Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 220757 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 357 AM EDT Sat Apr 22 2017 .SYNOPSIS... A frontal boundary will stall out across our area through Saturday with several waves of low pressure moving along it. Will see a wet pattern with periods of moderate to heavier showers through Monday morning. The front moves southeast by Sunday night into Monday, with high pressure building in Tuesday. && .NEAR TERM /THROUGH TONIGHT/... As of 320 AM EDT Saturday... Forecast still looking wet, but timing of front moving southward into NC has slowed and will have to wait for deep convection and any sfc wave to move east of the piedmont this afternoon to sink the front further south. With this in mind, have upped temperatures a little in the southern 2/3rds of the CWA, despite the fact that we will be mostly cloudy to overcast with occasional showers. With increase in temps, thunderstorm chance start to increase further north. CAMs showing some decent deep convection forming along/near the Blue Ridge by early afternoon thine shifting east to the piedmont by 21z. The Storm Prediction Center has bumped the marginal risk northward to give most of the CWA an isolated svr threat. Severe parameters point toward potential supercell/multicell structures given the frontal boundary and shear in place, though thermodynamics and shear are not that great, cannot rule out isolated wind damage threat, especially southeast toward Danville/Yanceyville. The bigger concern will be heavy rainfall. Fortunately most of the rain yesterday and this morning has not been constant. Heavier amounts did fall over the Mountain Empire into the NC mountains, and this will need to be watched later today. At the moment think enough of a break between this mornings activity which is light, and convection this afternoon to allow the rain to soak into the ground and cause any streams to recede. As convection forms, a flash flood watch may be needed if storms train any, though storm motion looks to be accelerated enough to prevent long duration heavy rainfall. Look for a southward shift in showers this evening, then as next upper trough moves east into West TN while sfc low shifts to east TN a gradual shift northward may take place though all of the CWA should see rain in this time frame. Any thunderstorms will be fading after early evening in the south. With wedge front sinking southward this evening, an uptick in northeast winds will occur, especially in the piedmont/foothills of VA. Lows tonight will be cooler in the mid 40s to lower 50s. Rainfall amounts today/tonight will average around 1 inch, with locally higher amounts likely to 1.5 to 2 inches possible. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/... As of 330 AM EDT Saturday... Guidance is in fairly good agreement continuing to advertise widespread significant rainfall through the weekend. A strong wedge of high pressure will remain in place as a slow moving upper closed low digs through the southeast US and drives low pressure through SC and GA Sunday into Monday. This will generate abundant isentropic lift as persistent low level southeasterly flow provides deep moisture transport over the top of the wedge. Multi-day QPF totals will be near 4 inches east of the Blue Ridge, with 2 to 3 inch amounts to the west. If this amount of rainfall occurs it will be sufficient to push area waterways out of their banks, but will hold off on any watches at this time until more exact timing and placement of heavy precipitation presents itself. The slow movement of the low as it spins off the coast will allow for only very slow improvement as precipitation gradually tapers off west to east Monday and Tuesday. Temperatures in the wedge will be a solid 10 to 20 degrees below normal on Sunday. Combined with a stiff northeast breeze and lots of rainfall, it will be feeling most unpleasant. Temperatures will slowly moderate through the first part of next week with readings approaching normal by Tuesday. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 312 AM EDT Saturday... A long wave upper trough will develop over the western and central United States Wednesday and Thursday. This will steer the remnants of our weekend storm system 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. 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 Wednesday through Friday with above normal temperatures. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... As of 140 AM EDT Saturday... Forecast will be of poor flying conditions, but this will not be for the entire taf period as models are hinting at slower frontal movement to the southeast thereby possibly keeping the DAN area and possibly LYH more into VFR this morning, with less rain coverage until this afternoon. Needless to say amendments are likely given off and on again rain. Thunder chances are there with slower movement and models are showing ROA/LYH/DAN and BCB as best overall for seeing thunderstorms in vicinity this afternoon. Could even see some stronger storms east of ROA. The front will slide southward tonight with winds shifting from southeast and southwest to northeast. The boundary overall will cause some wind shifts as will showers and storms, so again flying wx is poor despite a period of VFR at times. By Saturday evening, the models keep showers around with less thunderstorm threat, but show cigs sinking to IFR especially over all but BLF and possibly DAN. Will not sink it that far yet given rainfall may be enough to mix the lower cigs keeping them in the 1000-2000ft range. Extended Aviation Discussion... Weather pattern active into Monday with periods of rain/showers, heavy at times. Should expect mostly sub-VFR conditions 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...
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As of 330 AM EDT SATURDAY... After 4 to 5 days straight days this week with spotty mainly light rainfall 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 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 and portions of the Roanoke basins as the most likely to flood, suggesting a greater likelihood for moderate or higher flooding but the NAEFS has been consistently showing lower probabilities. The situation will be monitored closely on future shifts for a possible Flood Watch.
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&& .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...WP NEAR TERM...WP SHORT TERM...MBS LONG TERM...AMS/MBS AVIATION...DS/KK/WP HYDROLOGY...MBS/PC

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