Monthly Archives: August 2015

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 Friday, August 14, 2015

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 14, 2015
Valid: August 15-19, 2015 (Saturday – Wednesday)

MedRangeTable_20150815

MedRange_Cats_2

Summary:

A mid-level ridge will build into the Mid-Atlantic for the medium range period, acting as a block for approaching frontal boundaries, and allowing above average temperatures and humidity to persist through the end of the period and beyond. A High chance of USG ozone exists for Saturday, dependent on how fast the current air mass modifies (i.e., today’s persistence for tomorrow) and the fate of weak frontal boundaries (e.g., bay/sea breeze fronts, weak back door surface trough). Short and recirculating back trajectories and consistent air quality model guidance lend confidence to a possible exceedance forecast in the usual area of concern along the I-95 Corridor. An Appreciable chance for USG ozone will continue on Sunday and Monday, with the main questions centering on lower Sunday emissions and a possible shift to sustained onshore flow aloft at 500 m. The forecast for Tuesday and Wednesday is uncertain, but the global models are showing widespread light precipitation across the region both days due to weak 500 mb troughing/shortwaves. In addition, the southerly 500 m AGL flow will continue. These factors may be enough to limit ozone to the Good to low Moderate range. Looking ahead to the end of next week, the Bermuda High will extend westward beginning next Thursday, centered over the Southeastern US. Although the influence of the Bermuda High will keep temperatures and humidity above average in the Mid-Atlantic, the southern location of the high should allow southerly transport aloft to dominate. Thus, although warm, stagnant, and sunny weather will continue through at least next weekend, the risk for USG ozone should remain Marginal due to southerly flow aloft. Particles will increase as humidity rises and the air mass over the Mid-Atlantic modifies over the next few days, peaking in the upper Moderate range on Tuesday in the Pittsburgh metropolitan area.

This is the last medium range discussion for the 2015 summer season. The discussions will return in 2016 and run from mid-May to mid-August.

Discussion:

The weather models are in generally close consensus throughout the medium range period, but exhibit differences regarding precipitation on Tuesday and Wednesday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. The large trough that had settled over the eastern seaboard for the past couple of days has almost completely lifted out of the region, and a mid-level ridge building into the Mid-Atlantic is promoting abundantly clear skies and calm conditions. A center of surface high pressure will persist over WV throughout the period to varying degrees as the mid-level ridge (850 mb) slowly moves eastward. Whereas in previous days’ guidance, a weak back door cold front was expected to impact the northern Mid-Atlantic to some degree on Saturday-Sunday, and a stronger cold front was expected to move into the region on Monday-Tuesday and stall near the Mason-Dixon Line (MDL), this morning’s GFS and EC runs now show these fronts staying to the north and west of the Mid-Atlantic. The mid-level ridge now appears to dominate the Mid-Atlantic through the end of the medium range period and beyond, into at least next weekend, as the Bermuda High extends westward, centered over the Southeastern US.

At 500 mb, however, the weak trough/upper level shear axis is still evident in the EC and GFS guidance, which complicates the forecast. The shortwave energy dropping down on Saturday from the northern stream flow is still there today, but not quite as strong as in yesterday’s analyses. This feature is associated with a weak back door cold front at the surface. This front is even weaker today than in previous model runs, and now looks to only bring a chance of clouds and scattered precipitation to areas west and north of roughly I-81 on Saturday. There is still a question as to whether this boundary will act as a line of converging winds in the NMA on Saturday and Sunday, as discussed below. On Monday, the weak shortwave energy aloft will remain along a shear axis running approximately along the ridge of the Appalachian Mountains from the Mid-Atlantic to the Texas Gulf Coast. The persistent western US upper level ridge will try to build eastward at the beginning of next week, but the weak trough/shear axis will inhibit it. Small shortwaves dropping down from the northern stream flow on Monday will phase, to certain degree, with the shear axis energy, and dig out a very shallow trough over the Mid-Atlantic on Tuesday-Wednesday. These shortwaves will bring scattered precipitation to the Mid-Atlantic on Tuesday-Wednesday, as discussed below. At the same time, a much more substantial shortwave will drop down over the Plains and pull a cold front as far east as the Great Lakes before pulling northward. Consequently, this next cold front will not make it to the Mid-Atlantic next week, but instead the mid-level ridge will continue to provide warm and humid weather to the region.

The weekend continues to be of interest for USG ozone conditions. Obviously, persistent sunny skies, very light to calm winds, and temperatures in the low 90s °F set the stage for rising ozone across the region. The main forecast questions continue to be 1) the degree to which the air mass currently in place modifies, 2) the impacts of weak frontal boundaries, including sea/bay breeze fronts and a weak back door surface trough, and 3) air mass transport aloft, which appears to turn more southerly at 500 m AGL beginning on Sunday. Saturday is the primary day of concern, given recirculating back trajectories and higher Saturday emissions. Historically, fewer emissions of ozone precursors on Sundays limit rising ozone to the upper Moderate range, all other things being equal. The air quality models have been resolving upper Moderate ozone concentrations along and surrounding the I-95 Corridor for the past several days, lending confidence to the expectation that this previously very clean air mass will modify substantially by Saturday. The CMAQ-based air quality models (e.g., NOAA, BAMS, NCDENR) do have a high bias by this point in the summer, however, which should be taken under consideration. The BAMS, NCDENR, and NOAA air quality models agree in resolving USG ozone in the Baltimore metropolitan area and along the southern NJ coast on Saturday. Upper Moderate ozone is resolved in a wide area surrounding this sector of the I-95 Corridor, but there is less consensus regarding the extent of these conditions. Due to the very light southerly/southwesterly surface winds, Moderate ozone will most likely be limited to areas along and just to the east of I-95, where emissions will be highest. Moderate ozone is also likely along the eastern coast of MD and VA, where the high-res mesoscale weather models are resolving areas of surface divergence, and therefore subsidence and limited atmospheric mixing. A complicating factor is the weak back door trough, which is no longer a cold front. The 09Z SREF shows a medium to high probability of precipitation on Saturday afternoon for areas west and north of I-81, similar to yesterday’s analyses. The hi-res weather models are showing only increased cloud cover in these areas, which adds uncertainty to the likelihood of clouds or scattered showers making their way to the I-95 Corridor. The 06Z 12 km NAM and 13 km GFS do show a line of convergence over southeastern PA and central NJ, however, associated with the trough. As we saw a couple of weeks ago in the Philadelphia metro area, a weak semi-stationary boundary can push ozone well into the USG range on local scale. Trying to forecast where this trough will settle is extremely difficult, however. We will have to make our best assessment based on the 12Z model runs. The same thing can be said about the bay and sea breezes, which may be an issue on both weekend days, given the weak synoptic winds. Particles are expected to reach the Moderate range at scattered locations due to a slight increase in the humidity compared to today and the continuing calm conditions.

Another day of above average temperatures, clear skies, calm surface winds, and short, looping, southwesterly back trajectories leave room for concern regarding building ozone on Sunday. The NCDENR air quality model shifts its USG concerns to the DC metropolitan area for Sunday, while the BAMS guidance looks almost identical to Saturday’s. Regardless, upper Moderate conditions are expected to extend along and east of the I-95 Corridor as very light southwesterly surface winds persist throughout the region due to the surface high pressure centered over WV. A question for Sunday is the fact that back trajectories at 500 m AGL turn due south, inland from the Atlantic coast. We have seen previously this summer that southerly flow aloft can completely suppress ozone to the Good range. Although this transport signal is somewhat weak, it may be sufficient to keep ozone out of the USG range. On the other hand, Saturday’s weak surface trough in the vicinity of PHL may linger, and sea/bay breezes are a threat as well. Particles are expected to continue to increase into the Moderate range at scattered locations as calm conditions and another uptick in humidity allow the buildup of PM2.5.

Monday will be another day of temperatures in the low to mid 90s °F, increasing dew points, light southwesterly surface winds, and surface high pressure throughout the Mid-Atlantic. The main difference is that lower level (500 m AGL) back trajectories are fully south along the Atlantic coast of NC. This is most likely due to the fact that the mid-level ridge will continue to build eastward, placing the Mid-Atlantic on its western edge by early next week. The NC air quality models appear to be responding to this change in upper level flow, as they begin a downward trend in ozone on Monday, continuing into Tuesday, with only isolated Moderate ozone at coastal locations. The BAMS models keep upper Moderate ozone all along the I-95 Corridor, however.

Tuesday and Wednesday have considerable uncertainty, with continued Moderate ozone very likely, but less chance for USG ozone. The next cold front that was expected to stall near the MDL on Tuesday-Wednesday will now stay well to the north and west of the region, allowing above average temperatures and summer-time humidity to continue. However, the weak shortwave energy/troughing aloft will keep the chances for scattered clouds and rain showers in the forecast. Both the EC and GFS show scattered light precipitation across the region on Tuesday and Wednesday, which may be enough to again limit ozone to the Moderate range. In addition, back trajectories at 500 m AGL will continue to be due south, from coastal NC.

Looking ahead, the Bermuda High will extend westward beginning next Thursday, centered over the Southeastern US. Although the influence of the Bermuda High will keep temperatures and humidity above average in the Mid-Atlantic, the southern location of the high should allow southerly transport aloft to dominate. Thus, although warm, stagnant, and sunny weather will continue next week, the risk for USG ozone should remain Marginal beyond the end of the medium range period, due to southerly flow aloft.

– Eherts/Huff

 

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