Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 151403 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1003 AM EDT Sun Apr 15 2018 .SYNOPSIS... High pressure offshore will give way to a strong cold front that will approach from the west today before passing across the region this evening, and offshore by early Monday. A line of showers and thunderstorms will precede the boundary later today into tonight before drier air arrives overnight. Much colder air on strong northwest winds including mountain snow or rain showers follows the front for early next week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 1000 AM EDT Sunday...A very active day in store for the Blacksburg forecast area especially this afternoon into the mid to late-evening hours, including the threat for severe thunderstorms and localized flooding. SPC`s 13z SWODY1 has expanded the area of Enhanced Risk across a larger extent of the Piedmont. This now includes a 30% probability of damaging winds with 25 miles of a point for the expanded area. The area in the Enhanced Risk shows the best overlay of greatest instability and very strong low and deep- layer vertical wind shear per the overnight SPC HREF guidance. I`ve tried to re- shape PoPs/Wx and severe weather attributes based on the latest SPC outlook, which meshes well with forecaster thinking. Since the wind shear is quite high areawide but forecast instability decreases with northwestern extent, did include severe thunderstorms with damaging winds areawide, but opted for Chance severe north/west of the Blue Ridge and Likely severe into the Piedmont and foothills. This roughly aligns with the SPC categorical risk areas. Forecast instability may only run from about 500 J/kg or less west of the Blue Ridge, but as temperatures reach the mid 70s values of 1000 J/kg MUCAPE may be realized in the Piedmont/foothills. The greatest severe thunderstorm risk continues to be from damaging straight line winds, but a few short-lived spin- up tornadoes can`t be ruled out as QLCS advances north- northeast. The most favored area for such is east of the Blue Ridge and into Southside/upper NC Piedmont where surface winds are already backed/southeasterly and where progged 0-1 km SRH values are progged to be greater than 200 m2/s2 per SPC HREF progs. Timing continues to remain around mid-afternoon (between 2-4 pm) in our southwestern corner of the CWA, between 5-9 PM in the Blue Ridge/southeast WV into the Blue Ridge foothills, and between 6-10 PM in central VA/Southside into the upper Piedmont. Due to the potential for high-impact weather, issued a general awareness Special Weather Statement to address the severe and hydrologic threats. Residents are asked to please keep close tabs on the weather for this afternoon and tonight. More to come later... Previous discussion from 330 AM follows... Very dynamic system still on track to affect the region mainly this afternoon and evening as a closed upper low becomes negatively tilted to the west while swinging a surface cold front in from the west overnight. Strong moisture transport along with deep shear and moisture convergence likely to lead to a double threat of severe and flooding before the main moisture axis exits east after midnight. Latest guidance continues to show a multiple band type scenario starting in the far west by early afternoon as a pre-frontal band of deep convection heads east reaching the Blue Ridge by late in the day. Short term solutions show this to be the main QLCS type feature including embedded bowing segments and possible discrete cells both within and just ahead of the main line. Second broken line likely to arrive with the actual front during the evening with another round of heavy rain possible. However some of the latest guidance has shifted the better severe potential east of the mountains where a little more instability likely with better rotational threat with perhaps a weak meso-low over southern sections per latest upgrade to an enhanced severe threat. Heavy rain also a concern given such high level to Pwats and possible training espcly west from a couple bands before lift shifts east of the ridges. This may be enough given potential higher rainfall rates southwest and moist soils northwest to prompt a bit higher flash flood threat. Thus hoisting a watch across the far west for this afternoon/evening. Does appear that convection passing to the south could initially limit northward moisture advection but think that the Cstar effect will be overcome as the flow aloft turns more south/southeast under the negatively tilted system. Appears FFG high enough to limit the flash flood threat out east but still will keep heavy rain mention. Otherwise slowed pops down to start this morning given current trends and very compact nature to the band of showers to the west. Then blended an area of categorical pops east with damaging wind wording espcly south/east this afternoon/evening. Highs tricky pending clouds and timing of showers with 60s/70s west and mostly 70s to near 80 east, where if get more heating then will see a much higher severe threat. 850 mb front punches in from the west overnight leading to drying out east and onset of upslope far west as temps aloft fall below 0C. However will take a while for the boundary layer to cool enough to support snow, so thinking only light accumulations on the ridges at best. Will turn blustery espcly west by morning as deepening cold advection kicks in with this perhaps pushing added showers east to the Blue Ridge late. Winds may be close to advisory levels late but since likely only on the far western ridges will keep added wind headlines out for now. Lows ranging from mid/upper 30s far west to around 50 southeast.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 445 AM EDT Sunday... The line of strong to severe thunderstorms will quickly shift off to the east of the region after midnight Sunday night, generally exiting the eastern part of the CWA Monday morning. Strong cold advection will ensue through the day Monday on the heels of increasingly gusty northwest winds. With the upper low and surface low becoming vertically stacked, it will deepen and move very slowly northeast away from the region. This will result in an extended period of gusty northwest to north winds that could last Wednesday, at which point an upper-level disturbance will moving toward the region from the west. MAV MOS guidance has been highlighting and extended period of 20+ sustained winds in the Tuesday through Wednesday period for several runs now, especially from Roanoke to Jefferson to Boone. A High Wind Warning may be needed in a few spots through this corridor, but a Wind Advisory certainly seems likely across much of the CWA west of the Blue Ridge. Another issue, mainly across western Greenbrier, but potentially areas further south as well toward Jefferson and Boone, may be candidates for a Winter Weather Advisory Tuesday. With the upper low deepening and slowing its forward progress and linger across PA Tue-Wed, this will bring a prolonged period of upslope northwest flow to the Alleghanys. Snow accumulations at the higher elevations may be in the 2-3 inch range with locally amounts up to 4 inches, with 1-2 inches possible south toward Boone over a two day period. A Winter Weather Advisory for part or all of this area, but especially western Greenbrier, is not out of the question at all. Temperatures are expected to be well below normal through the entire period with highs in the 40s mountains to the 50s Piedmont with lows in the 30s to lower 40s. With the frost/freeze program already in place across the Piedmont, we may need to consider a Freeze Advisory for parts of the Piedmont during this time frame. There appears right now to be too much wind for the need of any Frost Advisories. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 510 AM EDT Sunday... Active weather pattern expected to continue through the period with at least two potential active weather systems to impact the area during this time frame. The late weekend/early week deep vertically stacked upper low will continue to linger across the eastern Great Lakes and PA/NY into Wednesday before finally shifting slowly off toward the Canadian maritimes by Thursday. Meanwhile, a fast moving relatively zonal mid-level short wave will be tracking across the mid-latitudes and reaching the Ohio Valley late Thursday, quickly moving into the prior position of the early week upper low. Moisture appears limited with this system and remains mostly north of our area. Two concerns will be an increase in or renewed period of gusty winds as low pressure deepens across PA/NY with this secondary upper low and the possibility of more snow showers across the Alleghanys as this upper low skirts to our north. Nonetheless, most of the precipitation with this system should remain north of I-64 and points further north toward the WV/MD/PA border. Finally, a much stronger southern stream Pacific-based system will move through the south central U.S. during the weekend and reach the southeast U.S. by Sunday. This looks to be a much more potent system for our area and again has the potential to bring healthy rainfall, possibly thunderstorms, and perhaps even some threat of winter weather to western higher elevations. The pattern in place next week will leave much of the time frame with below normal temperatures with a brief warmup noted between the departing early week system and the fast moving system approaching late Wednesday. Look for lows in the 30s and 40s and highs in the 50s west to 60s east much of the week, or roughly about 10 degrees below normal. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... As of 710 AM EDT Sunday... Mix of MVFR to VFR cigs will prevail this morning with periods of low clouds including spotty showers pushing north ahead of the cold front to the west. Then expecting deteriorating conditions from west to east this afternoon as the leading band of showers/storms works east. Appears best timing for heavier rainfall arrival will be around KBLF/KLWB in the 19-20z time frame, KBCB/KROA 20-22z, and KLYH/KDAN 21-23z. Thunder is possible but not expected to be widespread so maintained heavier showers including VCTS at all sites for now. Gusty winds with the leading edge of showers/embedded storms may have a quick shift from south- southeast to west-southwest for less than an hour with gusts over 40 kts possible. Moderate to heavy showers expected as well, so mainly looking at sub-VFR visibilities with localized IFR within the heavier embedded storms along the line(s) through this evening. Expect the majority of the showers/storms to have exited eastern sections shortly after midnight with upslope driven rain/snow showers returning over the far west late tonight. Appears a return to VFR will be possible within downslope flow east of the mountains by early Monday with MVFR to IFR cigs lingering espcly KBLF/KLWB. Northwest winds to also become quite strong late tonight over the mountains behind the front with gusts to 25-30 kts possible by daybreak. Sub-VFR to persist across the southeast West Virginia corridor Monday with lingering snow/rain showers making for MVFR to IFR vsbys at times. Otherwise looking at mostly VFR elsewhere but under strong northwest winds that could gust to 25-35 kts over the region during the afternoon. Extended Aviation Discussion... Gusty northwest winds will continue Monday night through Tuesday along with mountain sub-VFR in low cigs espcly KBLF/KLWB. Finally looking at a return to overall VFR on Wednesday with a gusty southwest wind likely ahead of yet another approaching cold front. Next front arrives Wed night-Thu with chance of showers, with gusty winds and sub-VFR cigs again Thursday in the mountains. && .HYDROLOGY... As of 350 AM EDT Sunday... Main concern with possible training of multiple convective bands over the mountains this afternoon and early evening where guidance has between 1-3 inches of rain in a 6-8 hour window. Rainfall rates could also be better than a couple inches per hour if convection does indeed deepen enough especially to the south. Since this similar scenario has lead to flash flooding with past events, decided in coordination with surrounding offices to hoist a flash flood watch for parts of the west for this afternoon into early this evening when guidance indicates the best potential for heavy rain. Some concern for convection to perhaps limit moisture transport early on before the system negatively tilts allowing for deepening south/southeast flow to win out. However this only supported by limited number of solutions with the latest GFS/ECMWF more in support of the heaviest rain near the watch area. This could lead to some river flooding as well into early next week so something to watch pending locations of heavy rainfall and overall totals at this point. && .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Flash Flood Watch through this evening for VAZ007-009-010-012- 015-016. NC...Flash Flood Watch through this evening for NCZ001>003-018>020. WV...Flash Flood Watch through this evening for WVZ042>044-507-508.
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&& $$ SYNOPSIS...JH NEAR TERM...AL/JH SHORT TERM...RAB LONG TERM...RAB AVIATION...JH/WP HYDROLOGY...JH

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