Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 141752 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 152 PM EDT Sat Apr 14 2018 .SYNOPSIS... High pressure offshore will continue to extend across the region today into tonight. A cold front will push east into the area by later Sunday possibly bringing strong storms with heavy rainfall to the region Sunday into Sunday night. Cooler than normal temperatures under gusty northwest winds are expected much of next week after the cold frontal passage. && .NEAR TERM /THROUGH TONIGHT/... As of 1005 AM EDT Saturday... Only minor changes to the forecast for today based on current conditions and trends in the Hi-Res guidance. Latest thinking still holds for the rest of the day. Upper ridging including surface high pressure offshore will remain dominant today with continued warm southwest flow in place. Some low level moisture should begin to advect into southwest sections early this morning before mixing out with mainly high/mid clouds expected to prevail for much of the day. Cant totally rule out an isolated shower popping up across the far west but too iffy to include much pop so basically keeping things dry/warm through the afternoon. Given such a warm start with many locations still in the 60s, and expected sunshine within the warm advection regime, appears will see highs on the warm end of guidance so bumped up temps a few degrees today. This should allow values to reach around 80 east and 70s west outside of the higher elevations. Front aloft will approach from the west overnight allowing a better veering profile to take shape with inclusion of a bit more low level southeast trajectory late ahead of this feature. Models again show moistening but mostly in the low levels where guidance has been overdone with moisture depth lately. Think best chances of showers will come after midnight but mostly southern Blue Ridge westward similar to a 00z GFS/ECMWF blend. Thus reduced pops some overnight as well with only isolated pops east and mid range chances along/west of the I-77 corridor. Otherwise should see increasing clouds, but still quite mild per mixing/clouds with lows mostly upper 50s to mid 60s. && .SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
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As of 425 AM EDT Saturday... Potential for Severe Thunderstorms and Heavy Rainfall Remains in Place for Sunday Afternoon and Evening... A deep increasingly negatively tilted upper low will rotate out of the Mid-South into the Mid-Atlantic through this period, bringing a notable threat of severe weather and heavy rainfall to the region. Very moist plume, on the order of 150mi wide with PW values in excess of 1.5", will surge inland up as far north as southwest/south central VA ahead of the front providing the buoyancy for deep convection. While forecast lapse rates are unsurprisingly weak given extensive cloud cover and pre-frontal rain showers, surface-based thunderstorms are expected ahead of the front as boundary-layer moisture will be seasonally high. Frontal convergence and large-scale forcing for ascent will generate an expanded corridor of frontal convection that should be ongoing at the start of the period. This activity will spread/develop into the Middle Atlantic region ahead of the ejecting short wave by afternoon/evening. Forecast soundings favor organized rotating updrafts but the predominant storm mode should be linearly forced along the cold front, essentially a strongly and narrowly focused QLCS. While it is not overly apparent from the pattern or the various model simulated radars that there will be much in the way of any discrete storms that evolve ahead of the line could exhibit supercell characteristics which would suggest at least some threat for tornadoes. In coordination with SPC, there appears to be at least a 5% tornado threat into the NC and VA Piedmont. Otherwise, damaging winds will be the primary concern with the squall line, and certainly any bowing segments supported by rear inflow notches/jets would likely deliver damaging wind gusts. The main threat will be south of Roanoke and southward toward Winston-Salem and points further south, then translate east to east-northeast to the Virginia coastal plain during the evening. Cities such as Martinsville, Reidsville, King, Caswell, Danville, South Boston, and even toward Appomattox could be the areas with the highest tornado threat, while damaging winds could affect much if not all of the CWA as the squall line/QLCS marches across the CWA, albeit more organized east of the Blue Ridge. Another concern with this strong weather system will be threat for training south to north individual shower/thunderstorm cells with heavy rainfall, that could lead to flooding or even flash flooding problems in certain locations. Western NC mountains/foothills into southeast VA toward Floyd and points east would appear to be the area with the greatest threat of such. Average rainfall amounts are expected in the 2 to 2.5 inch range, which given the fairly dry antecedent conditions should not be a major problem, except where cells train and rainfall amounts could potentially be double this. Overnight, the strongly negatively tilted upper low will begin to race off to the northeast and northern Mid-Atlantic quickly taking the threat of severe weather and heavy rainfall with it. Behind the front cold air will pour in the region on strengthening northwest winds. MOS guidance indicates at this point that a Wind Advisory will most likely be needed at some point in the Sunday - Tuesday time frame. Precipitation will taper to just showers and generally end east of the Blue Ridge. A ptype transition to a mix of rain/snow showers for the higher elevations west of the Blue Ridge, especially across the Alleghanys of eastern WV. Accumulations are expected to be an inch or less for the hilltops.
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&& .LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 500 AM EDT Saturday A relatively progressive upper air pattern is expected to evolve through the remainder of the week. A couple of additional upper lows are progged to impact the region, one around Thursday and another toward the weekend. Moisture is limited and the main storm track for these lows will be further north than the first system this weekend. However, most models do indicate some potential for showers with each of these systems, a better chance with the weekend one. With nominal cold air in place, these could again be snow showers in the mountains with minor accumulations mostly less than 1 inch possible in areas such as Western Greenbrier. The frequency of low pressure systems tracking across the U.S. will also keep conditions a bit on the windy side. Temperatures will generally be a bit below normal and somewhat on a roller coaster with the frequency of strong zonal systems tracking west to east across the U.S. Ahead of the cold fronts temperatures will be near to slightly above normal (e.g., Wed), while behind the cold front temperatures will be below normal with some light freezing temperatures possible mainly west of the Blue Ridge. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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As of 145 PM EDT Saturday... VFR conditions under high pressure will continue into the overnight hours. Some stratocu is moving into the areas east of the mountains this afternoon, with some VFR ceilings possible. Low level moisture will increase ahead of a frontal system that is approaching from the west. Most TAF sites will see ceilings drop to MVFR or potentially worse during the overnight hours into tomorrow. Also cannot rule out a chance of a stray shower overnight and tomorrow morning in the mountains, however most guidance suggests this activity will be restricted to the southwest of the TAF sites during the forecast period. Winds will remain gusty today until around sunset when a capping inversion sets up. Expect winds to stay strongest overnight around BLF where the strongest pressure gradient will exist. Winds should pick up again after daybreak at the other locations after sunrise however with uncertainty regarding how fast the temperatures will warm with prodigious cloud cover have only brought back in wind gusts at BLF. At the end of the TAF period, we will be seeing a line of potentially severe storms entering into the western boundary of the forecast area. If these storms have not yet reached BLF, confidence is increasing that they will shortly after the expiration of this TAF period. Confidence in sky conditions remaining VFR through the first half of the TAF period is high. Afterward, confidence in both the timing and category are low. Medium to high confidence in wind forecast. Extended Aviation Discussion... Isolated to scattered thunderstorms will be possible on Sunday afternoon, including strong to severe storms along a line late in the date through Sunday evening. Late Sunday night, winds become gusty from the northwest in the wake of the front. Flight conditions trend to VFR east of the Blue Ridge with lingering sub-VFR across the mountains into Monday. Gusty to very gusty northwest winds will continue into Monday night through Tuesday. Expect overall VFR on Wednesday with a gusty southwest wind likely ahead of yet another approaching cold front.
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&& .FIRE WEATHER... As of 330 AM EDT Saturday... Still no wetting until Sunday, which may come in the form of downpours of heavy rain associated with showers/thunderstorms from a strong cold front. Dewpoints will be significantly higher from this afternoon into Sunday giving the air a more summery feel with higher humidity. However could still see areas across the northwest where humidity levels remain below 35 percent and coincide with gusty winds to produce a low end enhanced fire weather threat. Thus will headline within the Fire Weather Forecast but run without an Enhanced fire danger statement for now. Winds will also remain gusty on Sunday, with some strong gusts with the frontal passage. Ventilation and dispersion will generally be good to excellent limiting smoke management issues. && .HYDROLOGY... As of 1015 AM EDT Saturday... Hydrologic ensembles from GEFS and North American (CMC-GFS) models continue to advertise rapid rises on nearly all the major rivers in southeast West Virginia, southwest Virginia and northwest North Carolina in response to the Sunday rain event. Most of the rises are within-bank to near Action stage with possible Minor flood stage on a few of the more susceptible locations, mainly the Dan River and lower Roanoke. As usual there is considerable run-to-run variation in the forecast rainfall amounts driving these forecasts but most are clustering around 2 inches storm total with the usual terrain enhancement effects which impacts the upper Dan River basin (VA Blue Ridge) most effectively. There also will be run-to-run variation in forecast river levels. Forecasts through Sunday morning will be using forecast rainfall amounts, then the forecast Monday morning will be largely based on observed rainfall amounts. Forecasts from Saturday morning for the New and Greenbrier are showing rises to just above action stage on the Greenbrier at Alderson. The James River with 48-hours of rainfall showed a sharp rise within-bank rise Sunday night and Monday, and takes Buchanan just above action stage. On the Roanoke and Dan Rivers sharp rises are forecast Sunday night and Monday, with Roanoke reaching action stage Monday. Danville and Paces on the Dan river will rise close to flood stage Monday night and South Boston is forecast to rise above flood stage Monday night, cresting on Tuesday. Then there is the National Water Model (NWM) which uses the single run QPF solutions from the GFS, the latest available from 06Z/2AM Saturday run which focuses most of the heavier rainfall east of the Blue Ridge. These simulations suggest the potential for flood stage to be exceeded along the Dan River and lower Roanoke (below Smith Mountain Lake) with rises to below bankfull along the James, New and upper Roanoke rivers. The response of some rivers in the NWM appears to be highly sensitive to QPF variations as the GFS. It remains to be seen how well the model performs with the actual rainfall which continues to be problematic with potential effects of upstream thunderstorms and local convection likely playing a role. In addition, the flash flood threat cannot be discounted as high rainfall rates are likely with convective elements but with a fairly progressive system it will take some persistent training to cause flash flooding. WPC forecast from early Saturday morning was showing a slight risk for excessive rainfall (exceeding flash flood guidance) on Sunday across much of the Appalachians. The 3-hour flash flood guidance currently ranges from about 1.75 inches in the wetter mountain areas up to 3 inches or more in parts of the drier VA piedmont where moderate drought conditions continue to exist. Given these fairly high numbers, some advisory level flooding such as ponding of water and minor runoff issues seems likely with this event, but not widespread flash flooding. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...AMS/JH SHORT TERM...RAB LONG TERM...MBS/RAB AVIATION...JH/JR FIRE WEATHER...JH/PM/WP HYDROLOGY...PC/RAB

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