textproduct: Blacksburg

SYNOPSIS

Wintry weather early this morning across mountain locations should quickly end by mid day. All rain should be out of the area by the early afternoon as the cold front quickly pushes off the coast. High pressure will quickly pass to the north Sunday night and early Monday before a low pressure system moving across the Southeast will bring the next chance for a wintry mix during the Monday night and Tuesday morning timeframe. High pressure returns through the middle of the work week.

NEAR TERM /THROUGH TONIGHT/

As of 200 AM EST Sunday...

Key Messages:

1) Winter Weather Advisory Continues through 12pm EDT.

2) Mainly a light icing event that is quick moving, changing to rain by Sunday mid day.

Light showers and wintry mix along a frontal boundary looks to continue through the mid morning hours for mountain locations and areas along and north of 460 across the Piedmont. Most mountain locations are expected to see under a tenth of an inch of freezing rain through noon, with many areas already seeing light snow and sleet thus far tonight. As a 40-50 knot southwesterly jet aloft warms temperatures across this layer, these snow/sleet showers are expected to transition to mostly freezing rain for mountain locations through the remainder of the frontal passage. Showers during this timeframe will also likely be fighting dry air in the low levels, which will help keep temperatures cooler at the surface through the morning hours before warm air advection finally wins out. By noon, most rain/freezing rain showers should be out of the area as a quick moving high pressure system pushes through the Ohio valley Sunday and Monday before the next system arrives late Monday night.

Temperatures this morning should hover around their current values of around freezing, with a gradual warming expected through the early morning hours. High temperatures Sunday afternoon are expected to eventually climb into the the mid to upper 40s areawide.

Forecast confidence is high on some light icing, but lower on snow/sleet.

Temperatures tonight should cool into the low 20s west of the Blue Ridge, and mid to upper 20s east of the Blue Ridge behind the cold front.

SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/

As of 130 AM EST Sunday...

Key Message:

1. Increasing confidence in a winter weather event to impact much of the region Monday night and Tuesday, especially across the mountains and northern sections of the Piedmont. 2. Dry Tuesday night through Wednesday night. 3. Temperatures around a little below normal.

A look a the 29 Nov 00Z NCEP Ensemble Mean 500mb Geopotential Heights shows for Monday/Monday night a deep closed low pressure system will be centered just north of Hudson Bay. As associated long wave will extend southwest from the low center into central CONUS. As such, southwest flow aloft will prevail across our region. A shortwave trough will be over British Columbia heading towards the Pacific Northwest. A shortwave ridge will be over the Gulf of Alaska. For Tuesday/Tuesday night, the trough over center CONUS is expected to make headway eastward, reaching the Tennessee Valley and southern Appalachians by the early evening. Upstream, the shortwave trough over BC moves southeast into western CONUS and begins to amplify. For Wednesday/Wednesday night, the trough over western CONUS maintains its relative position. However, arctic shortwave trough heads south on the west side of the closed Canadian low pressure into the Upper Mississippi and western Great Lakes region. Southwest flow will prevail across our region.

At the surface for Monday/Monday night, the center of surface high pressure crosses our region during the morning hours. By the late afternoon, its center is expected to be closer to NY/NJ. Also by the late afternoon, an inverted trough is expected to extend from the Lower Mississippi Valley northeast into the Tennessee Valley. Monday night, this inverted trough is expected to cross our region and start deepening into a closed low. For Tuesday/Tuesday night, this same closed low is expected to move northeast and be centered off the coast of the Delmarva Peninsula by the evening hours. High pressure will be centered over much of the Mississippi Valley. For Wednesday/Wednesday night, the high pressure shifts east, with its center over the Carolinas by the evening. A weak cold front will be over the Great Lakes and Ohio Valley, while another area of high pressure will be centered over the Dakotas.

A look at the 29 Nov 12Z NAEFS Ensemble Situational Awareness data shows 850mb temperatures for Monday will start the day within a range of -4C to +1C, north to south across the region. During the day warm air advection provides early evening values of -1C to +2C, ne-sw across the area. Warm air advection continues Monday night into Tuesday morning yielding a range of +2C to +6C, nw-se. On Tuesday, temperatures decrease during the day from northwest to southeast. By the early evening, expect values of -6C to 0C, nw-se, across the region. Through the night the temperatures gradient slackens a bit with values of -4C to 0C, n-s, by daybreak Wednesday. On Wednesday, temperatures inch higher with early evening values of 0C to +2C, n-s. Wednesday night, warm air advection brings a surge of warmer air across the Piedmont, with a tightening of the gradient in general across the region. Early Thursday morning values are expected to range from -1C to +5C, nw-se.

The above weather patterns offer the following forecast. After a dry day on Monday, the interaction of an approaching upper trough through the center of the country, the development of an inverted trough over the Tennessee Valley, and isentropic advection of moisture into our region, will yield a precipitation event which will begin Monday night, and not conclude until the axis of the upper trough is east of the area Tuesday evening.

Initially Monday night within the lower levels we will have sub- freezing air across northern parts of the area and slightly above freezing air in the south. At the surface, temperatures will be falling through the 30s during the evening hours with dew point depressions around 10 to 15F. Evaporative cooling should help initially enhance the cooling process, all the while aloft, warm air advection will be taking place. Eventually, warm air advection will reach the surface, yielding warming surface temperatures through the night, continuing into the daytime Tuesday through the early afternoon. After this time, both the approach and eventual sunset, and the passage of the the upper trough will start to allow for falling temperatures, and a loss of any warm nose aloft.

The result of all of this is expected to be light snow across the northern half of the region early Monday evening, with a mix of rain, freezing rain, and or sleet for the southern half. As the night progresses, the snow areas will retreat north with more of the region having the potential for freezing rain and/or sleet. By late Monday night, only the farthest north parts of the area will see snow, and that could easily be mixed with other wintry p-types. The freezing rain will also become more concentrated across the north, and also southern parts of the Blue Ridge, and much of the rest of the southern half of the area experiences a cold non-freezing rain.

On Tuesday, by the the late morning, much of the region will be experiencing a plain cold rain except for portions of southeast West Virginia and the southern Alleghany Highlands. Here, temperatures may stay just cold enough for conditions to still be snow and/or freezing rain. As we progress into and through the afternoon, look for area of light snow to increase across the mountains as temperatures fall and winds shift northwest. Ending rainfall is expected elsewhere.

Tuesday night, precipitation is expected to end across all but portions of southeast West Virginia. Here, some lingering flurries may continue through the evening hours. Otherwise, dry conditions are expected for the area. Dry conditions with temperatures around a slightly below normal are expected through Wednesday night.

Confidence in the above weather scenario is moderate.

LONG TERM /THURSDAY THROUGH SATURDAY/

As of 130 AM EST Sunday...

Key Messages:

1. Dry with seasonable temperatures Thursday. 2. Friday, potential wintry mix but, low confidence on details. 3. Saturday, potential cold rain day with some light mix in the north, but low confidence on details. 4. Temperatures trending milder through the period.

A look at the 29 Nov 00Z NCEP Ensemble Mean 500mb Geopotential Heights shows for Thursday/Thursday night, the axis of an arctic shortwave trough will progress from the Great Lakes region into New England. This axis will be over our region around or a little after the early evening. For Friday/Friday night, the trough over the area on Thursday is expected to reach the Canadian Maritimes by Friday evening. However, a second shortwave trough associated with a closed Canadian low pressure will be progressing through the western Great Lakes. A longwave trough/potential closed low, remains parked over southwest CONUS. For Saturday, the shortwave trough to our west on Friday across the area. Exact timing is questionable as the ensemble solution washes out the feature too much.

At the surface for Thursday/Thursday night, high pressure over the Dakotas heads southeast and becomes centered over the Lower Ohio Valley by the evening hours. An inverted trough develops near Brownsville, TX along a front which extends into the Gulf of America. For Friday/Friday night, the high pressure progresses east from the Ohio Valley to off the New England coast by the early evening. Low pressure rides east along a frontal boundary in the northern Gulf to reach the area of the Florida Panhandle by early Saturday morning. For Saturday, details with the ensemble washout within the averaging. However, there are hints that high pressure will be over our region, while a front, and possible low riding along it, will be somewhere over the SE US.

A look at the 29 Nov 12Z NAEFS Ensemble Situational Awareness data shows 850mb temperatures for Thursday falling during the day and reaching a range of -4C to +1C, nw-se by the early evening. For Friday, values trend upward, reaching a range of -1C to +2C, n-s. ON Saturday, values inch upward to a range of 0C to +4C, with a nw-se orientation.

The above weather patterns offers the following forecast. High pressure will maintain control on our weather pattern on Thursday with dry conditions and near normal temperatures. For Friday into Saturday, confidence on details slides downward a lot. High pressure will generally be situated over or just north of the region. Additionally, a couple of southern streams system will have the potential of bringing precipitation northward into our region, a region which will be occupied by generally cold air in association with the high pressure. This could set the stage for a Friday and Saturday which will have the potential for good precipitation coverage across the area. The one on Friday will have colder air off the surface and at the surface to work with for the potential for wintry weather as compared to Saturday. Our forecast will reflect that scenario, but know that confidence is low, and this far out in time there is plenty of opportunity for adjustments in precipitation type, timing and amount as we get closer to the days in question.

Confidence in the above weather scenario is low to moderate for Thursday and low Friday and Saturday.

EXTENDED AVIATION OUTLOOK

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-Thursday, slower at BLF Wed.

RNK WATCHES/WARNINGS/ADVISORIES

VA...Winter Weather Advisory until noon EST today for VAZ007- 009>020-022>024-032>035-045>047. NC...Winter Weather Advisory until noon EST today for NCZ001-002- 018. WV...Winter Weather Advisory until noon EST today for WVZ042>044- 507-508.