Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 28, 2016
Valid: June 29-July 3, 2016 (Wednesday-Sunday)

Summary:

The medium range continues to be influenced by a broad upper level trough positioned over southeastern Canada, which will be reinforced by successive two shortwaves. This pattern is generally not conducive to significant ozone formation. The first shortwave, which is pulling a cold front across the Mid-Atlantic today, will be slow to depart on Wednesday, and will remain stalled along the eastern U.S. coast. Most of the forecast region will be post-frontal, allowing for mid-level cooling and clean, northwesterly surface winds across the northern and central Mid-Atlantic. The chance for an ozone exceedance on Wednesday will remain Slight as a result. Widespread clearing is expected on Thursday, as surface high pressure pushes in from the Ohio River Valley. A fast northerly transport pattern, however, will limit rising ozone and keep the chance for an ozone exceedance Slight once again. There is considerable uncertainty in Friday’s forecast, primarily due to uncertainty regarding precipitation. The weather forecast models show a northward surge of precipitation Friday afternoon, associated with a retrogressing stationary front across NC, but the extent and strength of this precipitation remains a question. If the I-95 Corridor remains mostly sunny through the afternoon, ozone levels will rise rapidly, further enhanced by high anthropogenic emissions due to the start of the Fourth of July weekend. The chances for an ozone exceedance will rise to Marginal on Friday as a result. On Saturday, a new cold front will push southward into central Mid-Atlantic and join the stalled front, causing widespread clouds and precipitation south of the Mason-Dixon Line. However, a shift to westerly transport for the I-95 Corridor, paired with post-frontal clearing will keep the chances for an ozone exceedance Marginal on Saturday. On Sunday, another weak area of surface high pressure will build into the region, promoting sunny skies and subsiding winds throughout much of the Mid-Atlantic. A shift to northwesterly back trajectories for the I-Corridor, however, should limit ozone to the Moderate range along the I-95 Corridor. Therefore, the chance for an ozone exceedance will remain Marginal.

Discussion:

The weather forecast models remain in close agreement regarding the synoptic pattern through the medium range, except for slight differences with the location and intensity of a cold front impacting the Mid-Atlantic late Friday and into Saturday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad upper level trough currently scooped out across southeastern Canada and the northeastern U.S. will remain in place on Wednesday, with several shortwaves embedded within its flow. In particular, the shortwave pulling a cold front through the Mid-Atlantic today will remain overhead through 12Z Wednesday, allowing the cold front to remain stalled along the eastern U.S. coast. By 18Z, this shortwave will begin to lift northeastward toward the Canadian Maritimes, allowing the front to clear the northern and central Mid-Atlantic, but the southern extent of the front from will remain stalled across NC. Weak surface high pressure will build in from the Ohio River Valley and extend across the northern and central Mid-Atlantic by 12Z Thursday and remain in place through Friday. The closed circulation aloft will slowly churn down across ON throughout Thursday, reinvigorating the broad trough in place. The NAM currently shows a deeper and more slowly progressing cutoff, but this solution is unlikely given that this same NAM bias has been observed in the last few upper level shortwaves which have impacted the region. The GFS and EC are in agreement, however, quickly progressing the cutoff eastward on Friday and through the weekend. This forecast discussion follows the GFS and EC for the most part. This cutoff will pull another cold front, albeit faster and weaker, in from the northwest into the Mid-Atlantic by roughly 18Z Friday, with the front reaching the Mason-Dixon Line by 12Z Saturday. The stalled front still positioned across NC will shift back northward as its associated shortwave energy gets absorbed into the cutoff, whose axis will directly be over the Northeast on Saturday. These two frontal boundaries will pinch off the lingering surface high pressure from Friday, and conjoin into a single stalled front positioned in the vicinity of the NC/VA border during the latter half of Saturday, where it will remain in place through Sunday. Another weak surface high will begin to build in from the Great Lakes behind this second cold front.

On Wednesday, the cold front is expected to reach the eastern seaboard, allowing most of the Mid-Atlantic to be post-frontal. The only exception will be the southern Mid-Atlantic, where the front will remain stalled, triggering clouds, scattered showers, and some isolated diurnal thunderstorms. The northern Mid-Atlantic will experience clearing and afternoon sunshine, but mid-level cooling will moderate temperatures. 06Z back trajectories for the I-95 Corridor are short and westerly, but should be cleaner than typical westerly transport, given that locations upstream are post-frontal today. Furthermore, persistent northwesterly surface winds should be sufficient to usher slightly cooler and drier air into the northern Mid-Atlantic, thus limiting rising ozone levels. The 06Z air quality models reflect this cleaning trend, with the entire northern Mid-Atlantic in the Good range for ozone. The only area worth keeping an eye on will be along and east of the I-95, where convergence along the nearby front could cause locally enhanced areas of ozone, though this would only support Moderate ozone conditions. Regardless, the chance for an ozone exceedance will remain Slight for Wednesday.

Weak surface high pressure pushing into the northern and central Mid-Atlantic on Thursday will encourage another clear day for much of the forecast region. The stalled front remaining across NC will continue to support the formation of clouds and scattered showers through the day, especially during the late afternoon. The EC, NAM, and 03Z SREF all keep this precipitation across southern VA and NC of the MDL through 00Z, with only the GFS showing it stretching northward into the central Mid-Atlantic, as is expected with its consistent wet-bias. Another day of intense late June sunshine, paired with subsiding surface winds, will allow for ample ozone production along the I-95 Corridor. This will be tempered, however, by a shift to faster and northerly back trajectories. The current air quality guidance keeps the I-95 Corridor under Moderate ozone levels for Thursday. As a result, the chance for an ozone exceedance will remain Slight on Thursday.

There is a healthy dose of uncertainty for Friday’s forecast, due to discrepancies amongst the models regarding the precipitation forecast. The NAM and GFS show a northward surge of precipitation during the afternoon, likely associated with the stalled front across NC, which will shift slightly northward during Friday morning as discussed above. This surge is supported by the 03Z SREF and the 06Z 13km GFS, showing development of precipitation as far north as PA by 18Z Thursday. The EC is much drier, showing only scattered showers developing in NC at this time. How far north this precipitation can extend will determine how high ozone levels can rise along the I-95 Corridor. 06Z GFS back trajectories continue to recirculate for the I-95 Corridor, but have more of an onshore component today, compared to yesterday’s runs. It is also important to consider that Friday will be the beginning of the Fourth of July weekend, during which anthropogenic emissions are anomalously high due to increased travel. If an EC-like solution verifies, another day of intense sunshine will allow for ample ozone production, with the possibility of isolated exceedances along the I-95 Corridor. However, the guidance today seems more supportive of afternoon precipitation throughout the Mid-Atlantic, with the air quality models still only resolving Moderate ozone along the I-95 Corridor. Given the uncertainty, as well as the growing chance for clouds and precipitation, the chance for an ozone exceedance will rise, but only to Marginal on Friday.

On Saturday, the second cold front will push southeastward across the in the central Mid-Atlantic and collide with the stalled front remaining across NC, triggering widespread clouds and showers south of the MDL. The northern Mid-Atlantic will be the only portion of the forecast region to experience clearing, with temperatures climbing well into the 80s °F. Back trajectories for the I-95 shift westerly, and weak westerly surface winds will push pollutants from the I-95 Corridor eastward, rather than clean out the area. The main forecast question will be how quickly the front progresses during the morning. If the front slows down, as can often happen with weak cold fronts such as the one expected to impact the Mid-Atlantic on Saturday, the northern Mid-Atlantic may experience enough clouds and scattered showers to temper rising ozone. However, given the conditions expected, the chance for an ozone exceedance will remain Marginal on Saturday for locations along and east of the I-95 Corridor.

On Sunday, the stalled front will remain near the VA/NC border, allowing for another day of clouds and precipitation across the central and southern Mid-Atlantic. Another area of weak surface high pressure will push into the northern half of the forecast region, promoting sunny skies and light surface winds north of the MDL. Back trajectories for the I-95 Corridor turn northwesterly and originate from north of Lake Erie, which should bring cooler and drier air into the northern Mid-Atlantic. However, another day of air mass modification should allow ozone levels to rise quickly along the I-95 Corridor. As a result, the chance for an ozone exceedance will remain Marginal for Sunday.

-Brown/Ryan