Author Archives: Gregory P. R. DeBoe

A Weather Forecaster’s Perspective on Air Quality Forecasting

Since my first day as an undergraduate meteorology student at Penn State, I was exposed to weather forecasting through the Penn State Campus Weather Service club. There are two branches of the club: the communications branch and the forecasting branch. The communications branch provides daily radio broadcasts for clients and video forecasts that are uploaded online. The forecasting branch prepares five day forecasts for various regions across Pennsylvania, which are also uploaded online for the public to access. Included in the five day forecasts are high and low temperatures, precipitation, a brief description of the forecasted weather conditions, and a forecast discussion. I spent most of my time as a member of the forecasting branch. I learned the basics of weather forecasting from upperclassmen who had been a part of the club for a few years. By my junior year, I became a shift manager. As a shift manager, I supervised students to make sure all of the forecasting zones were covered. I also helped teach new members the basics of forecasting, as the upperclassmen did when I was a new member.

My weather forecasting experience also stems from my participation in WxChallenge, the national collegiate weather forecasting competition, as a member of the Penn State team during my junior year. The Penn State team has won the competition the past four years. The competition consists of making forecasts four days a week for ten cities across the United States. Each forecast includes four variables: high and low temperatures, maximum sustained wind speed, and the amount of precipitation to the nearest inch. In the 2014-2015 forecasting season, I placed in the top 50 forecasters in the competition out of a total of 1900 forecasters.

In the summer of 2015, I transitioned to working as an Air Quality Forecasting Intern in the Penn State Air Quality Forecasting Office, learning about operational ozone and PM2.5 forecasting. I had talked to a few of my professors about how to incorporate chemistry into my meteorology degree, and they guided me toward a focus on air quality. I took a few classes that concentrated on air quality and environmental policy, and I found them very interesting. I knew that focusing in air quality was the path that I wanted to take. I found the air forecasting internship through a classmate, fellow intern Lexie Herdt. This internship was the first experience I had dealing with air quality outside of a classroom setting. I found that air quality forecasting is similar to weather forecasting, but there are small differences that distinguish them.

One key difference I noticed was not having to decide on an exact value for temperature or the amount of expected precipitation. As a weather forecaster, most of my time was spent trying to decide on a single value to forecast for temperature or precipitation. For example, to make a perfect forecast in WxChallenge, every variable had to be narrowed down to a single value. If the temperature was off by a few degrees Fahrenheit or the amount of precipitation was off by a few hundredths of an inch, then error points were assigned to the forecast. The most accurate forecasts were given the fewest error points. So getting the forecast exactly right was essential to doing well in the competition. When transitioning to air quality forecasting, I found that narrowing down the temperature or precipitation forecast to a single value is not as essential as it is in weather forecasting. Knowing a range of expected temperatures (e.g., upper 80s °F to low 90s °F) or intensity of rainfall (e.g., light or heavy) is sufficient to make an accurate air quality forecast. For example, if a storm system moving through an area was expected to produce widespread rainfall for an entire day, then I would expect that the atmosphere would be cleaned out and that clouds would block ozone formation. Knowing the total amount of rainfall, whether it’s only 1 inch or 5 inches, is not essential. Dealing with a temperature forecast in a small area can be difficult as it can vary greatly in a very small distance. Determining a range of temperatures works well to make up for this issue when making an air quality forecast.

Having the public affect the air quality forecast is another difference I found. Typically, the public is impacted by the weather forecast. For example, if there is going to be a heat wave or a crippling snowstorm, then the public will have to adjust their plans accordingly, whether it involves travel or outdoor activities. With air quality forecasting, the public can impact the forecast through, for example, holiday travel and fireworks. Typically, the highest volume of traffic and vehicle emissions occur during the work week, Monday through Friday (and sometimes on Saturday, too). As a result, the highest ozone levels are seen during the work week, all things being equal. On average, ozone levels are lower on the weekends due to the lower vehicle emissions. However, on holiday weekends, such as Labor Day or Memorial Day, more people are likely to travel and vehicle emissions are higher than on a corresponding average weekend day. The higher concentrations of pollutant precursors from increased holiday travel emissions can lead to higher ozone levels than what is typically expected. We saw this in Philadelphia this past Independence Day holiday weekend, when ozone exceeded the NAAQS by 1 ppb at one monitor on Sunday, July 5; this exceedance was almost certainly attributable to the higher holiday emissions.

Fireworks celebrations can impact an air quality forecast by causing increased concentrations of particles. When a firework explodes, fine particles are expelled into the air from the smoke associated with the firework. During a typical fireworks show there are hundreds of explosions. This can cause a buildup of particles, especially if the wind is very calm and the smoke plume is stagnant. If the wind is light, it can blow the smoke plume away and cause a buildup of particle concentrations downwind. The impact of fireworks tends to be more localized rather than region wide, but they still have to be taken into account as part of the air quality forecast.

In addition to learning a new skill with air quality forecasting, I improved my weather forecasting skills considerably. Before my internship, I would only have to look at all of the weather observations and models a few days a week, especially on a synoptic scale. During my internship, I looked at observations and model guidance every day. My communications skills improved as well from writing the technical 5-day medium range air quality forecast discussion and the short 3-day air quality discussion for the public. Despite there being small differences, my improvement shows that weather forecasting and air quality forecasting have a strong connection.

Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday, August 13, 2015

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 13, 2015
Valid: August 14-18, 2015 (Friday – Tuesday)

MedRangeTable_201508014

MedRange_Cats_2

Summary:

A somewhat complex and competing synoptic pattern during the medium range period, with a ridge at 850 mb and weak troughing at 500 mb, adds uncertainty and makes the air quality forecast challenging. The main forecast questions are 1) how quickly the current air mass in place modifies, 2) the fate of a weak back door cold front on Saturday-Sunday, and 3) the direction of air mass transport, particularly on Sunday and Monday. Friday may be a possible transition day, depending on how quickly the air mass in place modifies, with scattered Moderate ozone likely. The day of most interest appears to be Saturday, when light surface winds, clear skies, rising temperatures, and the possibility of an afternoon sea breeze will increase the chances for USG ozone to Appreciable. Similar weather conditions along with the uncertainty about the weak back door cold front moving through the northern Mid-Atlantic keep the chances at Appreciable on Sunday as well. The BAMS and NC air quality models are showing upper Moderate and pockets of USG ozone at locations along the I-95 Corridor on Saturday and Sunday; they have been consistent with this trend for several days, which gives more credence to the possibility of upper Moderate to USG ozone over the weekend. Potential complicating factors are afternoon sea breezes and the fact that the weak back door cold front could provide a line of surface convergence and push ozone concentrations higher than expected. Uncertainty in the precipitation forecast ahead of the next approaching cold front drop the chances for USG ozone to the Marginal range on Monday and Tuesday. The GFS solution is faster and would limit significant ozone production on Monday, while the EC solution would lead to another day of upper Moderate to USG ozone. Clouds and precipitation appear more widespread on Tuesday, which return ozone to the Good to low Moderate range.

Discussion:

The weather models are in generally close consensus throughout the medium range period, but show some discrepancies on Monday and Tuesday. The 00Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. A somewhat complex and competing synoptic pattern, with a ridge at 850 mb and weak troughing at 500 mb, adds uncertainty and makes the air quality forecast challenging for the medium range period. Currently, a large ridge over the western US and a large trough over the eastern US, with its base reaching to the Gulf coast, are the dominating upper level features. The upper level trough will begin to retreat northward this afternoon, allowing surface high pressure to extend into the Mid-Atlantic. The center of surface high pressure will settle over WV on Friday, where it will remain through Sunday. As the upper level trough retreats, a mid-level ridge (850 mb) will quickly build eastward over the Mid-Atlantic, allowing temperatures to rapidly rise into the low 90s °F Saturday through Tuesday. The center of the mid-level ridge will sit over the Mid-Atlantic on Saturday and Sunday. On Friday, a bundle of small shortwaves will crest the upper level ridge and move from ON into the Great Lakes. These shortwaves will pull a weak back door cold front into the northern Mid-Atlantic (NMA) on Saturday, with a weak trough at mid-levels (850 mb). The weather models are continuing to have trouble resolving this front. It certainly appears to be very weak. In this morning’s analysis, WPC has the front becoming a surface trough by the time it reaches PHL on Sunday, compared to previous analyses, which kept the front together before dissipating it on Sunday. As a result, considerable uncertainty persists regarding the impacts of this front on air quality. Beginning on Saturday evening, the upper level western US ridge tries to build eastward across the entire CONUS, but continues to inhibited by weak upper level troughing lingering over the eastern US. In particular, a stronger area of shortwave energy over the Ohio River Valley (ORV) will form an upper level closed low and prevent this upper level ridge from becoming very prominent east of the Mississippi River. By 18Z Monday, shortwave energy moving eastward in the northern flow will phase with the upper level closed low and establish a shallow trough over the eastern US. This shallow trough will reach as far south as TN and remain in place through Tuesday. This trough will bring the next, weak cold front into the Mid-Atlantic on Tuesday. The GFS is about 12 hours faster than the EC in moving this trough and accompanying cold front eastward, which leads to uncertainty in the precipitation forecast for the end of the medium range period.

Skies will be mostly clear and surface winds will be light as surface high pressure centered over WV dictates the weather for the Mid-Atlantic on Friday. Localized back trajectories indicate that the air mass will be fairly stagnant. The main forecast question for Friday is the degree of air mass modification. As the upper level trough retreats and the mid-level ridge builds eastward, flow at the surface and aloft will be very light. Moderate ozone is expected along the I-95 corridor in response to the clear skies and light winds. The NOAA and BAMS air quality models are supporting this idea by showing scattered low to mid Moderate ozone along the I-95 Corridor. The NCDENR model is by far the most aggressive for ozone tomorrow, with pockets of USG ozone in southwestern Washington, DC and southern NJ. At this point, the NC model seems overdone, considering how low ozone concentration were yesterday, and seem to be so far this morning. A potential complicating factor is an afternoon sea breeze, which is possible along the DE and NJ coasts due to the light synoptic wind field. This sea breeze would affect mainly locations in DE and NJ and push ozone to the mid to upper Moderate range, more in line with the NC model guidance. PM2.5 concentrations will remain in the Good range on Friday due to persistent relatively low humidity. There may be a few locations in western PA that reach into the Moderate range in response to the light surface winds.

Saturday is the main day of interest during this period as light surface winds, recirculating back trajectories, clear skies, and temperatures continue to increase. The locations with the best chances for upper Moderate ozone concentrations will be along the I-95 Corridor in MD, DE, PHL, and NJ, due to close proximity to emissions. Today is the third consecutive day that the air quality models are showing Moderate ozone along the I-95 Corridor on Saturday, and the models have steadily increased concentrations each day, which lends confidence to an upper Moderate ozone forecast for Saturday. Today, the air quality models (BAMS and NCDENR) are showing upper Moderate all along the I-95 Corridor. The NC model is also showing pockets of USG in the DC metropolitan area and PHL/ southern NJ. An afternoon sea breeze is also possible on Saturday due to the light synoptic winds. The sea breeze would affect mainly DE and NJ locations. With conditions already prime to see upper Moderate ozone, a sea breeze could be the tipping point to push ozone into the USG range along its line of convergence. The surface high pressure over WV may not provide enough subsidence to limit vertical mixing as it is not very prominent in the upper levels of the atmosphere. Based on today’s analysis, the chances for USG ozone on Saturday is Appreciable. If the air quality models continue the trend tomorrow of increasing ozone on Saturday, and the air mass in place shows signs of modification, then the chances will increase to High. PM2.5 concentrations will likely reach the Moderate range at scattered locations in response to the light surface winds and a slight uptick in humidity.

The main forecast question for Sunday is the fate of a back door cold front moving through the NMA. The weather models are showing scattered clouds and rain showers along the frontal boundary as it moves southward into PA Saturday evening into Sunday morning roughly west and north of I-81. This front should weaken as it reaches PHL by 12Z Sunday, limiting the chances for clouds and precipitation. The WPC forecast has the back door cold front weakening into a surface trough as it reaches PHL, which means the front may act as line of surface convergence. The weather conditions on Sunday will be similar to Saturday as light winds, clear skies, and rising temperatures continue. Back trajectories are still short, reflecting the location of the mid-level ridge aloft, but shift more southwesterly, from the vicinity of western VA. These conditions suggest that upper Moderate to USG ozone conditions are possible on Sunday. The BAMS air quality models are giving credence to this forecast by showing upper Moderate ozone in the same locations as Saturday (MD, DE, PHL, and NJ) as well as pockets of USG in DC and NJ. If the back door cold front weakens and dissipates by the time it reaches PHL, then ozone concentrations may stay in the upper Moderate range. Usually, ozone concentrations are limited to the upper Moderate range on Sundays, all other things being equal, due to decreased vehicle emissions. PM2.5 concentrations will likely continue in the Moderate range at scattered locations due to increasing humidity and light surface winds.

There is still considerable uncertainty in the forecast for Monday and Tuesday as the next cold front approaches the region from the northwest. Monday will see similar weather conditions to Sunday, which should lead to another day of Moderate ozone along the I-95 Corridor. The air quality models are responding by showing Moderate ozone along I-95 in MD, DE, and NJ, although the NC model backs off ozone compared to previous days, with only isolated Moderate. Back trajectories are still slow, but shift more southerly on Monday, which may be enough to limit rising ozone along the I-95 Corridor.

The main question for Tuesday’s forecast is the fate of the approaching cold front. The GFS is faster than the EC in moving this cold front eastward from the Plains into the Mid-Atlantic on Monday. The GFS is showing scattered clouds and rain showers in the NMA and central Mid-Atlantic around 18Z Monday while the EC keeps the clouds and precipitation out of the region until Tuesday. The GFS solution should keep help to limit significant ozone production while the EC solution would lead to another day of upper Moderate ozone. Ozone concentrations should be limited to the Good range on Tuesday as the cold front reaches the NMA and promotes widespread cloud cover and precipitation. Given the relatively weakness of the front, particles will likely remain in the Moderate range through the end of the period due to rising humidity and persistent light winds.

– DeBoe/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Wednesday, August 12, 2015

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 12, 2015
Valid: August 13-17, 2015 (Thursday – Monday)

MedRangeTable_201508013

MedRange_Cats_2

Summary:

Steadily increasing temperatures and persistent sunny skies will promote rising ozone along and east of the I-95 Corridor, with concentrations peaking on Saturday and an increase to Appreciable chances for USG ozone. Clean mid-level air will limit concentrations mostly to the Good range on Thursday, but shortening back trajectories and rising temperatures will allow areas of Moderate ozone to form along and east of the I-95 Corridor on Friday. Despite model discrepancies regarding a back door cold front moving into the northern Mid-Atlantic, the skies over this usual area of concern will remain clear and allow ozone to continue to rise on Saturday. Recirculating back trajectories, a possible sea breeze, and surface high pressure centered over WV will aid in increasing concentrations. Despite persisting warm and sunny conditions, onshore, and looping back trajectories, Sunday’s chances for USG ozone will be limited to Marginal. The fate of the weak back door cold front on Sunday is still uncertain and will have to be monitored in the coming days. If the front continues to linger around PHL, the light converging winds may be enough to increase ozone into the upper Moderate range. Monday will see a continuation of the warm and relatively humid conditions and onshore looping back trajectories, again limiting ozone to the top of the Moderate range. PM2.5 will rise steadily throughout the period as humidity increases and winds remain light and west/southwesterly, plateauing in the mid-Moderate range over the weekend.

Discussion:

The weather models are in generally close consensus throughout the medium range period, but show some discrepancies on Saturday and Sunday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. A large upper level trough – reaching as far south as GA – is still prominent over the East Coast, with an equally large ridge over the western US. At the surface, however, a large area of surface high pressure centered over IL will extend eastward tomorrow, allowing clear skies to dominate the weather in the Mid-Atlantic. This surface high will slowly move eastward, with its center reaching WV by 12Z Friday. This surface high pressure is expected to remain centered over the region through Monday, although its impacts in the northern Mid-Atlantic (NMA) may be compromised by a weak surface low over the Great Lakes Saturday into Sunday morning. As in previous analyses, a broad area of shortwave energy aloft stretching from the southern Plains to New England will prevent the western US ridge from building eastward over the Mid-Atlantic on Sunday and Monday.

Thursday will be a warm but dry day, influenced both by the cool mid-level air and increasing surface pressure. Despite very light westerly surface winds and clear skies, ozone will stay mostly in the Good range due to fast northwesterly back trajectories from interior ON and the clean air mass in place over the region. Some isolated areas of Moderate ozone are possible along and downwind – to the east – of the I-95 Corridor. Particles will stay in the Good range for most of the region as low humidity continues on Thursday. Locations in the southern Mid-Atlantic (SMA) where the humidity is the highest may see PM2.5 concentrations reach into the Moderate range.

The models have come into closer consensus for much of Friday, but diverge around 18Z. Focusing on the WPC’s decision to side with an EC/GFS consensus, the backdoor cold front moving into PA on Friday evening will slow and weaken as it approaches the state. The GFS analyzes a faster and stronger front, as it did in yesterday’s analysis, and shows clouds starting to build into northern PA around 18Z Friday. The EC shows the front dissipating sufficiently as to not promote notable cloud cover at all. Regardless of which solution verifies, since the cloud cover likely will not reach the I-95 Corridor, Moderate ozone concentrations are expected downwind to the east and northeast of the interstate. The light southwesterly surface winds and increased dewpoints will allow Moderate PM2.5 to spread northward and engulf most of the Mid-Atlantic on Friday.

Saturday is the main day of interest during this period as temperatures continue to rise, surface winds remain light and southwesterly, back trajectories become local and recirculating, and skies stay mostly clear over the I-95 Corridor. It is also another day of continuing model discrepancies, with the GFS keeping the cloud cover over much of the NMA from Friday afternoon through early Sunday morning. Regardless, temperatures are expected to persist just above average and light southwesterly surface winds will allow dewpoints to continue to rise. Shorter, recirculating back trajectories will allow the air mass to continue to modify and promote Moderate ozone formation along and downwind of the I-95 Corridor. The lack of strong synoptic forcing may also allow a sea/bay breeze to form, which would increase concentrations higher into the Moderate range. Particles will continue to rise higher into the Moderate range as the air mass continues to modify and humidity increases.

Conditions on Sunday will be very similar to those on Saturday. The remnants of the weak back door cold front are expected to reach PHL by 12Z Sunday. The front will have weakened considerably by the time it reaches PHL, decreasing the chances for clouds and precipitation along its boundary. Instead, the front would act as a line of convergence at the surface, which would push ozone concentrations into the upper Moderate range or low USG range, depending on how dirty the air mass will be. The BAMS air quality models are giving some credence this forecast by showing upper Moderate to USG ozone in parts of MD and NJ. Some of this is expected to be an artifact due to overrunning from the local waters, but the presence of the weak frontal boundary makes us hesitant to completely rule out high ozone concentrations. More localized onshore, looping back trajectories may limit ozone precursors in the usual areas of concern along the I-95 Corridor. This clean transport would keep ozone concentrations in the Moderate range. The 06Z 36km NCDENR air quality model is leaning towards this solution as it keeps ozone in the Good range in most of the Mid-Atlantic. The NCDENR is only showing an area of Moderate to low USG ozone in northeastern NJ. Historically, Sundays this summer have been unable to reach USG ozone concentrations, regardless of prime synoptic and mesoscale set-ups. With discrepancies in the air quality models, the chances for USG ozone will remain Marginal on Sunday. We will have to continue to monitor the situation, possibly increasing the chances for USG ozone on Sunday to Appreciable. PM2.5, on the other hand, will linger in the Moderate range as relatively high dew points and light winds continue.

Monday will be another day of continuing heat and humidity, allowing ozone and particles to continue to rise as well. The center of high pressure previously stationed over WV will move eastward out to sea, allowing short, onshore, looping back trajectories to persist. Although these trajectories will be a limiting factor to ozone production, concentrations will most likely persist in the mid- to upper Moderate range. Particles will follow the same pattern as humidity and light westerly winds persist over the Mid-Atlantic, with Moderate concentrations expected.

– Eherts/DeBoe/Ryan

Mid-Atlantic Medium Range Air Quality Discussion, Issued Tuesday, August 11, 2015

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 11, 2015
Valid: August 7-11, 2015 (Wednesday – Sunday)

MedRangeTable_201508012

MedRange_Cats_2

Summary:

Back trajectories from the Ohio River Valley coupled with clear skies and light winds associated with surface high pressure centered over WV will increase the chances of USG ozone to Marginal for the end of the medium range period, but there is considerable uncertainty in the forecast. A seasonably cool and much drier air mass filtering into the region behind today’s cold front will keep air quality mostly in the Good range for Wednesday and Thursday. A mid-level ridge will build over the region through the end of the period, allowing a return to above average temperatures. Weak troughing aloft at 500 mb will limit rising temperatures to the upper 80s °F, however. Friday looks like the day with the best chance for widespread Moderate ozone, particularly along the I-95 Corridor, due to westerly transport aloft and light southwesterly surface winds. A key forecast question for the weekend will the fate of a weak cold front moving through the northern Mid-Atlantic on Saturday. The global models are showing clouds and precipitation associated with the front, which would limit significant ozone production. However, this front may weaken faster than model guidance is showing as it moves into a warm air mass. This could result in the cold front acting as a line of convergence, possibly pushing ozone into the upper Moderate range, particularly on Sunday. The other main forecast question will be the flow aloft; 500 m AGL back trajectories suggest that the flow will shift southerly for Saturday and Sunday, with an onshore component. This could limit any rising ozone to the Good to low Moderate range.

Discussion:

The weather models are in generally close consensus, but they begin to diverge on Friday, which adds considerable uncertainty to the air quality forecast for the end of the medium range period. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, the dominating upper level features are a western US ridge centered over the Rockies and a longwave trough over the eastern US. This longwave trough is bringing a cold front through the Mid-Atlantic today. The 09Z WPC surface analysis shows the cold front currently in northwestern PA, and it is draped southward along the western periphery of the Appalachian Mountains. This front will clear most of the region by Wednesday morning but will linger along eastern NC before moving completely offshore by Thursday morning. Seasonably cool and much drier air will filter into the Mid-Atlantic behind the front for Wednesday and Thursday. Beginning on Thursday afternoon, the longwave trough will retreat northeastward and allow the center of surface high pressure to settle over WV/western VA on Friday. As the upper level trough retreats, it will leave a weak trough behind over the Southeastern US, while the flow over the northern and central Mid-Atlantic (NMA, CMA) becomes more zonal. A mid-level (850 mb) ridge will build over the Mid-Atlantic on Friday, bringing a return of warmer air to the region through the end of the period. Also on Friday, a shortwave will crest the upper level ridge in the northern stream flow and drop down into the Great Lakes region, digging out a shallow trough, which will pull a weak cold front into the NMA on Saturday. The GFS is stronger and faster than the EC in regard to this shallow trough over the Great Lakes, and as a result, the GFS has a stronger cold front than the EC. The front has rotated slightly compared to previous days’ analyses, and is now looking less like a back door cold front and more like a traditional cold front. The differences in the model guidance translate into uncertainty regarding the impacts of this front on air quality in the NMA and CMA on Saturday and possibly Sunday, as discussed below. On Sunday, the western US ridge will build northeastward into the the Great Lakes. A broad area of shortwave energy stretching from the Gulf of Mexico to New England will attempt to coalesce into a longwave trough over the eastern US on Sunday. The EC and GFS both have this feature, but to varying degrees. This weak trough will prevent the western ridge from building over the Mid-Atlantic on Sunday.

Today’s cold front will continue moving through the Mid-Atlantic today and clear the NMA and CMA by Wednesday morning. The front will remain in eastern NC for much of Wednesday, reaching the east coast of NC by 00Z Thursday. 850 mb model analyses definitively show Canadian air briefly filtering into the Mid-Atlantic on Wednesday and Thursday. The air mass will be seasonably cool, with about average temperatures for this time of year. The air will be much less humid, however, with dew points in low 50s °F by Thursday. Surface winds on Wednesday will be sustained northwesterly, shifting more westerly and diminishing on Thursday. There is a chance for scattered clouds, rain showers, and thunderstorms in western and central parts of the NMA on Wednesday as the upper level trough sits overhead, but most of the region should see mostly sunny skies, improving to full sun on Thursday. Back trajectories are north/northwesterly both days, with the strongest flow on Thursday (from interior ON). Given the arrival of the new air mass, which is expected to be low in ozone and PM2.5 precursors, mostly Good air quality is expected region-wide. There is a chance for isolated pockets of Moderate ozone on Thursday, given the lighter surface winds. The air quality models keep ozone in the Good range across the board through Thursday.

The air quality forecast is uncertain for Friday-Sunday. A mid-level (850 mb) ridge will build over the region, allowing temperatures to rebound to roughly 3-5 °F above average (i.e., upper 80s to around 90 °F). The center of surface high pressure will move eastward on Friday and sit over WV/western VA through Sunday, leading to mostly clear skies and light southerly surface winds. These conditions should promote rising ozone, but there are some mitigating factors that lend considerable uncertainty to the forecast. The main question mark is the weak frontal boundary that will move into the NMA on Saturday. The global models are showing clouds and precipitation associated with this front, but the strength and coverage are still uncertain. The faster and stronger GFS brings clouds and rain down to MD/DE on Saturday, while the EC keeps them north and west of I-81. The fact that both models have some rain suggests that the front will be strong enough to generate cloud cover. It will weaken and dissipate on Sunday, however, somewhere in the vicinity of southeastern PA. WPC has the front reaching PHL by 12Z Sunday. However, the cold front will be moving into a warm air mass. This may slow down the front and dissipate it more quickly, impacting the chances for precipitation. There is also the possibility that the dissipating front will act as a line of convergence, which could allow for explosive afternoon ozone formation, similar to what occurred in the PHL metro area on July 28-29. Another question is the direction of air mass transport aloft. Back trajectories ending at PHL on Saturday and Sunday morning indicate southerly onshore flow at 500 m AGL. This may be sufficient to keep a flow of clean maritime air moving into the region. There is also a question as to the strength and direction of surface winds, particularly east of I-95. By the weekend, more of a south/southeasterly surface flow may be established, which would also help to inhibit ozone formation.

Given the information we have today, Friday seems like the day with the highest chance for more widespread Moderate ozone, particularly at locations north and east of the I-95 Corridor, due southwesterly surface winds, Ohio River Valley (ORV) back trajectories, rising temperatures, and sunny skies. The BAMS-CMAQ and NCDENR air quality models are showing Moderate ozone along I-95, including DC/BAL, ILG/PHL/TTN, and NYC metro. There is also a hint of a coastal sea breeze, which would enhance ozone production. Continued Moderate ozone certainly seems possible into the weekend, with the best chance at locations farther to the south and east in the NMA and CMA, which will be less impacted by the possible clouds and precipitation associated with the cold front on Saturday. The BAMS and NCDENCR air quality models are somewhat split for Saturday, with only the BAMS-CMAQ showing mid-to-upper Moderate ozone along the I-95 Corridor. Sunday could be anywhere from Good to upper Moderate, depending on air mass transport (clean) or a lingering weak line of convergence associated with Saturday’s cold front (dirty). PM2.5 concentrations will likely stay in the Good range for most of the region through Friday due to the continued low humidity. Locations in the southern Mid-Atlantic may PM2.5 rise into the Moderate range in response to the light surface winds and higher humidity.

-DeBoe/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Tuesday, August 6, 2015

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 6, 2015
Valid: August 7-11, 2015 (Friday – Tuesday)

MedRangeTable_20150807

MedRange_Cats_2

Summary:

A strong surface wave moving through the Mid-Atlantic tonight into Friday morning will promote widespread cloud cover and heavy rainfall for most of the region, effectively cleaning out the atmosphere and leading to generally Good air quality. Clear skies throughout the weekend will promote localized ozone formation, especially along the I-95 Corridor, but temperatures in the low 80s °F, lower vehicle emissions and onshore flow will limit concentrations to the lower end of the Moderate range. The best chances for scatted Moderate ozone will be on Sunday. The weather models are not in consensus regarding the next cold front, which will impact the region sometime in the Monday-Tuesday period. As a result, the forecast is uncertain beyond Sunday, but generally Good to Moderate ozone and low Moderate PM2.5 seem most likely.

Discussion:

The weather models are in generally close consensus in regard to a strong surface wave moving through the Mid-Atlantic on Friday, but they begin to diverge on Saturday. The 00Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. The mesoscale models are continuing to confine the bulk of the rain associated with the surface wave arriving today to areas along and south of the Mason-Dixon Line (MDL). However, the model are not in complete consensus regarding the coverage, magnitude, and northern extent of the wave’s precipitation. Aloft, the shortwave energy associated with the surface wave is over IL this morning and will move eastward into the Mid-Atlantic this evening into Friday morning. The surface wave will ride along the stalled frontal boundary, which is currently draped across the VA/NC border. The surface wave will move off of the Atlantic coast on Saturday morning and will move northward along the eastern seaboard. At this time, the weather models agree to move a shortwave eastward from the northern Plains into the Great Lakes. The EC continues to move this shortwave eastward into New England on Sunday, while the GFS and NAM dissipate it. This leads to differences in the upper level flow. The EC’s solution develops a westerly flow at 850 mb while the GFS and NAM solutions form a mini-ridge at 850 mb, leading to a light southwesterly to southerly flow on Sunday. Overall, the main takeaway for Sunday is that the persistent western US ridge tries to build eastward over the Mid-Atlantic, but it won’t get very far. On Monday, the global weather models are re-developing a broad upper level trough over the northeastern US with its base reaching as far south as KY. The GFS is slightly faster in moving the axis of this trough eastward, placing the axis to the east of the Mississippi River on Monday. In contrast, the EC places the trough axis over the Mississippi River. The upper level trough will amplify on Tuesday as its base stretches southward into GA.

The strong surface wave currently over KY will move eastward into the Mid-Atlantic along a stalled frontal boundary tonight into Friday morning. The models are in general consensus on the fate of the surface wave, but still disagree on the details, which makes the precipitation forecast somewhat uncertain. Today the models are continuing the trend from yesterday to push the wave further southward along the VA/NC border. This surface wave will promote widespread clouds and moderate to heavy rainfall across most of the region, with the more southerly track keeping locations north of the MDL mostly dry. As a result, Good ozone is expected at locations along and south of the MDL. The air quality models are having trouble resolving ozone guidance for tomorrow. They all develop areas of Moderate ozone at various locations, from the I-95 Corridor (NOAA) to southern NJ/DE/eastern shore MD (NCDENR). This seems unlikely given the expected widespread cloud cover, which should reach northward to at least PHL/TTN, and breezy onshore surface winds. Thus, a few locations north and east of the MDL may see ozone reach into the low Moderate range on Friday, but they should be isolated.

Hourly PM2.5 surface concentrations this morning are hovering in the single digits to low teens ug/m3 across most of the region, with the exception of the Hickory, NC area, which is seeing hourly concentrations into the 50s ug/m3. We can’t tell from surface or satellite observations what the cause of this spike in particle concentrations is in Hickory, but there is an AQA issued for the area, so a fire seems the most likely culprit. Aside from this isolated location in NC, the heavy precipitation moving through the Mid-Atlantic tonight into Friday morning will clean out the atmosphere and keep PM2.5 concentrations in the Good range on Friday.

Mostly clear skies across the Mid-Atlantic will return on Saturday as the surface wave moves off the east coast and the stalled frontal boundary moves southward into SC. Friday’s washout coupled with continued onshore surface winds will suppress significant ozone production, keeping concentrations in the Good range on Saturday. A few locations along the I-95 Corridor may see ozone rise into the low Moderate range as surface winds will die down inland and skies will clear. PM2.5 concentrations will likely stay in the Good range for another day due to Friday’s washout along with low humidity.

Clear skies will continue on Sunday as winds die down across the region. These conditions are conducive to rising ozone, but lower Sunday emissions coupled with average temperatures will limit concentrations to the low Moderate range. Locations along the I-95 Corridor have the best chances to see ozone reach into the Moderate range. PM2.5 concentrations will likely continue in the Good range as well due to the low humidity. Some locations in the southern Mid-Atlantic (SMA) where the humidity is higher and the surface winds are light may see PM2.5 concentrations reach into the low Moderate range.

The forecast for Monday and Tuesday is uncertain due to model differences in the arrival of the next cold front sometime early next week. The passage of front has slowed by a day since yesterday’s analysis, with the front now arriving in the Mid-Atlantic on Tuesday afternoon. The GFS has a more traditional frontal passage, albeit very slow, with a line of pre-frontal convection moving across the region from west to east beginning Monday afternoon and continuing to Tuesday evening. In contrast, the EC develops a very strong surface low over the SMA and brings it up the Atlantic coast as a nor’easter-like system. Thus, the precipitation forecast is uncertain for Monday and Tuesday. The GFS solution brings widespread precipitation through the region Monday evening into Tuesday morning, while the EC brings the bulk of the precipitation through Tuesday morning. Surface winds shift southerly on Monday, increasing humidity and cloud cover. Cloud cover will depend on whether the GFS or EC solution verifies, with the GFS keeping skies cloudy in the NMA and the EC with more clouds in the SMA. PM2.5 concentrations may rise into the Moderate range in response to the rising humidity. On Tuesday, both models have a washout across the region, although for different reasons. We will monitor the model trends to determine which solution is more likely; for now, the WPC is siding with the GFS’s “traditional” cold frontal passage on Tuesday.

-DeBoe/Huff