Area Forecast Discussion
Issued by NWS Blacksburg, VA

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078 FXUS61 KRNK 212307 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 707 PM EDT Fri Apr 21 2017 .SYNOPSIS... A cold front will travel southeast this evening into tonight. This front will stall near the North Carolina and Virginia border Saturday, with a secondary low moving across the area Sunday. This will keep us in a wetter pattern through the weekend into Monday. && .NEAR TERM /THROUGH SATURDAY/... As of 635 PM EDT Friday... Adjustments to the forecast this evening will reflect primarily changes to geographical location of the best chance of additional showers and thunderstorms through the evening hours. This readjustment is based upon trends on regional radars of the shower and thunderstorm activity, and expected movement through the next few hours. The best coverage is expected to be along and near the crest of the Blue Ridge from roughly Floyd, VA and parts southwest into the Northern Mountains and Foothills of North Carolina. This is expected to transition to more of a west to east oriented location of the best coverage generally along and south of Route 460. Have also made minor tweaks to hourly temperature, dew point, and winds based upon the latest observations and expected trends the next few hours. As of 312 PM EDT Friday... A cold front will travel southeast across the region this afternoon into tonight and stall on Saturday. Scattered showers and thunderstorms will develop ahead and along the boundary in the warm unstable air. The Storm Prediction Center on SWODY1 highlighted a marginal risk for severe thunderstorms for most of the area. With CAPES around 1k J/kg, freezing level around 9 kft and a modest mid- level flow of 30-40 kts, some of the stronger thunderstorms could produce damaging winds and hail. As seen on SPC Mesoscale analysis the best instability was located in the piedmont. Shaped the pops tonight towards a blend of the HRRR AND NAM. The forecast area will see a decent coverage of showers tonight, with thunderstorm chances subsiding with loss of heating. Added some patchy fog overnight into Saturday morning. Low temperatures tonight will range from the upper 40s in the northwest mountains of Greenbrier county in southeast West Virginia to the lower 60s in the Piedmont. An upper level trof will dig into the southeast State and develop into a slow moving closed low by Sunday. as it drives a surface low east through South Carolina. A wedge of high pressure will surges into the region east of the Appalachians. For Saturday morning into Saturday afternoon, isentropic lift from the surface low over the lower Mississippi valley will bring widespread showers to the region with a few thunderstorms. Across southern portions of the forecast area, there will be some instability pool ahead of the wedge to fuel development of thunderstorms, and shear along the backdoor front will be sufficient to help organize the convection. The Day 2 Convective Outlook has placed southern portions of the forecast area in a marginal risk for severe weather with wind and hail, the primary threat. High temperatures Saturday will vary from the lower 50s in the north to the mid 70s along the southern Blue Ridge. Temperatures on Saturday will be highly dependent on the strength of the wedge and placement of convection. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... As of 312 PM EDT Friday... A series of shortwaves in the northern jet stream from Colorado to Montana will deepen the upper trough over the central United States eventually closing off an upper low over the Tennessee Valley on Sunday. This low continues to track southeast, reaching the Georgia coast by late Monday. At the surface high pressure will be wedged down the Appalachians on Sunday as a low tracks southeast out of the Tennessee Valley. Inflow off the Atlantic is strong on Sunday night and Monday. Upper jet supports good lift too on Saturday night and Sunday. Forecast precipitable water values are above normal but not excessively high, but will be watching the 1.25 to 1.5 inch values advecting into the area because this would increase the heavy rain threat. Any chance of thunder would be with elevated instability and lift above the wedge. Will be keeping maximum temperatures at or just below coolest guidance for Sunday and Monday. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 312 PM EDT Friday... A long wave upper trough will develop over the western and central United States Tuesday through Thursday. This will steer the weakened upper low up the East Coast. Wide spread in the long range guidance solutions after Thursday but overall looks like 500MB pattern is amplifying with troughing in the west and ridging in the east. By Tuesday the low will be off the southeast coast which will bring surface and low levels winds around to the northwest and therefore erode the wedge. Models have a cold front stalling through the central United States Wednesday through Friday, with the GFS on the east side of the differing solutions. Given the building southeast ridge at the end of next week WPC preference was closer the the 00Z ECMWF This keeps the Mid Atlantic region dry Tuesday night through Friday with above normal temperatures. && .AVIATION /23Z FRIDAY THROUGH WEDNESDAY/... As of 705 PM EDT Friday... Most locations still are experiencing VFR conditions outside of the isolated to scattered heavier showers and storms across the region. As the night progresses, look for a trend towards MVFR ceilings and then a mix of MVFR and IFR by tomorrow morning, with these lower flight levels continuing through the day. A cold front will stall across the area tonight, and a disturbances will head eastward along it during the course of tomorrow. Visibilities will be mainly VFR with pockets of MVFR associated with the heavier showers and storms and some late night and early morning patchy fog. Winds will be light and variable overnight into Saturday outside of the areas of stronger showers and storms. Anticipate gusty conditions along with the variable directions. Moderate confidence in ceilings, visibilities and winds during the taf period. Extended Aviation Discussion... Weather pattern active this weekend into Monday with periods of rain/showers, heavy at times. Should expect mostly sub-VFR condition when raining, but some VFR possible, especially north of a LWB-LYH line. Rain lingers into Monday as does lower cigs, and finally seeing some VFR returning Tuesday. Drier weather continues Wednesday into Thursday. && .HYDROLOGY... As of 300 PM EDT FRIDAY... After 4 to 5 days straight days this week with spotty rainfall mainly light across the area, we are still looking at the possibility of a more significant hydro event this weekend. Despite the semi-wet week, antecedent conditions are fairly dry across the eastern 2/3 of the CWA and generally about normal in the west. Moderate drought is still depicted on the U.S. Drought Monitor across much of the piedmont and Abnormally Dry over most of the Blue Ridge and surrounding area with near normal conditions in the west. This suggests that we can absorb more water than might be typical. 3- hour flash flood guidance from the River Forecast Centers ranges generally from about 1.5 inches in the west up to around 3 to 3.5 inches in the west, reflecting well the drier conditions east of the mountains. A slight risk for convective rainfall exceeding flash flood guidance was issued by WPC earlier for Day 2 and Day 3 (through 12z Monday). Pockets of minor advisory-type flooding cannot be ruled out in convective storms which will be more likely today and Saturday and further south in the CWA where instability may be much higher. Any training convection over the same basins could produce pockets of flash flooding. At this point the river forecast is a purely QPF based. Model QPFs have shown some decent run-to-run consistency over the last few cycles providing more confidence that this will fulfill expectations. Current WPC QPF in the day 1-3 period (today through early Monday) period is generally about 2 to 4 inches, with the bulk of it falling Saturday and Sunday and current WFO grids are close to these numbers. The prolonged nature of the rainfall (48 to 72 hours) will lessen the risk of serious river flooding (and flash flooding) as runoff will be more spread out and less efficient. The two best St. Louis University CIPS analogs to this event are April 9-12, 2003 and March 28-30, 2010 both of which featured upper lows over the southeastern U.S. Both events resulted in minor to moderate river flooding on the Dan River but not on the New, James, upper Tennessee or upper Roanoke Rivers. Ensemble river forecasts from the GEFS ensemble are also highlighting the Dan River as the most likely to flood, suggesting up to a 30 percent chance for moderate flooding but the NAEFS has been consistently showing lower probabilities. The situation will be monitored closely on future shifts for a possible Flood Watch. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK NEAR TERM...DS/KK SHORT TERM...AMS LONG TERM...AMS AVIATION...DS/KK/WP HYDROLOGY...PC

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