Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 26, 2015
Valid: May 27-31, 2015 (Wednesday-Sunday)

Summary:

Despite having a warm and humid summer-like air mass over the Mid-Atlantic region through the end of the medium range period, the chances for Code Orange/Red air quality have decreased substantially, with only a marginal chance on Thursday-Friday. Onshore transport of maritime air aloft essentially cleaned out the region yesterday, and that trend is expected to continue through Wednesday. The forecast for Thursday will hinge on the location of clouds and convection associated with the a weak cold front that is expected to stall in the vicinity of the I-95 Corridor; upper level transport from the Ohio River Valley and diminishing winds will keep the chances for rising ozone to the northern part of the Mid-Atlantic. A substantial cold front will move into the region on Saturday and stall near the Mason-Dixon Line on Sunday. Clouds and organized convection ahead of the cold front will limit chances for rising ozone over the weekend, as will a return on onshore transport aloft.

Discussion:

The weather forecast models are in relatively close agreement on the main features of the medium range period. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. The overall synoptic setup continues to be dominated by an upper level ridge extending over the western Atlantic; the Mid-Atlantic will remain on the western edge of the ridge through the end of the medium range period. On Wednesday, shortwave energy over the Plains/western Great Lakes will move northeastward into southern ON and reach southern QC by 12Z Thursday. This feature will pull a weak cold front through the northern Mid-Atlantic (NMA) on Wednesday and Thursday. There is considerable uncertainty regarding the strength, speed, and position of this front, considering the relatively weak forcing aloft and the fact that the front will be moving into a warm air mass. WFC transitions it from a cold front (today) to a surface trough (Wed) to a stationary front (Thu). The weather models are consistent in bringing the front across the NMA on Wednesday, reaching the I-95 Corridor by roughly 18-21Z. At this point, it will stall in the vicinity of I-95 through Thursday before dissipating on Friday morning. The main impact for air quality is that the models develop widespread clouds and convection across the region on Wednesday afternoon. The GFS and EC have a slightly stronger shortwave aloft, which translates into more precipitation at the surface that reaches farther east, to the I-95 Corridor. This stalled front acts as a focus for cloud and convection again on Thursday, generally centered slightly farther south, over the central part of the Mid-Atlantic. On Friday, the upper level ridge strengthens and a surface high moves west to east across NY and New England, leading to clearing skies and stagnating winds across the Mid-Atlantic. Aloft on Friday, a large short shortwave/vorticity maximum will drop down from west-central Canada and reach southwestern ON by Saturday morning. This vort max will amplify the semi-permanent upper level trough over eastern Canada and pull a substantial cold front into the NMA in the Saturday-Sunday time frame. The usual discrepancies between the global models remain, with the GFS being the fastest to move the vort max eastward and the EC about 12 hours behind. This translates into some question about how quickly clouds and convection ahead of the front will reach to the eastern Mid-Atlantic on Saturday, as discussed below. The front will reach the vicinity of the Mason-Dixon line on Sunday morning and either stall or sag very slowly southward.

The chances for ozone reaching the USG range in the Mid-Atlantic this week have decreased substantially. The onshore/maritime transport aloft effectively cleaned out the region yesterday, somewhat faster than I expected, given there was still strong subsidence aloft. Even though the ridge will persist all week, with a warm air mass in place, the combination of onshore flow aloft on Wednesday (and then returning for the weekend) and much higher chances for widespread clouds and convention associated with the weak frontal boundary on Thursday and Friday look to suppress ozone to the Good to Moderate range. Particles are relatively low today, but as winds subside on Thursday and Friday, concentrations should increase, especially with surface convergence in the vicinity of the weak cold front.

Wednesday will be similar to Tuesday, except with more chances for afternoon clouds and convection as the weak front moves through the NMA. It will be warm and humid, with breezy south/southwesterly winds. Continued onshore back trajectories over the NC coastline should keep ozone and particles in the Good to Moderate range.

Back trajectories will turn more westerly on Thursday, from the southern Ohio River Valley (ORV). Winds will also diminish, particularly in the central and southern parts of the region. This should drive up particle concentrations into the Moderate range, given the relatively high humidity. Ozone will of course depend on available afternoon sunlight, which will in turn depend on where the weak cold front stalls and acts as a focus for clouds and convection. At this point, Good ozone is expected along the I-95 Corridor but ozone may reach the Moderate range at locations to the west, such as southwestern PA. The BAMS air quality models predict scattered Moderate ozone along the I-95 Corridor and some isolated Moderate PM2.5 in western PA/MD.

A strengthening ridge on Friday will help to keep skies clear and winds light. This will lead to more widespread Moderate ozone, possibly limited by onshore surface flow to the lower Moderate or Good range along the coast. The BAMS air quality models develop a more widespread area of higher Moderate to low USG ozone over eastern PA/northern NJ, likely in response to subsidence and afternoon sun.

The forecast for Saturday and Sunday is uncertain due to questions about the speed and progression the cold front. The GFS, being fastest with the front, brings precipitation to the I-95 Corridor on Saturday afternoon, while the EC is slower. Back trajectories are onshore both days, however, which may be sufficient to keep air quality in the Good to low Moderate range, regardless of any widespread clouds and convection that may develop ahead of the cold front on Saturday and in the vicinity of the stalled/slowing front on Sunday.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 25, 2015
Valid: May 26-30, 2015 (Tuesday-Saturday)

Summary:

With a warm and humid summer-like air mass persisting over the Mid-Atlantic region, there is an appreciable chance for Code Orange/Red air quality through the end of the medium range period. The main factors which limit higher chances for Code Orange ozone are the impact of maritime-influenced transport aloft and the chances for clouds and convection essentially every afternoon. The chances for afternoon clouds/precipitation are greatest on Thursday due to the expected arrival of a cool front from the west, which will progress to roughly the central part of the region before dissipating. The next substantial cold front will arrive in the Saturday to Sunday period, but it is uncertain how quickly it will progress into the Mid-Atlantic region.

Discussion:

The weather forecast models are in overall agreement on the synoptic features of the medium range period, but there are some slight differences, especially later in the period. The 00Z ECMWF, 00 CMC, 12Z NAM, and 06Z GFS were consulted for this analysis. The overall picture has not changed for the Mid-Atlantic. An upper level ridge will remain in place over the region through the end of the week, with surface high pressure anchored offshore. As a result, an unseasonably warm and humid summer-like air mass will persist over the Mid-Atlantic through the medium range period. The next strong, air mass-changing cold front will not impact the region until Saturday/Sunday at the earliest. The models are in close agreement on shifting the upper level ridge axis eastward on Tuesday, which will place the Mid-Atlantic on the far western edge of the ridge. A series of shortwaves will move though the flow essentially every day; the models disagree slightly on the timing and placement of these small shortwaves. The bottom line for air quality is the increased chance for clouds and convection every afternoon. The models all move a strong area of shortwave energy across the Great Lakes and into southern ON/QC on Wednesday and Thursday, although there are small differences with the timing and location. This feature will slowly move a cool front into the northern Mid-Atlantic (NMA) from the west on Wednesday before the front dissipates roughly over central PA on Thursday. The main effect of this feature is expected to be greater chances for organized convection on Thursday afternoon. On Saturday, a large shortwave/vort max drops down from north-central Canada and amplifies the semi-permanent upper level trough over eastern Canada. This feature will bring the next significant cold front into the Mid-Atlantic, sometime in the late Saturday to early Sunday time frame. The global models show the usual discrepancies regarding frontal passages, with the GFS the fastest with the vort max and surface front, followed by the EC, and then the CMC. The main effect for air quality on Saturday will be how fast an organized line of convection reaches the eastern Mid-Atlantic, as discussed below.

Given the eastward location of the upper level ridge, back trajectories continue to show an onshore component over the NC/SC coastlines for essentially the entire period, with the strongest maritime transport influence on Tuesday and Wednesday. As has been the case since last week, the air quality models (e.g., NOAA/EPA, BAMS, NCDENR) continue to keep air quality essentially in the Good range across the region on Tuesday and Wednesday. Presumably this is in response to the maritime-influenced transport pattern, although we did see that the models were biased roughly 10-15 ppb low for ozone over the weekend. The air quality models do show scattered Moderate ozone and PM2.5 conditions on Thursday and Friday across the NMA, mostly in the vicinity of the I-95 Corridor, likely in response to lighter winds later in the week.

Tuesday will be warm and increasingly humid, with mostly sunny skies and breezy south/southwesterly winds in the afternoon, mainly along the eastern part of the region. The main forecast question will be the impact of maritime-influenced upper level transport. We have seen this transport pattern suppress ozone formation in the past, even under full sun and high temperatures ≥ 85 °F. The degree to which ozone rises this afternoon across the region will give us an idea of the possible impact of the maritime-influenced transport. Particle concentrations may be suppressed as well, particularly at coastal locations, where concentrations are in the Good range this morning (i.e., Delaware) and are only very slowly rising despite the increasing humidity.

Wednesday, Thursday, and Friday have increased chances for afternoon clouds and convection, especially on Thursday due to the arrival of the cool front. The convection looks to be more likely across the western and central parts of the region, with most models keeping it to the west of the I-95 Corridor on Wednesday and Thursday. Winds will continue out of the south/southwest, with breezy conditions along the coast on Wednesday and then diminishing wind speeds on Thursday and Friday.

Saturday’s forecast is very uncertain given the possible approach of the next significant cold front. The GFS is the fastest with the front, and places widespread precipitation across most of the region by the afternoon, including the I-95 Corridor. The CMC is the slowest of the global models and keeps most of the region dry on Saturday.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, May 24, 2015
Valid: May 25-29, 2015 (Monday-Friday)

Summary:

High chances for Code Orange/Red air quality in the Mid-Atlantic region continue on Monday and Tuesday, with appreciable risk for the rest of the period, but uncertainty persists. An upper level ridge will remain over the Mid-Atlantic through the end of the medium range period, which will keep a very warm and humid air mass over the region. The ridge axis will be centered over the Mid-Atlantic on Monday and slide eastward on Tuesday, placing the Mid-Atlantic on the western edge of the ridge for the remainder of the period. The main limiting factors for Monday and Tuesday will be the speed with which regional ozone rises and the impact of onshore transport over the NC/SC coastline. Increasing chances for afternoon convection are the main sources of uncertainty for Wednesday through Friday.

Discussion:

The weather forecast models are in general agreement on the synoptic features for the medium range period, but there are some slight differences. The 00Z ECMWF, 00 CMC, 12Z NAM, and 06Z GFS were consulted for this analysis. The main issue for air quality continues to be the upper level ridge that will remain in place over the Mid-Atlantic through the end of the medium range period, which will keep a very warm and humid air mass over the region. The ridge axis will be centered over the Mid-Atlantic on Monday and slide eastward on Tuesday, placing the Mid-Atlantic on the western edge of the ridge. The NAM and GFS are slightly faster to push the ridge eastward compared to the EC and CMC. The ridge will weaken Wednesday through Friday as a series of shortwaves move through the flow. In particular, the global models bring a strong shortwave across the Great Lakes on Wednesday and into southern ON/QC on Thursday, which will possibly pull a cool front into the northern Mid-Atlantic. The main impact of the shortwaves will be increasing chances for clouds and convection, especially on Wednesday and Thursday. Due to the eastward placement of the ridge, back trajectories continue to be a major factor limiting possible rising regional ozone, with onshore transport over the NC/SC coastline beginning on Monday at low levels and continuing at all levels on Tuesday and Wednesday.

Memorial Day will be warm and increasingly humid. The location of the upper level ridge will keep any shortwaves moving through the flow to the north of the Mid-Atlantic. As a result, Monday will be mostly sunny across the region. Light south/southwesterly winds will pick up in the afternoon, especially along the coast. Back trajectories at 1500 and 1000 m AGL are slow and westerly from the Ohio River Valley (ORV), while low level trajectories at 500 m AGL are onshore from over the NC coastline; all of the trajectories show strong subsidence. The forecast questions for tomorrow will focus on the degree to which low level transport aloft of maritime air may suppress rising ozone levels, particularly along the I-95 Corridor. Memorial Day holiday emissions are another factor, as will be the rapidity with which ozone rises today across the region and upwind in the ORV.

Tuesday will be slightly warmer and more humid than Monday, with increased chances for afternoon clouds and thunderstorms. The 09Z SREF paints a high probability of precipitation across most of the western Mid-Atlantic in the afternoon, but the 12Z 4 km nested NAM only shows isolated thunderstorms. The EC and CMC keep the region essentially dry on Tuesday. Back trajectories all have an onshore component, however, hooking to the southeast over the NC/SC coastline. As has been the case for the past several days, the air quality models have a clean out for Tuesday and Wednesday, with only the NCDENR model indicating isolated Moderate ozone and PM2.5 in eastern PA. Despite the uncertainty in possible clouds/precipitation and onshore transport, chances for at least isolated USG ozone remain high given the very warm conditions, normal weekday emissions, and a possibly modifying air mass.

Substantial uncertainty remains in the forecast for Wednesday through Friday. The global models all have increasing chances for clouds and precipitation, given the warm and humid conditions and the series of shortwaves moving through aloft. If the weak frontal boundary predicted by the global models verifies, it will act as a focus for clouds and convection on Wednesday and especially Thursday. Back trajectories shift westward, away from the Atlantic, on Thursday and Friday, however, with slow localized transport on Friday. In response, the air quality models have scattered Moderate ozone along the northern I-95 Corridor on Thursday. Thus, despite increased chances for afternoon convection later in the period, chances for USG ozone remain, given the persistent warm and humid conditions, increasingly modified air mass, and more westerly back trajectories. The next true air mass-changing cold front will not impact the region until next Sunday at the earliest.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, May 23, 2015
Valid: May 24-28, 2015 (Sunday-Thursday)

Summary:

There is high chance for Code Orange/Red air quality in the Mid-Atlantic region beginning on Monday, but uncertainty still remains, especially for the end of the period. An upper level ridge will build over the Mid-Atlantic on Sunday and remain in place through the end of the period. The main question for air quality will be the position of the ridge; the Mid-Atlantic will be on the western edge of the ridge, which is not the preferred positon for high regional ozone. As a result, back trajectories turn fully onshore over the NC/SC coast by Tuesday and continue through the end of the period. There is also uncertainty about the chances for potentially widespread afternoon convection on Wednesday and especially Thursday, which would also limit rising ozone. Thus, Monday and Tuesday are the days at most risk for USG ozone in the Mid-Atlantic, with the possibility continuing for Wednesday and Thursday but with more uncertainty.

Discussion:

The weather forecast models are in close agreement on the synoptic features for most of the medium range period, but they diverge slightly by the end of the period. The 00Z ECMWF, 00 CMC, 12Z NAM, and 06Z GFS were consulted for this analysis. The big story is the upper level ridge that will build over the Mid-Atlantic from the Great Lakes on Sunday and persist through the end of the period. The question for air quality will be the position of the ridge. The ridge axis will be over the Mid-Atlantic on Monday and then continue to gradually shift eastward, with the axis reaching the Canadian Maritimes on Tuesday, which will put the Mid-Atlantic on the western edge of the ridge through the end of the period. This will place the ridge east of the preferred position for high regional ozone across the Mid-Atlantic. By Thursday, the global models bring several strong shortwaves over the Mid-Atlantic, which will disrupt the ridge but not displace it. The models differ on the timing and location of the shortwaves.

Sunday will be the beginning of a warming trend that will likely include the first heat wave of the season. Southwesterly flow around the center of surface high pressure offshore in the Atlantic will bring warmer and more humid air into the region. It will be sunny with southwesterly winds gusting to around 15 mph in the afternoon. Recirculation aloft will promote rising ozone and particle concentrations. With full late May sun, ozone will reach into the Moderate range across the I-95 Corridor. Particle concentrations are well inside the Good range for most of the Mid-Atlantic today, given the clean and very dry air mass in place. The exception is western PA, where concentrations this morning were in the Moderate range. There is widespread smoke across Central Canada from wildfires, and HMS analyzed this smoke extending as far east as MI and Lake Erie. It is possible that parts of the Ohio River Valley as well as the Pittsburgh area are seeing some of that smoke this morning, which is increasing particle concentrations faster than otherwise expected, in conjunction with stagnation and subsidence associated with surface high pressure moving overhead. The NOAA surface smoke model shows some additional transport of smoke from the Canadian wildfires into PA/NJ tomorrow, which may impact surface PM2.5 concentrations in the northern Mid-Atlantic.

Memorial Day will be warm with increasing humidity. A warm front will lift across the region, but the previously-forecasted back door cold front will not drop down from ON/QC late Sunday into Monday. As a result, Monday should be mostly sunny, with southwesterly winds gusting to 15-20 mph in the afternoon. Back trajectories at 1000 and 1500 m AGL will be westerly, from the Ohio River Valley. Low level transport (at 500 m AGL) will be onshore across the NC coastline, however, which may limit rising ozone along I-95. Nevertheless, with a strong ridge in place and ample afternoon sun, there is a high chance for isolated USG ozone in the region. An additional limitation will be holiday emissions, which tend to have the same overall magnitude as a normal weekday, but are not concentrated during morning and evening rush hours.

Tuesday will be warmer and more humid than Monday, with the potential for an Appalachian lee trough to form in the vicinity of the I-95 Corridor. The 09Z SREF and operational NAM and GFS models fire up afternoon convection across the western Mid-Atlantic, possibly in response to the lee trough. Enough sun will be present, especially to the east of I-95, for a high chance of USG ozone to continue. The main limiting consideration will be the position of the upper level ridge, which turns transport aloft at 1500, 1000, and 500 m AGL fully onshore. In addition, surface winds take on more of a southerly component. This may be sufficient to limit rising regional ozone.

The chances for USG ozone will continue on Wednesday and Thursday, given the persistent ridge and rising temperature and humidity. Wednesday will likely be the beginning of a heat wave, with temperatures reaching into the low 90s °F. Given the summer-like air mass in place, the weather models develop convection in the afternoons, particularly on Thursday, as several shortwaves move overhead. This increases the uncertainty in the ozone forecast for late in the period, as does continued onshore back trajectories from the NC/SC coast. Similar to yesterday, the extended air quality models (BAMS and NCDENR) show a clean out for Wednesday and Thursday, with Good air quality across the board, possibly in response to the transport pattern and/or greater chances for widespread afternoon precipitation.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 22, 2015
Valid: May 23-27, 2015 (Saturday-Wednesday)

Summary:

There is an appreciable chance for Code Orange/Red air quality in the Mid-Atlantic region beginning on Monday, but uncertainty still remains. A brief surge of cool, dry, and clean Canadian air on Saturday will keep air quality in the Good to low Moderate range across the region. Sunday is a transition to a potential period of deteriorating air quality. An upper level ridge will build over the region, with steadily increasing temperatures and humidity through the end of the period. It now appears that any clouds and rain associated with a weak frontal boundary across the northern Mid-Atlantic on Monday will stay to the north of the region, increasing the chances for rising ozone. However, the upper level ridge will be located east of the preferred position, which could introduce maritime air transport, particularly on Tuesday and Wednesday. This transport pattern may suppress ozone formation along the I-95 Corridor late in the period, despite temperatures in the 90s °F and mostly sunny afternoon skies.

Discussion:

The weather forecast models are in agreement on the overall synoptic features for most of the medium range period, but they diverge slightly on Tuesday and Wednesday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. Saturday will be the transition day to a new upper level pattern. A closed low rotating through the semi-permanent upper level trough over eastern Canada will reach northern New England by 12Z and the Canadian Maritimes by 00Z Sunday. As this energy moves eastward, an upper level ridge will build over the Plains, which will extend eastward over the Mid-Atlantic on Sunday. The ridge will link up with the Bermuda High in the Atlantic Ocean and remain in place over the Mid-Atlantic through the end of the medium range period, although the ridge will continue to gradually shift eastward. A series of shortwaves are still expected to drop down from northern Canada and rotate around the eastern Canadian trough axis through the end of the period, but the trend to keep them farther north continues today, which has implications for Monday’s precipitation forecast. One of the Canadian shortwaves is still expected to push another, weaker back door cold front into the northern Mid-Atlantic (NMA) late Sunday into Monday, but this weak front looks to stall slightly farther north in the NMA compared to yesterday’s model runs. As a result, the 09Z SREF, 06Z 12 km NAM and the other operational models all keep most clouds and all of the rain north of the Mid-Atlantic on Monday. Aloft, the upper level ridge will shift slightly north and east on Monday compared to Sunday’s position, with the ridge axis settling over the Mid-Atlantic. By Tuesday and Wednesday, the global models shift the ridge farther east, placing the Mid-Atlantic on the western periphery of the ridge. This will locate the ridge east of the preferred position for high ozone across the Mid-Atlantic. There are slight differences between the global models in regard to the ridge placement; the GFS is the fastest to move the ridge eastward and the EC is the slowest, but they all put the Mid-Atlantic on the far western periphery of the ridge by Tuesday, which will impact the upper level transport pattern, as discussed below.

On Saturday, a brief surge of cool, dry, and clean Canadian air will arrive in the wake of Friday’s back door cold front. The front will reach to roughly southern NC by 12Z Saturday. Surface high pressure will move across the Mid-Atlantic during the day and shift offshore in the evening. Full sun, stagnating winds, and Memorial Day weekend emissions will allow ozone to rebound, but with a clean and cool air mass in place, generally Good to low Moderate conditions are expected. Particles should also remain generally in the Good range in the clean, very dry, and unmodified air mass.

The warming trend will begin on Sunday as the ridge builds overhead and return flow around the surface high in the Atlantic brings warmer and more humid air into the Mid-Atlantic. Mostly sunny skies, light winds, and recirculating transport aloft will push ozone into the Moderate range at locations along the I-95 Corridor, with particles lagging in the still relatively dry air mass.

Memorial Day will be warm and humid. The weak back door cold front will stall in the vicinity of central PA/NJ on Monday morning and then move back north as a warm front during the day. The SREF and operational models have been trending toward keeping rain and clouds associated with this front to the north, which suggests that there will be sufficient sun on Monday for ozone to rise to the upper Moderate range across most of the region. An Appalachian Lee trough is expected, which will act as a line of convergence across MD/VA/NC. However, since the upper level ridge will be situated east of the preferred position, low level back trajectories (@500 m AGL) are southeasterly, indicating the possibility of some maritime air transport. Farther aloft (@1000 m and 1500 m AGL), back trajectories on Monday are westerly from the Ohio River Valley, a source region for ozone precursors. The forecast question for Monday will focus on the possible impact of onshore transport at low levels compared to rising temperatures and the presence of an Appalachian lee trough.

Tuesday and Wednesday will be hot and humid, with mostly sunny skies but the chance for isolated afternoon convection driven by diurnal heating. By this point, the Mid-Atlantic will be situated along the far western edge of the upper level ridge, and as a result, back trajectories all shift to the Southeastern US, with a continued maritime component, particularly at low levels. We’ve seen this pattern in the past suppress ozone formation along the I-95 Corridor, even with temperatures in the 90s °F and clear skies. Of note is that all of the operational air quality forecast models (BAMS-MAQSIP and -CMAC, NCDENR) keep ozone fairly low through the medium range period, despite the hot and humid weather, with some scattered Moderate conditions on Monday but Good ozone and particulate matter across the board on Tuesday. My guess is that the models are responding to the transport pattern, but CMAQ-based models in particular have a bias toward under-forecasting early in the season, which makes them unreliable.

-Huff