Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: May 31, 2017
Valid: June 1-5, 2017 (Thursday-Monday)

Summary:
Unsettled weather will persist as a quasi-permanent upper level trough over the eastern U.S. dominates the forecast, keeping the chances for an exceedance Slight through the medium range period. Thursday may be the driest and most pleasant day, as seasonable and less humid air filters into the region behind a weak cold front. Unsettled conditions will return to the NMA on Friday but the extension of Bermuda high pressure across the SMA will bring sunny skies for most of the day. The forecast for Saturday and beyond is fairly uncertain, given the differences in the weather models regarding the recharging of the quasi-permanent upper level eastern low and the track of its corresponding surface low. A lingering frontal boundary will remain across the CMA on Saturday, keeping chances for scattered showers in the forecast, but a weak ridge should promote mostly sunny skies across much of the region. Sunday and Monday look wet as the next surface low moves into the region.

Discussion:
There is agreement between the weather models on placement of major atmospheric features until 00Z Sunday but they diverge after that. Both the GFS and ECMWF have the same general trend in synoptic features through the end of the medium range period, however. The models consulted for this analysis were the 06Z NAM, 06Z GFS and 00Z ECMWF. Thursday, an upper level trough with a closed circulation will continue to progress slowly eastward, with the closed low at its center moving across southern ON. This feature will spin off shortwaves over the northern Mid-Atlantic (NMA) between 06Z Thursday and 00Z Friday. As this upper level feature moves eastward, an extension of the Bermuda High will begin to build into the southern Mid-Atlantic (SMA) and will be aided by mid-level trough moving over the Midwest, starting 06Z on Friday. The upper level trough, which will continue to progress eastward, with its center moving over eastern QC and northern New England, will bring another round of shortwaves to the NMA starting around 18Z Friday and will continue until 18Z Saturday.

The models diverge beginning on Saturday regarding the redeveloped of the semi-permanent eastern upper level trough. The GFS and EC both reinvigorate the trough, but they differ regarding the timing and strength. On Saturday, all three models show a strong shortwave diving down from SK/MB and beginning to dig out a new longwave trough. The EC phases this disturbance with the remnants of the previous upper level trough, strengthening to form a new longwave trough over the eastern US with a closed circulation over eastern ON by 00Z Monday. The GFS is slower to phase the two systems, keeping the western shortwave separate and forming a small closed low over MB by 06Z Sunday. Similar to the EC, the GFS will phase this closed low with remaining energy from the previous trough, however, the trough formed from the merger will be much weaker and not form a closed low during the medium range period. At this time, WPC is favoring a blend of the model, but with a bit more preference to the EC. Regardless of the exact details, the reinvigorated upper level low will move across the Midwest/Ohio River Valley late in the period, pushing the Bermuda High extension back out to sea, bringing shortwaves and unsettled weather to the majority of the Mid-Atlantic.

Despite the Bermuda High building over the SMA through Saturday, today’s air quality models are not developing much in the way of Moderate ozone. They have isolated Moderate at different locations across the region through the period, with little to no consensus, which does not add confidence to their guidance. They are likely reacting to the unsettled nature of the synoptic pattern.

Thursday may be the clearest day of the period, with mostly sunny skies across the region. The exception may be far eastern NC and VA, where a weak frontal boundary will linger. Previous model runs have suggested potential unsettled weather for the NMA in the morning through afternoon hours, but today’s model runs are trending toward dry and clear skies throughout the day. It will be post-frontal, with a less humid and presumably clean air mass filtering into the region. Remnant dilute smoke continues to linger today across the Gulf Coast and especially Florida, due to numerous local fires, Saharan dust, and transported smoke from seasonal burning in Mexico and Central America. This smoke is not expected to impact the Mid-Atlantic through the end of the medium range period, but forecasters in NC should continue to monitor conditions, since they are closest to the sources of smoke and dust. The ample early June sunshine should allow some locations to reach the Moderate range for ozone, but there is only a Slight chance for an exceedance, given the post-frontal seasonably cool and dry condition, with breezy surface winds.

Friday will be similar to Thursday in terms of sensible weather, with seasonable temperatures and less humidity across most of the region. Shortwaves will be moving over the NMA on Friday resulting in scattered cloudy conditions and rain showers, with the potential for isolated thunderstorms in late afternoon and evening. These unsettled conditions will be focused in the NMA since the Bermuda High extension will be promoting clear skies and lighter winds in the SMA. The exception again may be along the coastal SMA, where the semi-permanent lingering frontal boundary will remain. A Slight chance for an exceedance will continue, with more widespread Moderate ozone possible across the SMA, which will be under the extension of the Bermuda High.

The forecast for Saturday and beyond is fairly uncertain, given the differences in the weather models regarding the recharging of the quasi-permanent upper level eastern low and the track of its corresponding surface low. Previous days’ model runs had more clouds and showers across the region on Saturday, but beginning yesterday and continuing today, the trend is toward drier weather. WPC, following a blend of the EC and GFS, keeps the lingering frontal boundary across the region on Saturday, but brings it farther north, across the central Mid-Atlantic. A weak mid-level ridge will move over the NMA, north of the cold front and ahead of developing surface low pressure in the Midwest. The weather models are split on precipitation associated with this front, which may move slowly northward as a warm front on Sunday, associated with the low pressure system that will begin to move into the SMA later Saturday night. The chance for an exceedance will remain Slight on Saturday, but it may be the best day for more scattered Moderate ozone, depending on how sunny it is.

The surface low pressure will begin to impact much of the Mid-Atlantic on Sunday as it strengthens over the Great Lakes. The lingering frontal boundary will begin to push northeastward as warm front in the morning hours, resulting in abundant clouds and precipitation for much of the northern and central parts of the region. Clouds and precipitation will progress eastward in the afternoon to evening hours, impacting the entire NMA but 00Z Monday. Mostly sunny skies are expected in the SMA on Sunday, however, this clearing will allow convective rain showers and thunderstorms to possibly take place between 18Z Sunday and 00Z Monday. Widespread unsettled will put the Mid-Atlantic at Slight risk of an exceedance.

The surface low will be move slowly into the NMA on Monday morning, bringing widespread clouds and rain to the NMA. This system will continue to move eastward into the afternoon and evening hours, bringing a cold front down into the SMA by 00Z Tuesday. The timing of this cold front will determine how much rain and clouds the SMA will get on Monday. As mentioned above, there is uncertainty in the timing and strength of the low pressure system between the models. If the system progresses slower than expected, Monday could be a mostly sunny day for the SMA resulting in possible Moderate ozone levels. Despite the uncertainty there remains a Slight risk of exceedance for the Mid-Atlantic region on Monday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: May 30, 2017
Valid: May 31- June 4, 2017 (Wednesday-Sunday)

Summary:
Unsettled weather will persist through this week. A semi-permanent upper level trough with a closed circulation will once again dominate much of the weather through the medium range period. As result, a Slight chance of an exceedance is expected, with scattered Moderate conditions possible in the southern Mid-Atlantic, mainly through the end of the work week. At this time, the mixture of smoke and Saharan dust that is currently bringing borderline USG PM2.5 to FL is not anticipated to move northward into the southern Mid-Atlantic. A series of shortwaves interacting with a frontal boundary in the southern Mid-Atlantic and an approaching cold front will bring clouds and precipitation across the region on Wednesday. Behind the cold front, high pressure will bring clearing skies for both Thursday and Friday, with the best chance for dry weather on Thursday. Moving into the weekend, unsettled conditions will return on Saturday, mainly north and west, due to the arrival of another cold front. Differences in the GFS and EC regarding the strength and track of the next low pressure system result in low confidence in timing and strength of precipitation on Sunday, but the trend is toward more widespread clouds and rain.

Discussion:
There is general agreement between the weather models for the placement and progression of major atmospheric features until 06Z Saturday. The models consulted for this analysis were 06Z NAM, 06Z GFS and the 00Z ECMWF. The forecast period will start at 12Z Wednesday with an upper level trough over the eastern U.S., containing a closed low at its center over northern ON. This Canadian upper level low will spin off weak shortwaves through the northern Mid-Atlantic (NMA) and will slowly progress eastward through 06Z Thursday, remaining centered over ON. Westward of this closed low, a mid-level ridge will progress eastward across the upper Mid-West and will be flattened by the closed low begins on Thursday. The center of the upper level low will elongate over ON/QC by 12Z Thursday as the longwave trough contracts northeastward. At this time, the EC and GFS begin to slightly differ from the NAM with the speed of eastward movement of the Canadian closed low. The NAM keeps the center over ON/QC through the end of its run at 18Z Friday but the GFS and EC continue its eastward progression, with the center over central QC by 00Z Friday. Both the GFS and EC are in agreement that the Canadian upper level low will begin to weaken as it continues to move eastward throughout the day Saturday. Starting 06Z Saturday, the models diverge on the details, but they agree on the big picture, which involves redeveloping the upper level trough over the Great Lakes/upper Midwest over the weekend and forming a corresponding low pressure system at the surface. The EC has the much stronger set of features. During Saturday, it drops a strong shortwave down from the Canadian Prairies and reinvigorates the upper level trough, now stretched across southeastern Canada. In contrast, the GFS shows a very weak trough forming over the Midwest by 18Z Saturday with the help of shortwaves over OK/TX. These differences in the upper level pattern translate into different locations of the associated surface low. At 12Z Sunday, the GFS shows the surface low moving through the Ohio River Valley, bringing a much weaker, more east/west oriented front through the NMA. By 12Z Sunday, the EC shows a stronger surface low than the GFS, but located in MI. The WPC favors a blend of the GFS and EC for Saturday/Sunday, suggesting more widespread clouds and precipitation across the Mid-Atlantic region, with periods of heavy rain possible on Sunday. Regardless of the details, the trend is for another closed low/upper level trough to set up over the eastern U.S., which will continue the recent stretch of unsettled weather.

On Wednesday, shortwaves spinning off from the Canadian upper level low will interact with a lingering frontal boundary draped across the central and southern Mid-Atlantic. As the day progresses, a weak cold front will approach from the west. These two features will create scattered unsettled weather across the entire Mid-Atlantic. The 09Z SREF reflect this, with a high probability of precipitation across the entire region by 21Z. It appears much of the precipitation will be convective in nature, so rain and clouds are expected to be more localized than widespread. Despite this, there is only a Slight chance of an exceedance for the Mid-Atlantic with the potential for isolated areas of Moderate ozone in the southern Mid-Atlantic (SMA), where the most clearing is likely.

The weak cold front will continue to push through the central Mid-Atlantic (CMA) and SMA early Thursday morning but will be closely followed by a surface high. As a result, Thursday may be the sunniest day of the week, with mostly clear skies across the region. A presumably clean air mass will build into the region behind the weak front. A few weak shortwaves will linger over the NMA between 12 and 18Z, bringing a few clouds, but skies should remain mostly sunny. Once again, there is only a Slight chance of an exceedance for the Mid-Atlantic, with isolated areas of Moderate ozone most likely in the SMA.

Friday will be a mostly pleasant day across the Mid-Atlantic as the surface high slowly sinks southeastward across the central part of the region. This feature will keep skies mostly sunny through most of the afternoon. In addition, a switch to southwesterly return flow will allow warmer and more humid air to advect into the region. There is a chance for isolated, more instability-based convection in the late afternoon. Overall, Friday is the most likely day for more widespread Moderate ozone, but given the generally progressive pattern, only a Slight risk of an exceedance will continue.

Unsettled weather is poised to return for the weekend, keeping the chances for an exceedance Slight both days. A weak cold front will move into the NMA on Saturday. The models are not in close agreement on the start of precipitation or its intensity, but there is consensus that precipitation and clouds will be present by 00Z Sunday across northern and western parts of the region. Sunday is the wildcard of the week with timing of fronts coming into question. The GFS is about 18 hours slower than the EC the passage of the weekend cold front, and brings the center of low pressure over the NMA by 00Z Monday. The EC keeps the center of low pressure further north, bringing most of the precipitation through the Mid-Atlantic between 12 and 18Z, with lingering precipitation in the SMA. Regardless of the exact details, Sunday looks wet for most of the region.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 26, 2017
Valid: May 27-31, 2017 (Saturday-Wednesday)

Summary:

The weather pattern will remain generally unsettled through the medium range period, but today’s models are trending toward a less amplified upper level pattern, with a somewhat complex pattern of weak fronts and uncertain timing of shortwaves.  As a result, there is substantial uncertainty regarding the exact timing and location of clouds and precipitation, especially over the Memorial Day weekend.  On Saturday and Sunday, the weather will be dominated by waves of low pressure moving along a frontal boundary draped somewhere across the central Mid-Atlantic.  A stronger cold front, which was expected to move through the region on Tuesday, now has weakened substantially.  At the end of the period, a semi-stagnant pattern will develop, with the Bermuda High trying to build into the region from the south but running up against persistent troughing centered over ON/QC.  Overall, only a Slight chance for an exceedance is expected through the period, given chances for clouds and precipitation over the Memorial Day weekend, and then only a slow warm-up to around average temperatures on Tuesday and Wednesday.

Discussion:

There is a general agreement in the weather models regarding the key synoptic features of the medium range period.  Today’s model runs are trending toward weaker features at 500 mb, however, especially at the end of the period.  This translates into a synoptic pattern with weak fronts and uncertain timing of shortwaves.  As a result, the forecast remains unsettled, but there is substantial uncertainty regarding the timing and location of clouds and precipitation, especially over the Memorial Day weekend.  At the end of the period, a semi-stagnant pattern will develop, with the Bermuda High trying to build into the region from the south but running up against troughing centered over ON/QC.

The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF.  On Saturday, a series of upper level shortwaves will move across the northern Mid-Atlantic (NMA) throughout the day.  All of the models bring a more substantial shortwave through the region, developing a wave of low pressure along a frontal boundary that will move into the central Mid-Atlantic.  The placement and strength of the shortwaves all vary between the models, which adds uncertainty to the exact development and progression of clouds and precipitation associated with the front and surface waves.  A weak upper level ridge will build over the NMA ahead of an upper level trough with a closed circulation, centered over MB by 00Z Sunday.  The weak ridge will continue to progress eastward and will pass over the NMA between 06Z and 18Z Sunday.  Following this upper level high, a strong shortwave will move through the Mid-Atlantic on Sunday; like Saturday’s shortwave, the weather models are not in consensus on the strength and location of this feature either, which adds uncertainty to the precipitation forecast.  But the models are consistent in keeping the frontal boundary draped across the central Mid-Atlantic, with additional waves potentially moving along it.

The main story for the end of the period is the weakening upper level trough, which now appears will remain centered over southern Canada.  The models are in closer agreement today that the closed low will remain well to the north of the Mid-Atlantic, centered in ON on Monday.  The GFS and EC diverge slightly on Tuesday and Wednesday regarding the placement of the upper level low in eastern ON.  Both models have a weaker trough over the eastern US, but the GFS splits off a vorticity lobe and keeps the trough contracted farther north, while the EC extends the trough farther south.  Both models agree that shortwaves will continue to rotate through the trough axis and pass over the central and northern Mid-Atlantic, but once again, timing and strength is highly uncertain at this time.

Despite the uncertainty regarding the exact details of the precipitation forecast, ample clouds and scattered precipitation will be enough to keep an exceedance in the Slight range for Saturday.  A wave of low pressure, associated with upper level shortwaves, will ride along a frontal boundary, moving from the Ohio River valley into the central Mid-Atlantic.  This feature will bring unsettled weather to much of the Mid-Atlantic.  The deterministic and hi-res model guidance is not in consensus on the locations with the highest chance for precipitation, but it seems most likely at locations in the vicinity of the Mason-Dixon Line (MDL), where the front is likely to track, moving from west to east during the day.

More shortwaves will be moving overhead on Sunday, impacting the majority of the Mid-Atlantic.  Waves will move eastward along the stalled boundary across the central part of the Mid-Atlantic, causing scattered showers early in the day.  Widespread precipitation is expected for many parts of the northern and central Mid-Atlantic between 18Z Sunday and 06Z Monday.  Cloudy conditions and onshore flow should be enough to keep exceedance Slight for the forecast region.

On Monday, there will be another round of shortwaves, originating from the closed upper level Canadian low, moving over much of the Mid-Atlantic.  The uncertainties in timing of these shortwaves are evident in the models, which translates into questions about the precipitation forecast.  Nevertheless, clouds and precipitation should be enough to keep exceedance Slight for another day.

On Tuesday, the primary cold front associated with the Canadian upper level closed low is expected to move into the NMA.  This feature is much weaker in today’s model guidance.  The front appears likely to either dissipate or stall somewhere across the region, oriented more west/east than north/south.  Due to the weakening front, the models are currently showing clear skies for much of the NMA, but there is the possibility for showers and storms that will be convective in nature.   The 06Z air quality models developed isolated areas of low Moderate ozone along the I-95 Corridor, likely response to the potential for clear skies.

Wednesday looks similar to Tuesday, with a trend toward clear skies, especially for the SMA, where the influence of the Bermuda high will be strongest.  Temperatures are not expected to rise much beyond average, however, given the cool pool extending southward from the trough over eastern Canada.  The main forecast questions for the end of the period will be the northward extend of the southeastern mid-level ridge, as well as location of any frontal boundary that separates the two main air masses between the ridge to the south and the trough to the north.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 25, 2017
Valid: May 26-30, 2017 (Friday-Monday)

Summary:
After a brief break in the rain and clouds on Friday, unsettled weather will continue to be the norm across the Mid-Atlantic, ensuring only a Slight probability of an exceedance through the medium range period. The semi-persistent upper level trough that has been dominating the eastern CONUS will finally move into New England on Friday, allowing a mid-level ridge to build in the southern Mid-Atlantic. This brief pattern change will promote clearing skies, allowing for the best chance for some ozone formation during the medium range period. Waves of low pressure developing along a warm front on Saturday, however, will promote clouds and scattered precipitation across the central and southern Mid-Atlantic, which will clean out any potentially rising ozone that may build on Friday. This trend will continue on Sunday, as low pressure developing in the Ohio River Valley will bring scattered heavy precipitation and clouds to the Mid-Atlantic. On Monday and Tuesday, there is some uncertainty in the timing of the next cold front, but with another substantial upper level trough moving overhead, unsettled weather is expected.

Discussion:

There is close agreement among the weather models regarding the placement and timing of the large scale features until late in the medium range forecast period. Models consulted for this analysis were the 00Z ECMWF and the 06Z NAM and GFS. On Friday morning, the upper level trough that has dominated the eastern CONUS for the last few days will be centered over New England at 12Z and will quickly pass into the Canadian Maritimes by 00Z Saturday. At the same time, a mid-level ridge will build over the lower Mississippi Valley and progress northeastward, reaching southern Mid-Atlantic (SMA) by 00Z Saturday. Upstream, an upper level trough/closed low will be centered over SK/MB on Friday morning. This closed low will broaden slightly as it slowly moves eastward, reaching southeastern MB by 00Z Sunday. The mid-level ridge will continue to build northeastward into the central Mid-Atlantic on Saturday morning but will meet resistance from the mid-level troughs to the northeast and northwest, with a frontal boundary dividing the two major air masses (cool and moist to the north, dry and warm to the south). Aloft, several small shortwaves will move through the upper level flow over the Mid-Atlantic, helping to develop waves of low pressure along the weak frontal boundary draped across the central Mid-Atlantic. On Sunday, the closed low in Canada will begin to intensify and contract its center over southwestern ON. Several shortwaves rotating around the axis of the closed low will begin to dig out a longwave trough over the central CONUS. Behind this amplifying upper level trough, a ridge will be building over much of the western CONUS starting 00Z Sunday, which will help to reinforce the amplifying synoptic pattern over the CONUS. This morning’s EC and GFS runs have come into closer agreement on the track of the Canadian closed low, keeping it spinning slowly over southern ON through the end of the period. At the same time, additional shortwaves will continue to rotate around the axis of the closed low’s associated longwave trough, amplifying the trough over the central and eastern U.S. However, at mid-levels and the surface, the EC and GFS diverge regarding the timing of a weak cold front that will slowly move through the region in the Monday-Tuesday timeframe. The GFS is faster than the EC, bringing the front through the region on Monday, while the EC is about 24 hours slower. The main difference translates into the timing of precipitation associated with the frontal passage; the GFS is wetter on Monday, while the EC has more widespread precipitation on Tuesday. We are siding with the EC for now, following the WPC preference.

Friday, much of the Mid-Atlantic will see a break from the widespread heavy rain of the past few days as the semi-persistent upper level trough/closed low exits to the northeast. The mid-level ridge will building into the SMA will keep skies clear in the southern and central portions of the region. In contrast, clouds and scattered showers will linger in the northern Mid-Atlantic (NMA), mainly through the morning, but gradual clearing is expected by the late afternoon. Given the relatively clean air mass in place, there is only a Slight chance of an exceedance, with higher ozone levels across the SMA.

The recent trend in the models towards a cloudier Saturday has continued in today’s guidance. A warm front moving into the central part of the region will flatten the mid-level ridge and bring unsettled weather back to much of the Mid-Atlantic. Waves developing along the frontal boundary will bring rain showers to the central and southern Mid-Atlantic in the afternoon. The NMA should remain mostly dry but will be partly cloudy with light winds. With precipitation and cloud cover, there will only be a Slight chance of an exceedance, with higher ozone across the NMA.

A surface low located in the Ohio River Valley on Sunday will progress eastward, bringing clouds and scattered rain showers to the central and southern Mid-Atlantic during the morning to afternoon hours. The NMA will see scattered showers and cloudy conditions for much of the afternoon and into early Monday morning due to upper level shortwaves moving overhead. These conditions, along with onshore flow will keep the chances for an exceedance in the Slight category for Sunday.

There is uncertainty in the forecast for Monday and Tuesday, given the disagreement between the EC and GFS regarding the timing of the next major cold front. Rain showers and periods of heavy rain are expected for much of the region, but the timing of this is still uncertain. Another upper level trough will be building over the region for the end of the period, making unsettled conditions likely both days. Following the EC for now, Monday will be partly to mostly cloudy but generally dry, except for showers across NC. Then on Tuesday, following the slow progression of the cold front through the region, more widespread precipitation is expected. Despite the uncertainty regarding the precipitation forecast, a Slight chance of an exceedance will continue on Monday and Tuesday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 24, 2017
Valid: May 25-29, 2017 (Thursday-Monday)

Summary:
A persistent progressive and unsettled weather pattern will continue for the Mid-Atlantic during the medium range period, allowing mostly Good air quality to persist. A strong upper level trough will continue to dominate the region on Thursday before it exits the region to the northeast early Friday. Friday afternoon there will be a mid-level ridge building northward into the region, bringing relief from rain and clouds across the southern and central Mid-Atlantic. This reprieve will be short lived due to increasing confidence in the weather models that the Memorial Day weekend will be unsettled. Waves of low pressure will develop along a warm front moving into the central Mid-Atlantic on Saturday, bringing clouds and scattered precipitation mainly in the afternoon and evening. Elsewhere, in the northern and southern Mid-Atlantic, Saturday may be the clearest day of the period. The next low pressure system will form in the western Ohio River Valley on Sunday and pull a cold front into the Mid-Atlantic on Monday, leading to cloudy and rainy weather for the end of the holiday weekend. There is uncertainty as to speed and strength of this system, but nonetheless, continued Good to low Moderate air quality is expected on Sunday and Monday.

Discussion:

There is general agreement among the weather models for the first part of the forecast period. The 00Z Wednesday ECMWF was unavailable, so the 06Z GFS and NAM were consulted for this analysis. The trough currently dominating the eastern CONUS will begin to recede to the north and east on Thursday morning, contracting over the northern Mid-Atlantic (NMA) by 00Z Friday. At the same time, the closed low over SK will essentially spin in place, while a weak upper level ridge builds between the two troughs. On Friday, the eastern trough will contract further and quickly move northeastward into New England and the Canadian Maritimes, as the upper level ridge progresses into the Great Lakes. Concurrently, a mid-level ridge will advance into the southern Mid-Atlantic (SMA), moving southwest to northeast tough Friday into early Saturday morning. The overall pattern for the end of the period from yesterday’s models continues in today’s runs. On Saturday, the weak upper level ridge moves over the Mid-Atlantic, while the mid-level ridge tries to build northward as a warm front lifts into the central Mid-Atlantic. The NAM and GFS both have small shortwaves moving through the upper level flow over the northern and central Mid-Atlantic, which translate into scattered precipitation along the warm front on Saturday afternoon and evening. On Sunday, the next major low pressure system will form in the western Ohio River Valley, which will impact the Mid-Atlantic on Sunday and Monday. There is some uncertainty regarding the track of the low pressure system, tied to the track of the Canadian upper level closed low. Today’s GFS run brings the closed low southward into the northern Plains Sunday afternoon, whereas yesterday’s 00Z EC run kept it farther north. As a result, the GFS develops a stronger and much deeper trough over the eastern U.S. on Sunday. This difference is important to watch since it may translate into a slightly different outcome for Monday. Since the GFS has the trough extending further south, it brings a stronger cold front through the Mid-Atlantic region on Monday compared to yesterday’s EC run. Regardless, Sunday and Monday look unsettled for the Mid-Atlantic.

Unsettled conditions will bring widespread Good air quality to the Mid-Atlantic on Thursday. A center of low pressure associated with the strong eastern upper level trough will pass through the Mid-Atlantic early Thursday morning, moving northeastward throughout the day. This system will bring heavy showers and cloudy skies to most of the region in the morning and afternoon hours. As the low pulls away, precipitation will become more scattered and skies will clear from south to north in the late evening.

The upper level trough will be exiting the Mid-Atlantic region to the northeast early on Friday, resulting in scattered precipitation and lingering clouds across the NMA. The building mid-level ridge will bring mostly clear skies to the SMA. Some scattered Moderate ozone may occur, mainly in NC, but Good air quality is expected elsewhere.

The trend we saw yesterday, with the weather models increasing unsettled weather for the central and southern parts of the Mid-Atlantic on Saturday, continues in today’s model runs. A warm front will lift across the central Mid-Atlantic, and waves of low pressure may develop along it, triggered by upper level shortwaves moving over the region. Precipitation is most likely in the afternoon through evening hours and will be scattered in nature. At this point, much of the NMA and SMA look clear, so scatted Moderate ozone is possible in those locations, with Good ozone in the areas affected by the clouds and precipitation.

Conditions for Sunday and Monday are somewhat uncertain due to the differences in the weather models regarding the upper level trough that will progress over MB/ON and over the Great Lakes/Upper Plains regions at the end of the period. WPC is putting more weight on today’s GFS solution, bringing the center of vorticity further south on Sunday, thus developing a stronger surface low and bringing more widespread unsettled weather to the Mid-Atlantic region on Monday. If the GFS solution verifies, the cold front will reach I-95 by 12Z Monday and then continue slowly through the eastern part of the region, reaching the far southeastern Mid-Atlantic by 12Z Tuesday.

-Enlow/Huff