Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 241430 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1030 AM EDT Mon Apr 24 2017 .SYNOPSIS... Wet weather will persist into Tuesday, as a slow moving area of Low Pressure moves from the southern Appalachians to along the South Carolina coast by Tuesday morning. && .NEAR TERM /THROUGH TONIGHT/... AS of 1030 AM EDT Monday... Flood watch remain in effect until 8 pm this evening for much of the forecast area. Upper low over the southern Appalachians this morning is slowly moving to southeast and will push of the South Carolina coast by daybreak Tuesday. Flooding continues in western portions of Flood watch this morning, but the good news is that the rain has ended or is lighter. For this morning, the heaviest rain is located east of the Blue ridge mountains. Watching on latest radars, convection to our south lift north. The HRRR and NAM are pushing moisture northwestward this afternoon into tonight. With a deep southeast to easterly fetch of moisture into our forecast area, expected periods of rain to continue in the east. The rain will be heavy at times. While in the west, rain amounts will be lighter. Adjusted temperatures with latest surface obs and trended the late morning into this afternoon towards cooler GLAMP. With clouds and rain, expected little temperatures changes today. Adjusted pops and weather for latest radar trends this morning. Trimmed backed pops and intensities of rain in the west. More changes later this morning... As of 400 AM EDT Monday... No changes to the flood watch, with still some model differences in placement of highest rainfall threat.No change to Near Term Forecast. Rain...Heavy at times pretty much sums it up. Upper low over the southern Appalachians has ground to a halt, showing only little movement the last 12 hours and is forecast to move very little today per being cut-off from the mean upper level flow. Overall projection is for this feature to drift ever so slowly to the South Carolina coast by daybreak Tuesday. This will maintain a deep southeast to easterly fetch of moisture into our forecast area resulting in a persistent rain with periods of heavy rain. As such will maintain the Flood Watch for most of our forecast area along and east of the VA/WV border and into NC. As the rainfall continues to add up, expect streams to respond with small stream flood warnings morphing into larger scale river flood warnings. River Forecast Centers will be doing a lot of number crunching this morning, so watch for revised river forecasts and warning headlines. Rainfall today is expected to be the heaviest along and east of the Blue Ridge where the southeast upslope wind component will add orographic lift. Additional rainfall of 1 to 2 inches is likely (in some cases on top of the 2 to 4 inches that has already occured). Heaviest rain thus far has been along and just west of I-77 from Tazewell and Marion VA, south into Sparta and Wilkesboro NC. Focus today is expected to shift farther northeast into the headwaters of the Dan and Roanoke River basins. Temperatures today and tonight will change very little, cool northeast to easterly low level wind persisting until the low drifts off the coast Tuesday. && .SHORT TERM /TUESDAY THROUGH THURSDAY/... As of 400 AM EDT Monday... We`ll have one more day of cloudy and rainy/showery conditions on Tuesday. However, trend will be toward above-normal, more springlike temperatures looking ahead into the midweek period. Slow-moving, vertically-stacked cyclone should be roughly centered over the coastal NC/SC border based on general consensus reflected in 21z SREF mean/00z global model progs. Little significant change noted in the projected evolution of the cyclone, that being deamplificaton and gradual northeastward progress. Moisture plume on the eastern periphery of the cyclone, associated with PWAT values of up to 1.3" will continue to be drawn back westward, supporting a continued period of rain particularly for the central Virginia Piedmont and into the Southside of Virginia. Weak/nebulous forcing elsewhere across the CWA supports periods of light showers mainly during the first half of the day. As upper level low approaches the Chesapeake Bay and deeper moisture plume is shunted eastward/northeastward later in the day, PoPs then begin a taper to slight chance/lower Chance range areawide. An additional tenth to two-tenths of an inch of rain are possible in central Virginia associated with the most persistent rain band tapering to a few hundreths from the Blue Ridge foothills west. Thus a continued wet outlook but the steadiest and heaviest rains will have since ended. Aside from southeast West Virginia into western Smyth and Tazewell Counties in Virginia where at least some partial clearing is anticipated toward afternoon, for many it will continue to be another relatively dreary day with highs only in the 60s. Greater clearing anticipated on Tuesday evening with some radiational cooling under clearing skies in the upper 40s to mid 50s. Trend toward warmer temperatures remains on track starting Wednesday with building 500 mb heights and mostly sunny skies. Our 850 mb temperatures steadily rise Wednesday into Wednesday night to values around +14 to +16C. Much milder temperatures well into the 70s to low 80s anticipated for Wednesday, with lows Wednesday evening into the low 50s to around 60. These are some 10 to 15 degrees milder than 1980-2010 climatological normals. For Thursday, focus shifts to large-scale troughing evolving across the central Plains states and the ejection of a deamplifying mid- level trough embedded in deep south-southwest flow, its associated surface cold front expected to move from the Ohio Valley to the foothills of the western Appalachians Thursday. I`m hesitant to introduce slight to low Chance PoPs any earlier than Thursday afternoon given the amplified southerly mid-level flow that may act to slow the cold front down than currently depicted in most model guidance, and we also are capped to any convection out ahead of this. Generally expect mostly sunny skies through much of the day, albeit with increasing cloudiness later in the day toward Thursday evening. Even warmer conditions anticipated on Thursday; aside from the higher elevations, many areas stand a good shot at reaching the low to mid 80s particularly east where clouds will be slower to advance. These are a couple degrees cooler than MEX MOS guidance which would argue for some upper 80s values in the Piedmont and Southside. && .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... As of 300 PM EDT Sunday... 500MB heights rise Friday over the southeast United States but with an overall deepening upper trough over the central United States. A deep closed low develops over the Rockies by the end of the week which will keep much of the east downstream in broad southwest flow. At the surface a low pressure system tracks into the Great Lakes. The front with this system stalls in the Ohio and Tennessee Valleys brushing our far western counties Thursday night and early Friday. Thereafter, differences become apparent between the 00z GFS and ECMWF on how far north and east a warm front associated with the next developing Plains system extends. Generally maintained continuity with prior forecast for the upcoming weekend until these details can be better agreed upon. In any event, temperatures appear to remain above normal. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... As of 739 AM EDT Monday... Continued poor flying conditions through the TAF period mainly due to low ceilings predominantly into the IFR to MVFR range. Significant improvement in ceilings not anticipated until later Tuesday morning. Periods of rain, heavy at times with MVFR visibilities, will continue to spiral north- northeast. Coverage and intensity of rain should begin to diminish to a VFR visibility by tonight. Winds northeast 4-8 kts. By tonight, strong northeasterly low- level jet combined with inversion from persistent wedge will lead to low-level wind shear at most terminals overnight. Extended Aviation Discussion... Look for gradual improvement from west to east with better flying wx for by midweek, though another front may bring scattered MVFR showers/storms toward late Thursday in the mountains. Drier weather will return for Friday. && .HYDROLOGY... As of 300 PM EDT Sunday... No changes to the flood watch, with still some model differences in placement of highest rainfall threat. High-res models and analogs along with pattern recognition favor the southern Blue Ridge from Roanoke to Boone, east to the piedmont of VA/NC as those areas which appear to be in line to receive 2-3" through early Tuesday. This amount of rain combined with the 1 to 2+ inches that has already fallen since Friday should lead to hydro issues on rivers, as well as smaller creeks and streams. Looking at the analogs this pattern favors April 5th, 1993, and March 27th, 1993 which produced some minor to moderate flooding along portions of the Dan and Roanoke River mainly downstream of Danville and Altavista. Still will have to see how this sets up as we had been dry prior to this event. WPC has kept the risk of excessive rainfall along the Blue Ridge. Deep convection seems limited, thereby will have to rely on upslope component to enhance rainfall rates. Saturated ground may also result in downed trees and rock or mud slides. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flood Watch until 8 PM EDT this evening for VAZ007-009>018- 022>024-032>035-043>046-058-059. NC...Flood Watch until 8 PM EDT this evening for NCZ001>006-018>020. WV...Flood Watch until 8 PM EDT this evening for WVZ042. && $$ SYNOPSIS...PM NEAR TERM...KK/PM SHORT TERM...AL LONG TERM...AMS AVIATION...AL/PM HYDROLOGY...KK/MBS/WP

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