Area Forecast Discussion
Issued by NWS Blacksburg, VA

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459 FXUS61 KRNK 240130 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 830 PM EST Thu Jan 23 2020 .SYNOPSIS... The cold front associated with a storm system over the mid section of the country will cross the area tonight providing a focus for moderate to heavy rain. Some of the rain may begin as a period of freezing rain early Friday before transitioning to all rain. && .NEAR TERM /THROUGH FRIDAY/... As of 800 PM EST Thursday... After analyzing several models and BUFKIT soundings, having a difficult time supporting winter precipitation for any length of time or significant accumulation Friday morning. However, given that the advisory is out and all have planned accordingly, not wise to make wholesale changes at this point. Went carefully through the hourly temperatures to make adjustments to better match the non-diurnal trend in place overnight. Namely, temperatures will likely reach minimums around mignight, then begin to rise slowly under the thick blanket of clouds and general southeast wind flow. Then, as the precipitation reaches the western part of the CWA near or just before daybreak, temperatures will bottom out at their coldest levels as the initially dry air mass saturates and results in temperatures lowering to the dewpoint, which should result in a brief period of temperatures near or just below freezing in the 09Z-14Z period. Once the temperatures fall, they will be slow to rise, but likely will rise just enough or remain close enough to 32F to limit ice accumulations on anything other than elevated objects such as trees and power lines. Really do not anticipate major road problems at this point, especially any treated or previously treated roadways. Ice accumulations have been lowered across the board and confined mainly to the southern Shenandoah Valley where temperatures have the potential to remain at or below freezing the longest into the day. In addition, the Winter Weather Advisory was pulled back to end at 18Z as opposed to 23Z. Even the coldest models do not support holding onto the WSW that late into the day. All temperatures really should be above freezing by noon at the latest, although northern areas and deeper valleys could hover around 33-34F for several hours into the early afternoon. The bigger concern by evening may be the potential for a line of heavy rain showers resulting in pockets of minor flooding and also the potential for gusty winds as yet another negatively tilted trough brings the chance for a QLCS rotating up from the southwest. However, in comparison to the severe wind event of two weekends ago, CAPE with this event, given the much colder temperatures than were in place on the 11th, is negligible. Otherwise, timing of pops generally good, perhaps a bit fast in the early part of the forecast cycle, but otherwise seem to catch well the heaviest rain and best chances for rain. Finally, references to moderate freezing rain were reduced to just light freezing rain. Previous Near Term Forecast Discussion... .. As of 300 PM Thursday... Increasing clouds and dry conditions persist through the remainder of Thursday. Meanwhile, a low pressure system continues to push eastward across the U.S. As it enters our area tonight, it will still be fighting dry air at the surface and will have to fight to saturate this layer, so any precipitation starting off will be light. The expectation is still for precipitation to enter in from the SW after midnight. As this precip enters the area, it will mainly start as rain. Shortly after starting though, evaporative cooling looks to cool air at the surface, bringing temps below freezing. The Blue Ridge is still the prime area of possibility for this given increased cooling from upslope flow, but we`ve gone ahead with highlighting parts of the foothills, as well as parts west of the Blue Ridge, mainly north of 460, where for a very short period of freezing rain will be possible. By 10-11 AM, temps look to be warm enough for it to be all rain. The plan is to split a winter weather advisory to emphasize the greater risk/amounts of ~0.1" of ice accretion along the Blue Ridge, and another for areas east and north of Roanoke with one to allow the better description of just trace amounts of ice. Areas like the foothills will be such a brief window, that we currently plan cover it with an SPS as it warrants. Changing the subject, with the system will also be a stout low level jet that will be humming along at 50-60kts. This favors strengthening winds across the higher elevations (ridge crests) west of the Blue with the potential for damaging winds on the west side of the Appalachian Divide. This sort of set-up drives some pretty healthy wind gusts above 4000 feet in elevation, or lower if it can mix down. In addition, downsloping into the TN valley typically takes place. Tazewell County typically has wind damage issues in this sort of set-up. A wind advisory will be issued to cover this wind risk from Watauga Co, north to Tazewell and into western Greenbrier. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... As of 240 PM Thursday... A cold front will move east Friday night and push off the coast. The main batch of rain will move out of eastern Virginia. An isolated thunderstorm may be possible in the southern portion of the forecast area. In the west, temperatures will fall enough in the mountains to see the rain showers change to snow showers. Low temperatures Friday night will range from mid 20s in the mountains to the upper 30s in the piedmont. The closed upper low over the Great Lakes Region will slowly travel east Saturday into Saturday night. Winds will turn toward the west and increase Saturday morning. Boundary layer mixing should tap into the 850 mb winds around 35 knots and bring decent gusts down to the surface Saturday afternoon. Upslope rain and snow showers will continue through Saturday along the western mountains from Boone to Lewisburg, while the Piedmont should remain dry. High temperature Saturday will vary from the lower 30s in the northwest mountains of Greenbrier county in southeast West Virginia to the lower 50s in the piedmont with some sunshine. Snow showers and snow flurries will linger in the western mountains Saturday night. Low temperatures will generally be from the mid 20s in the west to the lower 30s in the east. Clouds will linger along and west of the Blue Ridge Sunday and Sunday night. Upslope precipitation should taper off to snow showers mainly for southeast West Virginia. High temperatures Sunday will range from the upper 20s in northwest mountains to around 50 degrees in the piedmont. Snow flurries will continue in northwest Greenbrier county Sunday night. Look for low temperatures Sunday night into Monday morning to feature readings from the mid 20s in the mountains to the lower 30s in the piedmont. Moderate confidence in the short term period. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 240 PM Thursday... The base of the upper trough will pivot east Monday. A reinforcing shot of cold air from the northwest will continue isolated snow showers in western Greenbrier Monday. Richer cloud cover will continue in the western mountains Monday into Monday night. While a downsloping winds will provide more sunshine in the east. High pressure will build overhead Tuesday and slide east Tuesday night. Cloud cover will gradually increase Wednesday ahead of the next system that takes aim to move in late next week. By Thursday into Friday, the GFS and ECMWF start to diverge. The ECMWF develop a coastal low off the North Carolina Thursday which lifts northeast into Friday. The GFS becomes drier with a more zonal flow, then upper ridging builds in. Used a model blend for Wednesday into Thursday and kept highest pops in the west. Moderate confidence in the long term period. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... As of 800 PM EST Thursday... VFR conditions are expected to continue to persist through midnight at most if not all TAF sites with just a mid-high level overcast in the 100-150 range. The lower levels of the atmosphere are dry from the recent Canadian air mass and will require precipitation late tonight/early Friday to saturate and result in lowering ceilings/visibilities. Ceilings will transition quickly through MVFR in the 06Z-10Z time frame, then to IFR or worse at most TAF sites by 12Z-14Z. Precipitation is expected to be mostly rain and for that matter all rain at DAN, LYH, and ROA. For BCB, BLF, and LWB, included some brief winter precipitation, mainly in the form of -FZRA, but a brief period of PL or even SN is possible at BLF and especially LWB where the cold air is a bit deeper than further south and east. Areas of fog will also be prevalent through the day, especially by afternoon. By late afternoon or early evening Friday, expect a narrow squall line with brief heavy rain and possibly gusty winds to move through the region from southwest to northeast and this has been reflected in the BCB, BLF, and LWB TAFs. Confidence in this reaching areas further east is low, so it has not been included there at this time. Winds will remain northeast to southeast overnight at speeds of 6-10kts with low end gusts possible in most areas as the night progresses, but especially at BLF and also LYH. Strong and gusty southeast winds will become more prevalent early morning at BLF as the low-level jet moves into that region from the southwest. /Confidence in Forecast Parameters/ Ceilings - Moderate to High, Visibilities - Moderate to High, Winds - Moderate to High Freezing Rain Threat - Low to Moderate. Extended Aviation Discussion... Once the front passes, conditions will improve east of the mountains Friday night. However, anticipate lingering MVFR cloud bases across the mountains into Sunday with snow showers. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Winter Weather Advisory from midnight tonight to 1 PM EST Friday for VAZ011-013>020-022>024. Wind Advisory from midnight tonight to 6 PM EST Friday for VAZ007-009-010-012-015. NC...Winter Weather Advisory from midnight tonight to 1 PM EST Friday for NCZ001-002-018. Wind Advisory from midnight tonight to 6 PM EST Friday for NCZ001-018. WV...Winter Weather Advisory from midnight tonight to 1 PM EST Friday for WVZ044-507-508. Wind Advisory from midnight tonight to 6 PM EST Friday for WVZ042-043-508. && $$ SYNOPSIS...PM/RR NEAR TERM...PM/RAB/RR SHORT TERM...KK LONG TERM...KK AVIATION...PM/RAB/RR

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