Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 142023 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 323 PM EST Tue Feb 14 2017 .SYNOPSIS...
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Low pressure will move east along the Gulf Coast states tonight into Wednesday, all while a cold front enters and then moves across our region. High pressure will gradually build into the area Thursday into the start of the holiday weekend. Weak low pressure will bring the next chance of precipitation to the area on Sunday.
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&& .NEAR TERM /THROUGH WEDNESDAY/...
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As of 230 PM EST Tuesday... Tonight, low pressure will track eastward along a warm front that is positioned across the Gulf Coast states. Precipitation on the northern extent of this feature is expected to move eastward through at least the southern half of our region starting this evening. By midnight, the activity is expected to be east to Danville, VA, and north as far as roughly route 460 in WV and VA. This activity will occur about twelve hours in advance of a cold front that will sweep through the region during the day Wednesday. As winds back in advance of the cold front, look for a small jog north of the main precipitation shield to encompass territory north of route 460, especially in areas east of Roanoke, VA. Low level temperature profiles support primarily a rain versus snow forecast tonight, with the snow confined to primarily the mountains, with measurable snow in the one to two inch range at elevations at or above roughly 4000 ft MSL. Coating level amounts are expected at elevations down to approximately 3000 ft MSL. Portions of the Northern Mountains of North Carolina may see a layer of melting and refreezing yielding some pockets of sleet as well. The cold front is expected to sweep through the region late tonight through the morning hours on Wednesday. This feature will shunt the moisture associated with the southern stream system out of the area. In its wake, increasing northwest flow will yield upslope rain/snow showers across portions of southeast West Virginia, south into the Northern Mountains of North Carolina Wednesday morning, with decreasing coverage Wednesday afternoon. The prevailing northwest winds will also help to erode cloud cover Wednesday across the Piedmont by noon, with most of the mountains seeing more sun than clouds by the mid-afternoon. Low temperatures tonight will range from the upper 20s to mid 30s across the mountains with upper 30s to around 40 across the Piedmont. Highs on Wednesday will range from the upper 30s to lower 40s across the mountains with mid to upper 40s across the Piedmont.
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&& .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
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As of 310 PM EST Tuesday... Pronounced mid-level low to our north and east induces another (brief) shot of colder temperatures for Wednesday night and Thursday; however, we start to moderate on Friday as mid-level heights begin to rise. For Wednesday Night: As frontal zone pushes south and eastward, surface ridge builds in and with focus for PoPs restricted to the upslope areas in western Greenbrier and northern Summers counties. This is all snow for these upslope areas, as 850 mb temperatures crash to values -8 to -12C thru 12z Thursday under rather strong cold thermal advection across the entire forecast area. Focused highest PoPs (Likely) along the western portions of Greenbrier County, generally lowering to Chance by early morning. This coincides with partially saturated -12 to -18C dendritic snow growth region, but limited QPF to around a few hundreths in western Greenbrier. However, the better moisture is confined further northeastward. This leads to accumulations from a coating to 1" in eastern Greenbrier and northern Summers Counties to 1-2" western Greenbrier. Northwesterly winds will increase as well overnight in the cold advection; however, based on current indications and per local post-frontal cold-season wind guidance research, it appears that wind gusts may be largely sub-Advisory except possibly in the NC mountains and perhaps into Carroll and Grayson Counties in VA along the Blue Ridge. Will continue mention in HWO for now, but outside of these areas wind gusts of 25-35 mph appear most likely. Factoring in teens to low 20s temps in with breezy to gusty northwest winds along/west of the Ridge, wind chills look to fall through the teens into the single digits overnight hrs. From the Roanoke Valley into the foothills and Piedmont, conditions should be marginally warmer with lows in the mid/upper 20s. For Thursday: Deep 500-mb cyclonic gyre and associated 1000-500 mb thermal trough establishes itself across the northern and eastern portions of the forecast area. Cold advection aloft generally weakens as we advance into the afternoon hours, but that will still render warming temps due to sunshine ineffective to some extent. Temperatures will certainly climb but I`ve opted for a very slow rise in areas along/west of the Blue Ridge to highs in the lower to mid 30s; in the foothills, temperatures look to warm into the 40s before falling. For Thursday Night: 500-mb low and associated cyclonic flow over our area begins to ever so slowly propagate northeastward. An 850-mb baroclinic zone then sets up across far western parts of the forecast area and into the eastern Tennessee Valley, separating much colder air to the northeast with the building anomalous warmth southwest of it. Today`s 12z NAM and GFS have shown a weak, narrow ribbon of vort energy that ripples along the baroclinic zone, on the southwestern periphery of the 500 mb low Thursday night; this may trigger a redevelopment of snow showers in southeast West Virginia. Shown slight to low Chance- level PoPs for snow showers Thursday night in these areas; however, I`ve also extended isolated flurries downstream into the Alleghany Highlands and into the southern Shenandoah Valley. Best chance of any limited accumulation would however be in southeast West Virginia aided by an upslope component. Lows in the 20s to near freezing across the area. For Friday and Friday Night: Moderation out of the cooler temperatures begins in earnest on Friday continuing into Friday night. 500 mb low loses its influence, with mid-level height rises replacing it leading to mostly sunny skies and a clear evening. By Friday evening, our 850 mb temperatures return to positive levels (+2 to +5C). Looking at upper 40s to near 60 for highs and lows in the mid/upper 30s to near 40 along ridgetops governed by radiational cooling.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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As of 310 PM EST Tuesday... Global models and their ensemble means remain in good agreement that the background mid-level height pattern will feature height rises in the mean throughout much of the period. Aside from a southern-stream deamplifying mid-level low Saturday night, precipitation chances through the rest of the period are nil. It would appear that the next significant chance of precipitation after Saturday night is not until Tuesday night or Wednesday. Anomalously warm temperatures are expected through the forecast period. 850 mb temperatures "cool" to +4 to +6 on Sunday, but are then around +8 to near +10C into midweek. Though lows will trend above normal as well, highs are well above normal. The anomalous warmth is reflected nicely as high probabilities for above normal temperatures in CPC`s 6-10 day temperature probabilistic outlooks. In addition, MEX MOS guidance are some 10 to 15 degrees above normal on both the highs and the lows after Sunday. Just another illustration of how anomalously mild the period projects to be, as MEX MOS guidance typically trends toward climo in the latter time ranges. Used SuperBlend as a starting point, but modified these somewhat using MOSGuide and bias-corrected highs/lows to better show the diurnal range. Bias-corrected temperatures are sure to do quite well in the latter ranges of the period. Monday looks to be the warmest day of the stretch as forecast highs are in the 60s to low 70s. Given the generally dry forecast period, the anomalously warm temperatures and the likelihood that sunshine will steepen low- level lapse rates and lower afternoon dewpoints, I suspect that fire weather would be a focus particularly after Saturday night, at least in a general sense. Fuels should dry out, even though winds are very light. Something to keep mind of and will be better refined as we continue into the upcoming weekend.
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&& .AVIATION /20Z TUESDAY THROUGH SUNDAY/... As of 1245 PM EST Tuesday... VFR conditions will prevail through the early evening. After that time, ceilings will gradually lower to MVFR in the west and low end VFR in the east thanks to increasing moisture in the low levels, and the onset of light precipitation. The precipitation will fall as a mix of mainly light rain and light snow, temperature and elevation dependent. A few locations near the Northern Mountains of North Carolina may also experience some light sleet. A cold front will cross the area late tonight into Wednesday morning. Look for winds to become gusty from the northwest, and the bulk of the precipitation to exit the area to the east. Lingering upslope rain and snow showers will be possible across the preferred regions of southeast West Virginia, south into the Northern Mountains of North Carolina. By the end of the TAF period, gusts across the mountains will be in the 20 to 25 kt range. Clouds in the east will clear early. Pockets of MVFR ceilings will continue where rain/snow showers are more probable. Extended Aviation Discussion... Gusty northwest winds will continue across the area into Thursday or Thursday night. Patchy MVFR cigs possible between KBLF-KLWB. Upper level shortwave ridge moves over the area Friday into Saturday. Look for light winds and VFR conditions all locations. Weak low pressure moves through the region Saturday night into Sunday. Pockets of sub-VFR conditions return along with light precipitation. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...DS NEAR TERM...DS SHORT TERM...AL LONG TERM...AL AVIATION...DS

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