Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 5, 2014
Valid: June 6 – June 10, 2014 (Friday-Tuesday)

Summary:

As high pressure moves overhead, the weekend will be warm and sunny. Friday will have good air quality throughout the region due to clean flow from Canada and deep mixing due to cool air aloft. Saturday will see scattered regions of Moderate ozone due to stagnant conditions and the retreat of upper level cool air. PM2.5, however, should stay in the Good range as humidity remains low. Warm air advection and a rise in humidity Sunday will increase both PM2.5 and ozone to the Moderate range. For Monday and Tuesday, a cold front is expected, which will create convection and precipitation, and a decrease in ozone and PM2.5 to the Good range.

Discussion:

There is good agreement between the weather forecast models through the weekend. There are differences on the timing of the onset of precipitation Monday with the ECMWF and NAM beginning precipitation about 12 hours earlier than the GFS, but the Tuesday forecasts are quite similar. As a result, there is higher than average confidence in the weather and air quality forecasts.

The upper level trough that is affecting our region today will move east with the axis of a weak ridge reaching the I-95 Corridor by early Sunday. All the forecast models agree on this large-scale feature but diverge beginning late Sunday on the timing and strength of a series of short wave disturbance that will form in its wake. The ECMWF and NAM have several weak disturbances strung out to our west from PA to KN late Sunday, while the GFS concentrates most of the energy in a single strong trough near IL. This difference will have more implications by mid-week as they develop a “cut off” upper level low in very different locations. For the medium range, however, the only air quality relevant issue is the timing of precipitation on Monday. We lean to the earlier onset (1200 UTC Monday) predicted by the NAM and ECMWF.

Friday will be a gorgeous and clean summer day. By 12Z, the cold front will pass through the region and leave behind an area of high pressure and clear skies. The advection of upper level cool air will increase mixing and reduce air pollutant concentrations. Clean and dry air will flow in from Canada reducing both PM2.5 and ozone concentrations to the Good range.

There are differences between the models on the timing of warm air advection on Saturday. As we are leaning to the NAM/ECMWF solution overall, we will follow a slower onset of warm air aloft on Saturday. While transport aloft continues from Canada, concentrations of both ozone and PM2.5 are expected to rise due to increased sunshine, light near surface winds, and limited vertical mixing. PM2.5 will stay in the Good range due to clean and dry northerly flow from Canada, but there will be scattered areas of Moderate ozone throughout the Mid-Atlantic.

As the center of high pressure moves slowly eastward through the mid-Atlantic Sunday, skies will be clear with temperatures in the low to mid 80’s and rising humidity. Rising humidity and periods of calm Sunday will allow PM2.5 to reach the Moderate range in the north, while it will stay in the Green range in the south. Ozone will rise further in the Moderate range.

Warm air aloft advects over the southern part of the region starting Monday morning, stabilizing the atmosphere. Winds will become light southerly. We expect clouds with rain and possible thunderstorms for both Monday and Tuesday. Concentrations of ozone and PM2.5 should stay in the high Good range for both days.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 4, 2014
Valid: June 5 – June 9, 2014 (Thursday-Monday)

Summary:

As low pressure slowly moves through the northern Mid-Atlantic region, precipitation and convection will clean out the area. Good air quality is expected for Thursday. High pressure will arrive Friday along with a clean air mass from Canada. Good air quality Friday with a slowly modifying air mass over the weekend as high pressure moves overhead. Saturday is expected to show scattered regions of Moderate air quality due to increased sunshine and light winds. Sunday and Monday will show an increase in both ozone and PM2.5 from stagnant air and increased moisture.

Discussion:

Models are in close agreement until Friday. However, on Friday, the NAM and ECMWF both show cool air arriving faster than the GFS at 850 mb. This would call for earlier ventilation of the boundary layer and cleaner air quality conditions. The GFS then shows the cooler air retreating faster than the other two models. By 18Z Saturday they generally agree that most of the cool air aloft has retreated, allowing warmer air to advect over the area. At 500 mb, the ECMWF and the GFS are quite similar in timing and location of shortwaves until they begin to diverge on Sunday morning.

A low pressure system, and its associated trough, will slowly move through the northern Mid-Atlantic Thursday. Precipitation will begin early on Thursday and clear the northern mid-Atlantic by mid-afternoon. Warm air aloft will linger over the southern mid-Atlantic along with precipitation and cloud cover. With rain and clouds likely, concentrations of both ozone and PM2.5 are expected to be in the Good range.

Friday will be a beautiful day as an upper level ridge from a high pressure system over the Great Lakes region extends over the Mid-Atlantic. A clean air mass will flow south from Quebec, which will reduce PM2.5 and ozone. There will be few to scattered clouds throughout the day and cool air aloft. The NAM persists in developing clouds along with a small disturbance running along the ridge but the other models are not in agreement on this. We suspect clouds will be limited Friday. In the southern part of the region, however, there is a chance for rain, as warm humid air will linger. This will further reduce ozone concentrations. PM2.5 and ozone will remain in the Good range for Friday.

The high pressure will move overhead Saturday. It will be clear with the exception of the southern mid-Atlantic. Northerly transport from Quebec will continue so that the incoming air mass will be clean. In addition, vertical mixing will be deep due to cool air aloft. However, winds will be near calm for most of the day Saturday. Thus, PM2.5 concentrations will be in the Good range as the air mass should not moisten and modify very quickly. Ozone will stay in the Good range for the most part with scattered regions of Moderate.

A low pressure system from the Southwest will move northward to the Great Lakes region by 12Z Sunday, bringing its attached warm front through the Mid-Atlantic. There will be a rise in dew point and temperatures and a chance for clouds and rain showers later in the afternoon. Sunday is expected to see a rise in both PM2.5 and ozone to low Moderate/high Green. By Monday, the low pressure system will move northeastward to Quebec bringing its cold front through the Mid-Atlantic. This will bring on a chance for rain showers, although it is uncertain how strong they will be. Ozone and PM2.5 concentrations could drop depending on precipitation intensity.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 3, 2014
Valid: June 4 – June 8, 2014 (Wednesday-Sunday)

Summary:

As a cold front slowly moves through the Mid-Atlantic region PM2.5 will rise into the low Moderate range and O3 will be in the Good to low Moderate range. Good air quality Thursday due to convection and precipitation. High pressure will begin to move into the region Friday along with northerly clean flow from Canada. Good air quality Friday with a slowly modifying air mass over the weekend as high pressure moves overhead. Moderate air quality possible Saturday and likely on Sunday as mixing decreasing.

Discussion:

The forecast models diverge on the location and timing of a short wave disturbance moving in from the west on Thursday. However, these differences won’t have a huge impact on air quality concerns as all models predict significant precipitation during the day Thursday across the mid-Atlantic. They agree on the timing of the arrival of precipitation, roughly 1200 UTC Thursday but they vary as to the location of the heaviest precipitation and the ending time. The GFS moves the shortwave more southward while the ECMWF is more to the north and the NAM is well to north of both. We will consider the NAM an outlier due to its recent consistency with sending the shortwave more northward than both the ECMWF and the GFS. In any event, all models agree on ridging and fair weather for the weekend.

For Wednesday, a weak cold front will slowly cross the mid-Atlantic. Back trajectories predict clean air from a source region in southwestern Ontario. The models all agree that Wednesday looks to be mostly sunny and dry until late. The models show warmer air aloft in the southern part of the region, which translates to more stable conditions and stagnant air. PM2.5 concentrations will reach the low Moderate range in the southern part of the region and the high Good range in the northern part. Ozone concentrations will be in the Good to low Moderate range. Highest ozone will occur east of the I-95 Corridor with magnitudes depending on the possibility of late day recirculation from the sea breeze.
By 12Z Thursday, the low pressure system from Ohio moves to the Delmarva Peninsula (we are discounting the more northern NAM track). Although the models show differences, as noted above, they agree that there will be precipitation starting early Thursday morning and lasting until the afternoon. Precipitation will be strong enough to bring PM2.5 concentrations back into the high Good range while clouds and rain keep ozone concentrations in the Good range as well.

The center of the low and its associated cold front will move through New England Friday with a region of high pressure centered over Indiana. Northerly air will flow from Quebec bringing in cleaner air conditions. There is expected to be lingering precipitation in North Carolina for most of Friday. Concentrations of both ozone and PM2.5 will remain in the Good range.

The region of high pressure will move overhead by Saturday 12Z. Good PBL mixing is expected on Saturday with good to low moderate air quality. As we work into June, the long day length and sunny skies may allow scattered locations to reach the moderate range for ozone.

A low pressure system from the Midwest will move towards the Mid-Atlantic on Sunday but high pressure will remain in control as it will not pass through until Sunday night or Monday morning. Warm air advection will reach the area by 18Z Sunday, limiting mixing and leading to moderate ozone and PM2.5 concentrations.

-Catena/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 2, 2014
Valid: June 3 – June 7, 2014 (Tuesday-Saturday)

Summary:
This week will see a rise and fall in PM2.5 and ozone concentrations. Due to differences between the weather forecast models of the time and location of precipitation, there is still uncertainty in the air quality forecast. As high pressure moves away from the Mid-Atlantic region on Tuesday, an upper level Canadian low pressure system will extend its associated trough over the region, creating showers and thunderstorms in the afternoon. Ozone and PM2.5 will be in the high Good/low Moderate range. As the cold front from the Canadian low pressure stalls just to the west of the Mid-Atlantic region, sunny and warm conditions will occur on Wednesday, leaving ozone and PM2.5 concentrations in the Moderate range. Due to timing and location differences of precipitation in the models, air quality is still uncertain for Thursday, although it is expected to improve to the Good range. As for Friday and Saturday, a high pressure system will move overhead bringing the concentrations back into the Moderate range.

Discussion:

The weather forecast models are in good agreement through Wednesday but begin to diverge Thursday as a result of different treatments of a convectively active short wave trough exiting the Midwest. For air quality concerns, good confidence in the weather forecasts through Wednesday, much less Thursday and Friday.

In the large scale, an upper level trough moves to the western Great Lakes on Tuesday, then through New York State on Wednesday. Although there are some differences on the timing and position of the trough and its attendant precipitation (as will be detailed below in the dailies), precipitation and clouds will affect the I-95 Corridor from west to east on Tuesday. The next major feature will be a convectively active trough that will be located near IA on Wednesday and then move east. There are large differences in how the models handle this smallish-scale feature as you might expect. As of the 0600 UTC runs, the NAM appears to be an outlier in driving a powerful wave into OH on Thursday and the GFS appears to have some convective feedback problems, so we will side with the ECMWF for the Thursday-Saturday period.

Daily Forecasts: As noted above, the weather forecast models are in close agreement for Tuesday other than a few timing discrepancies. The GFS is about 3 hours ahead of the ECMWF and the NAM with respect to precipitation along the I-95 Corridor on Tuesday but all models have mid-late afternoon rain. Air quality will be in the Good/Moderate range depending on timing of precipitation.

Wednesday is expected to be dry with temperatures in the lower 80’s F, NW winds at the surface and reasonably good PBL mixing. This is likely to translate to Moderate air quality for both ozone and PM2.5.

Thursday, as noted above, is a highly uncertain forecast due to timing, extent and strength of convection associated with the short wave approaching from the Midwest. Going mostly with the ECMWF, we expect significant rain on Thursday improving air quality to the Good range.

The cold front associated with the active short wave will move to the southern part of the Mid-Atlantic region on Friday. Surface high pressure over the Great Lakes region will extend over the Mid-Atlantic creating stable conditions. On Saturday, the warmer air aloft retreats in the southern region, which will create less stable conditions and more PBL ventilation. However, high pressure overhead will create sunny skies and a rise in dew point. There will be scattered to few clouds with light winds for both days. Low Moderate air quality expected for both Friday and Saturday.

-Ryan/Catena

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 1, 2014
Valid: June 2-6, 2014 (Monday-Friday)

Summary:

Temperatures and humidity will increase on Monday as the axis of a weak upper level ridge briefly builds over the Mid-Atlantic region. Ozone will reach the Moderate range, with higher mixing ratios in the eastern part of the region. PM2.5 will increase into the low Moderate range at scattered locations in response to rising humidity. Widespread clouds and convection on Tuesday afternoon will limit rising ozone to the low Moderate range, except for the locations along and east of I-95, where afternoon sun will allow ozone to reach the upper Moderate range. Uncertainty remains in the air quality forecast for the remainder of the period due to questions about clouds and precipitation associated with a slowly moving and stalling frontal system that will impact the region Wednesday to Friday, but Good to Moderate conditions are expected.

Discussion:

The medium range period features a complex and unsettled weather pattern for the Mid-Atlantic region. The weather forecast models are in general agreement on the main features of the period, but differ in many of the details. An upper level ridge over the region on Monday will give way to a shortwave trough that will move across southern Canada, from the northwestern Great Lakes on Tuesday to the Canadian Maritimes on Friday (06Z GFS) or Saturday (00Z ECMWF). As the shortwave trough flattens the ridge over the Mid-Atlantic, a series of large and small shortwaves will zip across the region in the zonal flow aloft. These shortwaves will act as triggers for rain and convection across the Mid-Atlantic essentially daily from Tuesday through Friday. Since the operational models differ in the timing and location of many of these shortwaves, a great deal of uncertainty exists for the precipitation forecast. The WPC is keeping the chance for precipitation across the region during the medium range period in response to this uncertainty, with the chance for slightly higher amounts of precipitation in the northern Mid-Atlantic on Wednesday through Friday. The chance for higher precipitation in the northern part of the region is in response to a frontal system that will enter the region from the northwest on Wednesday, pulled by the weakening shortwave trough aloft over southern ON/QC. A large area of shortwave energy aloft will develop a low along this frontal system, over PA, on Thursday. This low will lift to the northeast on Friday, pulling the cold frontal boundary to approximately central VA, where it will stall though at least Saturday. This slowly moving/stalling frontal system increases the uncertainty in the air qualify forecast for Wednesday through Friday.

With the upper level ridge overhead on Monday, and the center of surface high pressure settling over NC, temperature and humidity will begin to increase. Warm air advection aloft and recirculating back trajectories will favor rising pollutant concentrations. With mostly sunny skies along the eastern Mid-Atlantic, ozone will rise into the mid-Moderate range. The weather forecast models have backed off on yesterday’s predictions of widespread clouds and afternoon thunderstorms in the western Mid-Atlantic. The 12Z run of the 4 km NAM has scattered rain showers in the morning followed by isolated thunderstorms in the afternoon. The 09Z SREF keeps the highest probability of precipitation in northwestern PA on Monday afternoon. As a result, ozone could reach the low-Moderate range in parts of the western Mid-Atlantic. PM2.5 will be on the rise into the Moderate range at scattered locations as well, in response to increasing humidity.

The uncertainty in the forecast begins on Tuesday. It will be warm and humid, with southwesterly flow both aloft and at the surface. Tuesday still appears to be the day with the best chances for widespread convection, with the entire region in the warm sector of the approaching weak low pressure system. The 4 km NAM develops a line of thunderstorms and moves it across the Mid-Atlantic from west to east in the afternoon, although the storms dissipate as they reach roughly I-95. The SREF has a high probability of precipitation across the region in the afternoon except along the east coast, roughly along and east of I-95. Thus rising ozone mixing ratios will likely be limited to the upper Good to low Moderate range for most of the region except the eastern coast, where clear skies through the late afternoon will give ozone the chance to increase into the upper Moderate range. The BAMS and NC air quality models support this idea, and have scattered Moderate conditions along the northeastern Mid-Atlantic. In the increasingly humid and modified air mass, widespread Moderate PM2.5 is likely.

The air quality forecast for Wednesday to Friday is highly uncertain, but Good to Moderate conditions are expected. Warm and humid weather will continue. With low pressure developing at the intersection of the warm and cold frontal boundaries over central PA on Wednesday, and then the slow progression of the cold front to roughly central VA on Thursday, where it will stall, widespread clouds and rain are likely in the vicinity of the front(s). With a series of shortwaves moving over the region, development of an MCS is not out of the question. Areas that experience clouds and heavy rain will observe Good air quality conditions. There will likely be a dividing line of cleaner air to the north of the frontal boundary and modified air to the south. But it is not clear to what extent the weak front will displace the warm, humid, and increasingly modified air mass that will be in place over the region. The air quality models keep the chances for Moderate ozone in the northeastern Mid-Atlantic on Wednesday and scattered-to-isolated Moderate PM2.5 in the northern and parts of the region through Thursday.

-Huff