Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 161445 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 945 AM EST Fri Feb 16 2018 .SYNOPSIS... A series of upper-level disturbances will continue to track from west to east across the region over the next few days. Meanwhile, a Canadian cold front will move slowly southward through the region today and tonight, then stall across the southeast U.S. Light to moderate rainfall will accompany the front today and tonight. Then, an upper-level disturbance will bring a chance of wintry precipitation to the area Saturday as the cold air settles in place. Then early next week, a warm front will move back north bringing unseasonably warm temperatures to the area that will last several days. && .NEAR TERM /THROUGH TONIGHT/...
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As of 945 AM EST Friday... A cold front will drop south across the area today into tonight. WSR-88d showed scattered showers moving south across the region this morning. Adjusted pops with latest radar images and trends for this morning, then shaped towards HRRR and Conshort blend for late morning into this afternoon. Modified temperatures with latest surface obs and their expected trends, then blend in GLAMP to capture falling temperatures with front. More changes later this afternoon. As of 420 AM EST Friday... Unseasonably warm air was in place this morning with low temperatures mostly in the 60s, even a few readings near 70 east of the Blue Ridge, such as Roanoke. Keep in mind that normal high temperatures for this area at this time of year are in the 40s west to 50s east, so we are starting out the day already 20-30 degrees above normal of even the high temperature! The warm air is largely the result of a strong summerlike subtropical ridge anchored over the Gulf of Mexico to the western Atlantic and a progressive zonal flow in place across the mid-latitudes, keeping the really cold air, like we saw so much of January, locked well to our north. However, some changes are on the way for the weekend that will remind us that winter is not finished yet. A cold front was approaching our western border this morning extending from PIT to CRW to LEX and southwestward. The front will move very slowly east and southeast today as it encounters the upper ridge to our south. Nonetheless, a short wave moving across the northern states will help to ensure that the front moves through the entire CWA and down into the southeast states before it stalls during the weekend. Light to moderate rainfall will accompany the front but the best dynamics will remain well north of the CWA and have already tracked through the OH Valley into PA and beyond. As the front becomes parallel to the upper flow and encounters the strong upper ridge, it will begin to lose a lot of the associated precipitation. Rainfall amounts through the day are expected to range from about 1/2 inch west to less than 1/4 inch Piedmont. An isolated thunderstorm is possible mainly this morning near the TN/southern WV border, but all-in-all the air mass will become unstable and capped with little additional threat of any thunder. As noted above, temperatures will be well above normal this morning and still warm well into the 60s and lower 70s eastern areas before the front arrives later this afternoon. In the west, look for morning high temperatures, slowly falling in the afternoon. So temperatures will remain well above normal again today, but not as warm as Thursday, and actually approaching more normal values late in the day behind the cold front. By morning, low temperatures are expected to be just below freezing for most areas west of the Blue Ridge, to the mid and upper 30s across the Piedmont. As the next upper disturbance tracks northeast from the Mid-South, this will bring about a threat for wintry precipitation as referenced in the "short term discussion" below.
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&& .SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/... As of 300 AM EST Friday... Guidance is in pretty good agreement with upcoming weather scenario for Saturday, but spatial distribution of ptype will still be difficult to pin down. Strong wedge will be in place across the region as short wave energy and isentropic lift stream up over the top. This will spread precipitation across the region from the southwest during the morning. Precipitation falling into the wedge with below freezing wet bulb temperatures will help deepen the cold air at the surface, while warm air aloft surges well above freezing. This set up will generally favor snow at the start, then a mix to sleet/freezing rain. Accumulations with mixed accumulations are quite tricky as sleet/freezing rain greatly reduce amounts on the ground. Expect snow/sleet amounts to be generally in the 1 to 3 inch range with a glaze of ice. Overall confidence on accumulations and ptype remain on the low side since it will not take much change in the temperature profile to swing things one way or the other, so will hold off on any headlines and wait to look at later runs of the meso models. As the system pulls away Saturday night, upslope showers with a wintry mix will linger west of the Blue Ridge into Saturday morning with a with some additional light accumulation of snow/ice possible. We get a brief break in the action on Sunday as high pressure moves in from the west and forms a wedge east of the Appalachains, but a warm front will push some rain showers back into the area late Sunday night continuing into Monday. There may be some ptype issues developing toward daybreak Monday with potential for freezing rain especially from the Alleghany Highlands through western Greenbrier, but at this time the threat looks rather limited. Will be watching evolution of this in later model runs closely. && .LONG TERM /TUESDAY THROUGH THURSDAY/... As of 400 PM EST Thursday... Yet another disturbance is expected to cross the forecast area Monday with another round of precipitation. Temperatures may be near critical for a few hours early Monday morning, but think increasing cloudiness Sunday night ahead of Mondays rain event will keep temperatures from radiating, thus supporting temperatures above freezing areawide. For now will entertain mainly rain. Any sort of wedge Monday is expected to break and give way to much warmer temperatures for the middle of the week. Warm front will move north of the area placing our region under warm, but moist southerly flow. Models suggest temperatures will climb back well into the 60s and testing 70 again for southside VA and into the piedmont of NC. Rain chances will increase again by Thursday as yet another disturbance approaches from the southwest. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... As of 715 AM EST Friday... Generally poor aviation conditions expected through much of the TAF valid period. Ceilings will average in the MVFR range at most sites through the period, with brief periods of IFR in -RA BR. Ceilings will begin to improve some after 00Z as drier air works southward into the area. Synoptic situation features a slow moving west-east oriented cold front accompanied by post- frontal showers and light rain. Visibilities will be VFR most of the period, except brief periods of MVFR in -SHRA BR. This front will drift to just south of the CWA by Saturday morning. Cold air will spread south and eventually into a wedge setup by Saturday afternoon. An upper-level disturbance will enhance isentropic lift into the wedge air mass and bring a period of wintry precipitation to much of the CWA by Saturday afternoon. Winds early will be WSW to SW 10-20 mph with gusts 20-30kts becoming NW 10-20 mph with low end gusts, decreasing some after 00Z. Medium confidence in ceilings through the TAF valid period. Medium to high confidence in visibilities through the TAF valid period. Medium to high confidence in wind speed and direction through the TAF valid period. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...RAB NEAR TERM...KK/RAB SHORT TERM...MBS LONG TERM...MBS/PM AVIATION...RAB

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