Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 20, 2014
Valid: June 21-25, 2014 (Saturday-Wednesday)

Summary:

A wave of low pressure will form along a frontal boundary across the central Mid-Atlantic on Saturday, bringing rain and thunderstorms to much of the region. Cloud cover and mixing will limit ozone and PM2.5 to the Good range. Sunday will see mostly sunny skies in the northern part of the region, allowing ozone to increase into the Moderate range at scattered locations, while ozone will be at the Good/ Moderate threshold for the southern half of the region, depending on cloud cover. PM2.5 will remain in the Good range across the region due to low humidity. Monday and Tuesday will feature sunny and pleasant conditions, which will increase ozone and PM2.5 to the Moderate range for most locations; the exception will be the western Mid-Atlantic on Tuesday, where afternoon precipitation may keep air quality in the Good range. Air quality on Wednesday will depend on the timing of the next cold front.

Discussion:

There is close agreement between the weather forecast models on the main features of the medium range period. In particular, the 06Z GFS and 00Z ECMWF are in much closer agreement today regarding the development and progress of an upper level trough and associated cold front on Tuesday and Wednesday. They agree to mainly zonal flow aloft for Saturday. Sunday, a weak upper level ridge will develop over the Great Lakes and spread slowly eastward. As the ridge moves eastward toward the Mid-Atlantic, an upper level closed low over MB will flatten the ridge through Sunday night, resulting in slight ridging aloft over the Mid-Atlantic for Monday. On Tuesday morning, shortwave energy will begin to dig out a longwave trough over the western Great Lakes. The GFS and ECMWF develop the trough and corresponding surface low in roughly the same location, but the ECMWF is slightly more amplified with the trough than the GFS. As a result, the ECMWF brings more precipitation into the western Mid-Atlantic on Tuesday afternoon compared to the GFS, but both models are consistent in developing precipitation over the Mid-Atlantic Tuesday afternoon through Wednesday.

The models show some minor variations in the evolution of smaller shortwaves associated with the longwave trough, which is to be expected at this time in the medium range forecast. For example, the models start to diverge Tuesday afternoon with the development of shortwave energy over the Midwest. The GFS and the ECMWF start to weaken the shortwave while the 00Z CMC intensifies it. The GFS spreads out the energy into smaller shortwaves, which pass through the Mid-Atlantic region starting around 18Z Wednesday. The CMC does not weaken the shortwave until Wednesday night and keeps it located west of the Mid-Atlantic, over the Great Lakes. This uncertainty in the arrival and intensity of the shortwave energy developing the longwave trough over the Mid-Atlantic affects the cold front expected to impact the region on Wednesday.

A cold front will move northward as a warm front and settle over central VA early Saturday morning. The front will stall as a stationary front and a wave of surface low pressure will move along the boundary overnight and through the day Saturday. The cold front will move southward to NC by Saturday evening. The wave and frontal boundary will spark showers and thunderstorms overnight and through Saturday evening, mainly in the central and southern region of the Mid-Atlantic, yet there is still a chance for the precipitation to reach the northern Mid-Atlantic also. Mostly cloudy skies associated with the presence of the frontal boundary will keep ozone in the Good range. The clean and dry air mass in the northern part of the Mid-Atlantic and precipitation and mixing in the southern Mid-Atlantic will limit PM2.5 to the Good range.

By Sunday, the frontal boundary will push south of the region, giving way to calm and sunny weather as a Canadian high pressure system extends its ridge over the Mid-Atlantic. Recirculation of air aloft along the I-95 Corridor will allow PM2.5 to accumulate. However, the existing air mass will likely be too dry for PM2.5 to increase into the Moderate range. There is a chance for ozone to reach the low Moderate in the northern part of the Mid-Atlantic, while ozone will be at the Good/ Moderate threshold for the southern half of the region, depending on cloud cover.

The surface ridge will still be over the region as the center of high pressure moves to the Atlantic on Monday. Sunshine and calm weather will continue. PM2.5 concentrations will reach the low Moderate range along the I-95 Corridor due to continued recirculation of air aloft. Afternoon sunshine and light winds at the surface will allow ozone to reach the Moderate range at scattered locations.

The surface ridge will be displaced eastward to the Atlantic by Tuesday with the arrival of the next cold front. The cold front will reach the western Mid-Atlantic by Wednesday morning and a pre-frontal trough will develop in the vicinity of central VA/NC. The approaching cold front and arrival of an associated warm front will increase the chances for precipitation beginning on Tuesday afternoon in the western Mid-Atlantic. PM2.5 and ozone will reach the Moderate range for most locations on Tuesday, with the exception of the western Mid-Atlantic, depending on rain and clouds. Wednesday’s forecast will depend on the timing of the cold front, with generally Good air quality expected.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 19, 2014
Valid: June 20-24, 2014 (Friday-Tuesday)

Summary:

Friday will have showers in the southern Mid-Atlantic and sunshine in the northern Mid-Atlantic as a cold front moves southward through NC. Air quality will be limited to the Good range with the chance for some areas of Moderate ozone in the northern half of the Mid-Atlantic. Saturday will see a chance for widespread scattered rain and thunderstorms as the cold front moves back northward as a warm front. Cloud cover and mixing will keep air quality in the Good range. As the front switches direction once again and follows its path back southward to SC on Sunday, rain and cloud cover will keep air quality in the Good range for the southern part of the region, while ozone and PM2.5 will reach the Moderate range at scattered locations in the northern Mid-Atlantic due to sun and stagnant air. There is some uncertainty in the forecast for Monday and Tuesday but air quality is expected to reach the Moderate range for both days.

Discussion:

The weather forecast models are in relatively close agreement for the medium range period, with slight differences beginning on Sunday that amplify by Tuesday. The 06Z NAM starts to diverge from the 06Z GFS, the 00Z ECMWF, and the 00Z CMC beginning 18Z Saturday at 500 mb so we will leave it as an outlier. By Tuesday, the GFS, ECMWF, and CMC have completely different solutions for a developing longwave trough over the Great Lakes/Mid-Atlantic that will pull the next cold front through the Mid-Atlantic. The GFS is the fastest and farthest east with the developing upper level trough, followed by the CMC, and then the ECMWF. These differences result in uncertainty in the precipitation forecast for Tuesday, as the GFS generates widespread showers and thunderstorms from west to east across the Mid-Atlantic, ahead of the approaching front. There is also a question of the timing of the frontal passage itself; WPC is leaning toward the slower solutions, with the cold front holding back to the west over the Great Lakes and slowly moving into the northern Mid-Atlantic on Wednesday and Thursday.

A cold front will slowly move south through NC on Friday, generating rain and thunderstorms in its wake across the southern Mid-Atlantic. Surface high pressure will move down into the Northeastern U.S. from interior ON, resulting in sunny and calm weather for the northern Mid-Atlantic. Northerly clean and dry flow aloft in the northern and central Mid-Atlantic and rainout of pollutants in the southern Mid-Atlantic will limit PM2.5 to the Good range for most locations. Ozone will also be in the Good range for most areas, with some scattered pockets of Moderate ozone possible in the northern Mid-Atlantic due to sunny skies and light winds.

On Saturday, the cold front will retrace its tracks northward as a warm front, reaching the vicinity of central/southern VA. There will be a chance of widespread scattered rain and thunderstorms with cloud cover that will last throughout the day mainly in the central and southern Mid-Atlantic. A wave of low pressure will form along the front. The forecast models differ slightly with the timing and strength of the wave, which introduces some uncertainty in the extent of the precipitation forecast. Regardless, air quality will likely remain in the Good range for Saturday, with the chance for isolated Moderate ozone in the northern part of the region, where skies will be mostly clear.

Sunday, the warm front will reverse its direction once again as a cold front and move southward to SC. The northern Mid-Atlantic is expected to stay dry while there will be a chance for showers in the lower half of the region. There will be some recirculation of upper level flow, resulting in accumulation of PM2.5 along the I-95 Corridor. As a result, PM2.5 concentrations will increase, but the absence of high atmospheric humidity should be enough to keep PM2.5 in the upper Good range. There is the chance for isolated Moderate ozone in the northern Mid-Atlantic due to increased sunshine and stagnant flow, while showers and cloud cover in the southern part of the region will keep ozone in the Good range.

Humidity will begin to rise on Monday and Tuesday as the surface flow turns southerly. Monday will likely be mostly dry and sunny, while chances for rain increase on Tuesday ahead of the next cold front, which will approach from the northwest. The possibility for Moderate ozone will remain on Monday and Tuesday, mainly along the I-95 Corridor. PM2.5 concentrations will continue to rise steadily in response to increasing humidity, likely reaching the Moderate range at scattered locations. If the GFS’s faster track for the cold front and associated precipitation verify for Tuesday, air quality conditions will return to the Good range. However, we favor the slower solutions of the ECMWF and CMC, which would keep Moderate air quality in the forecast for early next week.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 18, 2014
Valid: June 19-23, 2014 (Thursday-Monday)

Summary:
A cold front will oscillate between the central and southern Mid-Atlantic through the medium range period. It will split up the region north/south, air quality wise, due to different weather conditions brought on by the front. Convection and cloud cover in the northern Mid-Atlantic on Thursday will keep ozone in the Good range, while PM2.5 will linger in the Moderate range, whereas both ozone and PM2.5 will reach the Moderate in the southern Mid-Atlantic. Friday will see a reversal in ozone concentrations with Moderate ozone in the north and Good ozone in the south. PM2.5, however, will stay in the Good range in most locations. The chance for widespread scattered rain and thunderstorms daily Saturday-Monday will keep air quality in the Good to low Moderate range for most locations.

Discussion:
The weather forecast models are in close agreement until Sunday. The models agree on the development of an upper level ridge west of the region over the Great Lakes at the beginning of the medium range period. The upper level ridge will flatten into zonal flow as a closed low moves eastward over MB and ON. The uncertainty starts on Sunday morning between the GFS and the ECMWF. As the closed low dissipates, the ECMWF develops a more pronounced upper level ridge over MN and WI, while the GFS has more of a zonal flow. This creates uncertainty for Sunday’s weather forecast due to differences in location and timing of shortwaves generated by this upper level ridge which will impact the Mid-Atlantic. By Monday, the GFS and ECMWF are completely out of phase with each other.

A cold front will move to the VA/NC border by Thursday evening, sparking rain and thunderstorms in the northern part of the Mid-Atlantic for the second half of the day. Cloud cover and convection will reduce ozone concentrations to the high Good range in the northern Mid-Atlantic. However, because the front will not reach the southern Mid-Atlantic, ozone concentrations will reach the Moderate range mostly in NC. PM2.5 will remain the Moderate range due to continued atmospheric moisture.

The front will stall Friday in the vicinity of the VA/NC border and bring rain and thunderstorms mainly to the central and southern Mid-Atlantic. The arrival of a cleaner, drier, and cooler air mass in the northern Mid-Atlantic, along with light northerly flow, will reduce PM2.5 concentrations to the Good range, but ozone will reach the Moderate range at scattered locations due to mostly to partly sunny skies. Cloud cover and precipitation along the front will limit air quality to the Good range for the central and southern Mid-Atlantic.

The front will slowly move northward back up to the VA/MD border as warm front early Saturday morning, and by 12Z, the front will stall over northern Virginia as a stationary front. Waves of low pressure will form along the front, creating convection and clouds along this boundary. Recirculation of air aloft with rebounding humidity will bring PM2.5 back into the low Moderate range for the northern part of the region. The southern Mid-Atlantic will have lower concentrations of PM2.5 in the Good range. Widespread scattered rain and thunderstorms with cloud cover will keep ozone in the Good range, with the chance of isolated Moderate ozone in areas that see more sun.

The front will move back southward again as a cold front on Sunday, to the vicinity of the NC/SC border and stall through Monday. Due to great uncertainty between the GFS and the ECMWF, as stated above, the air quality forecast for Sunday-Monday will depend on precipitation and cloud cover forecast, but generally Good to low Moderate conditions are expected.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 17, 2014
Valid: June 18-22, 2014 (Wednesday-Sunday)

Summary:
Very warm conditions and sunny skies through the early evening on Wednesday will keep the chance for USG ozone in the forecast, with mid-to-upper Moderate PM2.5. A slowly moving cold front will spark afternoon showers and thunderstorms on Thursday, which will limit ozone to the mid-to-upper Moderate range, while PM2.5 will continue in the mid-to-upper Moderate range. There is uncertainty in the location of the frontal boundary and the chance for associated clouds and rain on Friday-Sunday. Good to low Moderate air quality is generally expected.

Discussion:
The weather forecast models are in relatively close agreement for the medium range period. An upper level ridge axis will be located to the west of the Mid-Atlantic, over the Great Lakes, through Friday. At that point, a closed low over the northern Plains will move eastward and flatten the ridge. The ECMWF and the GFS disagree slightly on the movement of the closed low. Both of the models deamplify the closed low as it moves east, but the ECMWF splits off a lobe of the vort max and stretches the energy across southern ON and QC by Sunday. In contrast, the GFS keeps the closed low compacted over southern ON and the northern Great Lakes. This difference creates uncertainty in the precipitation forecast for Sunday. The GFS calls for more widespread rain and thunderstorms all day on Sunday while the ECMWF has less of a chance for precipitation. If the GFS were to verify, Sunday would show improved air quality conditions. At the mid-levels, warm air advection aloft is the consensus through Friday. A primary forecast question for Friday-Sunday will be the location of a frontal boundary that will move into the central Mid-Atlantic from the north on Thursday. The boundary will set up around the Mason-Dixon Line on Friday and then oscillate and slowly move north as a warm front on Saturday and Sunday. Areas to the north of the front will see northerly flow and a cooler and less humid air mass, while locations to the south of the front will continue to experience a humid and modified air mass.

A surface trough will linger over the Mid-Atlantic on Wednesday ahead of an approaching cold front pulled by a Canadian low pressure system. The front will move southward to Philadelphia by Wednesday night and then likely stall. The ridge of high pressure aloft will promote westerly flow from the Ohio River Valley. Wednesday is expected to be the hottest day of the year so far with high humidity. These conditions, along with the approaching front, will create instability in the afternoon resulting in convection and precipitation, but the 09Z SREF, 00Z 4km NAM, and the 00Z 4km NMM/ARW models keep the convection to the north and west of I-95 through 00Z Thursday. As a result, there is a continued likely chance for another day of USG ozone along the I-95 Corridor and widespread Moderate PM2.5 across the region.

The cold front will stall around southern PA until Thursday night and then will slowly move southward through Friday morning, bringing on the chance for rain and thunderstorms to the northern and central Mid-Atlantic. The surface trough will move southward ahead of the front and will remain over the southeastern Mid-Atlantic. Westerly flow, light winds, and partly sunny skies will keep PM2.5 and ozone in the Moderate range for most locations. The chance for clouds and rain in the vicinity of the front will limit the chances for continued USG ozone. Instead, upper Moderate ozone and mid-to-upper Moderate PM2.5 is more likely, with the highest concentrations expected east of I-95.

The forecast for Friday-Sunday will hinge on the location of the frontal boundary and the impact of associated clouds and precipitation. On Friday and Saturday, there is the chance for scattered rain and thunderstorms to occur along the front, mainly in VA and NC for the second half of the day. Northerly flow aloft will bring in clean and less humid air to locations north of the front. This will call for improved air quality to the high Good to low Moderate range. Due to uncertainty in the precipitation forecast, Sunday is expected to keep PM2.5 in the Good to low Moderate range, while ozone will depend on cloud cover and precipitation.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 16, 2014
Valid: June 16-20, 2014 (Tuesday-Saturday)

Summary:
This week will be characterized by a rise in dew points and temperatures, a daily chance of rain and thunderstorms, and westerly flow aloft until Friday, when a cold front will pass through the northern part of the Mid-Atlantic. Both PM2.5 and ozone will reach the Moderate range for Tuesday and Wednesday, with a chance for isolated USG ozone, particularly along the I-95 Corridor. Northwesterly strong surface winds in the northern Mid-Atlantic will bring PM2.5 to the Good range for Thursday, while ozone will remain in the Moderate range. In areas south of the frontal boundary, air quality will remain in the Moderate range. As the front passes through Friday, an incoming dry and northerly air mass will clean out the northern part of the Mid-Atlantic, bringing both concentrations of ozone and PM2.5 to the Good range, while the southern part of the region, ahead of the cold front, will see Moderate air quality. The same conditions will apply for Saturday.

Discussion:
The weather forecast models generally all agree on the large scale features with the exception of a few small scale disturbances to hit the area. They agree on warm air advection aloft over the region for the week, resulting in a stable and stagnant atmosphere. They also agree to mainly zonal flow aloft for Tuesday with the development of a ridge west of the Mid-Atlantic. The models are generally in phase with each other until Friday afternoon when the ECMWF extends the upper level trough axis more southeastward. They then differ significantly and become out of phase for the rest of the medium range period. Both the GFS and the ECMWF move a closed low over the northern Plains by Friday, at which point the GFS keeps the feature stationary while the ECMWF spins off a vort max and moves it eastward across the Great Lakes. This difference results in a low confidence precipitation forecast for the Mid-Atlantic for Friday night into Saturday.

A warming trend will continue on Tuesday, with the highest temperatures of the year expected across the Mid-Atlantic, with highs reaching the mid-90s F. The development of a surface trough early Tuesday will result in a chance for scattered rain and thunderstorms. However, with above average temperatures at the surface and aloft, the atmosphere will be very stable, limiting mixing and leaving the air stagnant. Back trajectories will be westerly, from the Ohio River Valley, which is a source region for pollutants and pollutant precursors. This will allow PM2.5 to rise into the Moderate range. Ozone will also rise into the upper Moderate range at scattered locations, particularly along the I-95 Corridor. Depending on how quickly the air mass modifies, isolated USG ozone is possible, which is supported by the BAMS-MAQSIP and NC air quality models.

By 12Z Wednesday, a surface low pressure system on the Vermont/Canadian border will extend its cold front over northern PA. The surface trough will still be in place continuing the chance for scattered rain and thunderstorms. However, most of the rain will arrive in the late afternoon/evening and be limited to the northern Mid-Atlantic. Wednesday will continue the above average temperatures and dew points with light winds traveling from the Ohio River Valley. PM2.5 and ozone will remain in the Moderate range in most locations, with ozone in some areas along the I-95 Corridor reaching the upper Moderate range. The chance for USG ozone will depend on the speed of the advancing frontal boundary and associated clouds and precipitation.

The surface low will travel east northeastward slightly on Thursday, bringing the cold front southward, although it will remain in roughly central PA. Rain and thunderstorms will hit mainly in the second half of the day. There will be northwesterly flow from the Great Lakes region and strong surface winds in the northern and central Mid-Atlantic. PM2.5 will be reduced to the Good range while ozone will fall into the low Moderate range. In areas south of the frontal boundary, PM2.5 and ozone will remain in the Moderate range.

By Friday, the cold front will travel down to northern Virginia sparking rain and thunderstorms along the way, with high pressure in its wake. The incoming air mass will bring dry northerly flow from Canada, cleaning out the area behind the front. The northern part of the Mid-Atlantic will see cleaner conditions of air quality to the Good range while in the southern part of the region, ahead of the cold front, air quality will still be Moderate. The front will linger around central VA until Saturday as a stationary front resulting in convection in VA with a surface trough extending to NC. If the ECMWF forecast verifies, as discussed above, precipitation Friday night into all day Saturday will keep ozone and PM2.5 concentrations to the Good range. The exception will likely be NC, where the humid and modified air mass will remain in place and keep the chance for Moderate air quality. If the GFS forecast verifies, air quality will likely remain in the Moderate range across the region.

-Catena/Huff