Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 21, 2014
Valid: July 22- 26, 2014 (Tuesday-Saturday)

Summary:

The chance continues for scattered upper Moderate/USG ozone in the northeastern Mid-Atlantic on Tuesday and Wednesday as the westward extension of the Bermuda High sets up over the region, but uncertainty remains due to inconsistencies in model forecasts of key features. Air quality will return to the Good range for most locations Thursday-Saturday as a cold front passes through, triggering widespread rain showers and clouds, and a clean and dry air mass builds in behind the frontal passage.

Discussion:

The weather forecast models are in excellent agreement for the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS start out with a weak upper level ridge centered over the Mid-Atlantic. It will exit by Wednesday morning and will be immediately followed by an upper level trough over the Great Lakes, which will pull through a cold front at the surface Wednesday-Thursday. The ECMWF is slightly faster with the progression of the cold front compared to the GFS, which will create some uncertainty in the precipitation forecast Thursday-Friday. The models begin to diverge on Saturday at 12Z regarding the development of the next upper level trough that will impact the Mid-Atlantic in the second half of the weekend.

After trending toward a cleaner medium range forecast over the weekend, the weather and air quality models have switched course and are now indicating greater chances for upper Moderate to USG ozone in the northeastern Mid-Atlantic on Tuesday and Wednesday.

On Tuesday, a region of high pressure will sit over the northern Mid-Atlantic (NMA) as a stationary front remains over NC/SC/GA. The stationary front in the southern Mid-Atlantic (SMA) will bring on precipitation and cloud cover for mainly the second half of the day. The SMA will see air quality in the Good range. The NMA will see mostly to partly sunny skies, high temperatures approaching 90 °F, light surface winds, and southeasterly/southerly flow as the westward extension of the Bermuda High tries to establish itself over the region. There is a slight chance for scattered rain showers in the NMA, but nothing too significant, so mostly clear skies are expected. The main forecast questions for tomorrow in the NMA will be the degree of modification of the air mass and the impact of more southwesterly (vs. westerly) back trajectories. The BAMS ozone models have ramped the chances for USG ozone back up today, and are predicting widespread upper Moderate and scattered USG ozone in the northeastern Mid-Atlantic, specifically northern and central NJ. The NOAA ozone model has a similar pattern, with the highest ozone in the NJ/eastern PA area, but is much more conservative with peak ozone, keeping it in the upper Moderate range for an isolated area in central NJ. Given mostly sunny skies and light southerly surface winds, areas of upper Moderate to USG ozone are certainly possible north of I-95. The back trajectory forecast for tomorrow is more southwesterly than the southerly (and cleaner) transport seen in yesterday’s analysis, and shows some recirculation, which is another factor suggesting the chance for areas of upper Moderate to USG ozone. Increasing levels of atmospheric moisture will allow PM2.5 to rise into the Moderate range in the NMA.

The center of surface high pressure will move over western NC on Wednesday as the Bermuda High briefly takes hold over the region. It will be hot and humid, with high temperatures in the low-mid 90 °F. However, an approaching cold front from the northwest will push the high pressure out of the region by the end of the day and generate rain and thunderstorms in the afternoon and evening. The main forecast questions for Wednesday will be the speed of the front and development of convection ahead of it. The front is expected to move into the NMA in the afternoon and reach southeastern PA by 12Z Thursday. The operational models and the 09Z SREF develop widespread clouds and precipitation ahead of the front but keep it west of I-95 until after 00Z Thursday. Also, the upper level trough overhead will allow for some mixing and southerly surface winds will increase by the end of the day. As a result, the Mid-Atlantic will see Good air quality for most locations with the exception of the northeastern part of the region, along I-95, which may see another day with a chance for upper Moderate to isolated USG ozone.

The cold front will continue to move southeastward through most of the region on Thursday and reach northern NC by Friday morning. The progression of the front will generate potentially heavy rain and thunderstorms in the central and southern Mid-Atlantic, lasting from around noon until the evening. The front will bring in dry northwesterly flow behind it along with dry air. Air quality will be in the Good range for most locations.

High pressure will build into the NMA on Friday and Saturday. By Friday morning the cold front will reach NC and continue to move southward during the day. Model discrepancies on Friday, as discussed above, create uncertainty for the precipitation forecast. However, due to the high pressure overhead, most of the NMA should stay dry and sunny. The frontal boundary in NC will create rain and clouds for Friday in the SMA. Most of the region will be dry on Saturday with mostly sunny skies. The arrival of the dry and clean air mass from Thursday’s cold front will keep air quality in the Good range for both days, with the chance for scattered Moderate ozone in the NMA on Saturday.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, July 20, 2014
Valid: July 21- 25, 2014 (Monday-Friday)

Summary:

The chances for upper Moderate/USG ozone early in the period continue to trend downward due to the faster arrival of the next cold front. Although temperatures will reach the low 90s °F by Wednesday, clouds and precipitation and a cleaner transport pattern aloft are expected to limit ozone to the low to mid Moderate range through Wednesday, with Moderate PM2.5 as well. Widespread rain and the arrival of a clean and dry air mass will return air quality to the Good range for Thursday and Friday.

Discussion:

The weather forecast models are in good agreement this morning regarding the synoptic pattern for the medium range period. The 00Z ECMWF, the 06Z GFS, and the 12Z NAM were used for this analysis. The upper level flow over the Mid-Atlantic will turn zonal on Monday and transition to weak ridging on Tuesday. A series of shortwaves will crest the strong ridge over the western U.S. on Tuesday and begin to dig out a trough over the western Great Lakes. This trough will amplify and shift eastward on Wednesday, and then build over the Mid-Atlantic on Thursday and Friday, pulling a cold front through the northern Mid-Atlantic (NMA) in the late Wednesday to early Friday time frame. At the mid-levels, a ridge will try to build over the region Tuesday and Wednesday before being displaced by the upper level trough.

Monday-Wednesday are still the days of most interest in the period, but the chances for upper Moderate to USG ozone continue to trend downward today, due to the progressive development of the upper level trough and faster arrival of the next cold front. Although the mid-level ridge over the region will increase temperatures and dew points through Thursday, high temperatures are trending downward, with highs only reaching the low 90s °F in the NMA on Wednesday (the hottest day of the period), compared to the mid- 90s °F in yesterday’s analysis. In addition, the GFS and ECMWF agree that the next cold front will begin moving into the NMA on Wednesday afternoon/evening, which will trigger a line of showers and thunderstorms across the region, according to the 12Z versions of the12 km and 4 km NAM as well as the 09Z SREF. The BAMS air quality models have continued to back off on their previous predictions of USG ozone along the I-95 Corridor as well. This morning’s runs keep ozone in the Good range across the region on Tuesday and Wednesday.

Surface high pressure will move back over the region on Monday, resulting in a period of stagnation overnight and then light surface winds shifting from E/SE to S by the end of the day. Shortwaves moving over the region will result in a mixture of sun and clouds. Flow aloft will be onshore (E) at lower levels (e.g., 500 and 1000 m AGL) and veer to the SW at 1500 m AGL. The 06Z NOAA ozone model does indicate an isolated area of upper Moderate/USG ozone in northern NJ and northeastern PA, likely in response to the light onshore flow and closer proximity to the center of surface high pressure. Given the onshore winds and partly cloudy skies, Good to low Moderate ozone is expected elsewhere in the NMA. An area of shortwave energy interacting with the persistent stationary front along the Gulf Coast and Southeast will trigger clouds and rain that will keep air quality in the southern Mid-Atlantic (SMA) in the Good range. PM2.5 will be in the Moderate range at inland locations, roughly west of I-81. Smoke from the central and western Canadian wildfires is still located aloft over the northwestern parts of the Mid-Atlantic; the NAAPS model continues to keep the smoke north of the region through the end of the period, so it is not expected to impact surface air quality.

Winds become southerly on Tuesday as a return flow sets up around the center of surface high pressure, which will be located off of the NJ coast. It will be warmer and more humid, with high temperatures in the upper 80s °F in the NMA. Mostly sunny skies and light winds in the NMA will promote rising ozone levels, but the transport aloft will have an onshore (S/SE) component, which will likely limit ozone to the low-to-mid Moderate range. As noted above, the BAMS ozone models keep conditions in the Good range across the region. Afternoon clouds and rain, per the SREF, look to keep ozone in the Good to low Moderate range in the SMA. PM2.5 will likely reach the Moderate range across the region due to increasing humidity.

Although Wednesday will be hot and humid, the mesoscale models develop a line of rain and thunderstorms in the afternoon, triggered by the cold front approaching from the NW. Back trajectories are again more S/SE, with some onshore (maritime) component, which will continue to limit rising ozone mixing ratios. The model trend is toward a faster cold front passage, compared to previous days’ analyses, which is resulting in more clouds and precipitation on Wednesday. Thus, Moderate ozone is probably limited to along and east of I-95, with little chance of USG ozone. PM2.5 will fall into the Good range in the western part of the region as the advancing precipitation cleans out the atmosphere; Moderate PM2.5 will linger along the eastern part of the region until the cold front arrives late Wednesday into Thursday morning.

Thursday and Friday will feature Good air quality across the region as the cold front moves to the SMA by Friday morning and a cooler and dry Canadian high pressure system builds in behind the front.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 19, 2014
Valid: July 20- 24, 2014 (Sunday-Thursday)

Summary:

Light onshore flow and a mix of sun and clouds will keep air quality in the Good to low Moderate range for Sunday and Monday. Tuesday-Thursday will be very warm and humid as a mid-level ridge builds over the region. The possibility for scattered upper Moderate to USG ozone remains along I-95 primarily for Wednesday, but chances are lower today given the more rapid development of an upper level trough that will pull the next cold front into the region on Thursday.

Discussion:

The air quality picture for the medium range period is much less clear today, given some discrepancies in the weather forecast models and a move away from the definitive arrival of upper level ridging late in the period, as seen in yesterday’s analysis. The 00Z NAM, GFS, and ECMWF were used for today’s analysis. The period begins on Sunday with weak troughing aloft over the eastern U.S. as the center of surface high pressure slides northeast to the Canadian Maritime coastal waters. On Monday, the NAM and GFS lift the weak upper level trough north and east and transition to weak ridging over the eastern U.S. by 12Z Tuesday. In contrast, the ECMWF keeps the trough over the Mid-Atlantic through about 18Z Tuesday, anchored by small shortwaves that rotate through the base of the trough. These discrepancies are reflected at the mid-levels, as the GFS is faster than the ECMWF to establish a mid-level ridge over the Mid-Atlantic on Tuesday. The models come back into relatively close agreement on Wednesday regarding an upper-level trough that will develop over the Great Lakes and then amplify and slide eastward into the Mid-Atlantic on Thursday, pulling a cold front through the northern part of the region.

On Sunday, air quality will remain in the Good range along the eastern Mid-Atlantic due to light onshore flow and along the western Mid-Atlantic due to persistent clouds and rain showers. The NOAA and BAMS air quality models develop scattered Moderate ozone along/north/west of I-95, which seems possible if the onshore surface winds remain light and sufficient afternoon sun can break through the clouds. The air quality models also predict Moderate PM2.5 at inland locations, roughly between I-79 and I-95, likely in response to the stagnant conditions away from the coast. Smoke from the numerous wildfires in western and central Canada remains aloft over the northern Mid-Atlantic this morning, and it does not appear to be affecting surface air quality. The NAAPS model keeps the smoke well to the north of the Mid-Atlantic through the medium range period, but we will continue to monitor the smoke plumes in case this situation changes.

Temperatures and dew points begin an upward trend on Monday as surface high pressure moves back overhead and mid-level ridging tries to establish itself over the region. The persistent stationary front along the Southeast and NC coastline will act as a trigger for clouds and scattered rain, which will keep air quality in the Good range across the southern Mid-Atlantic through Wednesday. A mix of sun and clouds in the northern Mid-Atlantic, with continued light onshore flow, will keep air quality along the Good/Moderate threshold.

Tuesday and Wednesday are the days of most interest in the medium range period, but given the evolution of the mid/upper level forecast conditions, the chances for USG ozone are less than they appeared to be yesterday. To that end, the BAMS air quality models have backed off from yesterday’s predictions of upper Moderate/USG ozone along the I-95 Corridor for Tuesday. This morning’s runs keep ozone mostly in the Good range on Tuesday and bring concentration up only to the mid-Moderate range on Wednesday. This change is likely due to the weather forecast models coming into agreement on the developing upper level trough over the Great Lakes on Wednesday that will flatten the mid-level ridge over the Mid-Atlantic.

Surface winds and flow aloft will shift to the south/southwest on Tuesday, with some recirculation at lower levels (e.g., 500 m AGL). There is uncertainty in the precipitation forecast given that the ECMWF keeps the weak trough over the region through mid-day, which would result in more rain and clouds and a cleaner forecast. At this point, there is a chance for scattered Moderate ozone along the I-95 Corridor on Tuesday, given mostly sunny skies, light winds, and high temperatures rising into the upper-80s °F.

Wednesday has the greatest chance for widespread Moderate ozone across the northern Mid-Atlantic, with a chance remaining for isolated USG ozone along I-95. It will be the hottest and most humid day of the period, with high temperatures reaching the mid-90s °F as the mid-level ridge extends over the region. The weather forecast models develop some isolated instability showers and thunderstorms in the afternoon, given the warm and humid conditions and the approach of the next cold front from the northwest, which could limit rising ozone levels. PM2.5 will reach the Moderate range across the northern Mid-Atlantic in the humid and increasingly modified air mass.

The cold front will move across the northern Mid-Atlantic on Thursday, triggering widespread clouds, showers, and thunderstorms that will return air quality to the Good range for most locations.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 18, 2014
Valid: July 19- 23, 2014 (Saturday-Wednesday)

Summary:

Ozone will reach the Moderate range at scattered locations along the I-95 Corridor on Saturday due to the influence of surface high pressure centered over New England. PM2.5 will also be in the Moderate range at inland locations, particularly the Susquehanna Valley and western MD. Sustained onshore flow on Sunday will return air quality to the Good range for most locations. Monday-Wednesday are the days of most interest for the period, as weak ridging aloft will allow the chance for ozone to rise into the upper Moderate/USG range in the northeastern Mid-Atlantic, depending on the chances for daily precipitation, which is uncertain at this time.

Discussion:

The 00Z ECMWF, 06Z NAM, and 06Z GFS start out the period with an upper level shortwave over IN/OH carving out a weak trough over the eastern U.S. This trough will move eastward on Saturday, and several smaller shortwaves will rotate through the Mid-Atlantic, along the base of the trough. The trough will gradually lift through the region until Monday night, when the flow aloft turns zonal. Starting Tuesday morning, weak ridging develops over the eastern U.S. The ECMWF and the GFS have come into agreement today on the development of this ridge. On Wednesday, potent shortwaves moving over the top of the ridge into the Great Lakes will begin to flatten the eastern ridge. The models diverge on the placement and speed of the shortwaves and the subsequent trough they develop, with the GFS featuring faster eastward movement. By Wednesday night, the ECMWF shows a more negatively tilted trough axis, centered roughly over WI/IL, while the GFS has the trough centered over southern ON/MI. This developing trough will eventually pull the next cold front through the Mid-Atlantic at the end of next week. For now, we follow WPC and the NWS and favor the slower ECMWF solution.

Surface high pressure will sit over New England on Saturday as a stationary front lingers along the GA/SC border up through the Southeast coast. Precipitation created by the approaching shortwaves aloft will keep ozone in the Good range in the western Mid-Atlantic. The eastern Mid-Atlantic will remain mostly to partly sunny and dry due to the influence of surface high pressure, with early morning stagnation and light onshore winds in the afternoon. Some clouds may develop due to the passage of shortwaves and positive vorticity advection associated with the weak trough aloft. Whether they impact the air quality forecast will depend on the altitude of the clouds. Guidance suggests they will be mid-to-high level clouds along the eastern Mid-Atlantic, which would allow ozone to reach the Moderate range at scattered locations, particularly along/north/west of I-95. PM2.5 will be in the Moderate range at inland locations, particularly the Susquehanna Valley and western MD. Areas along the western Mid-Atlantic that experience precipitation overnight and through the day on Saturday will see PM2.5 fall into the Good range. Smoke from the central/western Canadian wildfires has spread into the northern Mid-Atlantic today, as analyzed by the NOAA HMS Fire and Smoke team. The remnant smoke doesn’t appear to be impacting surface air quality, but we will continue to monitor it through the weekend.

As the center of surface high pressure pulls away to the Canadian Maritimes on Sunday, a more sustained onshore flow will develop, both aloft and at the surface. The weather models are showing poor run-to-run consistency regarding an area of low pressure that is expected to develop roughly over the Delmarva coastal waters, along the stalled frontal boundary draped along the Atlantic coast, some time on Saturday night into Sunday morning. Most of the guidance keeps precipitation associated with the low offshore, but there is a chance that coastal locations in the NMA could see some rain and clouds on Sunday. Regardless, ozone should be in the Good range for most locations, due to the onshore flow. PM2.5 will remain in the Moderate range at inland locations.

Monday-Wednesday will be the days of most interest during the medium range period, especially Tuesday and Wednesday. As the weak ridging aloft spreads over the Mid-Atlantic, surface high pressure will reestablish itself over the region, and a warming trend will take high temperatures into the low 90s °F by Wednesday, with rising humidity. The persistent stationary front will linger along the Atlantic coast through at least Tuesday, which may keep air quality in the Good to low Moderate range in the southern Mid-Atlantic (SMA), particularly NC. The BAMS air quality models develop a widespread area of high Moderate to USG ozone in the northeastern Mid-Atlantic, along the I-95 Corridor, on both Monday and Tuesday. This seems a bit aggressive at this point; more likely, Monday will be a transition day, given that high temperatures are only forecast to reach the low-to-mid 80s °F, and flow will still be onshore. On Tuesday, surface winds will shift south/southwesterly, with slow and localized westerly transport aloft, giving a greater chance for ozone to reach the USG range at isolated locations. The weather forecast models are generating instability showers/thunderstorms for Tuesday and Wednesday, while there is uncertainty regarding the speed of the developing upper level trough over the Great Lakes, as discussed above. Thus uncertainty in the precipitation forecast makes the air quality forecast uncertain as well. PM2.5 will respond to the rising humidity and should reach the Moderate range across the region by Tuesday and Wednesday.

-Catena/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 17, 2014
Valid: July 18- 22, 2014 (Friday-Tuesday)

Summary:

A center of high pressure will station itself over the northern Mid-Atlantic (NMA) on Friday. Sunny skies, light to calm winds, and an afternoon sea breeze could develop isolated upper Moderate/USG ozone along I-95, but consistent cloud cover predictions from the weather models add uncertainty to this forecast. PM2.5 will reach the Good/Moderate threshold due to low atmospheric moisture and light winds. As the center of high pressure moves northeastward on Saturday, it will extend its ridge down the Mid-Atlantic, keeping most of the region dry. As a series of shortwaves pass through the region, mostly cloudy skies will prevail, limiting ozone to the lower Moderate range for most locations. PM2.5 will reach the low Moderate due to increasing dew points. Sunday looks to be the cleanest day of the period due to sustained onshore flow. Monday and Tuesday will see a gradual rise in temperatures and dew points but the air quality forecast is still uncertain; Good to Moderate air quality is expected.

Discussion:

The weather forecast models are in close agreement for the medium range period. Friday will start out with a weak upper level trough over the Mississippi River Valley, which was created by the development of shortwaves moving in from the southern Plains. The trough will move into the Mid-Atlantic, where it will stay until Monday afternoon, when it will begin to flatten into zonal flow for the rest of the period. The 00Z ECMWF and 06Z GFS diverge on Tuesday, with the GFS developing weak ridging over the eastern U.S., while the ECMWF continues the zonal flow aloft.

High pressure overhead on Friday will result in mostly sunny skies and very light to calm winds, with a switch in the predominant surface flow from northwesterly to northeasterly/easterly. The 06Z 4 km and 12Z 12 km NAM this morning also indicate a line of convergence along central NJ, associated with a sea breeze front. These conditions have been conducive for isolated upper Moderate/USG ozone several times so far this summer along the I-95 Corridor in the northern Mid-Atlantic (NMA). However, for the past few days, including today, the weather forecast models have predicted cloudiness on Friday, although this morning’s GFS run is somewhat slower with the eastward movement of the clouds. The models may be responding to rising motion from the upper level trough to the west of the region. Given that the center of surface high pressure will be directly over the NMA, actual conditions will most likely be sunnier than predicted by the models. The NOAA and BAMS air quality models are consistent in predicting Good ozone for Friday, most likely due to the cloud cover forecast. Nevertheless, with light to calm winds, high pressure overhead, an afternoon sea breeze, and short back trajectories, there is a chance that ozone will reach the upper Moderate/USG range along the I-95 Corridor in the NMA. There will be a chance of late afternoon rain showers on the very southwestern side of the Mid-Atlantic as a series of shortwaves approach the region aloft, which will lead to Good ozone in this part of the region. PM2.5 will be limited to the Good range due to low atmospheric moisture. Remnant smoke from wildfires burning in western and central Canada has reached the Great Lakes region this morning, which is upwind of the Mid-Atlantic for Friday. The smoke appears to be aloft at this point, and not impacting the surface significantly, and the NAAPS smoke model keeps the impact of the smoke north of the Mid-Atlantic on Friday. However, given the high pressure center over the region, the impact of the smoke plumes will need to be monitored.

The shortwaves aloft will travel through the region on Saturday, triggering clouds and precipitation along the western Mid-Atlantic. The center of high pressure will move northeastward and will extend its ridge down through the Mid-Atlantic, keeping the rest of the region dry. PM2.5 will reach the Good/Moderate threshold due to rising dew points and morning stagnation. The passing of the shortwaves will leave the skies mostly cloudy. Ozone will reach the Moderate range in locations with the most sun, yet most of the region will see ozone in the Good range. Given the forecasted surface winds, the chance for mid-to-upper Moderate ozone along I-95 will need to be monitored, but at this point, it looks like there will be sufficient clouds to limit ozone to the lower Moderate range.

The surface ridge will remain over the Mid-Atlantic on Sunday as a stationary front will move northward as a warm front along the Southeast coast. The precipitation forecast is still somewhat uncertain due to weather forecast model discrepancies. The weather models, particularly the GFS, want to develop a wave of low pressure along the stalled frontal boundary, but it’s not clear when this will occur. The GFS develops the wave off of the Delmarva coast as early as Sunday morning, which brings clouds and precipitation along the NMA coastline. The ECMWF keeps the chance for rain showers mainly along the southwestern part of the Mid-Atlantic and the Southeast coast. Regardless of the exact precipitation forecast, air quality will be reduced to the Good range for most locations due to sustained easterly winds, with the exception of the chance for Moderate ozone in inland areas.

On Monday, the center of high pressure will move farther northeastward over the Atlantic before returning back overhead on Tuesday. Temperatures and humidity will gradually rise. Substantial uncertainty exits for the air quality forecast due to questions about when/where a wave of low pressure will form along the stationary front off of the East Coast, as well as the differences in flow aloft, as discussed above. For now, we expect Good to Moderate air quality, with the highest chance for mid-to-upper Moderate ozone in the NMA along I-95, particularly if the GFS forecast verifies.

-Catena/Huff