Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 22, 2015
Valid: July 23 – 27, 2015 (Thursday – Monday)

Summary:

As an upper level ridge approaches from the west, clear skies and light winds will prevail across the region for much of the medium range period. The exception will be in the Carolinas on Friday, when a developing coastal low will bring clouds and showers to the southern Mid-Atlantic. Ozone and PM2.5 will increase steadily throughout the week, although the weekend dip in precursor emissions will limit peak concentrations of both to the Moderate range. The forecast models are not in agreement with respect to cloud amount on Monday so that Moderate air quality remains a possibility.

Discussion:

The weather models are in very good consensus throughout most the medium range period, but begin to diverge on Sunday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. A large upper level trough over New England and an upper level ridge over the Plains are currently maintaining a zonal flow over the Mid-Atlantic. This will keep a sprawling area of surface high pressure over the region for the next couple of days as the ridge continues eastward. As a coastal low develops off the NC coast on Friday, the southern Mid-Atlantic (SMA) will see clouds and periods of precipitation. By the time the ridge reaches the Northeast on Saturday, it will weaken considerably compared to its current state. Behind it, a small dip in the flow will begin to intensify into an upper level trough over western ON. Sunday is the first day with a chance for widespread clouds and showers as the trough moves to the north of the region. The GFS intensifies the trough into a closed low moving eastward through southern QC, while the EC keeps the trough in Canada relatively weak and well to the north of the US. A blend of the two models brings a very weak cold front eastward through the Mid-Atlantic on Monday morning, with pre-frontal clouds and showers overnight Sunday and throughout Monday.

Highs in the mid-80s F and dew points in the 50s F will make for a very pleasant Thursday throughout the Mid-Atlantic. With a surface high pressure center developing over WV, westerly winds will diminish throughout the day. Although these calm, sunny conditions are favorable for the formation and buildup of ozone and PM2.5, back trajectories from the Great Lakes and the recent frontal passage will limit pollutant concentrations. In the usual areas downwind of the I-95 Corridor, ozone and particles will likely reach the Good/Moderate threshold.

Friday’s weather will be very similar to Thursday’s, with temperatures and dew points remaining almost constant. Surface winds will see a shift towards northwesterly in the central and northern Mid-Atlantic (CMA, NMA) as a coastal low develops off of Cape Hatteras. This system will keep clouds, showers, and Good air quality in Friday’s forecast for the SMA. Elsewhere, ozone and PM2.5 concentrations will continue to rise into the Moderate range as clear and calm conditions prevail.

Despite the overnight passage of a weak backdoor cold front through the NMA, temperatures and humidity will remain virtually unmoved throughout the region on Saturday. The region will see a third day of clean back trajectories from the Great Lakes, limiting maximum ozone and particle buildup to the Moderate range despite full sun and light surface winds. As the coastal low moves out to sea, the area of Moderate ozone will extend southward from the I-95 Corridor into the Carolinas. Following the SMA washout, PM2.5 will be limited to the Good range along the southern coast. Throughout much of the rest of the Mid-Atlantic, particles will linger around the Good/Moderate threshold due to a lack of commuter emissions.

With the upper level ridge overhead, a widespread area of surface high pressure will keep skies clear and winds calm throughout the region on Sunday morning. Southerly winds in the CMA and NMA will increase humidity, increasing particle concentrations from Saturday. Increasing clouds ahead of an approaching cold front will limit Moderate ozone concentrations to the I-95 Corridor.

The humidity will return in full force on Monday, combining with the workday commuter emissions to bring PM2.5 higher into the Moderate range. The medium range model disagree on the strength of the upper level disturbance passing to the north. The GFS has a stronger solution which suggests clouds are likely Monday. However, the EC has a weaker trough displaced further north, which will allow for more sun. This difference will impact the ozone forecast much more than the particle forecast. For now, we expect Moderate ozone on Monday.

– Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 21, 2015
Valid: July 22 – 26, 2015 (Wednesday – Sunday)

Summary:

Cleaner and drier air will filter into the mid-Atlantic in the wake of a cold front passing through the region today. This new air mass will limit ozone and PM2.5 to the Good range for most of the region on Wednesday. Persistent high pressure will be the rule for the region beginning Thursday. Light winds and clear skies will increase the chances for USG ozone to Marginal for the weekend. As of now, clean back trajectories from Canada should cap ozone production, limiting higher chances for USG ozone.

Discussion:

The weather models are in very good consensus throughout the medium range period. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. A large trough currently over the ON/QC border will continue to slowly move eastward, reaching the Atlantic Ocean on Saturday. This trough will pull a cold front through the Mid-Atlantic today. The cold front will sweep from the northwest to the southeast, reaching SC by 12Z Wednesday. Behind this cold front, an upper level ridge will begin to build into the region. The ridge axis will move eastward from the Plains on Wednesday to the western periphery of the Appalachians on Saturday, maintaining its strength as moves. A weak back door front will reach MD on Friday and dissipate further south Saturday. Weak pressure gradients will be the rule over the mid-Atlantic after the front dissipates.

A cold front will pass through the Mid-Atlantic today, sweeping from northwest to southeast. Scattered convection Tuesday evening across the northern and central Mid-Atlantic (NMA, CMA) will help clean out the atmosphere ahead of the cold front. A drier, cleaner air mass will filter in behind the front on Wednesday. Surface high pressure will also build in behind the front leading to clear skies and steady NW winds across most of the region. The new air mass will limit ozone to the Good/Moderate range with PM2.5 concentrations, currently below 10 ug/m3 upwind behind the front, limited to the Good range.

Surface high pressure will dominate the weather on Thursday, leading to clear skies and slowly diminishing winds throughout much of the region. Scattered shortwaves moving overhead in the southern Mid-Atlantic (SMA) will promote clouds and convection in NC on Thursday, limiting ozone production. Throughout the rest of the region, clear skies and light winds will enhance ozone production, but peak concentrations will be limited to the Good/Moderate threshold for another day. The air quality models are showing upper Good ozone for much of the region, with a few locations in eastern VA seeing Moderate ozone. PM2.5 concentrations should be limited to the Good range on Thursday due to the continuing low humidity with the exception of a few locations in NC and the western Mid-Atlantic.

Clear and calm conditions over the region on Friday and Saturday, allowing ozone to reach into the Moderate range. The main uncertainty will be the possibility of slightly cleaner air behind the weak back door front Friday in the NMA. Emissions on Friday will be more spread out in the afternoon, allowing for a higher potential for late afternoon ozone production. The air quality models are responding by showing Moderate ozone just to the west of the coast on Friday and in DC and Baltimore on Saturday. Clean back trajectories from Canada should prevent ozone from reaching the USG range on both Friday and Saturday. PM2.5 will also reach into the low Moderate range on Friday and Saturday due to light winds. Back trajectories originating in Canada and continuing low humidity will help limit concentrations from rising beyond the mid to upper Moderate range.

Surface high pressure will continue on Sunday, allowing clear skies and light winds to continue for much of the region. Lower Sunday precursor emission will limit ozone to the Moderate range. Increasing humidity and the light winds will help keep PM2.5 concentrations in the Moderate range. The precipitation forecast for Sunday is still uncertain as the fate of the approaching frontal boundaries is to be determined. There will likely be scattered clouds and precipitation in the NMA.

-DeBoe/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 20, 2015
Valid: July 21 – 25, 2015 (Tuesday – Saturday)

Summary:

Tomorrow’s cold front will bring drier and cleaner air to the mid-Atlantic. Air quality will slowly deteriorate throughout the remainder of the work week as an upper level ridge builds in from the west. Cleaner, drier air behind the front will drop ozone and PM2.5 into the Good range on Wednesday. Persistent high pressure over the Northeast will allow clear skies and calm winds to prevail over the Mid-Atlantic throughout the upcoming week, promoting the steady formation and buildup of ozone and particles. Favorable weather conditions for deteriorating air quality will peak on Saturday, making it the most likely day for a USG forecast.

Discussion:

The weather models are in very good consensus throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. A large trough currently over ON will continue eastward over the next couple of days, digging southward as it does so. This feature will be reflected at the surface via a cold front that will sweep west to east through the Mid-Atlantic tomorrow, finally bringing a drier air mass into the region. The front will continue southeastward through Saturday, slowing as it penetrates the Southeast. A ridge currently aligned with the western edge of the Rockies will continue to strengthen and build in behind this trough, with its axis reaching through the Great Plains by 00Z Thursday. In the Mid-Atlantic, zonal flow aloft will slowly shift southeasterly as the ridge slowly builds in from the west. Mostly sunny skies will prevail over the Mid-Atlantic following the passage of tomorrow’s cold front, with little chance for precipitation until Sunday.

Despite the passage of a cold front currently over eastern PA, Tuesday will be another hot and humid day across the Mid-Atlantic. Westerly surface winds and back trajectories originating from the Ohio River Valley (ORV) will set the stage for an atmosphere rich in ozone precursors. A dissipating stationary front along the Mason-Dixon Line (MDL) will act as a focus for convergence for the buildup of ozone and PM2.5. A cold front will push west to east through the Mid-Atlantic tomorrow, reaching the coast by 00Z Wednesday. Impacts will include building clouds ahead of and along the front, as well as cooler, drier air filtering in behind it. Convection is likely west of the I-95 Corridor in the NMA and will be widespread in the SMA. The resulting cloud cover will limit ozone formation, with Moderate ozone in areas that see periods of afternoon sun ahead of the front, particularly along and east of the I-95 Corridor. Stagnant morning air will help particles to peak in the lower Moderate range at scattered locations along and east of the interstate as well.

Frontal convection overnight Tuesday will help to clean the atmosphere of pollutant precursors by Wednesday morning. Seasonally average temperatures, dew points in the 50s F, and Canadian back trajectories will combine to limit both ozone and particles to the Good range despite mostly sunny skies.

Thursday’s weather will be very similar to Wednesday’s, but with diminishing surface winds as the weather pattern in the Mid-Atlantic settles. With the cold front continuing to drop into the Carolinas, a sprawling area of high pressure will spread into the Northeast from the west. This will promote lighter winds and continue to maintain clear skies, allowing the air mass over the region to begin to modify and allow ozone to rebound to the Good/Moderate threshold downwind of the I-95 Corridor. Particles will build just into the Moderate range in the western Mid-Atlantic, where the center of surface high pressure is expected to settle and result in the calmest winds.

Friday will be another day of deteriorating air quality as high temperatures persist in the mid-80s F and dew points linger around 60 F. Back trajectories will shorten compared to the past few days, with the air aloft originating from MI. Full sun and light winds will allow ozone to form, unhindered, throughout the day. Northwesterly surface winds will contain the areas of highest ozone to locations downwind of the I-95 Corridor in the eastern Mid-Atlantic. The highest particle concentrations will spread eastward from the center of surface high pressure in the ORV, with Moderate PM2.5 likely across the entire Mid-Atlantic south of central PA.

A small ridge will be over the mid-Atlantic on Saturday. This will be the day of poorest air quality in the period. A resulting area of surface high pressure will engulf the entire Mid-Atlantic, promoting calm surface winds and abundantly sunny skies. However, northerly back trajectories and the lack of weekday commuter emissions will keep ozone and particles from increasing much compared to Friday with Moderate air quality expected.

– Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, July 19, 2015
Valid: July 20 – 24, 2015 (Monday – Friday)

Summary:

The forecast models point to lower chances of precipitation on both Monday and Tuesday. As a result, there is a high chance of Code Orange ozone on Monday, mainly east of the I-95 Corridor. Slightly cooler and less humid on Tuesday but cloud cover extent is uncertain so that we continue with an appreciable risk of Code Orange ozone. Cooler and much less humid from Wednesday to Friday with Moderate air quality.

Discussion:

The forecast models are in good agreement with respect to the key air quality relevant issues over the medium range. The forecast for the larger scale pattern remains similar to yesterday’s model runs with a closed upper level low moving slowly from ON to QC and a trough developing over the eastern US. The main change in the model forecasts relate to the short term (Monday and Tuesday) and point to a lower chance of significant precipitation. This, in turn, has prompted a slight increase in the chances for USG ozone on both Monday and Tuesday.

On Monday, a weak cold front will reach the I-95 Corridor, on roughly a TTN-HGR line during the morning rush hour, and then stall and dissipate more or less in place. Previous model forecasts developed convection along this boundary but today’s runs of both the parameterized and convection-allowing forecast models show convection developing only west of the I-95 Corridor. With weekday emissions returning Monday, there is a high chance of Code Orange ozone. Near surface winds will be more westerly Monday so that the location of peak concentrations will be east of the I-95 Corridor. Particle concentrations rose into the Moderate range overnight and, with little change to the air mass, upper Moderate concentrations are expected on Monday.

There will be a step down in temperature and humidity on Tuesday with increasing clouds as a stronger cold front approaches from the west. The key forecast issue will be the timing and extent of cloud cover and convection. The forecast models have decreased the chances of precipitation on Tuesday. Although we expect concentrations of ozone and PM to decrease on Tuesday, there is still an appreciable risk of widely scattered Code Orange ozone.

We return to more seasonable weather on Wednesday with lower temperatures and humidity. Steady northwest flow and sunny skies will keep ozone in the Moderate range through Friday. Particles will drop further due to lower humidity with Good to low Moderate concentrations expected.

-Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 18, 2015
Valid: July 19 – 23, 2015 (Sunday – Thursday)

Summary:

There is a high probability of scattered Code Orange ozone on Sunday lingering into Monday although convection, possible both early and late in the day, may limit ozone Monday. Particles will be in the Moderate range. A closed low forming over eastern Canada will bring seasonable weather for the remainder of the medium range period with Moderate air quality expected.

Discussion:

The forecast models are in good agreement through the medium range period. A closed upper level low will form over Hudson’s Bay on Sunday and then slowly move eastward to northern QC by Thursday. A series of disturbances will rotate around the low with the strongest disturbance moving through the Great Lakes on Tuesday bringing an end to a short period of very warm weather in the mid-Atlantic. Sunday and Monday will be the days of most interest for air quality concerns as it will be quite warm, mostly sunny and very humid.

For Sunday, all the ingredients for high ozone levels are expected to be in place: temperatures in the mid-90’s F, light and variable winds, particularly in the morning and early afternoon hours, westerly transport and moderately strong cap on convection. However, in recent years we have seen very few Sunday USG occurrences due to reduced emissions both locally and regionally. As a result, we expect only scattered USG ozone on Sunday.

Monday will remain very warm and, with the addition of weekday emissions, prime for another USG ozone day. However, the forecast models continue to bring early morning convection through the mid-Atlantic that will serve to take the edge off the carry over of pollutants and their precursors. The WPC has a weak cold front just west of the I-95 Corridor early Monday and passes it through the region during the day. This could act as a trigger for afternoon thunderstorms. The risk of USG ozone continues Monday although slightly less enhanced than Sunday. PM will be in the mid-upper Moderate range both days.

A secondary cold front associated with the Great Lakes trough will pass through the region Tuesday. The remainder of the medium range will feature normal temperatures, slightly drier conditions and steady west to northwest flow. This will translate into Moderate air quality through Thursday.

-Ryan