Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 10, 2015
Valid: August 11-15, 2015 (Tuesday – Saturday)

Summary:

Good air quality is expected for the beginning of the medium range period as a clean and dry air mass moves in from Canada behind tomorrow’s cold front. By the end of the period, however, a return to above average temperatures, mostly sunny skies, and light surface winds will promote rising ozone, although levels are not expected to exceed the Moderate range. On Tuesday, widespread clouds and showers will inhibit ozone production. For Wednesday and Thursday, the air mass change will promote Good ozone despite sunny skies and light surface winds. Friday and Saturday are days of Marginal interest due to rising temperatures and a slowly modifying air mass, although several factors will be present that should limit any ozone increases, including onshore back trajectories at lower levels and the approach of a backdoor cold front on Saturday. Particles will persist in generally the Good range through Friday as humidity falls behind the front.

Discussion:

The weather models are in very close consensus throughout the medium range period, having closed the timing discrepancy regarding this weekend’s frontal passage. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. An upper level closed low over the James Bay is currently digging a trough into the Great Lakes region, which will continue to strengthen, move eastward, and dip into the Carolinas by 12Z Tuesday. A cluster of strong shortwaves in the base of the trough will develop a surface low that will skirt just north of the Mid-Atlantic on Tuesday, pulling a strong cold front through the entire region. Clouds and showers will impact the Mid-Atlantic starting Monday night (associated with the leading warm front) and continuing through Tuesday morning, with another round of pre-frontal convection ahead of the cold front on Tuesday afternoon. Periods of heavy rain are expected in the central and northern Mid-Atlantic (CMA, NMA) on Tuesday morning. A bought of cleaner and drier air will filter into the CMA and NMA and persist through Thursday. The upper level trough will slowly lift north- and eastward Wednesday through Friday, allowing an area of surface high pressure to build eastward into the Mid-Atlantic. The close proximity of surface high pressure will dominate and keep skies clear until the next cold front approaches on Saturday. The weather models are in much closer agreement today regarding this next front. Due to the weak nature of the backdoor front, it will begin to dissipate and result in only cloud cover and scattered rain showers on Saturday for the NMA and northern parts of the CMA.

Tuesday will be a warm and humid day with breezy south/southwesterly surface winds (turning westerly behind the cold front), cloudy skies, and plenty of rain for the Mid-Atlantic. Southerly looping onshore back trajectories will combine with these factors to make for a day of mostly Good air quality. The BAMS and NOAA air quality models are resolving a swath of Moderate ozone in eastern DC/BAL metropolitan areas. They are likely responding to some pockets of clearing shown in the high-res mesoscale weather models during the afternoon hours, between the two rounds of precipitation. This guidance is likely over-predicting, seeing as the models have been doing so for the past few days and that periods of morning rain will rid the atmosphere of precursors. Since PM2.5 takes longer to respond to air mass changes, areas that receive only light rain in the western Mid-Atlantic (WMA) will see scattered locations reach the top of the Good range.

Temperatures will rebound quickly on Wednesday back into the mid-80s °F, although dew points will drop slowly as the Canadian air mass filters in behind the front. The arrival of the cooler and drier air mass is slightly slower in today’s analyses, with the full impact of the air mass change not occurring until Thursday. As a result, isolated Moderate ozone is possible, particularly along the coastline, where the cleaner air mass will reach last. Particles should remain in the Good range as well in response to the decreasing humidity.

Surface high pressure centered over IL will continue to expand eastward on Thursday, promoting clear skies and calmer conditions across the east coast. This is the day that the Canadian air mass fully makes itself felt over the region. Northwesterly back trajectories from interior ON and the coolest and driest weather of the week will promote Good air quality across the board despite sunny skies and light northwesterly winds.

Friday will be another day of calm and sunny conditions as the surface high pressure continues to build eastward, with the center settling over WV. Temperatures will quickly rebound into the upper 80s °F, but humidity will remain relatively low. Ozone will increase into the Moderate range in some locations as the air mass slowly modifies – specifically, downwind of the I-95 Corridor. On Friday, light southwesterly surface winds place these areas of interest in DC, BAL, southwest PA, NJ, DE, and along the eastern shore of MD. Although it is too far out to forecast with confidence, the lack of strong synoptic forcing on Friday will increase the chances for a possible sea breeze. This feature would greatly increase the chances of high ozone concentrations along and east of I-95.

Saturday will be a pre-frontal day for northern part of the region as a weakening backdoor cold front reaches PA in the morning. Temperatures will continue their upward trend from Friday, with highs approaching 90 °F. There are a few features that should limit any rising ozone to the Moderate range. Back trajectories at 500 m AGL hook onshore, which we have seen this summer tends to very effectively cap rising ozone. In addition, the weak backdoor cold front will move into the NMA. Scattered afternoon clouds and showers will decrease chances for Moderate ozone in the CMA and NMA. In NC and parts of VA, continuing high pressure and mostly clear skies may allow ozone to reach the Moderate range, depending on air mass transport aloft. Increased humidity ahead of the frontal boundary may allow for scattered Moderate PM2.5 concentrations in PA and NJ.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, August 9, 2015
Valid: August 10-14, 2015 (Monday – Friday)

Summary:

There is high confidence in overall Good air quality through Thursday as a strong cold front approaches on Monday and moves eastward through the region on Tuesday. Pre-frontal clouds and rain will encroach into the region west to east early enough on Monday to limit ozone production across the Mid-Atlantic. Widespread rain will reach the western flank of the Mid-Atlantic overnight Monday into Tuesday morning, with a Tuesday morning washout for eastern parts of the region. Behind the front on Wednesday, the first true Canadian air mass in recent memory this summer will build in on northwesterly flow. This cool, dry, and clean air will keep ozone and particles in the Good range on Wednesday and Thursday. Temperatures will quickly rebound to average values on Friday as the center of high pressure settles over the region. There is uncertainty in the cloud cover forecast for the northern Mid-Atlantic on Friday, associated with the impacts of a back-door cold front. In the event that the front remains to the north on Friday, a return to Moderate ozone is expected under sunny skies and westerly transport.

Discussion:

The weather models are in very close consensus throughout most of the medium range period, but show some discrepancies on Friday regarding the next cold front to potentially impact the region. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a more zonal upper level flow is temporarily established over the Mid-Atlantic. An area of shortwave energy over the Northern Plains will get caught up in a trough digging out in northern ON, which is expected to dip into the Mid-Atlantic starting Monday night. The main area of shortwave energy in this trough over southern ON will be reflected at the surface as an area of low pressure located over Lake Ontario at 12Z Tuesday. At this time, the quickly amplifying upper level trough will reach well into GA. The frontal boundaries associated with the surface low – a warm front closely followed by a cold front – will sweep eastward across the Mid-Atlantic on Tuesday, preceded by widespread clouds and showers. 500 mb analyses for 12Z Wednesday show the large area of shortwave energy in this trough sprawled over the western New England/Canada border; 850 mb analyses show cooler, drier Canadian air spilling into the Mid-Atlantic behind the front; and surface analyses show a large area of surface high pressure centered over IA extending into the Mid-Atlantic. As the energy in the upper level trough begins to lift north- and eastward on Thursday, surface high pressure will continue to dominate the weather in the Mid-Atlantic. The trough will continue to lift northeastward on Friday and further allow surface high pressure to build into the region, but model differences arise regarding an approaching back-door cold front and its pre-frontal impacts. Both the EC and GFS show this front, but there are the usual timing differences. Whereas the faster GFS brings increasing cloudiness and rain showers to the northern Mid-Atlantic (NMA) on Friday ahead of the approaching front, the EC slows this boundary enough to keep Friday clear across the region. Due to the question of cloud cover, this discrepancy has an impact on the air quality forecast – however, since the day in question is the fifth day of the period, there is time for the models to come into closer agreement.

Monday will be a seasonably warm day across the region, with increasing cloudiness from west to east beginning around 15Z as the warm front lifts into the region, with the cold front close behind. The cloud cover will limit ozone to the Good range for most locations, with the exception being coastal areas of the Mid-Atlantic, where the clouds will reach last. This will allow those locations to see another day of mostly to partly sunny skies before the front reaches the Atlantic coast. Despite periods of sun along the coast, breezy southeasterly surface winds should be sufficient to limit ozone to the Good range for most locations. The air quality models are split this morning, with the NOAA model bringing ozone to the upper Moderate range along the northern I-95 Corridor (PHL to NYC) and mid-Moderate range to the south and west (BAL, DC, and PIT). In contrast, the BAMS and NCDENR models keep ozone in the Good range for tomorrow. The NOAA model has been overforecasting for the past several days, so we tend to discount its guidance for tomorrow, and go with generally Good ozone for the region. In the event that cloud cover is not as widespread as expected, some pockets of low-to-mid Moderate ozone are possible. Particles will increase across the western and central Mid-Atlantic, especially in PA, WV, and western MD, where humidity will be highest ahead of the cold front. Since rain is not expected until the overnight hours, PM2.5 will have ample time to rise into the Moderate range in the calm morning and light afternoon inland surface winds.

Rain will move eastward through the region overnight Monday into Tuesday morning, with the heaviest precipitation expected between 06-15Z Tuesday. Clouds will linger over the entire region throughout the day on Tuesday, inhibiting ozone production and keeping concentrations in the Good range. Particles will drop into the Good range as well, providing the strength of the rain is consistent across the region – as model guidance currently suggests. Southerly, onshore back trajectories lend confidence to this clean forecast for Tuesday.

The cold front will quickly move through the region. Wednesday will be a dry and mostly sunny day, with dew points dropping more than 10 °F compared to Tuesday. With the cold front now well to the east and out to sea, cool, dry, and clean Canadian air will filter into the region on northwesterly winds (at the surface and aloft) throughout the day. Because of this, the abundantly sunny skies and building surface high pressure will have little impact on the ozone forecast, and it will stay Good for another day. Particles should also remain in the Good range in response to the arrival of the clean and very dry summer air mass.

Surface high pressure centered over IL will continue to extend eastward and keep skies mostly clear over the Mid-Atlantic on Thursday. The Canadian air mass will have fully moved into the region at this point. Below average temperatures and fast, northerly back trajectories from interior ON will keep ozone and particles in the Good range across the eastern seaboard.

Temperatures will quickly rebound to average values on Friday as the upper level trough retreats northeastward. Atmospheric humidity will remain relatively low, however. There is some question about the arrival of the next cold front from the north. The GFS shows a faster approach of this cold front compared to the EC, and the GFS has pre-frontal cloud cover and light showers impacting the NMA on Friday. If this cloudy forecast does not verify, Moderate ozone seems likely, given a shift to westerly transport at the surface and aloft. However, the air mass over the Mid-Atlantic will still be sufficiently clean and unmodified to limit rising ozone to the low-to-mid Moderate range. Particles may increase into the Moderate range at scattered western locations due to westerly transport from the Ohio River Valley, but persistent low humidity will keep most locations in the Good range.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, August 8, 2015
Valid: August 9-13, 2015 (Sunday – Thursday)

Summary:

Overall Good air quality is expected throughout most of the medium range period as a strong cold front approaches on Monday and passes through the region on Tuesday. Mostly sunny skies and light northeasterly surface winds on Sunday will allow for Moderate ozone at the usual areas along and downwind (west) of the I-95 Corridor and in southwestern PA. Although lower Sunday emissions should keep rising ozone in the Moderate range, there is a Marginal chance for isolated USG ozone just to the west of I-95; as we saw yesterday, light northeasterly surface winds can promote isolated pockets of locally high ozone production in the afternoon. Particles will linger in the Moderate range in the western Mid-Atlantic, where easterly winds will be the lightest and humidity will be the highest. Monday will be an increasingly cloudy and breezy day as a warm front lifts northward, with the bulk of the pre-frontal rain sweeping eastward through the region on Tuesday. A seasonably cool, dry, and clean air mass is expected to arrive behind the front for Wednesday and Thursday, for Good air quality.

Discussion:

The weather models have come into closer agreement since yesterday, and are in very close consensus throughout the medium range period. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, a large upper level ridge is well established with its axis through the Mississippi River Valley, and its northeasterly edge is reaching into the Mid-Atlantic. The ridge will swiftly recoil back out of the Mid-Atlantic as areas of disorganized shortwave energy crest the ridge and impact the Northeast, keeping scattered clouds and showers in the forecast for Sunday. Large shortwaves over northern ON will begin to dig out a large upper level trough on Sunday afternoon, reflected at the surface via a widespread area of low pressure. As this low pressure gradient strengthens and moves eastward, it will establish an extensive frontal boundary that will stretch from the James Bay to IA by Monday morning. Clusters of small short waves in the base of the trough will promote some cloud cover and showers in the Mid-Atlantic on Monday as a warm front moves northward at the surface. More widespread, heavy, pre-frontal showers will begin later in the day as the cold front associated with the Canadian low approaches. As the trough slowly moves eastward, shortwave energy in its base will continue to promote widespread clouds and rain along the frontal boundary on the east coast through Wednesday morning. Skies will clear quickly behind the front on Wednesday as cooler, drier Canadian air filters into the region on northwesterly winds.

Sunday will be a day of scattered cloud cover throughout the Mid-Atlantic as an unsettled area of dilute shortwaves moves overhead, with the thickest clouds expected along a north-south axis through central PA to central VA. The high-res weather models are resolving areas of light scattered rain showers in areas of central PA, MD, and VA later in the day. The usual areas of concern along and downwind of the I-95 Corridor and in southwestern PA will continue to see mostly sunny skies, however, and therefore Moderate ozone. Light easterly surface winds will push the bulk of the emissions just to the west of I-95, placing the areas of highest ozone around the DC and Baltimore metropolitan areas, which is supported by the air quality models. Due to the lower Sunday emissions, ozone is not expected to rise higher than the Moderate range, although there is a Marginal chance for isolated USG ozone just west of I-95, given the third day in a row of light northeasterly surface winds and mostly sunny skies. Particles will follow a similar pattern, with Moderate concentrations likely in the western part of the Mid-Atlantic due to increasing humidity and lighter inland winds.

A southerly wind shift on Monday as a warm front lifts northward through the region ahead of an approaching cold front will increase humidity, and therefore PM2.5, west of the I-95 Corridor. Ozone, on the other hand, will be inhibited due to mostly cloudy skies moving from west to east. Along and east of I-95, where the clouds will reach last, the southerly winds should be sufficiently breezy and onshore to hinder ozone formation and buildup. Faster, northerly back trajectories lend confidence to the Good ozone forecast.

The cold front moving through the region on Tuesday is substantial, and will generate widespread clouds and precipitation. Despite analyzed areas of Moderate ozone in the air quality models in southern MD and the surrounding areas on Tuesday, the long-lasting and widespread nature of the clouds and precipitation suggests this guidance is overdone. The rain is expected to move gradually westward across the region starting late Monday night and continue through early Wednesday morning. This leaves little opportunity for ozone to form, especially considering the onshore looping back trajectories. In areas where only light rain falls, the high pre-frontal humidity will allow pockets of Moderate PM2.5 to form, particularly in the western Mid-Atlantic.

Wednesday will be a calm and sunny post-frontal day in the CMA and SMA, while clouds may return to the NMA after a brief period of morning sun. The Canadian air mass behind the front is expected to be cool, dry, and clean, limiting ozone to the Good range across the region. By 12Z Wednesday, the southern extent of the cold front is expected to be draped over the eastern Carolinas. Along and just behind this frontal boundary, ozone will be limited by the clouds, while particles will linger in the Moderate range until the front fully clears the region.

By Thursday, the upper level trough will dip to the FL/GA border, with the ridge in the west reaching well into AB. A large area of high pressure will build eastward into the Mid-Atlantic, with abundantly clear skies expected over the entire region. Despite this, air quality likely will rise just barely into the Moderate range – if at all – due to the continuing clean air mass, dry air, below average temperatures, and fast back trajectories from the James Bay.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 7, 2015
Valid: August 8-12, 2015 (Saturday – Wednesday)

Summary:

Overall Good air quality is expected to prevail throughout the medium range period in the Mid-Atlantic due to a relatively progressive pattern and only intermittent periods of calm, sunny conditions. The recent washout in the central and southern Mid-Atlantic, along with breezy northeasterly winds, will limit ozone mostly to the Good range on Saturday. Because of the anticipated sunny skies and lighter winds inland, the exception will be in areas of increased precursors directly downwind of the I-95 Corridor. Sunday will be another dry day, but possible cloud cover in the northern Mid-Atlantic and lower Sunday emissions will keep ozone mainly in the Good to low Moderate range once again. Throughout the entire weekend, particles are only expected to reach into the Moderate range at inland locations where winds will be lightest and humidity will be higher. Monday will see the arrival of the next disturbance, bringing afternoon clouds and showers eastward across the region, with Good ozone expected for most locations. Due to the light nature of the precipitation, as well as southerly shifting surface winds, PM2.5 will likely continue in the Moderate range in the western Mid-Atlantic. Immediately following Monday’s disturbance – a relatively weak surface low – an extensive cold front will sweep west to east across the eastern seaboard, bringing widespread clouds and showers to the region on Tuesday. Model discrepancies lend uncertainty to the exact precipitation forecast, but regardless, the passage of the cold front will continue the trend of Good ozone. PM2.5 will thrive on the increased humidity ahead of and along the front, depending on the strength of the precipitation. Skies will clear quickly behind the frontal passage on Wednesday morning, but a cooler, drier, and clean Canadian air mass is expected to arrive behind the front, which will return air quality to the Good range across the region.

Discussion:

The weather models are in generally close consensus throughout the beginning of the medium range period, but begin to differ on Monday. However, the discrepancies mainly occur aloft, and they do not have a large impact on the air quality forecast or its confidence. The 06Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. Currently, an upper level trough axis along the eastern seaboard with an associated area of shortwave energy over the Carolinas is the source of widespread showers and cloud cover. While the showers are limited to coastal areas south of the Mason-Dixon Line (MDL) this morning, the clouds spread from the Atlantic coast to the Appalachians. As the trough continues moving eastward today, the area of shortwave energy will organize enough to give some structure to its resulting surface low that will move northeastward and away from the coast by 12Z Saturday. This will allow an upper level ridge currently centered over the Plains to briefly build into the Mid-Atlantic for Sunday, which will be a clear and very calm day throughout the region. By Monday morning, a cluster of shortwave energy over the Great Lakes is resolved by all of the global weather models, and develops a surface low, but there are discrepancies regarding its strength and exact extent of associated precipitation across the Mid-Atlantic. Since the timing of this system is virtually identical for the GFS and the EC, the confidence of the air quality forecast is not impacted substantially by this difference in shortwave characteristics, with the bulk of precipitation likely remaining west of I-95 through the late afternoon. The models have come into closer consensus on the precipitation forecast for Monday and Tuesday, with the previously drier EC actually surpassing the GFS on the extent of the rain. The EC has completely backed off on yesterday’s solution of forming a nor’easter like coastal low and now has a traditional cold frontal passage on Tuesday, similar to the GFS. Both models analyze a large upper level trough digging southward into the northeastern US in a winter-like fashion, with the southernmost edge of the trough reaching as far south as southern GA on Tuesday. All in all, the models show a very similar solution for the end of the period, with a widespread area of pre-frontal rain moving eastward across the entire Mid-Atlantic from 18Z Monday to 06Z Wednesday. This looks to be a substantial cold front, with both the EC and GFS showing a pool of cooler Canadian air filtering into the Mid-Atlantic on Wednesday behind the frontal passage.

The clouds and showers currently impacting the central and southern Mid-Atlantic (CMA, SMA) will linger along the NC coastline through Saturday morning. Except for these showers and some scattered, topographically-driven showers in western MD and WV, Saturday will be a mostly sunny day throughout the region. Back trajectories from eastern New England, breezy northeasterly winds along the coast, and the recent washout will limit ozone to the Good range for most of the Mid-Atlantic. The air quality models haven’t been handling the impacts of the surface wave well, with substantial over-forecasting of ozone yesterday and (one hopes) today along I-95. This lends much less confidence to their guidance for tomorrow, which features scattered Moderate ozone downwind of I-95 and in the Pittsburgh metro area. Nevertheless, the I-95 Corridor will act as the usual area of increased precursor buildup on Saturday. Combined with the abundant sunshine and lighter surface winds inland, there is a chance for isolate Moderate ozone along and downwind of the interstate. Particles may see some buildup in these areas as well, but it will be limited to the low Moderate range due to the low humidity.

Sunday will be a calm and mostly sunny day as surface high pressure prevails over the Tennessee River Valley (TRV). However, some of the high-res mesoscale weather models are resolving some pop-up afternoon showers along the eastern edge of the Appalachians in the CMA and northern Mid-Atlantic (NMA), as well as widespread cloud cover. Depending on the optical thickness of these clouds and the characteristics of the northerly air building into the region on the western flank of the coastal low, ozone will likely be limited to the mostly Good range. The exception, according to the air quality forecast models, is a small pocket of possible low Moderate ozone in DC/along the southern coast of MD. We suspect that, given the full sun and very light to calm surface winds, area of Moderate ozone may be more scattered than suggested by the air quality model guidance. The calm winds will increase PM2.5 across the region, with the highest concentrations in the western Mid-Atlantic where humidity will be the highest.

Increasing clouds throughout the day on Monday ahead of an approaching surface low will limit ozone to the Good range for most of the region. Swaths of low Moderate ozone are possible east of I-95, where the clouds will reach last, but potentially breezy southerly surface flow may limit ozone to the Good range in these locations as well. The southerly will raise dew points and promote Moderate PM2.5 from the I-95 Corridor to the edge of the Appalachians, where only light rain will fall on Monday.

The surface low that sourced Monday’s rain will move across southern PA overnight Monday into Tuesday morning, reaching the PHL metropolitan area by 12Z Tuesday. At the same time, a low pressure center will pass well to our north – along the northern NY/Canada border – and drop an extensive cold front through the region immediately following the passage of the surface low. Although there are discrepancies between the global weather models as to the exact extent of the resulting precipitation, the combination of these two rain-promoting disturbances will keep clouds and at least scattered showers over the region through Tuesday night. The lack of sunlight will inhibit ozone production, keeping it in the Good range across the board. Particles may linger in the Moderate range in scattered locations until a cooler and drier Canadian air mass builds in on Wednesday. Areas that see heavier rain will observe PM2.5 concentrations dropping into the Good range.

Skies will be clear across the eastern seaboard by 12Z Wednesday behind the passage of Tuesday’s cold front. Dew points will decrease throughout the day as northwesterly surface winds filter in cleaner, drier air behind the front. Based on the 850 mb model guidance, Good air quality is expected across the entire Mid-Atlantic on Wednesday. However, if the air does hold some precursors, the full sun and light winds may be sufficient to allow Moderate ozone downwind – to the southeast – of the I-95 Corridor. Overall, however, Tuesday’s washout will be enough to limit concentrations to the bottom half of the Moderate range.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

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

Summary:

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

Discussion:

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

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

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

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

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

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

-DeBoe/Huff