Area Forecast Discussion
Issued by NWS Blacksburg, VA

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765 FXUS61 KRNK 171015 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 515 AM EST Wed Jan 17 2018 .SYNOPSIS... Low pressure was along the Mid Atlantic and Southeast Coast this morning and will drift northeast today and tonight. Arctic high pressure over the Central and Southern Plains will move into the Gulf Coast region by late Thursday. Temperatures will moderate over the weekend ahead of the next low pressure system approaching the eastern United States on Monday. && .NEAR TERM /THROUGH TONIGHT/... As of 430 AM EST Wednesday... Winter storm warning has been expanded and Winter Weather Advisory has been expanded. Well defined short wave seen on water vapor loops will close off over the Tennessee Valley this morning then track east across Virginia tonight. Not much synoptic scale isentropic lift or upper diffluence/deformation over the area but strong vorticity advection and good low level moisture convergence. Models have slowed down system compared to runs on Tuesday so higher QPF is likely. Area radars showed higher reflectivities filling in over northern North Carolina and southern Virginia at 4AM. Ceilings have lowered and several observation sites were reporting visibilities of 2 to 4 miles which would indicate a heavy enough snow for accumulation. Snow ratios will average about 15:1 more much of the day so have increased snow amounts in line with WPC guidance. This puts Pittsylvania and Rockingham Counties in a Winter Storm Warning. The Winter Weather Advisory has been expanded as a buffer around the Warning. There will be a sharp gradient in snowfall through the Advisory area with amounts near one inch on the northwest fringe and localized amounts around three inches closer to Southside Virginia and adjacent sections of North Carolina. Hi-Res models show a majority of the accumulating snow between 15Z/10AM and 21Z/4PM. Have gone with a non-diurnal temperature trend between the evaporative cooling of temperatures as the snow picks up and the cold air advection as the the 850MB temperatures dropping into the -5 to -15 range by the end of the day. Have added a Winter Weather Advisory for a large part of the mountains for tonight where wind chill will easily will be from zero to -10. Minimum temperatures tonight may need to be lowered to account for the fresh snow cover. && .SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/... As of 430 AM EST Wednesday... The upper low generating the snow across the area today will be well off the Mid-Atlantic coast by the beginning of this forecast period. Very cold Arctic high pressure will be centered across the Southern Plains and moving toward the Midsouth. The core of this very cold air mass, running around -18C at 850mb, will be rapidly diminishing and lifting out of the area by afternoon as the low lifts out. It lifts out so quickly that 850mb temps have surged to +4C as quickly as 18Z Thursday. Not sure the surface will warm quite as quickly, but this cold air mass will (1) not be as severe as it has been in the Midwest where temperature have been well below zero for two consecutive mornings, and (2) like the first two weeks of bitterly cold temperatures that lingered across the eastern U.S. With this in mind and the core of the Arctic air mass already six degrees warmer than when it went over Kansas City yesterday, low temperature have been raised a few degrees for Thursday morning, leaving mainly teens across most of the forecast area. However, expected snow cover of 1-4 inches across the region, will lead to efficient cooling overnight despite a fair amount of wind. On the other hand, winds will be strong enough when combined with the low temperatures in the teens to issue a Wind Chill Advisory for the entire area west of the Blue Ridge, with wind chill readings nearing warning criteria in western Greenbrier county. Winds will diminish fairly quickly during the day Thursday as high pressure builds in from the southwest. With the warming at 850mb and near full sunshine, eastern areas will recover into the 40s by late afternoon with 30s western areas. The remainder of this period will be quiet with a marked warming trend. 850mb temperatures will hover in the 0C to +5C range through the period. With near full sunshine and west winds as the surface high moves to our south through the period, temperatures will be able to recover fairly quickly to the 50s as soon as Friday with the Piedmont nearing 60 for Saturday and 50s in the west. It will be a pleasant change from the bitter cold this first half of January! && .LONG TERM /SUNDAY THROUGH TUESDAY/... As of 455 AM EST Wednesday... Heading into the weekend, an upper level trough will begin to deepen over the Four Corners region and eject into the Central Plains states, all while broad upper level ridging builds over the southeast U.S. into the Mid-Atlantic region. 850mb temperatures will continue to hover in the relatively mild 0C to +5C range across the area through the weekend, then as the upper ridge builds over the eastern U.S. and the western U.S. trough shifts into the central U.S., 850mb temperatures will rise toward +10C across much of the area. Surface temperatures will remain well above normal through the weekend with lows mostly above freezing in the 40s and highs in the 50s west to 60s east, roughly 15-20 degrees above normal for a change. Moisture will also be on the increase, especially the western sections. Sunday night through Monday night, the Central Plains trough is expected to close off, and lift rapidly northeast into and through the Great Lakes region, and then into southeast Canada. Its associate surface cold front is progged to progress through our region Monday afternoon and evening. Anticipate a well defined line of showers to cross the area during this time frame, along with the potential for some gusty winds based upon an associated 40 to 50 kt jet crossing the area both pre- and post- frontal passage. It is setting up to potentially be a QLCS scenario, followed by notable cold air advection in the wake of the system. A mix or change over to snow is expected in the west Monday night. By Tuesday, the system is expected to east of the region, with low level winds backing rapidly during the course of the day, thus shutting off any northwest flow upslope in the west early in the morning. Temperatures Monday into Tuesday will trend cooler, but still average about five to ten degrees above normal for this time of year. 850mb temps are not expected to drop below -10C even behind the next cold front. && .AVIATION /10Z WEDNESDAY THROUGH SUNDAY/... As of 1210 AM EST Wednesday... Ongoing light snows have mainly affected Bluefield and Lewisburg this evening associated with Arctic frontal zone. By around 10z, expect flight categories to begin to improve toward BKN/OVC VFR and VCSH at most of the TAFs except for Lynchburg and Danville. Recent model guidance has trended toward more snow and longer period of reduced visibilities in the eastern and southeastern portion of the Blacksburg forecast area. Would expect IFR type visibilities at Lynchburg in snow until 16z. The longest duration of sub-VFR (3/4 SM to 2 SM) more likely at Danville through 20z. Greatest operational impact including snow-covered runways more likely at these two TAF sites. Limited operational impact elsewhere. After 20z Wednesday, conditions then begin to trend VFR though northwesterly upslope stratus may dip toward MVFR levels at Bluefield. As Arctic front advances southeastward, look for a northwesterly wind shift to around 6-11 kts areawide. Slow decrease in winds anticipated by Wednesday evening. Extended Discussion... Dry air will restore conditions to VFR by Thursday under high pressure. VFR conditions should persist into the weekend with warming temperatures. Moisture will return to west Sunday night into Monday. && .EQUIPMENT... As of 545 PM EST Tuesday... NOAA Weather Radio, WXL60 (Roanoke Transmitter) which broadcasts at 162.475 MHz is off the air. No estimated back in service time. Verizon is testing the circuit which normally carries the signal to the site. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Wind Chill Advisory from 6 PM this evening to 11 AM EST Thursday for VAZ007-009>020. Winter Weather Advisory until 10 AM EST this morning for VAZ007-009-010. Winter Weather Advisory until 3 PM EST this afternoon for VAZ032>035-043-045>047. Winter Storm Warning until 5 PM EST this afternoon for VAZ044- 058-059. NC...Wind Chill Advisory from 6 PM this evening to 11 AM EST Thursday for NCZ001-002-018. Winter Weather Advisory until 3 PM EST this afternoon for NCZ003-004-019-020. Winter Storm Warning until 5 PM EST this afternoon for NCZ005- 006. WV...Wind Chill Advisory from 6 PM this evening to 11 AM EST Thursday for WVZ042>044-507-508. Winter Weather Advisory until 10 AM EST this morning for WVZ042>044-507-508. && $$ SYNOPSIS...AMS NEAR TERM...AMS SHORT TERM...DS/RAB LONG TERM...DS/RAB AVIATION...AL/PM EQUIPMENT...PM is the U.S. government's official web portal to all federal, state and local government web resources and services.