Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 10 2014
Valid: June 11 – June 15, 2014 (Wednesday-Sunday)

Summary:

A back door cold front will serve as a focus for clouds and showers through Friday. Air quality will be in the Moderate range Wednesday due to partly sunny skies, humidity, and showers likely to hold off until evening along and east of the I-95 Corridor. Thursday and Friday will see a reduction in both ozone and PM2.5 to the Good range due to strong convection and clouds throughout the day. High pressure will move into the area on Saturday setting the stage a beautiful weekend with sunny skies and quiet weather. Moderate air quality will develop from these conditions.

Discussion:

The weather forecast models are once again in good agreement with each other and are internally consistent run-to-run. A deep trough over the central US will slowly move east and weaken. Near the surface, a stationary front will remain stalled over the mid-Atlantic until a cold front arrives very early Saturday.

The differences in the models, as noted in earlier discussions, are the timing, location and extent of precipitation. The forcing mechanism for these showers will be small (mesoscale) disturbances causing instability in a very humid air mass near the stalled frontal boundary. The GFS is, overall, the wettest model with significant rain forecast for Wednesday. The other models, including the 4 km versions of the NAM are less aggressive with rain. By Thursday, all models agree on widespread rain and convection.

The stationary front stalls over the Pennsylvania and Maryland border Wednesday, slightly moving southward as a back door cold front by evening. There will be afternoon precipitation mainly in the northern part of the region but will cover the entire region by the evening. Warm air transport aloft will limit mixing and create a stagnant atmosphere. PM2.5 will rise into the Moderate range along with ozone as there will be partly sunny skies between rain and thunderstorms. The exception will be west of the I-95 Corridor where rain will occur earlier.

The warm front will drift north to Maryland Thursday. A chance of scattered showers in the morning, but rain and thunderstorms are more likely to occur in the afternoon and evening. Clouds and convection will bring ozone and PM2.5 down into the Good range.

As the low moves its warm front northward into NY on Friday, a lee-side trough will develop west of the I-95 Corridor. There will likely be low level convergence along the trough and it will serve as a focus for convection and precipitation. Weather will be similar to Thursday with greater chance of showers and thunderstorms during the second half of the day. Ozone and PM2.5 will remain in the Good range.

As the cold front passes through the Mid-Atlantic early Saturday, a region of high pressure will move in west of the area. The upper level short wave will flatten out by late afternoon Saturday, leading to sunny weather. This pleasant weather pattern will last through Sunday as the high pressure moves over New York. The incoming dry air mass from the Northwest will reduce the PM2.5 to the Good range. Ozone, however, will stay in the Moderate range both days due to increased sunshine.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 8, 2014
Valid: June 9 – June 13, 2014 (Monday-Friday)

Summary:

An unsettled weather pattern is expected through the medium range. Temperatures will be in the low-mid 80’s F with partly to mostly cloudy skies and a chance of rain each day. The best chances for sustained rain will be Monday and Friday. Air quality will oscillate between Good and Moderate levels depending on the extent of clouds and precipitation at any location.

Discussion:

The weather forecast models are in good agreement through the medium range period with respect to the large scale features. Unfortunately for air quality concerns, the key air quality relevant weather factors will be embedded in mesoscale features that are more difficult to forecast beyond the short range.

A weak short wave will cross the Mid-Atlantic early Monday while a deeper trough to the west will “cut off” and stall over IL and move very little Tuesday and Wednesday. Finally, a “kicker” (short wave disturbance) from the northern Plains will push the remnants of the trough eastward Thursday, reaching the Mid-Atlantic early Friday. With southerly flow at low levels, a very humid air mass will be in place through the period so that any minor disturbances embedded in this generally unsettled weather pattern have the ability to produce precipitation and clouds. The weather forecast models differ on the timing and extent of precipitation after Monday and aren’t very well able to resolve these features. As a result, confidence in the day to day air quality forecasts depends on our confidence in the day to day precipitation forecasts. That is to say, not very good after Monday.

On Monday, an upper level disturbance will bring rain to the Mid-Atlantic. As noted above, there are differences in timing, location and extent of rain but cloud cover will be significant across the region. Ozone will be in the Good range, PM2.5 will rise along with humidity and should reach the Moderate range except where rain is heaviest.

Tuesday looks drier and while some locations will see afternoon showers, there will be enough sun to push temperatures into the mid-80’s F and ozone will reach the Moderate range. PM2.5 will continue in the Moderate range as well.

Wednesday and Thursday will be classic “chance of showers” days with very humid air in place and a warm front (associated with the trough stalled to our west) drifting northward through the mid-Atlantic. The precipitation forecasts at this range can’t be trusted so the best bet is to follow persistence with Moderate air quality both days. A cold front approaches on Friday with rain more likely. Ozone likely drops to the Good range but PM2.5 will remain Moderate.

-Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 9, 2014
Valid: June 10 – June 14, 2014 (Tuesday-Saturday)

Summary:

Tuesday and Wednesday will be characterized as mostly cloudy all day with a chance of rain due to the stalling of a warm front over the region. Ozone will reduce to the Good range while PM2.5 will increase to the Moderate range due to warm air advection. Thursday and Friday will both have a greater chance of convection and precipitation as the front travels through the region. The air quality will improve both days, yet PM2.5 will still remain in the low Moderate. As the surface low brings its cold front through the region on Saturday morning, sun will shine for the greater part of the day increasing ozone concentrations. Air quality will be in the Moderate range.

Discussion:

There is high confidence in the weather forecast models for the upcoming days. All the models are in agreement that a deep trough in the Southern Plains will move slowly eastward and weaken, crossing the region on Friday. Give or take some minor discrepancies, the NAM, ECMWF, and GFS all show very similar predictions at the surface, 850 mb, and 500 mb. On Tuesday, however, the ECMWF calls for less precipitation, which would lead to an increase in air pollutants.

A surface stationary/warm front will slowly move northward through the region on Tuesday creating humid conditions and mostly cloudy skies. There will be afternoon rain in the northern part of the region, and we expect better air quality to the north. An upper level ridge, however, will keep most of the Mid-Atlantic dry. Mostly cloudy skies and limited mixing will easily bring PM2.5 into the low/mid Moderate range for Tuesday. Scattered regions of Moderate ozone are expected as well.

The surface warm front will inch northward on Wednesday leaving the Mid-Atlantic region in the warm sector with temperatures and dew points rising to above average. The upper level ridge, still in place, should reduce the chance of rain, but there will still be localized showers and thunderstorms on Wednesday. PM2.5 will stay in the Moderate range, while ozone will be reduced to the high Good range.

By Thursday, the surface warm front will move southward over Maryland. The weather forecast models all agree that there will be significant divergence aloft over the entire region. With enough sun, and plenty of moisture in place, this combination will destabilize the atmosphere, and increase chances for convection and rain. There is a high chance for rain and thunderstorms all day Thursday. Ozone will remain in the Good range, while southerly flow and warm air advection aloft will keep PM2.5 in the Moderate range.

A good chance for rain and thunderstorms will continue on Friday as a trough extends over the Mid-Atlantic. There lies a good chance that convection will become organized over the prefrontal trough running parallel to I-95 Corridor. The air quality will improve bringing ozone and PM2.5 concentrations to the Good range. The low will carry its cold front through the region on Saturday leaving early morning showers and sunshine for most of the day. Air quality will reach the Moderate range.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 7, 2014
Valid: June 8 – June 10, 2014 (Sunday-Thursday)

Summary:

An approaching low pressure system will end the sunny weekend with clouds and a slight chance for rain Sunday night. However, due to humid and hot conditions all day, ozone and PM2.5 will reach the Moderate range. The low front will bring scattered rain and thunderstorms Monday, dropping the ozone to the Good range, but not enough to lower to PM2.5, keeping it in the Moderate range. There is still major uncertainty in Tuesday’s forecast due to weather forecast model discrepancies. Good to Moderate ozone and PM2.5 depending on timing and location of precipitation and cloud cover. Wednesday and Thursday will also bring on scattered rain and thunderstorms, limiting ozone to the Good range. PM2.5, however, should stay in low Moderate to high Good.

Discussion:

The weather forecast models are in good agreement on the large scale features. There are some differences regarding the amplification of the upper level trough in the southern Great Plains Monday and Tuesday with the ECMWF being the slowest (most amplified) and the NAM the fastest (less amplified). In any event, they all end up in concert at the end of the period with a progressive trough near IL 1200 UTC Wednesday.

Because the strongest synoptic scale forcing remains well to our west through the forecast period, most of the sensible weather (e.g., rain) will depend on smaller scale features associated with weak frontal boundaries and mesoscale features. As a result, we have low confidence in the timing and extent of cloud cover and precipitation after Monday and this leads to low confidence in the ozone forecast. With high humidly and light to moderate winds, PM2.5 should be relatively stable.

Sunday will be the warmest day and most conducive to ozone formation. However, the latest model runs suggest substantial high cloudiness building in late in the afternoon. This will take the edge off peak concentrations, particularly in the western mid-Atlantic. Rising humidity and light winds will allow PM2.5 to increase. Moderate concentrations for both.

Monday continues to look like cloudy and rainy with Good ozone but Moderate PM2.5. Note that there are major differences between the forecast models on timing of rain Monday and location (north to south) but all models have significant cloud cover.

Large differences on Tuesday with respect to rain although the GFS has deep PBL mixing which might trigger convection late in the day. This is the most uncertain day with ozone and PM2.5 Good to Moderate depending on location of rain and extent of cloud cover.

SE winds dominate Wed and Thursday, ECMWF still much wetter than GFS. Maritime tropical air mass will keep PM2.5 in the low Moderate to high Good (cleanest eastern mid-Atlantic), ozone looks to straddle the Good-Moderate line depending on rain chances.

-Ryan/Catena

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 6, 2014
Valid: June 7 – June 11, 2014 (Saturday-Wednesday)

Summary:

High pressure will be overhead Saturday and slowly move off the coast by Sunday night. Sunshine, clean and dry northerly flow, and the retreat of cool air aloft Saturday will bring scattered regions ozone into the low Moderate range and keep levels of PM2.5 in the high Good/low Moderate range. Sunday will be hot and humid with limited vertical mixing moving both ozone and PM2.5 to the Moderate range. A low pressure system from the west will approach Sunday night bringing scattered rain and thunderstorms throughout the rest of the week. Clouds and precipitation will decrease both air pollutants to the Good range. Warm air advection from the south will move in Tuesday and Wednesday with partly sunny skies. Ozone and PM2.5 should move back into the low Moderate for both days.

Discussion:

The weather forecast models are in good agreement this week with the exception of the timing of the onset of precipitation on Monday. The NAM and the ECMWF show precipitation starting near the I-95 Corridor as early as 1200 UTC Monday while the GFS still lags about 12 hours behind. We will rely on the NAM and the ECMWF for the timing of the onset of precipitation. The forecast models come back into agreement Monday afternoon and for the rest of the forecast period. Specifically, the models all agree on the development of an upper level disturbance in the southern Plains, which will affect our weather later next week. For air quality concerns, there is good confidence in the weather forecast models with the exception of Monday. That being said, the precipitation expected Tuesday and Wednesday will be convective and of the hit-or-miss variety so that local pollutant concentrations may be highly variable.

High pressure will move overhead Saturday, bringing a warm and sunny day. Transport aloft will continue to arrive from Canada, bringing in cleaner and drier air, but near surface winds will diminish by afternoon leading to sea/bay breeze recirculation and rising late day pollutant levels. Although the lingering dry air in the area will keep the PM2.5 down to the Good range, there is a chance for very low Moderate range PM2.5. Sunny skies and sea breeze recirculation will allow ozone to reach the low Moderate range.

Sunday will see an even greater rise in temperatures and dew points. High pressure will drift slowly eastward and significant stagnation is expected through the morning hours. A complex mish-mash of air mass boundaries (see the WPC forecast surface analyses) will move northward into the region late Sunday night, bringing a chance of cloud cover late in the afternoon and late night showers. Overall, Sunday will have stable and humid conditions, decreased mixing, bringing ozone and PM2.5 well into the Moderate range.

Following the ECMWF we expect showers beginning late Sunday at the earliest and continuing on and off until the end of the period. Monday should see a decrease in both PM2.5 and ozone concentrations to the Good range. Two major sources of uncertainty Monday will be the timing, strength and location of showers as well as the timing of the onset of sustained SSE low level winds.

South-southeasterly flow will bring warm air aloft, and maritime air near the surface starting Tuesday morning through the rest of the period. There will be scattered showers and thunderstorms throughout the day with sunshine in-between. Temperatures and dew points will rise to above normal. Both PM2.5 and ozone will rise to the Moderate range in locations where showers are not sustained in duration. Wednesday will have the same conditions as Tuesday with a chance of scattered showers and thunderstorms with partly sunny skies. The air quality will remain in the Moderate range.

-Catena/Ryan