textproduct: Blacksburg

SYNOPSIS

High pressure will pass overhead today on its way to push off the Atlantic coast tonight. This will be followed closely by areas of mixed wintry precipitation across the mountains and possibly the foothills early Sunday as a cold front approaches from the west. High pressure passes across the region on Monday. Low pressure moving across the Southeast will bring the next chance for a wintry mix during Monday night and Tuesday.

NEAR TERM /THROUGH TONIGHT/

As of 955 AM EST Saturday...

Key Messages:

1) Warmer this afternoon under mostly sunny skies and light winds.

2) Chance of wintry precipitation across the mountains and foothills late tonight.

Very little change needed to the ongoing forecast. Due to mostly sunny skies, after a cold start in the teens and lower 20s, temperatures should rebound into the upper 30s to lower 40s in the mountains, and mid to upper 40s in the Piedmont, before high clouds increase from the west late this afternoon.

Previous discussion...

Update: A Winter Weather Advisory has been issued from 2 am to noon on Sunday for the mountains and foothills for the potential of areas of freezing rain. Details below.

As high pressure overhead moves east today, expect warmer afternoon temperatures, particularly across the mountains, as winds shift more from the southeast. Highs today will be 5 to 12 degrees warmer over those occurring Friday along and west of the Blue Ridge, and will reach into the mid 30s to the mid 40s areawide. Clouds will increase in coverage beginning shortly after sunset, but it won't be enough to keep temperatures from falling quickly below freezing early in the night given the dry air situated across the lower Mid-Atlantic.

Moisture will begin to cross the central Appalachians after midnight Sunday morning as a cold front approaches from the west. With temperatures below freezing as the moisture arrives, there is strong consensus in the latest weather data that precipitation will likely start as a brief period of snow, before transitioning to a mixture of sleet and freezing rain along and west of the Blue Ridge before dawn. As the moisture continues to move in, temperatures will gradually warm from west to east, and precipitation will change over to rain for most locations after sunrise. That stated, there will be zones along the higher ridges that linger in a layer of subfreezing temperatures into early afternoon, where freezing rain may persist longer.

As precipitation will remain light during the morning hours of Sunday, icing accumulations from freezing rain are expected to remain below 0.10 of an inch. However, given the very chilly air that's been present since Wednesday night, there is a chance that ice will adhere to roadway surfaces, particularly along bridges, overpasses and elevated roadways along and west of the Blue Ridge. This will result in an increased probability of hazardous travel conditions for Sunday morning until temperatures warm up enough to allow the icing to melt.

SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/

As of 115 AM EST Saturday...

Key Points:

1) Light wintry mix expected northern sections Sunday morning.

2) Dry Monday.

3) Potential for more robust wintry mix Monday night through Tuesday.

4) Temperatures slightly below normal for this time of year.

A look a the 28 Nov 00Z NCEP Ensemble Mean 500mb Geopotential Heights for Sunday/Sunday night depict a deep closed low centered just north of Hudson Bay. Its associated trough axis is expected to extend south into the Lower Ohio Valley around evening time. Across western CONUS, a shortwave trough is expected from WY southwest into Baja California. An amplified ridge will be positioned just offshore the Pacific Northwest. For Monday/Monday night, the trough which was over the Ohio Valley on Sunday progresses northeast and is expected to be centered over the Canadian Maritimes by Monday evening. Looking west, the trough which was over western CONUS shifts east into central CONUS, and merges with yet another shortwave trough in association with the closed Canadian low. Southwest flow is expected over our region. For Tuesday/Tuesday night, the central CONUS trough shifts east, reaching the Great Lakes to Gulf Coast States region by the early evening.

At the surface for Sunday/Sunday night, low pressure will progress through the Ohio Valley early Sunday morning, and reach southern Quebec by the evening hours. Its associated warm front, followed by its cold front, will cross our region during the course of the day. High pressure will be building south through the Upper and Mid Mississippi Valley. For Monday/Monday night, the Mississippi Valley high will progress east, and become centered over PA/NY around early Monday evening. An inverted trough is expected to develop across the Lower Mississippi Valley into the western Tennessee Valley by this same time. For Tuesday/Tuesday night, the inverted trough deepens into low pressure which is expected to head northeast and be off the mid-Atlantic coast by the early evening hours. High pressure becomes centered over the mid-Mississippi Valley.

A look at the 28 Nov 12Z NAEFS Ensemble Situational Awareness data shows 850mb temperatures for Sunday has temperatures mildest in the morning, ranging from around 0C to +3C, north to south across the area. By the evening, values range from -5C to +2C, along a nw-se gradient with a cold front crossing the region. For Sunday night, temperatures trend cooler by a couple of degrees before increasing and reaching a range of -2C to +2C, n-s by the early evening on Monday. For Tuesday, values start the day around 0C to +5C, nw-se. By the early evening, the nw-se gradient tightens to roughly -2C to +4C.

The above weather patterns offers the following forecast. Precipitation ongoing at the start of Sunday is expected to be primarily rain for the southern two-thirds of the region as warm air advection continues south of an advancing warm front. However, northern sections of the region will still have pockets of sub-freezing surface conditions, or a warm nose not quite warm enough for full melting to yield areas of freezing rain and/or sleet. The trend in the potential for light snow looks less likely, but not zero as timing of the warm air advection could be still be too quick. By the late afternoon, the cold front arrives along with northwest flow cold air advection. This process will remove any lingering warm nose features and allow for purely a snow versus rain scenario across the area. Best chances of light snow remain over southeast West Virginia and neighboring counties of southwest Virginia. The precipitation will exit the region from west to east late Sunday afternoon into early Sunday evening.

High pressure visits the region briefly on Monday for a dry and slightly cooler than normal day.

Our next, and potentially more potent wintry weather maker is currently on track for a late Monday night into Tuesday timeframe. An upper trough moving east through the central CONUS is expected to help usher a surface trough/low northeast from the Gulf Coast towards and across our region -- all while receiving support from the larger synoptic scale trough. Moisture and warm air advection will enter the region starting Monday night. This moisture will encounter a boundary layer airmass with a mix of readings both above and below freezing. As the night progresses, these boundary layer values will trend slowly higher, some exceeding 32 degrees, others not. This paints the picture of a potential wide mix of precipitation types for the region Monday night, with the best chance of a cold rain across southeastern portions of the area, and the best chances for snow in the far northwest and north. In between these two extremes, a mixture of snow, freezing rain, and/or sleet are probable.

For Tuesday, the scenario may be similar to that which is expected on Sunday, just with higher qpf values. The warm air advection will continue such that a greater expanse of the southern half of the region changes to a pure cold rain with the north maintaining a wintry mix, but with less of a chance for snow. By the afternoon, after the passage of the main upper trough axis, we switch to cold air advection northwest flow. This would place us within a pattern conducive to a rain vs snow forecast, with the best snow potential over western and northern parts of the region. Precipitation is expected to conclude Tuesday evening.

Temperatures during this portion of the forecast are expected to average few degrees below normal.

Confidence in the above weather scenario moderate on the the timing of synoptic features and the precipitation types on Sunday. Confidence is low regarding precipitation types, timing and amounts Monday night into Tuesday night.

LONG TERM /WEDNESDAY THROUGH FRIDAY/

As of 130 AM EST Saturday...

Key Points:

1) Dry Wednesday and possibly Thursday for most of the region.

2) Low confidence in light rain/snow for the area on Friday.

3) Temperatures trending milder through the period.

A look at the 28 Nov 00Z NCEP Ensemble Mean 500mb Geopotential Heights shows for Wednesday/Wednesday night, the trough centered to our west on Tuesday progresses quickly to over southeast Canada by Wednesday evening. A broad longwave trough oriented ne-sw is expected over western CONUS. Another shortwave trough heads southeast on the western side of the stalled deep cut-off low near Hudson Bay. For Thursday/Thursday night, a shortwave trough heads through the Great Lakes region. A broad longwave trough over western CONUS become elongated eastward. A ridge axis moves onshore the Pacific Northwest. For Friday, the ensemble solution offers a solution of a very broad longwave trough covering much of central and western CONUS oriented ne-sw associated with stalled low pressure near Hudson Bay.

At the surface for Wednesday/Wednesday night, the center of high pressure moves over the Tennessee Valley. A weak cold front will be over the western Great Lakes region. For Thursday/Thursday night, a ridge of high pressure remains over our area, but the center of the high shifts to over the mid- Mississippi Valley. For Friday, a ridge axis remains over the region, but its center is lost to ensemble averaging.

A look at the 28 Nov 12Z NAEFS Ensemble Situational Awareness data shows 850mb temperatures for Wednesday of 0C to +2C, north to south across the area. For Thursday, values trend significantly colder to -5C to 0C, nw-se. On Friday, values inch higher slightly to -2C to +1C.

The above weather patterns offers the following forecast. While the ensemble mean solution places a west-east oriented ridge of high pressure over our region Wednesday through Friday, it is not a clear cut period of dry weather for the area. Within the ensemble, the day with the highest probability of being dry is Wednesday. For Thursday, some solutions have a northern stream system getting close enough to the region for some light precipitation for northwest parts of the area. For Friday, both a northern and a southern stream system potentially flirt with offering our region with some precipitation. Temperatures are expected to trend milder through the period.

Confidence in the above weather scenario is low to moderate.

EXTENDED AVIATION OUTLOOK

A wintry mix with sub-VFR is expected Sunday morning mainly along/west of a ROA-BCB line. Icing will be an issue for those terminals. Should see it change to rain between 14z-17z.

Expect MVFR, possibly IFR from ROA west Monday, with VFR east. Most sites except possibly BLF return to VFR Monday evening, with VFR for all Monday morning. Another system arrives with more wintry weather Monday night so expect sub-VFR cigs/vsbys late Monday night into Tuesday. Return to VFR for most by Wednesday, slower at BLF.

RNK WATCHES/WARNINGS/ADVISORIES

VA...Winter Weather Advisory from 2 AM to noon EST Sunday for VAZ007-009>020-022>024-032>035. NC...Winter Weather Advisory from 2 AM to noon EST Sunday for NCZ001-002-018. WV...Winter Weather Advisory from 2 AM to noon EST Sunday for WVZ042>044-507-508.