atlantic, with the second cold front pushing south of the Mason-Dixon Line and merge with the stalled front across NC, forming a new stationary front. Ample afternoon sunshine will encourage ozone production along the I-95 Corridor, but a clean northwesterly transport pattern aloft should tame rising ozone as it did on Thursday. The chance for an ozone exceedance will remain Marginal for locations in the vicinity on the stationary front, where near-front convergence could enhance ozone formation. Another area of weak high pressure will approach the region on Sunday, with another day of abundant sunshine expected across the northern and central Mid-Atlantic. Continued northwesterly transport, however, will have the same moderating effect, allowing the chance for an ozone exceedance to remain Marginal once again. There is some uncertainty in Monday’s forecast, due to questions regarding the track of upper level shortwave energy which could impact the northern half of the Mid-Atlantic during the afternoon. At this point in time, however, it appears that there will be sufficient afternoon sunshine for another day of ample ozone production along the I-95 Corridor. The chance for an ozone exceedance will remain Marginal for Monday as a result.
Discussion:
The weather forecast models remain in excellent agreement regarding the synoptic pattern through the medium range, with only slight differences near the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The upper level shortwave dug out over the northern Mid-Atlantic today will lift northeastward towards the Canadian Maritimes, with its axis reaching ME by 06Z Thursday, and pull the northern extent of its associated cold front away just off the Mid-Atlantic coast. A broad area of weak surface high pressure behind the front will build in from the Ohio River Valley, and envelop the northern and central Mid-Atlantic by 12Z Thursday. The southern extent of this front will remain stalled across NC on Thursday, and impact portions of the central and southern Mid-Atlantic for the entirety of the medium range. Beginning 06Z Friday, a closed circulation aloft will swing down across southern ON, pulling another cold front into the western edge of the forecast region by roughly 18Z. The NAM has come into better agreement with the GFS and EC in today’s guidance regarding the strength and track of the closed low, differing only slightly with the breadth of the cutoff. On Saturday, the closed circulation will progress slowly eastward, with its center only reaching southern QC by 18Z. The cold front will push southward and reach the Mason-Dixon by 12Z, and approach the stalled front still positioned across the southern Mid-Atlantic. By 12Z Sunday, these two frontal boundaries will merge into a single stationary front, which will orient itself in the vicinity of the NC/VA border. A mid-level ridge will build eastward from the Midwest throughout Sunday, and settle over the eastern U.S. by 00Z Monday. Concurrently, high pressure will build in again across the northern half of the forecast region on Sunday. The closed circulation aloft will fully lift northward away from the northeastern U.S. by 12Z Monday, returning the Mid-Atlantic to mostly zonal flow aloft. Upper level shortwave energy will ride this flow along the stalled front and stream into the Mid-Atlantic Monday afternoon. The EC is slightly faster and more southerly with the progression of this energy, bringing into the central Mid-Atlantic by 18Z Monday. The GFS, on the other hand, shows a slower, more organized which does not reach the northern Mid-Atlantic until 06Z Tuesday.
Approaching surface high pressure will promote clear skies and subsiding winds on Thursday across the northern Mid-Atlantic, primarily north of the MDL. The stalled front across NC will trigger widespread clouds and precipitation throughout the day, especially during the afternoon when diurnal heating and low level convergence near the front is expected to fire up convection and scattered thunderstorms. The weather forecast models are split in terms with the northern extent of this afternoon precipitation. The NAM and EC, as well as the 06Z 4km NAM, keep the majority of the precipitation across southern VA and NC through 00Z Friday, while the GFS shows the development of showers and thunderstorms up through the central Mid-Atlantic. Though the GFS has a known wet-bias, the 03Z SREF and the current WPC surface analysis shows a similar precipitation pattern, which lends credence to the GFS solution. Regardless, the majority of the central and southern Mid-Atlantic is expected to remain relatively clean for Monday, with the only concern being locations just north of the stalled frontal boundary, where near-front convergence could locally enhance ozone production. The northern Mid-Atlantic will experience more substantial sunshine, with temperatures climbing to the mid-80s °F, which historically is sufficient for ample ozone formation. However, 06Z GFS back trajectories for the I-95 are northerly and presumably clean. The main question will be if afternoon sun can promote enough local production to overwhelm a clean regional transport pattern. The 06Z air quality models have the clean transport winning out, with only patches of Moderate ozone modeled along the I-95 Corridor. At this point in the time, the chance for an ozone exceedance will remain Slight for the I-95 Corridor, as well as areas just north of the stationary front.
Friday is looking increasingly wet for the Mid-Atlantic, with growing support for the development of shower and thunderstorms across the eastern Mid-Atlantic during the afternoon. As surface winds shift westerly across the Ohio River Valley ahead of the approaching cold front, a lee trough will begin to develop, allowing winds downstream of the Appalachian Mountains to shift southerly. This southerly flow will advect heat and moisture northward into the central and northern Mid-Atlantic into the afternoon, destabilizing the lower levels of the atmosphere in these locations. Lifting ahead of the cold front, which will reach the western edge of the forecast region around 18Z, will be sufficient to trigger clouds and thunderstorms. All of the deterministic models reflect this, showing precipitation all along the eastern seaboard by 18Z Friday. The 03Z SREF also shows this pattern, with very high probabilities of precipitation downwind of the Appalachians at this time. The air quality guidance, however, are having trouble resolving the impacts of this late day convection. The BAMS-MAQSIP-RT and the NCDENR air quality model seem to be accounting for this precipitation, with only upper Good and low Moderate ozone throughout the eastern Mid-Atlantic. The BAMS CMAQ, however, shows widespread Moderate ozone conditions in these locations. This is not an entirely unfathomable solution, given that temperatures will be high enough that if convection did not develop or was more isolated than expected, ozone levels would rise rapidly especially along the I-95 Corridor. There is also the concern about anthropogenic emissions, which will be anomalously high due to increased travel at the beginning of the Fourth of July weekend. It is likely that that any ozone exceedances would be isolated, solely in locations which remain under mostly full sunshine during the afternoon. Given the uncertainty, the chance for an ozone exceedance will rise, but only to Marginal.
Saturday will be another post-frontal day for the northern Mid-Atlantic, reminiscent of Thursday’s frontal passage. The central and southern Mid-Atlantic will remain under persistent cloud cover, with afternoon showers developing along the new frontal boundary across the VA/NC border formed by the collision of the cold front and the old stationary front from earlier in the period. Ample afternoon sunshine will encourage ozone formation throughout the northern Mid-Atlantic, but slight mid-level cooling and northwesterly transport behind the front should be sufficient to tame rising ozone. The primary area for increased ozone concentrations will be along and just north of the stalled front, where nearby convergence could locally enhance ozone productions, as was the case on Thursday. The BAMS model currently shows a band of Moderate ozone in the vicinity of this front on Saturday, with a small area of USG ozone along the VA coast. The main forecast question will the placement of this stalled frontal boundary, which will determine where the highest ozone levels will be experienced across the forecast region. If the new stationary boundary sets up farther north, possibly due to a slower cold front, the chances for increased ozone would shift northward toward the I-95 Corridor. The chance for an ozone exceedance on Saturday will remain Marginal for locations in the vicinity of the stalled front.
On Sunday, the stationary front is expected to remain across northern NC, supporting another day of clouds and precipitation along and south of the frontal boundary. The northern and central Mid-Atlantic will experience full sunshine and lighter winds, as high pressure approaches the region from the west. Back trajectories for the I-95 Corridor remain northwesterly and relatively fast, however, which again should act to limit rising ozone. This leaves locations along and just north of the stalled front as the primary areas for increased ozone concentrations once again. The BAMS model is showing Moderate ozone near the front. The chance for an ozone exceedance will remain Marginal on Monday as a result.
Monday’s forecast will be largely dependent on the speed and track of upper level shortwave energy which is expected to stream into the Mid-Atlantic during the afternoon, along the stalled front. As mentioned above, the deterministic models diverge in their representation of this energy, with the GFS bringing it directly over the central Mid-Atlantic by 18Z. This solution would trigger clouds and thunderstorms across the central Mid-Atlantic, as well as the southern Mid-Atlantic which will still be under the influence of the stalled front. The EC solution, however, would spare the northern and central Mid-Atlantic of the influence of this upper level energy through 06Z Tuesday, allowing for another day of ample ozone production. High pressure settling directly over southeastern PA by 12Z will promote clear skies and subsiding winds, and back trajectories for the I-95 Corridor are short and westerly, all of which will work to amplify ozone formation. At this point in time, we will err on the side of caution and side with the EC solution. The chance for an ozone exceedance will remain Marginal for Monday.
-Brown/Ryan