Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 170511 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1211 AM EST Wed Jan 17 2018 .SYNOPSIS... A cold front will move through the region overnight into Wednesday morning bringing measurable snow to parts of the region and very cold temperatures to the entire area. High pressure will build into the region through the weekend, along with dry weather and milder temperatures. Another cold front will cross the area on Monday, bringing a line of moderate to heavy rain showers, some with very gusty winds. && .NEAR TERM /THROUGH TODAY/... As of 1030 PM EST Tuesday... New 00Z/7PM guidance is coming in. Confidence now above 75 percent that areas south and east of Danville will reach warning criteria (4 inches of snow or more). Per coordination with NWS Wakefield, a Winter Storm Warning is now in effect for Casewell, Halifax, Charlotte Counties. Remainder of the forecast area unchanged. There will be a sharp northwestern edge to the snow plume. As expected the snow is struggling to make it over the mountains from the OH/TN valley, the precip essentially drying up as the arctic front crosses the mountains. Still anticipating a wave of low pressure to develop along the surface front over North Carolina. Main impact from this feature will be over the Piedmont of NC and into southeastern VA...east of Danville and Lynchburg with a sharp gradient to the northwest where dryer/colder arctic air will be infiltrating the region from the northwest. Onset will be in the 3-5AM time frame and continue into the afternoon Wednesday before ending. Previous Discussion As of 815 PM EST Tuesday... A positive tilted trough draped over Ohio Valley southwest to the Ozark, will slowly push a cold front over the region tonight into Wednesday morning. This front will also be slowed by a series of reflections developing along the front, tracking to the northeast. The upper level trough will move over the mid Atlantic region Wednesday afternoon pushing the front and snow off the coast. Associated with this front will be snow, wind, and cold temperatures/wind chills. As the front drifted over West Virginia and western Kentucky today, snow amounts ranged from 2 to 5 inches. Slightly lesser amounts (2-4 inches) are expected in our extreme western counties (Tazewell VA to Greenbrier WV) tonight. Accumulating snow will taper to flurries during the early morning hours Wednesday, ending as early as sunrise for our western counties. As the front moves to the Blue Ridge this evening, jet dynamics develop south and east of the area overnight. This will create a precipitation minimum over counties along the Blue Ridge and immediate foothills, where less than an inch of snow is expected. Snow is expected to taper to flurries around sunrise, ending by late morning. Farther east, Highway 29 corridor and points east, the expectation is for snow to develop late tonight and linger longer into the day Wednesday. A wave of low pressure is forecast to develop along the front vicinity of the NC Piedmont. Best lift and model dynamics for snow production are forecast along a CLT-GSO/RAH-RIC line/corridor with the heaviest of the snow occurring south and east of Danville...between Danville and Raleigh. At the present time it looks like LYH-MTV will be the back edge of this moisture plume. A general 1-3 inches of snow is being forecast from Lynchburg-Danville with our eastern 3 counties (Charlotte-Halifax-Caswell) teetering with 4. These numbers may need to be adjusted slightly pending the next round of model ensemble guidance. Not out of the question that our eastern 3 counties may meet warning criteria (4 or more inches of snow across more than 50% of the county). If this turns out to be the case then warnings will be expanded into parts of southern/eastern VA. For this forecast package, there were very few changes to any headlines. Yadkin County was added for the potential for up to 2 inches of snow there and a buffer to the winter storm warnings farther to the south and east (e.g. the Triad). Advisories in the far western CWA and eastern CWA for snow remain in place through Wednesday morning. Unless the forecast changes overnight, a special weather statement will be issued for counties bordering the Blue Ridge for possible minor travel impacts. The map contains a large donut hole with respect to the Advisory Headlines for a reason. There will be a cooridor (I-81) right along the Blue Ridge including the Shenandoah, Roanoke and eastern New River Valleys that get very little snow accumulation from this event. As a general rule Advisories are generally not issued until amounts exceed 2-3 inches and warnings are not issued unless we are anticipating more than 4-5 inches...using those numbers as an average. A tight pressure gradient develops behind the front, but until clouds can clear, most of the strongest winds will remain above 3000 ft. As the inversion lowers towards morning across the mountains, winds will blow 15-25 MPH with gusts up to 35 MPH through the day. These winds will likely blow snow on to treated roadways. Wind coupled with very cold temperatures will also send wind chills values below zero across the higher elevations to the single digits in the mountain valleys Wednesday morning. Temperatures overnight will drop below freezing areawide. Temperatures over the foothills and piedmont counties warmed into the 40s Tuesday afternoon. Despite the warmer temperatures, this should not impact the snow accumulation potential as wetbulb temperatures are well below freezing. With dewpoints in the teens/20s, readings air temperatures will quickly wetbulb below 32 degrees once the snow starts later tonight. Arctic air on the west side of the frontal boundary will enter the mountains overnight with temperatures dropping into the 10F- 20F range. Cold air will continue to surge in during the day tomorrow. Mountain temperatures will warm very little with highs ranging from 15F to 25F. Cold air will make it east of the Blue Ridge resulting in nearly steady or falling temperatures for the foothills and piedmont into the 20s. Downsloping winds and sun in the afternoon should allow temperatures to warm into the upper 20s to lower 30s Wednesday. Areas with a 2-3 inch snow pack will be cooler by 3F- 5F. && .SHORT TERM /TONIGHT THROUGH FRIDAY/... As of 130 PM EST Tuesday... At start of Wednesday night, the center of the upper level low/trough partially responsible for Tuesday night and Wednesday`s snowfall, will be directly overhead. As its shifts east overnight, gusty northwest winds will help maintain some upslope snow showers and flurries over portions of southeast West Virginia. However, the trajectory will be a little bit more north than true northwest, so the impacts of the upslope will not be maximized. Regardless, much drier air is progged to quickly enter the region thus minimizing what uplsope there will be. During the day Thursday, the prevailing flow will back to the west and the overall synoptic pattern will be a near zonal flow across the area. This pattern will continue through Friday will gradually increasing heights/temps as a shortwave ridge starts to build over the central potions of the nation. Temperatures will trend milder during this portion of the forecast with readings averaging some five degrees above normal by Friday. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... As of 1215 PM EST Tuesday... 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 upper level ridging builds over our part of the country. Concurrently, the low level temperature profile will gradually warm as southwest flow warm air advection is forecast. Also, moisture will increase across our region, especially the western sections. The result will be a period of increasing temperatures, with limited chances of precipitation. Only the far western sections of the area will have mention of some isolated showers both Saturday and Sunday afternoon. High temperatures by Sunday are forecast to range from the mid to upper 50s across the mountains with upper 50s to around 60 across the Piedmont. 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 may be a scenario very gusty showers crossing the, with decent 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. && .AVIATION /05Z 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...Winter Weather Advisory until 10 AM EST this morning for VAZ007-009-010. Winter Storm Warning until 5 PM EST this afternoon for VAZ058- 059. Winter Weather Advisory until 3 PM EST this afternoon for VAZ034-035-043>047. NC...Winter Storm Warning until 5 PM EST this afternoon for NCZ006. Winter Weather Advisory until 3 PM EST this afternoon for NCZ004-005-020. WV...Winter Weather Advisory until 10 AM EST this morning for WVZ042>044-507-508. && $$ SYNOPSIS...DS NEAR TERM...PM/RCS SHORT TERM...DS LONG TERM...DS 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.