Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 231416 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1016 AM EDT Mon Apr 23 2018 .SYNOPSIS... A deep low pressure system will drift from western Tennessee this morning across the mid-Atlantic by Wednesday. The arrival of this system will promote gusty east to southeasterly winds across the higher terrain from North Carolina through southeast West Virginia, as well as locally heavy rainfall along the Blue Ridge from Floyd County southward. As this system departs on Wednesday, two additional disturbances look to cross over or near our area Thursday into Friday. && .NEAR TERM /THROUGH TONIGHT/... As of 1010 AM EDT Monday... Update: No changes to existing headlines. Expect rain to move northeast into southern WV to the New River Valley to the NC piedmont by early afternoon. Rainfall amounts so far have been mostly under a quarter inch, and high-res models showing better enhancement of the southeast flow and upslope this afternoon after 18z/2pm. As far as the winds have had gusts over 50 mph in the higher terrain of NC with most 30-40 mph where the wind advisory exists. The stronger low level jet arrives this afternoon with the rain, but the rain will also inhibit really strong winds, but even some gusts 45-50 mph will cause isolated problems, small trees down, isolated power outages. Previous discussion from early morning... Keeping an eye on a deep low pressure system making its approach from far western Tennessee this morning, with the passage of this system across our area presenting two distinct forecast challenges through tonight. The fist challenge concerns gusty southeasterly winds across the higher terrain from northern North Carolina into far southwest Virginia and southeast West Virginia. The approach of this system has caused the pressure gradient to tighten through the night, a trend that will continue into late evening. Strongest gusts are expected along the east-facing slopes in this region where gusts could reach 50 mph at times during the afternoon and evening. By midnight, winds are expected to begin shifting more east to northeasterly, at which point there will be less of a downsloping enhancement to the wind speeds. That said, would not be surprised to see the wind advisory which is currently in effect be extended in time, perhaps to Tuesday morning, if wind direction takes longer than anticipated to make the shift. The second challenge is the heavy rainfall this system brings with it. The strong southeasterly wind flow around this system will draw Atlantic moisture rapidly up the Blue Ridge, where the sudden increase in elevation will locally enhance rainfall amounts. As such, areas within a few miles on either side of the Blue Ridge Parkway from Floyd County southward can expect 2 to locally 4 inches of rain through sunrise Tuesday. Rainfall amounts decrease significantly further away from the Blue Ridge, however the Piedmont can expect around an inch, while downsloping across far western Virginia and southeast West Virginia will cut rainfall amounts down to three quarters of an inch or less. Unless localized bands of heavy rain develop and continue to train across the same locations, the threat of flash flooding is not the primary concern. Rather, rainfall is expected to fall at a moderate and steady pace, especially this evening and tonight, leading to a possibility of minor flooding on smaller streams and rivers. As such, a flood watch remains in effect along the Blue Ridge of North Carolina and southern Virginia through Tuesday afternoon. With clouds increasing and rain moving into the area today, do not expect much warming to take place today across the mountains, where temperatures will hold in the 50s for afternoon highs. Low clouds and rainfall will steadily progress eastward across the Piedmont during the afternoon, though not before temperatures there have had a chance to warm into the low/mid 60s. Temperatures tonight will vary from the mid 40s across the mountains to the low 50s across the Piedmont. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... As of 310 AM EDT Monday... Rain, heavy at times will continue into Tuesday night. WPC day 3 excessive rainfall outlook places a marginal threat across eastern portions of the forecast area. Will be monitoring flooding issues both stream flooding, to potential main stem flooding. See hydro section below for details. PWATs and moisture transport remain high enhanced by the easterly upslope winds. Surface low pressure over Georgia Tuesday morning will translate east to off the Mid-Atlantic Coast Wednesday. Then, the coastal low lifts northeast to southern New England by Wednesday night. The moderate to heavy rain will continue into Tuesday afternoon, then taper off Tuesday night as easterly flow weakens. High temperatures on Tuesday will vary from the upper 40s along the Blue Ridge into the Alleghanys to the upper 50s in the far southwest. CAD may trim back temperatures even more across the north. There may be an isolated thunderstorm Tuesday afternoon and evening in the south as highlighted by the general thunder on Day Two convective outlook. However, with the lack of instability will hold off on added any thunderstorms at this time. Cloudy conditions with scattered showers are expected for Tuesday night. Low temperatures will be above normal with readings from the mid 40s to the lower 50s. The upper trough stays to our west and a northern stream front tracks toward us Wednesday. Northwest flow on Wednesday will return sunshine east of the Blue ridge while clouds will linger in the west. Decided to allow for an isolated to scattered showers in the west Wednesday night. High temperatures on Wednesday will range the mid 50s in the mountains to the lower 70s east of the Blue Ridge. Low temperatures Wednesday night will generally be from the upper 30s in the northwest mountains to around 50 degrees in the piedmont. && .LONG TERM /THURSDAY THROUGH SUNDAY/... As of 300 AM EDT Monday... Deterministic models are still quite different but are trying to merge on a solution for the extended period. Both the GFS and ECMWF have the cold front to our east Thursday morning with some drier weather initially. The difference in the models comes in on what happens with the southern stream system and its track. The ECMWF holds onto a drier solution compared to GFS, favoring high pressure over the area. The northern advance of the moisture on Thursday is kept to our south on the ECMWF. While, the GFS is a little faster and further north with the moisture with the weak piece of energy moving over the Gulf Coastal States into Friday morning. The 500 mb pattern shows a broad trough over the eastern half of the nation, with two distinct storm tracks into the weekend. Overall the ECMWF has us mainly dry with a few showers possible with a front cutting across Saturday. Meanwhile, the GFS shows a strong Gulf Coastal system Thursday with better southwest flow into the southern Appalachians. Then it develops a surface low across the central Carolinas Thursday evening, tracking it to the mid-Atlantic coast by Friday afternoon while a strong northern stream upper low forms over the lower Great Lakes, with a secondary front and shot of colder air moving in Friday night into Saturday. Lean toward the steadier ensemble means for this period which for has at least a chance of showers Thursday-Friday, with possibly cold enough air after the front Saturday morning to have snow showers in the Alleghanys. Expect temperatures to run at or just below normal this period. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... As of 745 AM EDT Monday... Surface observations indicate moisture associated with a deep low pressure system over the central Mississippi River Valley continues to work its way across the central Appalachians. MVFR/IFR ceilings have developed across much of the mountains, though VFR conditions remain east of the Blue Ridge. Forecast models continue to hint that the bands of rain arriving from the southwest will have to overcome the dry air that remains in place across the mid-Atlantic. This will eventually happen, however have kept TAF sites ROA/LYH/DAN in VFR territory into the afternoon until moisture deepens. By sunset, much of the forecast region is expected to be sub-VFR under a combination of low ceilings and variable visibility as bands of rain, which may be locally heavy, passes across the area. MVFR/IFR conditions will remain in place through the overnight. The other concern is gusty winds due to a tightened pressure gradient as the low approaches, especially across the downslope areas along the western ridges. BLF in particular will see some very strong winds with occasional gusts to near 40 MPH possible. Winds overnight will gradually shift more east- northeasterly as the low approaches. Wind speeds will then gradually diminish as the downslope enhancement decreases and the pressure gradient relaxes. Extended Aviation Discussion... Sub-VFR ceilings and visibilities and gusty winds in the west, will continue through tonight and early Tuesday. Tuesday afternoon and night, the low pressure responsible for the wet weather will shift east of the area. This will bring our winds to the northwest, and allow for the precipitation to trend showery in the west. The weather pattern will remain unsettled Thursday into Friday as a series of disturbances crosses the area, each with the potential for providing showery precipitation and localized sub-VFR conditions. && .HYDROLOGY... As of 500 AM EDT Monday... A prolonged period of strong upslope, moisture laden flow off the Atlantic will produce a very generous rainfall for most of the region today and tonight. Amounts of one to two inches will be common for most of the area. However, locations near the crest of the Blue Ridge from near Floyd, VA southwest into the Northern Mountains of North Carolina, amounts of three to four inches are more likely, with locally higher amounts. It will be this region, and downstream flow areas in parts of Patrick, Surry, and Wilkes Counties, that have the greatest potential for flooding in the nearer term. A flood watch remains in effect from 800 AM this morning through 400 PM Tuesday. Confidence in a significant upslope rain event is heightened by the Ensemble easterly wind component anomalies of 2 to 4 standard deviations during the time of concern. Looking a little bit further out in time, Model Ensemble River Forecast are focusing on the Dan River as a location to watch for possible minor river flooding once we reach the Tuesday into Wednesday time frame. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flood Watch through Tuesday afternoon for VAZ015>017-032. Wind Advisory until midnight EDT tonight for VAZ007-009-015. NC...Flood Watch through Tuesday afternoon for NCZ001>003-018-019. Wind Advisory until midnight EDT tonight for NCZ001-018. WV...Wind Advisory until midnight EDT tonight for WVZ042-043. && $$ SYNOPSIS...NF NEAR TERM...NF/WP SHORT TERM...KK LONG TERM...KK/WP AVIATION...JR/NF HYDROLOGY...DS is the U.S. government's official web portal to all federal, state and local government web resources and services.