Medium Range Air Quality Outlook Mid-Atlantic Region Issued: Sunday, July 6, 2014 Valid: July 7-11, 2014 (Monday-Friday) Summary: High pressure to our east will bring warm humid weather and a slight chance of Code Orange ozone on Monday. Rain will slowly move into the western mid-Atlantic late Monday. Rain to our west will limit ozone and PM transport all week. As a result, we expect Moderate air quality through Wednesday. A weak cold front arrives Thursday and stalls Friday. Discussion: The weather forecast models agree that a broad trough will develop over the eastern US during the coming work week. Before it settles into place, however, we will see sunny, warm weather conducive to ozone and PM formation. The forecast models differ on the strength and speed of the trough development with the NAM the most aggressive and fastest and the ECMWF the weakest and slowest. This discussion follows a consensus of the three main models. Two factors will be key for air quality during the short range (Monday-Tuesday). First, we expect sustained SW flow through Tuesday. There is no evidence from the SREF that there will be any stagnation through Tuesday and this will serve to limit pollutant accumulation. Second, rain will slowly advance into the western mid-Atlantic on Monday and become more widespread on Tuesday. Although there is a chance of rain reaching the I-95 Corridor as early as Tuesday, we don’t expect this to impact air quality until Wednesday. However, back trajectories will turn WSW today and continue through Tuesday. With rain and clouds in place to the west, the regional pollutant load is not expected to be overwhelming. As a result, while we can’t rule out an isolated instance of Code Orange ozone, we expect air quality to remain in the upper Moderate range Monday and Tuesday. A better chance of rain Wednesday although not widespread enough to reduce pollutant levels too much with Moderate air quality continuing. A weak cold front will bring slightly cooler weather and clouds on Thursday. The front stalls nearby Friday. We expect Good to Moderate air quality both Thursday and Friday.

-Ryan

Medium Range Air Quality Outlook Mid-Atlantic Region Issued: Saturday, July 5, 2014 Valid: July 6-10, 2014 (Sunday-Thursday) Summary: Moderate air quality will be the rule for most of the forecast period. The day of most interest will be Monday when a combination of warm temperatures, mostly sunny skies and limited mixing may allow isolated instances of Code Orange ozone. Discussion: The weather forecast models remain in good agreement so that there is high confidence in the weather forecast on the synoptic scale. Variations in air quality will hinge on smaller scale features best resolved in the short range forecasts. An upper level trough will cross through the northern tier states during the medium range period with the first significant ridge forecast to develop very late in the coming week. Rain associated with the trough will be limited to the western and southern mid-Atlantic for the most part. For Sunday, dry air will remain in place limiting PM accumulation. Deep PBL mixing will also limit PM as well as ozone formation. Back trajectories suggest there will be some recirculation along the I-95 Corridor. A short wave disturbance will reach western PA Sunday afternoon and may introduce mid-level clouds in the mid-Atlantic. As a result, expect Good PM concentrations with ozone reaching the Moderate range along the I-95 Corridor. Monday is the day of most interest for air quality concerns. Skies will be mostly clear as high pressure drifts slowly east to the Atlantic coast. Temperatures are likely to reach near 90 F with increasing humidity and boundary layer mixing will be limited. Forecast back trajectories suggest transport from locations just south of the Ohio River Valley although it is not clear yet how modified this air mass will be. The only factor limiting ozone formation will be steady SW winds. This points to upper Moderate ozone levels with a chance of isolated Code Orange. PM will be solidly in the Moderate range. More clouds are likely Tuesday as a cold front reaches extreme NW PA. A lee trough will form along the I-95 Corridor which may serve as a focus for locally high ozone levels. Steady SW winds should keep air quality in the Moderate range. The southern and western mid-Atlantic are likely to see more clouds and a chance of rain. The cold front dissipates over the Corridor on Wednesday with a weak secondary front arriving on Thursday morning. Clouds are likely with a chance of rain as well. Temperatures will fall to the mid-80’s F and air quality remains in the Moderate range. There is disagreement between the main longer range models regarding a broad ridge building over the eastern US beginning Friday. We will continue to monitor the forecasts for this possible poor air quality event.

-Ryan

Medium Range Air Quality Outlook Mid-Atlantic Region Issued: Friday, July 4, 2014 Valid: July 5-9, 2014 (Saturday-Wednesday) Summary: Good air quality with dry Canadian air on Saturday. This air mass will begin to modify on Sunday with Moderate air quality likely. Humid with southwest winds and Moderate air quality through Monday and Tuesday. Chance of rain on Wednesday. Discussion: Good agreement between the weather forecast models and good run-to-run consistency as well. The upper level trough over the mid-Atlantic today will move offshore along with Hurricane Arthur on Saturday. Zonal flow will be in place in the wake of the trough Sunday with a broad trough re-establishing itself early next week. The models agree on rain reaching the western mid-Atlantic Tuesday with a chance of convection along the I-95 Corridor by Wednesday. Steady northwest winds and a dry air mass in place on Saturday will keep air quality in the good range. Sunday will be the day of most concern, as high pressure of continental origin will link up with the Bermuda High. Although the continental air mass is not likely to have time to modify by Sunday, there will be a period of stagnation early Sunday that will allow ozone and PM concentrations to rise. We expect Moderate air quality on Sunday although we will continue to monitor the extent of air mass modification. On Monday, increasing southwest winds will develop ahead of a slow moving cold front in the Midwest. Humidity will begin to rise and this will push PM into the moderate range. With temperatures in the mid-upper 80’s F, ozone will be in the Moderate range as well. Westerly transport becomes established on Tuesday. A short wave in the westerly flow will bring rain and convection to the western mid-Atlantic. We expect dry conditions further east with Moderate air quality. Clouds and a chance of rain on Wednesday will continue Moderate air quality.

-Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 1, 2014
Valid: July 2- 6, 2014 (Wednesday-Sunday)

Summary:

A pre-frontal trough will develop ahead of an approaching cold front from the west on Wednesday resulting in afternoon rain and thunderstorms. Air quality will be limited to the Good range with the exception of Moderate ozone along the I-95 Corridor in PHL and MD. The cold front will move to the Mid-Atlantic and stall Thursday, calling for all-day rain and thunderstorms. Tropical Storm Arthur has formed off the coast of Florida and will move northward to SC. Air quality will remain in the Good range. The front will pass through Friday as the tropical system moves northward to Cape Hatteras. Good air quality is mostly likely although there is a slight chance for Moderate ozone as the tropical system will result in subsidence and calm weather for inland locations. The weekend will be beautiful and sunny as high pressure moves into the area and remains until Sunday. Northerly flow will keep air quality in the Good range.

Discussion:

There is high confidence in the weather forecast models for the medium range period. The 06Z NAM, 06Z GFS, and the 00Z ECMWF are in phase with each other regarding the eastward movement of the upper level longwave trough. The trough is positioned over the Great Plains at the beginning of the period and will pass over the Mid-Atlantic Friday night into Saturday morning. The development of an upper level ridge will follow the trough across the U.S, but will flatten out by Saturday morning over the Great Plains becoming more zonal for the weekend. Tropical Storm Arthur has formed off the coast of Florida and will travel along the East Coast to Cape Hatteras Friday morning and move northeast into the Atlantic by Saturday morning. The GFS keeps it closer to the coast on Saturday than the ECMWF. The impact on air quality remains uncertain at this forecast length.

As a cold front approaches from the west on Wednesday, a pre-frontal trough will develop just west of the I-95 Corridor, sparking widespread rain and thunderstorms, primarily in the second half of the day. Wednesday will be hot and humid with above average temperatures and scattered clouds throughout the day. Mixing from diurnal heating and cleaner air transported from southwesterly winds and late day convection will keep air quality in the Good range, with the exception of Moderate ozone along and west of the I-95 Corridor, mainly in PHL and MD.

By Thursday, the frontal boundary will move and stall over the Mid-Atlantic. A tropical system will develop off the coast of Florida and travel northward up to the shores of SC. All-day heavy rain and thunderstorms are expected with overcast skies. The front will bring in northwesterly flow, although southwesterly flow will reside in locations ahead of the front. Air quality will remain in the Good range.

TS Arthur will move northward up the coast to Cape Hatteras until 12Z Friday and will then travel northeastward into the Atlantic. Arthur’s impact on mid-Atlantic air quality will be strongest on Friday. At that time, however, a frontal boundary will be crossing the mid-Atlantic and the interaction between these two features will determine regional air quality. Tropical storms typically have a zone of subsidence at the periphery of their circulation and this can, in the right circumstances, result in poor air quality. In this case, however, synoptic scale upward motion, along with clouds and rain, from the advancing frontal boundary will limit the impact of this subsidence effect and we expect good air quality Friday.

As the cold front moves off the coast and the tropical system moves northeastward into the Atlantic, high pressure will move into the Mid-Atlantic and remain overhead for Saturday and Sunday. Northerly flow will bring in dry, clean air for both days, limiting air quality to the Good range for the weekend.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 30, 2014
Valid: July 1- 5, 2014 (Tuesday-Saturday)

Summary:

Sustained southerly flow and partly sunny skies will keep air quality in the Good range with the exception of Moderate ozone along the northern part of the I-95 Corridor. Scattered rain and cloud cover Wednesday with southerly flow will keep air quality in the Good range. Moderate pockets of ozone are expected along the northern part of I-95. Two frontal boundaries will briefly stall over the Mid-Atlantic Thursday. PM2.5 will be able to reach the Moderate range for most locations throughout the Mid-Atlantic while all-day cloud cover will limit ozone to the Good range. Friday and Saturday will see Good air quality as the fronts pass through and bring partly to mostly sunny skies and northwesterly wind although considerable uncertainty is present in the Friday forecasts due to the expected development of a tropical system off the southeast US coast.

Discussion:

The weather forecast models are in close agreement for the medium range period. The 06Z NAM, 06Z GFS, and the 00Z ECMWF all agree that the closed low currently over Manitoba will open up and pass through the Mid-Atlantic region by Wednesday morning with the upper level flow becoming more zonal by Saturday morning. The only major differences between the models are the timing and location of shortwaves to impact the area, creating an uncertainty in the precipitation forecast. The GFS is less aggressive than the other major models with the strength of the trough and its associated short waves.

Tuesday will be partly to mostly sunny with a slight chance of precipitation as a cold front approaches from the west. The day will be hot and humid with sustained southerly flow. Good air quality is expected for most of the region other than a chance for Moderate ozone along the northern part of the I-95 Corridor.

Wednesday will see scattered rain and clouds in the second half of the day as a pre-frontal trough forms along the I-95 Corridor ahead of a stationary front stationed over the Ohio River Valley. A secondary, and slightly stronger, frontal boundary will develop behind the stationary front. Wednesday will continue the hot and humid weather. Deep mixing and southeasterly flow will keep PM2.5 in the Good range. Ozone will stay in the Good range for most locations with the exception of along and west of the I-95 Corridor where it will most likely reach the Moderate range.

Overcast on Thursday with widespread scattered rain and thunderstorms all day due to the passage of both frontal boundaries. The two fronts will stall over the Mid-Atlantic for most of the day but will pass through overnight. West northwest transport aloft and convergence along the frontal boundaries will allow pollutants to accumulate, resulting in a high chance of Moderate PM2.5 for most locations. All day cloud cover will limit ozone to the Good range.

By Friday, the cold front will stall over the southern Mid-Atlantic. The key uncertainty Friday will be the possible development of a tropical system off the Florida coast, moving northeast to near Cape Hatteras (HAT) on Friday. The location and strength of the tropical system is still uncertain although the hurricane and weather models suggest that showers will occur along the East Coast for most of the day. Subsidence to the north and west of the storm will reduce cloud cover for the rest of the region. The tropical system is likely to head south of New England Saturday. High pressure will move overhead Saturday leaving sunny skies and calm weather. Transport aloft will continue to flow from the Great Lakes bringing cleaner air into the area. Friday and Saturday are expected to see Good air quality.

-Catena/Ryan