Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 150911 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 511 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/... As of 330 AM EDT Sunday... 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. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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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.
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As of 310 PM EDT Saturday 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.
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&& .AVIATION /09Z SUNDAY THROUGH THURSDAY/... As of 104 AM EDT Sunday... VFR conditions will continue to prevail tonight before low level moisture including sub-VFR cigs arrive from the south/west toward daybreak. Should be a mix of MVFR to VFR cigs after mid morning Sunday before the leading edge of showers moves into KBLF/KLWB in the 19-20z time frame. KBCB/KROA 20-21z, and KLYH/KDAN 21-23z. Thunder is possible but not expected to be widespread so maintained 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). Extended Aviation Discussion... Main area of heavier showers exits after 06z Monday in the east. Late Sunday night-Early Monday, 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. Next front arrives Wed night-Thu with chance of showers, with gusty winds and sub-VFR cigs 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... VA...Flash Flood Watch from noon EDT today through this evening for VAZ007-009-010-012-015-016. NC...Flash Flood Watch from noon EDT today through this evening for NCZ001>003-018>020. WV...Flash Flood Watch from noon EDT today through this evening for WVZ042>044-507-508. && $$ SYNOPSIS...JH NEAR TERM...JH/WP SHORT TERM...RAB LONG TERM...MBS/RAB AVIATION...JH/JR/WP HYDROLOGY...JH is the U.S. government's official web portal to all federal, state and local government web resources and services.