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

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 29, 2014
Valid: June 30- July 4, 2014 (Monday-Friday)

Summary:

Southerly winds increase Monday into Tuesday with warm, humid conditions and patches of
moderate air quality.  Rain is likely both Wednesday and Thursday as two frontal boundaries cross the mid-Atlantic.  Good air quality both days.  Drier and less humid Friday although there is high uncertainty after Thursday due to the possibility of a tropical system developing off the SE US coast later next week.

Discussion:

The weather features and forecasts discussed in yesterday’s medium rang discussion continue to be applicable today with some slight modifications.  This forecast is based on the 0600 UTC runs of the NAM and GFS and the 0000 UTC run of the ECMWF.  In the large scale, a closed upper level low now in central Canada will open and move slowly east during the period reaching the mid-Atlantic by Friday.  The forecast models agree on the strength and direction of movement of this trough but differ on the speed with the ECMWF consistently slower than the American models.  For the purposes of this discussion we favor a compromise between the three models.

For Monday, south-southwest winds will gradually build during the day.  Temperatures will be in the mid-80’s F with a mix of sun and clouds.  Rain will develop by afternoon in the western mid-Atlantic.   The air quality models suggest widespread Code Green, but given the time of year and uncertainty in cloud cover forecasts, some locations will reach the Moderate range for ozone.  PM concentrations will reach the low moderate range as humidity increases and winds transport air along the I-95 Corridor.

Tuesday will be a very warm, humid and windy day.  Forecast back trajectories suggest a maritime component to the incoming air mass.  Precipitation will creep closer to the I-95 Corridor.  The western mid-Atlantic will see good air quality due to rain while there will remain pockets of low moderate ozone and PM along and east of the I-95 Corridor.

Wednesday and Thursday look wet.  As noted above, the numerical weather models time the onset of heavier precipitation a bit differently but we believe this will have little impact on the daily air quality with Code Green both days likely.  The first, and weaker, cold front will dissipate along the I-95 Corridor on Thursday with a strong cold front following Friday.  Both of these boundaries should provide enough lift in the context of a very humid air mass to set off showers across the region.

Cooler and drier air on Friday with good to low moderate air quality.  By Friday, or perhaps as early as Thursday in the southern mid-Atlantic, we may have to deal with the passage of a tropical storm.  Lots of uncertainty with that system (to be called Arthur) but the consensus forecast is that it will meander off Florida then pass just east of HAT in the Thursday-Friday period.

-Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 28, 2014
Valid: June 29- July 3, 2014 (Sunday-Thursday)

Summary:

Surface stagnation on Sunday may push ozone into the mid-to-upper Moderate range at locations west and north of I-95 in the northern Mid-Atlantic, but generally Good air quality is expected elsewhere. A warming trend will begin on Monday, with hot and humid conditions expected on Tuesday and Wednesday. Despite the hot weather, breezy south/southwesterly surface winds and the chance for scattered to widespread afternoon thunderstorms will keep air quality in the Good to low Moderate range. Widespread precipitation on Thursday will end the period with Good air quality.

Discussion:

The weather forecast models are in very close agreement this morning on the main features of the medium range period. The 12Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. The models agree on the placement of a closed low over southern MB and a ridge axis over the Mid-Atlantic on Sunday. The closed low will slow move eastward to southwestern ON on Monday and then open up and deepen over the western Great Lakes on Tuesday, progressing steadily eastward through the end of the medium range period. The western periphery of the longwave trough will be over the Mid-Atlantic by Thursday. The area of low pressure associated with this upper-level trough will pull a cold front through the region on Wednesday, followed by a second, stronger cold front on Thursday.

The center of surface high pressure will move offshore of New England on Sunday. With the ridge overhead, it will be sunny and seasonably warm, with light surface winds. The exception will be the western Mid-Atlantic, which will be impacted by precipitation in the afternoon and evening, as shown by the 09Z SREF and 12Z 4km NAM. The key question for Sunday for most of the region will be the impact of light winds all day, with a period of stagnation overnight, due to the close proximity of the surface high pressure center. With mostly to fully sunny skies in the central and eastern parts of the region, areas of Moderate ozone seem likely. The forecast models (06Z NOAA, BAMS, and NC) are consistent in placing a zone of mid-to-upper Moderate ozone west and north of I-95 (e.g., PHL, northeastern PA, northern NJ). Northeasterly transport aloft from New England will act as a moderating influence, as will the relatively lower volume of Sunday emissions (compared to a weekday). PM2.5 should be in the Good range across the region due to the relatively dry air mass in place, with dew points in the mid-50s °F in the northern part of the region.

Warm air advection aloft on Monday will promote rising temperatures, humidity, and cloud cover. The western Mid-Atlantic will again see clouds and rain in the afternoon and evening, triggered by a shortwave aloft. Surface winds will pick up, particularly along the coast. The BAMS and NC air quality models are unanimous in keeping ozone and PM2.5 in the Good range regionwide, and this does seem likely, given the more widespread chance for clouds and the sustained southerly surface winds.

Tuesday and Wednesday will be hot and humid with breezy southwesterly surface winds. The chances for more widespread lines of showers and thunderstorms will increase, particularly on Wednesday as the first cold front approaches from the northwest. With the humid air mass in place, any storms that develop have the potential for periods of heavy rain. Generally Good air quality seems likely, with the chance for isolated Moderate ozone in areas that have sustained afternoon sunshine, and scattered Moderate PM2.5, given the humid conditions.

The ECMWF and GFS are in surprisingly close agreement on the timing of both of the fronts in the Wednesday to Friday time frame, although there are differences in the way the models handle the evolution of a semi-permanent low pressure system that will linger along the SC/GA/FL Atlantic coast through the medium range period and possibly move northward along the East Coast on Thurday/Friday. Thursday looks wet for most of the region, as the first and weaker of the two cold fronts stalls along the East Coast and acts as a focus for clouds and precipitation. Temperatures will return to more seasonable values, dew points will fall, and winds will turn northwesterly behind the front. Good air quality is expected.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 27, 2014
Valid: June 28- July 2, 2014 (Saturday-Wednesday)

Summary:

A high pressure system centered over New England on Saturday will generate onshore flow that will keep air quality in the Good range, with the exception of areas west of I-95 in the northern Mid-Atlantic, where ozone may reach the mid-to-upper Moderate range. Sustained onshore flow on Sunday and Monday will allow air quality to persist in the Good range. Temperatures and humidity increase beginning on Tuesday due to the influence of the westward extension of the Bermuda High. Diurnal heating will create the chance for scattered showers and thunderstorms on Tuesday and Wednesday, which will limit rising ozone and PM2.5 levels to the Good to Moderate range.

Discussion:

The weather forecast models are once again in similar agreement for the medium range period. However, the usual differences of the timing and location of shortwaves will create some uncertainty in the precipitation forecast. The 06Z NAM, 06Z GFS, and the 00Z ECMWF all agree on an the upper level ridge tilted over the Great Lakes at the beginning of the period. The ridge will slowly move through the Mid-Atlantic and will finally get pushed to the East Coast by the eastward movement of an upper level trough through southern ON early Tuesday morning. Starting Tuesday night, the ECMWF and the GFS start to diverge just a tad from each other. The ECMWF intensifies the upper level trough with additional shortwave energy moving through the axis of the trough. By Wednesday afternoon, the ECMWF then moves the trough eastward faster and broadens it, while the GFS amplifies the trough further south into the U.S., just west of the Mid-Atlantic region. This difference creates some uncertainty in the precipitation forecast for Wednesday.

A New England surface high pressure system on Saturday will result in mostly to partly sunny weather and onshore surface flow. The presence of a stationary front in southern SC will create the chance for rain and thunderstorms for the southern Mid-Atlantic. Flow aloft will be northeasterly, transporting cleaner air into the area. Sustained onshore winds, mixing, and scattered clouds will limit ozone and PM2.5 to the Good range for most locations. The 06Z runs of the NOAA, BAMS, and NC ozone models all indicate a band of mid-to-upper Moderate ozone along and west of I-95 in the northern Mid-Atlantic (e.g., DC, BAL, PHL, northern NJ). The models are probably responding to the onshore flow and a period of stagnation at the surface overnight into the morning. As a result, there is a likely chance for Moderate ozone west of I-95 along the urbanized northern corridor.

By Sunday, the development of a Bermuda High will continue the seasonably warm weather for mainly the northern half of the Mid-Atlantic. The southern half of the region will experience precipitation and cloud cover as a wave of low pressure forms along the remnant frontal boundary just off the coast of the Carolinas. Onshore flow will dominate, maintaining Good air quality across the region.

A slow moving shortwave will move over the Mid-Atlantic on Monday from the west and diurnal heating will trigger scattered afternoon rain and thunderstorms across the Mid-Atlantic. The northern Mid-Atlantic will have partly to mostly cloudy skies. The New England high pressure system will move eastward into the Atlantic Ocean, which will continue onshore flow, thus keeping air quality in the Good range.

Warm and humid conditions will begin on Tuesday and continue until the next cold front passes in the Thursday to Friday time frame. With the passing of the shortwave aloft, another day of diurnal heating, and the eastward movement of the approaching upper level longwave tough over the western Great Lakes, Tuesday will experience the chance for scattered rain and thunderstorms throughout the day across the region. The rise in temperatures and dew points could possibly increase PM2.5 to the Moderate range in scattered inland locations, but southerly flow aloft and at the surface should continue to keep PM2.5 concentrations in the Good range along the coast. Ozone will remain in the Good range due to cloud cover and mixing, with the chance for Moderate ozone along and west of I-95. The approach of the upper level longwave trough on Wednesday will shift flow southwesterly, which will help to promote more widespread Moderate air quality. However, given the warm and humid air mass that will be in place, there is a likely chance of scattered afternoon precipitation and cloud cover, which would limit rising ozone levels and possibly PM2.5 as well for areas that experience significant rainfall.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 26, 2014
Valid: June 27- July 1, 2014 (Friday-Tuesday)

Summary:

Sunshine and onshore flow on Friday will allow ozone to reach the Moderate range at isolated locations in the northern Mid-Atlantic west of I-95. Low pressure off the coast of the Carolinas will create cloud cover and precipitation, reducing ozone to the Good range for the southern Mid-Atlantic. Persistent onshore flow will limit PM2.5 to the Good range until Monday. Saturday’s weather will be similar to Friday’s, with Moderate ozone possible at isolated locations in the northern Mid-Atlantic and Good ozone in the southern region. The westward extension of the Bermuda High will build in over the Mid-Atlantic Sunday-Tuesday. Sustained onshore flow will keep ozone and PM2.5 in the Good range on Sunday. For Monday and Tuesday, increasingly warm and humid conditions associated with the Bermuda High will be tempered by onshore flow, keeping the chances for USG ozone low, with Good to low Moderate air quality most likely.

Discussion:

Other than the usual discrepancies of the timing and arrival of shortwaves, the weather forecast models are in good agreement for the medium range period. The 06Z NAM, 06Z GFS, and the 00Z ECMWF all show similar progression of an upper level ridge that will gradually move eastward over the Mid-Atlantic by Sunday. The ridge will stay overhead until it is displaced on Monday by a strong upper level trough moving across the southern Canadian Plains.

A surface trough will extend from New England down into the southern Mid-Atlantic on Friday, creating uplifting and instability. This will trigger rain and thunderstorms for the southern part of the region later in the day, resulting in cleaner conditions. Scattered clouds will extend into the northern part of the region although there is a less likely chance for precipitation due to the northeastern movement of the center of surface high pressure from southwestern PA to New England. Sunshine and onshore flow will allow ozone to reach the Moderate range at isolated locations in the northern Mid-Atlantic west of I-95, while clouds and precipitation will keep ozone in the Good range in the southern Mid-Atlantic. Onshore winds and mixing will limit PM2.5 to the Good range for most locations throughout the Mid-Atlantic.

Saturday will be similar to Friday’s conditions. The northern part of the Mid-Atlantic will experience mostly sunny weather and light winds due to the lingering high pressure in New England. Today’s model runs show less chance for a convergence zone inland compared to yesterday’s analysis, which reduces the chances for more widespread Moderate ozone. Instead, ozone may reach the Moderate at isolated locations west of I-95. An area of low pressure off of the coast of the Carolinas will remain for Saturday, resulting in continued cloud cover and precipitation for the southern part of the region, which will keep ozone in the Good range. Persistent onshore flow for Saturday will allow PM2.5 concentrations to remain in the Good range.

By Sunday, a surface ridge will develop down the Mid-Atlantic region from the surface high that will have moved just off the coast of New England, and by the end of the day, high pressure will transition to the westward extension of the Bermuda High. Low pressure will persist off the coast of the Carolinas, maintaining the chance for rain and thunderstorms in the southern Mid-Atlantic. Ozone and PM2.5 will be limited to the Good range across the region due to onshore flow aloft and at the surface.

With the Bermuda High in place, temperatures and humidity will increase on Monday and Tuesday. Diurnal heating both days will create some instability and the chance for afternoon rain and thunderstorms. Sustained onshore flow will continue on Monday, both aloft and at the surface. On Tuesday, the flow aloft will become more southwesterly, with persistent southerly winds at the surface. The main forecast question will be the relative influence of the competing features – the Bermuda High creating warm and humid conditions vs onshore flow bringing maritime air into the Mid-Atlantic. The BAMS and NC air quality forecast models are unanimous in keeping air quality in the Good range for Monday. At this point, mostly Good to low Moderate ozone seems likely both days. PM2.5 may rise into the Moderate range on Tuesday in response to rising humidity.

-Catena/Huff