Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 15 2014
Valid: June 16 – June 20, 2014 (Monday-Friday)

Summary:

A pre-frontal trough developing over the Mid-Atlantic Monday will create scattered rain and thunderstorms for the rest of the week. Monday will see an increase in ozone and PM2.5 concentrations to the Moderate range due to increased moisture, westerly transport, and mostly sunny skies. Air quality will remain in the Moderate range for the rest of the week as Tuesday, Wednesday, and Thursday will all have increased humidity, scattered rain and thunderstorms with mostly to partly sunny skies, and stagnant conditions. The cold front will pass through the northern part of the region Friday creating mixing and convection. Air quality will be reduced to the Good range.

Discussion:

The weather forecast models show multiple discrepancies after Monday. However, the models do agree to the start of warm air advection aloft Monday, which will result in a stable and stagnant atmosphere lasting the rest of the week. In regards to precipitation at the surface, Tuesday and Friday show great uncertainty in timing and location of precipitation, which could possibly affect ozone concentrations. The models at 500 mb also show differences in timing and location of small scale disturbances starting Tuesday, creating a chance for scattered showers and thunderstorms each day of the week. They do agree on mainly zonal flow aloft until Thursday morning when the ECMWF develops a trough over the Mid-Atlantic. This will result in Thursday rain and cleaner concentrations if the ECMWF were to verify. The models also agree on the development of a large ridge to the west of the region starting Wednesday morning. However, the GFS and the ECMWF start to diverge Thursday when the ECMWF extends the ridge farther north than the GFS, calling for more intense storms to hit the region Thursday.

Monday will see a rise in temperatures and dew points as the high pressure system moves offshore. A surface trough will develop down the region creating the chance for clouds and some mixing. Warm air advection aloft, however, will inhibit most mixing. There will be mostly Moderate ozone with some areas of Good. PM2.5 will also reach the Moderate range from westerly transport and light surface winds.

Tuesday will have a chance of scattered rain and thunderstorms as the pre-frontal trough develops down the region. However, as stated above, there is still great uncertainty in where and when these storms are going to hit. There will be scattered clouds throughout the day with light westerly flow and a steep rise in temperatures and dew points. PM2.5 and ozone will remain in the Moderate range.

Scattered rain and thunderstorms will mainly hit the northern part of the Mid-Atlantic and last throughout the day Wednesday as the pre-frontal trough lingers over the region. With mostly sunny skies in-between storms and southwesterly flow, PM2.5 and ozone will stay in the Moderate range with scattered regions of reduced ozone in the Good range.

The cold front will travel southward to northern PA by 12Z Thursday. Most of the rain will arrive by evening with some scattered clouds during the day. There will be warm air advection aloft calling for PM2.5 accumulation later in the afternoon keeping air quality in the Moderate range. By Friday, the cold front will move to the MD/VA border, bringing the pre-frontal trough south. This will result in scattered rain and thunderstorms throughout the day, yet timing and location are still uncertain. Mostly cloudy skies, convection, and mixing will reduce air quality to the Good range.
-Catena

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 14, 2014
Valid: June 15 – June 19, 2014 (Sunday-Thursday)

Summary:

Sunny and warm weather for Sunday and Monday as high pressure remains overhead. Northerly flow from Canada will keep PM2.5 in the Good range along with scattered regions of Moderate ozone for Sunday. With increasing humidity, sunshine, and a more stagnant atmosphere, air quality will reach the Moderate range for Monday. As a pre-frontal trough develops Tuesday morning and lingers for the rest of the period until Thursday, there is a daily chance for scattered rain and thunderstorms. Westerly flow and little mixing will keep air quality in the Moderate range for all three days, yet ozone could possibly reach Code Orange depending on the timing and arrival of storms and cloud cover.

Discussion:

The weather forecast models are in good agreement on the large-scale phenomenon but mostly disagree on the small mesoscale features after Monday. They strongly agree to sunny weather until Monday night when they start to diverge from one another. They also agree on warm air transport aloft starting Monday morning, lasting until the rest of the period. This will create a stable atmosphere allowing ozone and PM2.5 to easily accumulate.

The models have come in slightly better agreement for the upper air forecast for Tuesday. Zonal flow aloft will last for most of the day as an upper level ridge builds up to the west of the Mid-Atlantic. The GFS, however, develops a more pronounced trough over the Northeast U.S. starting early Tuesday, suggesting that small mesoscale disturbances will travel further south than predicted. The medium range models disagree on the timing and location of MCS’s to hit the area Tuesday through Thursday, however, creating a chance for precipitation and cloud cover each day, which could limit ozone accumulation.

Sunday will be warm and sunny with high pressure overhead. Northerly Canadian transport will keep the day dry and clean with a high atmospheric boundary layer. PM2.5 will stay in the Good range while scattered regions of Moderate ozone will develop throughout the Mid-Atlantic from cloudless skies.

High pressure will remain overhead for Monday with northerly flow transitioning to southwesterly recirculation from VA by the evening. Transport of warm air aloft will result in a stagnant atmosphere with light winds and little mixing. PM2.5 will reach the Moderate range along with ozone due to increased sunshine.

By Tuesday, a pre-frontal trough will develop down the Mid-Atlantic ahead of an approaching cold front. There will be a chance of showers and thunderstorms throughout the day with westerly flow from the Ohio River Valley. Tuesday will be hot and humid as temperatures and dew points will rise to above normal. Air quality will remain in the high Moderate range for Tuesday, although there is a chance for ozone to reach the USG range depending on precipitation and cloud cover.

The pre-frontal trough will linger over the Mid-Atlantic for Wednesday and Thursday. By 12Z Thursday, the cold front will move to northern PA. As stated above, there is still uncertainty in the timing and location of rain and thunderstorms to hit the region for both days. PM2.5 will stay in the Moderate range while ozone could reach the USG range depending on the intensity of the storms and when they arrive, mostly sunny skies, westerly flow from the Ohio River Valley, and light winds.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 13 2014
Valid: June 14 – June 18, 2014 (Saturday-Wednesday)

Summary:

High pressure will move overhead and remain for the weekend bringing dry northerly flow and sunshine. Ozone concentrations will reach the Moderate range while PM2.5 will stay in the Good range for both days. There is a chance for scattered precipitation Monday as a low moves northward west of the Mid-Atlantic. Monday will mostly be sunny with rising temperatures, resulting in Moderate air quality. Tuesday and Wednesday will see a rise in ozone, possibly to the USG range (Unhealthy for Sensitive Groups) and Moderate PM2.5 due to zonal flow aloft and westerly flow from the Ohio River Valley in the transport layer.

Discussion:

There is high confidence in the weather forecast models through Monday. They agree on an upper level trough passing through the Mid-Atlantic early Sunday with an upper level ridge developing in its wake. The NAM is a faster with the upper level ridge Sunday and we follow the medium range models instead. By 1800 UTC, Sunday all the forecast models come into agreement. Zonal flow is the consensus with a weak ridge building over the mid-Atlantic. By Tuesday, however, the medium range models diverge significantly. The ECMWF phases a short wave over the northern US with a stronger system in Canada with the result that an upper level trough drops into the eastern US and a back door cold front drops into the northern mid-Atlantic late Wednesday with rain likely. The GFS, on the other hand, keeps a ridge in place through the period. The GFS solution points to high ozone in the Tuesday-Thursday period while the ECMWF does not. As a result, we have little confidence in the forecast beyond Monday.

In the dailies, a cold front passes through the mid-Atlantic Saturday morning with a surface high building in later in the day. Other than along the NC coast, where clouds and rain may linger, there will be Moderate ozone due to strong early June sun. PM2.5 will stay in the Good range due to northwesterly clean and dry flow.

By Sunday morning, high pressure will move directly overhead although the back trajectories support a continued northerly flow of clean air from Canada aloft. Warm air advection aloft will start in the evening leading to reduced mixing and stagnant winds late in the day. Because of this, ozone will reach the mid Moderate range while PM2.5 will remain in the Good range due to an unmodified air mass.

The low will move northward to Canada Monday bringing its warm front northward west of the Mid-Atlantic. This could create scattered showers throughout the day although it will not have that much of an effect on the region regarding air quality. There will be sunny and warm weather Monday with the high still in place, and dew points will be on the rise. Forecast back trajectories predict recirculation on Monday that will increase both ozone and PM2.5. Ozone and PM2.5 will be well into the Moderate range on Monday with late day peaks likely.

By Tuesday, the low will move northeastward and a lee side trough will develop over the Mid-Atlantic region ahead of the approaching cold front. The medium range models suggest convection may develop in association with this trough (especially the ECMWF) but there is high uncertainty with this feature. There will be westerly flow aloft both Tuesday and Wednesday from the Ohio River Valley, indicating dirtier transport into the area. High pressure will remain overhead with the lee side trough still extending into the area. Both days are expected to have scattered rain and thunderstorms with partly sunny skies. PM2.5 will be well into the Moderate range for both days. There is a chance for ozone to reach Code Orange in the Tuesday-Thursday time frame but the model forecasts are not consistent enough at this time to have confidence in the forecast.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 12 2014
Valid: June 13 – June 17, 2014 (Friday-Tuesday)

Summary:

Deep convection with rain and thunderstorms will occur Friday as the cold front passes through the region. Both ozone and PM2.5 concentrations will be in the Good range. High pressure will linger overhead for the weekend bringing in northwesterly winds, a dry air mass and sunshine. Ozone concentrations will reach the Moderate range while PM2.5 will stay in the Good range for both days. There is still uncertainty on Monday and Tuesday regarding the timing and location of scattered rain and thunderstorms. PM2.5 will reach the Moderate range due to increased moisture and stagnant air. There will be scattered regions of Moderate ozone with peak concentrations depending on the occurrence of precipitation.

Discussion:

The weather forecast models are generally in good agreement on the large scale weather features through Sunday and diverge thereafter. However, there are some differences in the location of an upper level trough moving through the northern states. The ECMWF extends the trough further south into PA Saturday morning than the GFS and especially the NAM. The ECMWF solution implies morning rain in the northern part of the Mid-Atlantic and better air quality than either the NAM or the GFS. As a consequence of the difference in track of the trough, by Monday morning the GFS and ECMWF differ on the location and timing of the onset of warmer air aloft from the south and west. However, by 1800 UTC Monday, they agree that there will be warm advection aloft (WAA) over the entire region that will persist through the remainder of the forecast period.

As the upper air ridge axis moves east of the mid-Atlantic early next week, the weather forecasts become very uncertain. In fact, by late Tuesday, the GFS and the ECMWF are almost out of phase at 500 mb with the GFS predicting a rebuilt ridge over the eastern US – a high ozone pattern – and the ECMWF developing a trough over the Great Lakes.

In the daily forecasts, deep convection will occur along the pre-frontal trough over the Mid-Atlantic Friday. The convection is expected mainly in the second half of the day. As we will be located in the warm sector, heating should create some mixing and reduce any low level clouds. These conditions will keep air quality in the Good range.

The cold front will move parallel to the coast by Saturday morning, making room for a region of high pressure to move in. Saturday day will be sunny with dry northwesterly flow. With intense mid-June sun, ozone will reach the low Moderate range while PM2.5 will stay in the Good range.

Good weather will continue Sunday with the exception of a chance of afternoon and evening showers in VA and NC, as the cold front from Saturday becomes a stationary front and moves northward. Sunny and warm conditions will keep ozone in the Moderate range. Although we will see a slight rise in PM2.5, dry air and mixing will keep concentrations in the high Good range.

As stated above, there is an uncertainty in the timing and location of rain and thunderstorms for Monday and Tuesday. Both days are expected to have a chance of scattered storms with partly sunny skies in-between. Recirculation of back trajectories and a rise in dew points will increase PM2.5 to the Moderate range for both days. Ozone should reach the Moderate range yet it will depend on the arrival of precipitation and clouds.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 11 2014
Valid: June 12 – June 16, 2014 (Thursday-Monday)

Summary:

Thursday and Friday will see a reduction in ozone and PM2.5 concentrations. There will be scattered rain and thunderstorms both days as the Canadian low moves its attached stationary front and following cold front through the Mid-Atlantic. High pressure will move into the area Saturday, with sunny conditions. A chance for afternoon rain in the southern mid-Atlantic will continue until Monday, limiting ozone to the Good range yet there will be scattered regions of Moderate ozone in the northern part. PM2.5 will stay in the Good range for the rest of the period due to the incoming dry and clean air mass.

Discussion:

The weather forecast models once again agree on the large scale phenomenon yet still disagree somewhat on the smaller scale features. The timing and location of small shortwave disturbances crossing the region Thursday still differ from model to model. The ECMWF and the GFS both agree on the location and timing of the upper level trough in Canada above the Great Lakes region until 18Z Friday. The ECMWF brings the trough further southeastward as the GFS and the NAM bring it northeastward. The ECMWF extends it southward to the top of the Mid-Atlantic on 12Z Saturday while the GFS and the NAM have it up in Canada. If the ECMWF were to verify, this would call for a greater chance of morning showers on Saturday. By 18Z of that day all three models agree on the flattening of the trough over the region. The models again diverge on Monday with timing and location of shortwaves.

The stationary front will linger over the northern Mid-Atlantic all day Thursday and will serve as a focus for scattered rain and thunderstorms throughout the region. Onshore flow from the Atlantic will also clean out the region, reducing PM2.5 concentrations to the Good range. Ozone will be limited to the Good range due to low level clouds and convection all day.

As the stationary front moves northward as a warm front on Friday, a lee-side trough will develop parallel to the I-95 Corridor ahead of the approaching cold front. Significant convection will occur along the trough creating heavy rain and thunderstorms. Air quality will be well into the Good range.
By early Saturday morning the cold front will be east of the I-95 Corridor. A region of high pressure will move overhead providing calm and sunny conditions. There is a chance for evening rain in the southern region, however, bringing cleaner conditions in VA and NC. The cold front will bring in clean dry air, which will keep PM2.5 in the Good range. Due to increased sunshine, there will be scattered regions of Moderate ozone, mainly in the northern Mid-Atlantic.

The high pressure will remain overhead for Sunday and Monday as well. However, the chance for showers and thunderstorms remains in the afternoon for the southern region. By Monday night, the models agree on region-wide rain and thunderstorms. PM2.5 will stay in the Good range for both days. Ozone, however, will depend on the timing and intensity of the storms, but it is expected to reach the Moderate range.

-Catena/Ryan