Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 5, 2015
Valid: August 6-10, 2015 (Thursday – Monday)

Summary:

Generally Good to low Moderate air quality is expected throughout the medium range period due to two substantial disturbances. The first will arrive in the form of a surface wave along a frontal boundary stalled over northern NC, which will bring widespread clouds and rain to the Mid-Atlantic south of the Mason-Dixon Line from Thursday through Saturday afternoon. North of these impacts, the clean post-frontal air mass in place will limit ozone production. Following this large system, we will see two days of at least partial sun, with Moderate ozone possible around the southern I-95 Corridor on Sunday. Then the next cold front will move through the region on Monday. Although there is some uncertainty in the precipitation forecast this far in advance, widespread pre-frontal clouds and convection are expected, which will drop particles and ozone into the Good range, while Moderate air quality may continue east of I-95.

Discussion:

The weather models are in generally good consensus throughout the beginning of the medium range period, but begin to differ on Friday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large closed low aloft just east of James Bay has an associated trough reaching into the central Mid-Atlantic (CMA). An area of shortwave energy aloft over NE/KS will move eastward with the flow and begin to dig out its own mini trough. The global and mesoscale models have come into much closer consensus today regarding the fate of this area of shortwave energy, but some discrepancies remain. In particular, the global weather models begin to show slight differences on Friday regarding the exact shape and speed of this shortwave. However, resulting model discrepancies do not impact the air quality forecast until Monday. This shortwave feature will begin to impact the Mid-Atlantic on Thursday afternoon as a wave of surface low pressure, bringing widespread clouds to areas south of the Mason-Dixon Line (MDL) and associated rain showers to I-95. The models now agree in containing the major impacts of this disturbance to areas south of the PA/MD border; as a result, PA and north/central NJ will see little to no precipitation. Saturday will be a transition day following this rainy system as it moves northeastward up the Atlantic coast. The persistent upper level ridge in the western US will flatten and spread towards the Mid-Atlantic on Sunday, promoting another day of mostly sunny skies. But the eastward extension of the upper level ridge will not spread any farther, as the semi-persistent eastern Canadian upper level trough re-develops for early next week. On Sunday, the models agree that an upper level low will reach the MB/ON border, reflected at the surface as an area of low pressure in southcentral ON. This low will have an associated southwestward stretching frontal boundary that will reach from the Great Lakes to northern KS on Sunday. The WPC surface analysis shows this front moving southeastward through the Mid-Atlantic throughout the day on Monday, which proves a favoring towards the GFS global model solution, which shows an extensive area of pre-frontal precipitation moving through the entire Mid-Atlantic starting on Monday night.

Extensive cloud cover will spread over the entire Mid-Atlantic from west to east tomorrow, due to both a stalled frontal boundary along the VA/NC border and an approaching surface low riding along the stalled front. Rain showers are expected to spread west to east across the CMA and southern Mid-Atlantic (SMA) throughout the day, reaching the coast by overnight Thursday into Friday. Model consensus has grown closer over the past couple of days, resulting in increased confidence that the rain will stay to the south of the MDL. Back trajectories from the Upper Peninsula of MI, along with the widespread clouds and afternoon rain, will limit ozone to the Good range for most areas. The exception will be where there is surface convergence resulting from the frontal boundary near coastal VA and NC, where the clouds may not reach until after nightfall. Good PM2.5 is anticipated across the region, due to either cleaner, drier air filtering in to the north of the front or the widespread evening washout.

Despite model discrepancies regarding the speed of the eastward motion of the surface low, widespread clouds and rain showers are expected to continue throughout the region on Friday – again, south of the MDL. Despite shorter back trajectories from the Lake Ontario region, the continuing cloud cover will keep ozone mainly in the Good range across the eastern seaboard. The BAMS air quality models are resolving an area of low Moderate ozone in northwestern VA, where an extended afternoon break in the clouds is resolved in the high-res mesoscale weather models. This most likely will not verify due to the fact that a substantial area of rain will have just cleared the region, and that the mesoscale models do not show the sun coming out until 21 to 00Z. PM2.5 is expected to stay in the Good range as well, except for a small area just north of the clouds in the Susquehanna River Valley of PA, which is being resolved by the BAMS/NCDENR air quality models. Both models are responding to the increased humidity just to the north of the rain and cloud cover, where a lack of heavy precipitation (such as there is to the south) may allow particle concentrations to build into the Moderate range.

There is general model consensus that the center of surface low pressure will move off the Mid-Atlantic coast overnight Friday into Saturday and make its way toward New England. It will take its time, however, and clear skies are not expected to overtake the region until about 18Z Saturday. Additionally, the air at 500 m AGL will originate from near Cape Cod. Ozone is therefore expected to stay mostly in the Good range. As the surface low shifts towards a more northeastward track and moves parallel to the Jersey coast, northeasterly surface winds will increase across the region, particularly along the coast, lending confidence to the Good air quality forecast. Particles will remain in the Good range as well, due to the sustained northeasterly winds and recent washout.

Sunday will be the first sunny day for the medium range period ahead of an approaching cold front. Clear skies, warm and seasonable conditions, and a slight increase in humidity will promote scattered afternoon cloud cover. However, this will not be sufficient to hinder scattered low Moderate ozone in the usual areas surrounding the southern I-95 Corridor. A stretch of Moderate PM2.5 is also resolved in the air quality models in DC, eastern VA, and much of NC. This is most likely due to the fact that winds will be calmest there for the longest behind Friday’s coastal low.

Monday will be pre-frontal, and following the GFS model solution, it will most likely be another washout throughout much of the region, dropping ozone and particles solidly back into the Good range. A cold front approaching from the northwest will sweep through the NMA, passing through northwest PA around 12Z. Widespread precipitation associated with this front will scrub the atmosphere of any precursors that managed to build since the last frontal passage. The cloud cover is not expected to reach the coast before nightfall, meaning that ozone will have an additional day of sun to build into the Moderate range in areas east of the I-95 Corridor. The same will go for PM2.5, as a calm morning and continuing humidity allow particles to build into the Moderate range ahead of the rain. Major model discrepancies do exist, however, lending uncertainty to this forecast. Whether or not the widespread showers verify, the atmosphere in place will be clean enough to limit max ozone to the Moderate range.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 4, 2015
Valid: August 5-9, 2015 (Wednesday – Sunday)

Summary:

Cleaner and noticeably drier air filtering into the Mid-Atlantic behind today’s cold front, coupled with the passage of a strong surface wave on Friday, will limit the chances for USG ozone to Slight for the entire medium range period. Today’s cold front will progress through the region on Wednesday and stall across NC and southeastern VA through Thursday, promoting scattered clouds and precipitation. Pockets of Moderate ozone and PM2.5 are possible along and south of the stalled frontal boundary, with additional isolated Moderate ozone east of I-95. A strong surface wave is still expected to move through the Mid-Atlantic on Friday, although there is more uncertainty today regarding the strength, track, and timing of this feature. Nevertheless, the arrival of the wave late Thursday into Friday will promote widespread clouds and heavy precipitation on Friday across much of the region, effectively cleaning out the atmosphere for Good air quality. Friday’s washout coupled with onshore surface winds, scattered clouds, relatively low humidity, and slightly below average temperatures will keep air quality generally in the Good range throughout the weekend.

Discussion:

The weather models are in generally close consensus throughout most of the medium range period, but begin to show differences by Thursday regarding the track, timing, and magnitude of a strong surface wave. The 00Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. An upper level closed low currently sits over James Bay with the base of its trough extending southward to NC. This closed low will slowly creep eastward in the northern flow on Wednesday, reaching the Canadian Maritimes late Thursday evening. A cold front moving through the Mid-Atlantic today associated with the closed low will slowly move eastward and stall across NC on Wednesday morning. This frontal boundary will linger over the region through the end of the period, stalling along the NC coast through Thursday and then moving north and east, settling along the Atlantic coast through the weekend. On Thursday, the weather models have slightly different solutions for a series of shortwaves moving eastward from the Plains into the Tennessee River Valley (TRV). There is actually less consensus on this feature in today’s model runs compared to yesterday’s. The EC continues to show one large shortwave, while the GFS and NAM have more of a series of shortwaves. Consequently, the EC develops a stronger surface wave. The NAM is about 6 hours faster than the EC, while the GFS is weaker and slower than both the EC and NAM. The surface wave will impact the Mid-Atlantic on Friday, although given the lack of consensus in the global models, the timing and track is uncertain. The EC is the slowest model and places the strongest shortwave over the central Mid-Atlantic (CMA) at 18Z Friday, leading to widespread heavy precipitation across the region. The NAM is faster, but has equally widespread precipitation, while the GFS has less precipitation. The EC then proceeds to move the shortwave and accompanying surface wave northeastward on Saturday, deepening the surface wave as it moves along the east coast into new England. Another series of shortwaves will move eastward from the northern Plains into the Ohio River Valley (ORV) early Saturday morning. The GFS is the stronger solution as it develops a surface low over VA on Saturday, resulting in the GFS being the wetter solution. In contrast, the EC is much weaker with the shortwaves, keeping the forecast dry for Saturday for most of the region. The weather models diverge aloft on Sunday, generating some uncertainty, but mostly dry conditions are expected as Friday’s and Saturday’s surface waves move away from the region.

A cold front slowly moving through the Mid-Atlantic today will stall across NC and southeastern VA on Wednesday, promoting scattered clouds and precipitation in the southern Mid-Atlantic (SMA). Despite mostly sunny skies and temperatures hovering around the 90 °F mark, Good ozone is expected throughout most of the region as a clean and drier air mass will build in behind the passing cold front. Moderate ozone is possible at locations west of the stalled frontal boundary in NC and VA where pockets of clearing and converging winds will promote a buildup of ozone concentrations. The NOAA and NCDENR air quality models support this analysis, showing a swath of Moderate ozone stretching east/northeastward from western NC to southern VA. Moderate ozone is also possible east of I-95, in DE, the eastern shore of MD, and southern MD due to westerly surface winds blowing highway emissions eastward. All of the air quality models, including the BAMS, support this possibility. Hourly PM2.5 concentrations in NC, VA, and along I-95 are hovering in the mid to upper teens ug/m3 this morning. Drier air will filter into the region on Wednesday behind the passing cold front, lowering PM2.5 concentrations into the Good range. There may be a few locations along and south of the stalled front in NC that will see PM2.5 concentrations rise hover in the low Moderate range in response to the high humidity and converging winds.

Clean back trajectories from the Great Lakes, slightly below average temperatures, and the possibility of clouds will keep ozone in the generally Good range throughout the Mid-Atlantic on Thursday. The forecast question is the coverage of clouds ahead of an approaching wave of low pressure. The trend is for clouds to increase from west to east. The 06Z 12km NAM and global 00Z EC are keeping skies overcast for the region throughout most of the day on Thursday. In contrast, the 06Z 13km GFS is keeping skies clear over most of the region, in response to its weaker solution regarding the surface wave. The GFS solution would lead to chances for Moderate ozone, while the NAM and EC solutions would shut off ozone production. The WPC has forecasted a center of high pressure over western MD on Thursday, which is evident in the 06Z 4km NAM wind fields; this gives some credit to the GFS solution. The air quality models are hinting at isolated Moderate ozone in and around the DC metropolitan area. Moderate ozone and PM2.5 concentrations are expected at locations along the stalled front in NC where pockets of clearing and converging winds are expected. Otherwise, PM2.5 concentrations will stay in the Good range for most of the region as low humidity continues.

By Friday, the surface wave moving eastward through the TRV will reach the Mid-Atlantic, promoting widespread clouds and precipitation. Although there is still considerable uncertainty in the track and timing of the wave, precipitation will likely begin in the morning and last throughout much of the day. As a result, Good ozone is expected across the region on Friday. PM2.5 concentrations will likely stay in the Good range as well due to the precipitation being heavy and lasting long enough to clean particles out of the atmosphere.

Despite model discrepancies in the precipitation forecast for Saturday, slightly below average temperatures and a clean atmosphere from Friday’s washout will keep air quality in the Good range. The wetter GFS continues to drop precipitation across most of the region on Saturday. In contrast, the EC solution keeps the region dry as it moves Friday’s surface wave northeastward along the east coast. Slightly below average temperatures will continue on Sunday as surface winds and transport aloft (at 500 m AGL) turn onshore. These factors coupled with partly cloudy skies will continue the trend for Good air quality for Sunday.

-DeBoe/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 3, 2015
Valid: August 4 – August 8, 2015 (Tuesday – Saturday)

Summary:

Generally Good air quality is expected throughout the medium range period due to the passage of a cold front on Tuesday that will linger over the region, promoting unsettled weather. A large upper level trough reaching into NC will drop a cold front southeastward through the northern Mid-Atlantic early tomorrow morning, but it will stall along the I-95 Corridor through the evening hours. Clear skies will prevail behind the frontal passage, but a pool of clean air filtering in behind the front will limit the buildup of ozone and PM2.5 on Wednesday. Some areas of Moderate particles are possible on Thursday as humidity increases ahead of a substantial surface wave of low pressure arriving from the west. Resulting widespread clouds and showers are likely overnight Thursday through Friday afternoon as the wave moves through the Mid-Atlantic along the stalled front, inhibiting ozone production and scrubbing the atmosphere of precursors. A resurgence of cooler air behind the disturbance, as well as onshore surface winds and back trajectories, will keep air quality Good throughout the weekend despite a return to sunny skies.

Discussion:

The weather models are in generally good consensus throughout most the medium range period, but begin to show slight differences by Wednesday and larger ones on Thursday. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large upper level closed low is centered over the James Bay, with the associated trough stretching from ND to NC and off of the East Coast. This low aloft will inch eastward over the coming days, with a general model consensus showing it moving off the Canadian Maritimes on Friday morning. This trough will drop a cold front southeastward through the Mid-Atlantic early Tuesday morning, which the WPC surface analysis shows reaching the I-95 Corridor around 12Z. It will promptly stall over eastern MD as a surface low develops over eastern VA and moves off the coast throughout the day, allowing the front to later continue southward into the Carolinas on Wednesday morning. This frontal boundary will vary in strength and latitude throughout the coming week, keeping cooler conditions to its north, with a substantial surface wave moving along it on Thursday into Friday. By Wednesday, an area of shortwave energy aloft over MO becomes apparent, with slight differences in its shape and exact position growing into major forecast discrepancies by Thursday, with the GFS as an outlier at this point. The EC solution appears to be a good consensus of the GFS and NAM at first, where the NAM is the stronger of the two weather models in terms of the resulting surface low pressure and precipitation and the GFS is more disorganized. This surface low will move along the stalled frontal boundary and impact the Mid-Atlantic on overnight Thursday into Friday. Despite upper level discrepancies, both the EC and GFS bring an area of widespread precipitation through the Mid-Atlantic from 00Z Friday to 00Z Saturday. However, from Thursday into Friday morning the EC begins to grow towards the NAM solution as it strengthens into a closed surface low, while the GFS remains the weaker and more diluted solution. The tighter EC surface low will allow skies to clear more quickly behind it as it moves out to sea overnight Friday. This makes Saturday the biggest question mark of the medium range forecast, as there is an even split regarding the cloud cover forecast. It is also worth mentioning that the EC 850 mb solution brings a pool of cooler air as far south as NC on Friday afternoon, while the GFS keeps this cleaner mid-level air in PA.

Tuesday will be a slightly above average day in terms of temperature and humidity behind an early-morning cold front passage. This front is expected to reach and stall near the I-95 Corridor by 12Z, placing an area of surface convergence along the interstate during the morning rush hour. The front has weakened since yesterday, as now there is little convection or other precipitation expected to form in the vicinity of the boundary until the overnight hours. Even then, only scattered storms are resolved in the high-res mesoscale models along the southern coast of MD around 00Z Wednesday. Surface winds are expected to be light and relatively sustained ahead of and along the front, with southwesterly winds promoting the highest ozone along the frontal boundary just east of I-95. Otherwise, the cleaner air mass filtering in behind the front will limit ozone to the Good range. Particles will follow the same general pattern, with moderate consensus among the air quality models that concentrations will reach the Good/Moderate threshold along the northern I-95 Corridor.

Wednesday will be only marginally cooler but noticeably less humid than Tuesday as the cold front dips southward into NC. Overall Good air quality is expected behind the front, where prevailing westerlies and fast back trajectories from the Great Lakes region will limit ozone production despite clear skies. Pockets of Moderate ozone are possible in southern NC below the front, however. Particles are expected to linger around the Good/Moderate threshold for another day due to light morning and overnight surface winds.

With continuing clean trajectories from the western Great Lakes and a frontal boundary draped through VA, most of the Mid-Atlantic is expected to see Good ozone on Thursday due to mostly cloudy skies. A rainy surface low will produce widespread precipitation in the Ohio River Valley and move eastward along the frontal boundary, increasing cloud cover in the Mid-Atlantic later in the day. Model discrepancies lend uncertainty to the forecast regarding precipitation on Thursday, as the ECMWF shows widespread rain throughout the northern and central Mid-Atlantic (NMA, CMA) overnight Thursday while the GFS brings only scattered showers to the CMA and southern Mid-Atlantic (SMA). Although there is uncertainty regarding the precipitation forecast, both models bring a pool of cooler air aloft over the NMA by 00Z Friday. Because of this, regardless of which rain forecast verifies, Good ozone is likely on Thursday. The WPC surface analysis appears to favor the EC solution of a closed surface low and widespread resulting rain, which would clean out the atmosphere. Given the close consensus between the NAM and EC with this system, we consider the GFS an outlier at this point and favor the stronger surface wave with the rainier forecast. Moderate particle concentrations are expected ahead of the system as humidity rises and converging winds increase, particularly along the frontal boundary in central and northern VA/the DC metropolitan area, except in locations that see heavy rainfall.

Friday and Saturday will see very similar conditions in one sense, as temperatures will hover around 80 °F and dew points around 60 °F with generally east/northeasterly winds on both days. However, Friday is likely to be a washout for the majority of the region if the EC/WPC forecast verifies – which seems most likely at this time. Skies will clear quickly behind the rain as the system moves out to sea overnight, allowing for a clear and sunny Saturday as strong high pressure centered over Hudson Bay tries to edge southward. Onshore trajectories aloft as well as light and sustained surface winds will prevail throughout the Mid-Atlantic, limiting ozone production in an already clean atmosphere.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, August 2, 2015
Valid: August 3-7, 2015 (Monday – Friday)

Summary:

An upper level low moving slowly across southern Canada will spawn a series of weak frontal boundaries that will cross the mid-Atlantic during the week. Warm weather and mostly sunny skies will keep ozone in the Moderate range. Particle concentrations will move up and down as humidity and ventilation varies day to day but will be generally in the low Moderate range.

Discussion:

The forecast models (NAM, GFS and ECMWF) remain internally consistent and in good agreement with each other through the medium range resulting in high confidence in the weather and air quality forecasts. In the large scale flow, a closed upper level low just west of James Bay will drift to the east, reaching QC by Wednesday. Zonal flow will then set up for the remainder of the period with another deep trough forming to our west late Friday. At the surface, this pattern will be associated with a series of weak frontal boundaries moving through the mid-Atlantic. This will prevent any significant build-up of pollutants although sunny skies and humid air will keep air quality in the Moderate range for the bulk of the coming work week.

On Monday it will be warm, sunny and humid with strong southwest winds. Deep vertical mixing and steady winds will keep air quality in the Moderate range with highest concentrations along the I-95 Corridor.

The forecast models have slowed the approach of the first in a series of cold fronts and we now expect the front to reach the I-95 Corridor during the Tuesday morning rush hour. There will be significant convection west of the mid-Atlantic late Monday into the nighttime hours. For Tuesday, there is slight change of rain in the mid-Atlantic. Winds will shift west and remain strong while deep vertical mixing is again expected. Air quality will be at the Good/Moderate threshold with highest concentrations east of I-95.

A change in air mass will lag the frontal passage with drier air finally filtering in late Tuesday.   The next weak cold front is expected during the day Wednesday. Air quality remains in the Good to low Moderate range. Humidity returns Thursday with one more frontal passage Friday. Moderate air quality is expected both Thursday and Friday.

-Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, August 1, 2015
Valid: August 2-6, 2015 (Sunday – Thursday)

Summary:

Moderate air quality will be the rule for most of the medium range period. An upper level low over James Bay on Monday will move slowly east and several weak frontal boundaries will cross the mid-Atlantic. The day of most interest will be Monday as temperatures return to the 90’s F although strong winds should ventilate the region enough to keep peak concentrations in the Moderate range.

Discussion:

The forecast models have been consistent over the past several runs leading to higher confidence in both the weather and air quality forecasts through the medium range.

An upper level closed low will be centered west of James Bay early Monday and remain in place through Tuesday before slowly moving eastward to southern QC on Wednesday. As the low opens and moves further east Thursday, zonal flow will set up. This pattern is consistent with generally Good to Moderate air quality in the mid-Atlantic.

Surface high pressure over WV, with a coastal front over eastern NC will result in light westerly winds, mostly sunny skies and Moderate levels of both ozone and particles in the mid-Atlantic. By Monday, winds will shift to the southwest and increase in strength. Temperatures will rise to the low 90’s F and dew points to the upper 60’s F. This pattern is conducive to both ozone and fine particle formation. However, steadily increasing winds will keep air quality in the Moderate range.

A weak cold front will reach the I-95 Corridor on Tuesday morning. Although weak, this boundary will bring a drier air mass and steady westerly winds, along with deep vertical mixing. As a result, particle concentrations are likely to drop to the Good range while ozone concentrations will decrease but remain in the Moderate range.

The frontal boundary dissipates over the southern mid-Atlantic on Wednesday with another frontal passage expected early Thursday morning. Clouds, particularly south of the Mason Dixon Line, and a chance of showers on Wednesday will bring Good to low Moderate air quality. A cooler and slightly drier air mass after the front passes Thursday will keep air quality in the Good to low Moderate range.

-Ryan