Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 171512 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1012 AM EST Sat Feb 17 2018 .SYNOPSIS... High pressure over the mid-Atlantic today will shift off the New England coast this evening. Low pressure tracks from the Gulf Coast states to the Carolinas during this time. This system then exits east late tonight with high pressure building in Sunday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1012 AM EST Saturday... Winter weather advisories remain in effect until Midnight for light wintry mix. Made some minor adjustments to temperatures utilizing the latest surface obs, their trends and shaped towards a blend of NAM and GLAMP for this afternoon. Temperature profiles will be the key to precipitation type this afternoon associated with low pressure center. In general, light wintry mixture of snow, sleet, freezing rain and rain expected today into tonight. Shaped precipitation and weather towards the combination of NAM Nest and ISC continuity for this morning into this afternoon. Watching the area of rain noted on regional WSR- 88D radars and temperature trends for adjustments to winter weather advisories. At this time, accumulations of snow and ice look reasonable. Highest snow totals expected along and north of I64. The greatest icing anticipated along the Blue Ridge mountains. More changes later this morning... As of 320 AM EST Saturday... Temperatures still running a little milder out there, and 00z model analysis versus upper air soundings showed models initialized a degree to 3 degrees colder at 8h from RNK to BNA, however, colder air aloft is arriving behind the front. Kind of a nuisance event coming up today and timing of measurable precip as it moves into a cooler airmass will lead to what ptype occurs. Models vary on the thermal structure, but overall no changes are planned to where the current advisories are. Given that temps are trending a little warmer in the low levels, with a shallow colder layer above the surface, the another narrow warm nose above this cold layer and the fact the winds are not that strong out of the east, appears icing will not be as widespread, but still could see a tenth to two-tenths of an inch along the Blue Ridge especially from Roanoke to Meadows of Dan, with patchy icing elsewhere along the Blue ridge and isolated portions of the Alleghanys. Coupled with warmer and wet ground temperatures, icing will mainly be confined to trees and power lines, and with wind not much of an issue, not expecting too much impact on power, but isolated outages are not out of the question, mainly Roanoke/Floyd/Franklin/Patrick. Further north, colder air will allow for a mix of snow/sleet to stick around longer, but again appears accumulating snow/sleet will be confined to higher terrain, with less in the valleys and sleet will cut into totals as well. Cut back somewhat snowfall but have 2 to 4 inches at the higher terrain of Greenbrier/Bath and Amherst/Rockbridge through tonight, with some of the higher amounts also occuring as snow showers kick in behind the departing surface low off the coast with high pressure building into the mid-MS Valley. Given the speed of this event, overall impact is less. Still those traveling from the New River Valley and Blue Ridge north of Hillsville northward to WV into northern VA should be prepared for slick spots at times. Timing of precip shows rain/snow arriving into the NC mountains around 8-9am, spreading northeast thru midday. Most should see precip today but best coverage will be along and northwest of the Blue Ridge, and north of highway 460. Temperatures today will not move too much, may fall initially at precip onset, then steady out or rise a degree or two. So at dawn, temperatures will range from around 30 in the Alleghanys to Greenbrier higher ridges, to lower to mid 30s elsewhere along/west of the Blue Ridge and north of the Roanoke River in VA, to upper 30s to lower 40s southside VA into the NC foothills/piedmont. By 4pm, temperatures will be a little cooler along the Blue Ridge especially north of the NC/VA border with lower to mid 30s. Lower 30s also lingering into the Alleghanys and portions of the Shenandoah to New River Valleys, with mid to upper 30s elsewhere, but possibly seeing lower 40s across far southwest VA west of Tazewell and Marion. Overnight, temps slide down into the upper 20s to lower 30s, with clearing taking place east of the mountains, and snow showers tapering off by dawn, though will possibly see some freezing drizzle along the western slopes per model forecast soundings showing little if any moisture in the snow crystal growth temp zone.
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&& .SHORT TERM /SUNDAY THROUGH TUESDAY/... As of 356 AM EST Saturday... Sunday begins with high pressure building across the northern half of the Blacksburg forecast area. This high pressure area will shift low-level thermal gradient at least briefly southward. After early- morning stratus in the Greenbrier mtns into the far western mountain counties burns off, a pretty nice day appears on tap with mostly sunny skies expected to push temperatures from the mid-40s to mid/upper 50s. Aforementioned thermal gradient begins to return back northward as a warm front Sunday night (00z NAM/ECMWF camp) or early Monday (00z GFS). This warm front is on the leading edge of low-mid level height rises ahead of what will turn out to be a highly-anomalous subtropical ridge and surge of low-level Gulf moisture. Regardless of which solution(s) are chosen, greatest larger-scale lift passes us by to the northwest - more in the Ohio Valley. Sided toward the NAM/ECMWF idea for timing the front across, which begins to approach our far southwestern counties toward midnight. Will keep PoPs to no better than low-end Chance (30%-ish or less) west and south of Roanoke before midnight, and dry to the north/eastern tier of counties. Best chance for rain is really into the early morning/pre- dawn hours where PoPs increase to high Chance/lower Likely, lowest chances furthest east more removed from the best frontal forcing. What this will do is set up somewhat of a complicated temperature pattern where clouds and warm advection earlier in the eastern TN Valley/NC mountains will lead to early lows, whereas enough opportunity radiational cooling happens through a large part of the overnight across the Piedmont northwest into the Shenandoah/Highlands areas, before rising closer to early morning. Used a non-diurnal temperature curve to better capture these details, with lows mid-upper 30s mainly from I-64 to Lynchburg, and the lower to mid 40s further south and west. Biggest question mark for Monday is timing the warm front northeastward and the degree of post-frontal cloud clearing and its effect on mixing/temperatures. The warm front probably will still be making slow progress northeast across the northern half of the CWA (roughly along/north of Route 460) early Monday. Light showers most likely to continue until early afternoon, with leftover cloud cover likely to restrict mixing significantly in northeastern counties and the Piedmont. 850 mb temperatures are on the order of +10 to +12C range by mid-afternoon. BUFKIT soundings off the NAM and GFS at Lynchburg each show poor potential for mixing, which leads to highs in the mid 50s in the Piedmont closer to what the 00z MAV suggests. Better chance at temperatures reaching or exceeding 60 is from Roanoke southwest. All told, a rather low-confidence temperature forecast. If better mixing occurs than currently indicated, these temps probably are much too cold, perhaps by as much as 5-10 degrees closer to the 00z MET idea. In terms of rainfall with the warm front, amounts are generally a quarter inch or less and not sufficient enough to yield any hydrologic issues. Anomalous 591-dm 500 mb ridge becomes established off the Carolina coast Monday night, continuing into Tuesday. For Monday night, there`s a fair amount of RH that`s indicated in the GFS guidance which along with a light south-southwest breeze should keep temperatures from falling very far. Despite PWATs on the rise, ridging aloft should keep forecast area on the dry side. Overall a mild evening with temperatures in the 50s, with dewpoints also in the upper 40s to low 50s. Tuesday should be the warmest of the period, with 850 mb temperatures between +12 and +14C. Much like Monday, Tuesday high temperatures again will be heavily determined by how much sunshine/breaks in clouds is realized. Since it`s unlikely we see clouds clear any more than partly cloudy, will keep temperatures for now in the upper 60s to lower 70s, with potential for even milder. Even those lower values are still 20-30 degrees warmer than climo. It will also feel like an early-mid spring day as dewpoints rise into the upper 50s-near 60. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... As of 356 AM EST Saturday... Synoptic pattern features strong subtropical ridge of high pressure from the central/eastern caribbean up toward Bermuda. As a result, well above average mid-level heights are forecast across the eastern U.S. with values in the 582-588 DM range. Relative to climatology, this is 2 to 2.5 standard deviations above normal. This set-up favors much above normal temperatures for the Southeast States with the baroclinic zone forced to reside on the west side of the Appalachians from East Texas to the Ohio Valley and into the eastern Great Lakes. Repetitive rainfall is likely within this corridor during the week with our forecast area far enough east of the boundary to escape anything significant. Toward the end of the week, models hint that a surface front will impinge on the forecast area, but overall weather conditions for mid week feature above normal warmth and only a chance for showers pending any instability within the warm subtropical airmass. Models suggest potential for a backdoor front to impinge from the north-northeast Thursday, so somewhat cooler/wetter conditions are favored for the end of the week. && .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... As of 635 AM EST Saturday... Tricky aviation forecast as a wintry mix works in from the southwest later this morning and impacts all taf sites, except DAN/BLF which should stay all rain. Will see cigs sink to MVFR and lower through the day, with rain heading to BLF between 15-17z, and shortly thereafter at LWB/BCB/ROA where more more snow mixes in at LWB and sleet at ROA/BCB, and some snow. For most of the afternoon will see mainly sleet/snow mixed at LWB while as mixture of sleet and rain meanders from BCB to ROA/LYH, before ending this evening out east and around midnight in Blacksburg. Vsbys/cigs will deteriorate to IFR or lowers after 21z, and remain that way into the evening. Once the wind shifts to the west a clearing trend and rise in cigs will occur east of the mountains. Expect lower clouds to linger into 12z Sunday at LWB/BLF with a few snow showers or patchy freezing drizzle. There will be a corridor of some ice along the Blue Ridge and Alleghanys, so be aware of this. Extended Aviation Discussion... VFR returns again on Sunday under weak high pressure. A warm front lifts back north into the area on Monday with another round of sub-VFR ceilings and visibilities with rain, especially mountains. High pressure strengthens offshore resulting in a return to overall VFR Tuesday ahead of the next cold front well west of the Appalachians. Scattered MVFR showers are expected Wednesday into Thursday. && .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Winter Weather Advisory until midnight EST tonight for VAZ011- 014-017>020-022>024-032>035. NC...None. WV...Winter Weather Advisory until midnight EST tonight for WVZ044- 507-508.
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&& $$ SYNOPSIS...KK/WP NEAR TERM...KK/WP SHORT TERM...AL LONG TERM...AL/PM AVIATION...AMS/WP

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