Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 240813 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 413 AM EDT Wed May 24 2017 .SYNOPSIS...
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Wedge of cool air across the region will gradually erode today as a strong wave of low pressure slides east, pushing a trailing cold front through the area this evening. An upper level area of low pressure will then cross the Mid-Atlantic region Thursday resulting in more showers and storms. Weak high pressure will follow this system for Friday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 310 AM EDT Wednesday... Brief reprieve from the rain will be short lived as a digging strong upper low across the Midwest comes in tandem with surface low pressure sliding through the Ohio Valley by this evening. This will push a trailing cold front toward the region today and to the east late tonight as the system occludes. Very strong southwest flow aloft ahead of the upper cold pool to aid a swath of upper diffluence that will arrive from the southwest this afternoon along a possible triple point secondary wave. This basically overtop the residual wedge will help redevelop strong isentropic lift that should result in another axis of showers initially before the actual pre-frontal convergence arrives. Strong dynamics and deepening southwest flow should also help erode the wedge as the residual front to the south lifts north. However just how far any surface based instability gets still iffy with potential to see much higher severe potential south/southwest zones with more elevated storms farther north, espcly north of BCB/ROA to LYH and points north. Models also a bit slower to push main axis of lift east which could allow more erosion of the cool pool with strong shear developing over a strong CAPE gradient just south of Highway 460 per latest Cam/Nam solutions. Other issue remains with rainfall given wet soils and possible higher rates later on given more convective nature. Thus plan to hoist a flash flood watch for a few counties along the VA/NC border and far southwest where flash flood guidance is quite low and any downpour resulting in an inch or more in an hour could cause problems. This may have to be expanded on a short term basis farther north during the day pending trends. Otherwise slowing down high pops a bit through early afternoon before slinging in categorical coverage from southwest to northeast into this evening. Highs again tricky with potential to see northern sections stay in the 60-65 range while southern and southwest sections zoom up into the 70s pending where the front ends up as the convection arrives. For now bumped up values a notch given a warmer start in spots. Should finally see the shield of showers/storms swing northeast and mostly out of the area after midnight as a dry slot wraps in from the southwest. However some wrap around behind the pivoting upper low to the northwest may keep showers going espcly northern sections through the overnight but more spotty in nature by then. Otherwise remaining mostly cloudy with showers diminishing and a bit cooler west with lows mainly in the 50s with some lower 60s southeast.
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&& .SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/... As of 330 PM EDT Tuesday... A slow moving area of upper level low pressure will head eastward through the Ohio Valley Wednesday night into Thursday. A substantial stream of moisture will be advecting into at least the eastern portion of the area early Wednesday evening. As the evening progresses, this moisture axis will shift eastward in concert with the approaching upper level trough axis. Anticipate very good coverage of moderate rain showers and scattered storms to give way to decreasing coverage by late evening, and even less coverage after midnight. The activity across the region after midnight will be associated with cold pool lapse rate with the passage of the trough axis. On Thursday, this trough axis will still be passing overhead, have daytime heating to help fuel additional showers and storms, all while its associated surface front heads east of the region. Expect good coverage of showers and storms during the day with west to northwest wind becoming gusty by the afternoon. Thursday night, northwest flow continues to increase on the backside of the system. Precipitation will quickly decrease in coverage east of the Blue Ridge thanks to increased subsidence. Upslope rain scattered rain showers are forecast across parts of southeast West Virginia during the overnight. Some of the gusts at the higher elevations will be on the order of 30 to 40 mph. On Friday, drier air will continue to work its way into the area, but lingering upslope showers in the west will continue through at least mid-day. Although, coverage will be on the decline. By Friday night, the winds will have weakened as the pressure gradient continues to weaken. Just as conditions trend drier, we will start to watch our next system developing in the mid- Mississippi Valley by late Friday night, with associated leading moisture reaching western parts of the area by sunrise Saturday morning. Temperatures during this portion of the forecast will start slightly below normal but trend to readings near normal by Friday. && .LONG TERM /SUNDAY THROUGH TUESDAY/... As of 330 AM EDT Tuesday... Saturday into Sunday, our weather will turn wet again as low pressure tracks through the Ohio Valley. In advance of this system, a warm front will lift through the region on Saturday, followed by an associated Sunday afternoon and evening. Guidance differs on how promptly this front will exit the region, along with any associated lingering showers. What is more uniform in solution is the position of another closed upper low. Anticipate a slow moving upper low just north of the Great Lakes region to linger through at least Tuesday. Energy pinwheeling around this feature will interact with the remnants of the cold front from the weekend. Additionally, a cold front in association with this upper low will be heading toward our area late in the day Tuesday. The result will be a weekend that will have the potential to be on the wet side, with Monday and Tuesday having scattered activity. Temperatures during this portion of the forecast will average a little over five degrees above normal. && .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... As of 1250 AM EDT Wednesday... Expect overall poor flying conditions to prevail at all TAF sites this period. Confidence is high that most if not all sites will stay sub-VFR with periods of IFR or worse through the period. Exceptions at times may be KBLF/KDAN Wednesday afternoon as wedge/stable layer shifts east with thunderstorms possible. At the moment will keep sites on the IFR side with some MVFR at times overnight, but not a big window. Otherwise rain should become more spotty overnight with a possible break in coverage to mainly drizzle/fog by daybreak on Wednesday. Next round of showers and possible storms to return by Wednesday afternoon from west to east with continued overall sub-VFR into the evening. Will keep VCTS mention across the south and west while edging east into KBCB although trends appear slower. Extended Aviation Discussion... Rainfall should become a bit more showery by Thursday with sub-VFR mainly associated with the showers while periods of afternoon VFR possible outside of the convection. Friday will be drier with a better chance of VFR ceilings and visibilities. Another front reaches the area for Saturday with more showers and thunderstorms including sub- VFR conditions into Sunday. && .HYDROLOGY...
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As of 408 AM EDT Wednesday... Rainfall amounts the past 12 hours have averaged 0.50 to 1 inch over most of the forecast area, with lower amounts in portions of the NC foothills, and amounts approaching 2 inches along the Blue Ridge near Blowing Rock, NC/Wilkes County, as well as the Greenbrier Valley. Next round of convection/heavier rains will occur this afternoon, with most models favoring the same areas to get the higher amounts, with some minor differences, but generally along and either side of the Blue Ridge southwest of Roanoke, east to southside VA into the NC piedmont. Given wet antecedent conditions in much of the southern and southwest sections, hoisting a Flash Flood watch given potential for higher rate rainfall over wet ground into this evening. This may need to be expanded north and east later pending trends.
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&& .RNK WATCHES/WARNINGS/ADVISORIES...
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VA...Flash Flood Watch from noon EDT today through this evening for VAZ007-009-010-012-015-016-032-043. NC...Flash Flood Watch from noon EDT today through this evening for NCZ001>006-018>020. WV...None.
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&& $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...RAB LONG TERM...DS/RAB AVIATION...JH/MBS/WP HYDROLOGY...JH/MBS

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