Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 241037 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 637 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 400 AM EDT Wednesday... Really only one mostly dry day this period as a very active weather pattern continues across the region. The first upper low that has kept the wet/stormy weather in place for the first half of the week will be located over the region Thursday. The wedge will be gone and well to the northeast leaving the forecast area in the warm sector. As the core of the upper low moves over the region with cold -20C or better 500mb temps providing steep 7C 1000-500mb lapse rates, any pockets of stronger surface heating will quickly yield instability thunderstorms and the potential for mainly marginal severe hail. SPC has outlooked the eastern portions of the CWA Thursday for the hail threat as indicated, but would not be surprised with later outlooks to see this expanded further west as really feel much of the CWA will be under the threat for strong discrete hail producing cells Thursday afternoon. However, other severe weather threats should be less of an issue and the threat for thunderstorms should diminish quickly with the loss of daytime heating. Rainfall issues should be less of a concern with lower PWATs and a more scattered nature to the activity overall. Friday has the potential to offer one dry day to the area, however, it may not be totally dry in all areas. While the first upper low will lift out of the area, another one is quickly on its heals digging southward through the Midwest. This upper low will take several days to get here in full form, but will result in increasing lift and instability as we progress through the weekend. This next upper low will not become as deep as the former one and thus result in a baroclinic zone/frontal boundary setting up across the area through the weekend serving as a focus for convection. More on that in a minute. For Friday, very brief upper ridging is indicated, however, convection developing across the Ohio Valley could well reach the western part of the CWA toward evening and especially during the overnight hours. All synoptic-scale models now indicating quite a bit of convection reaching the area before daybreak Saturday. So, have had to introduce pops late Friday and especially into the overnight hours. This may include thunder, especially in the west. Saturday, convection during the afternoon has the potential to become quite vigorous along/near the baroclinic zone. The GFS seems a bit overdone with LIs near -8C, CAPES in the 2000-3000 J/kg range, with other models showing such values about half of this. There is also decent shear across the area with upper diffluence also a factor with a broad upper ridge to our south. The potential for severe appears to be fairly decent on Saturday. Location may be the main issue, that is will it be mainly along and north of the I-64 corridor, or perhaps a tad further south. An average location would put the maximum core of severe weather basically between I-64 and U.S. 460. All threats of severe weather appear possible with the advertised scenario and flash flooding may also be a concern once again given the nature of strong convective cells and the extensive areas of saturated ground in our CWA at the current time. Convection would well continue into the evening Saturday across parts of the CWA. Temperatures Thursday will remain cool with scattered convection and abundant cloud cover under the cold core upper low. Look for highs in the 60s west to 70s east with lows in the 50s. Temperatures Fri-Sat will be notably warmer as 850mb temps surge toward +20C across the area. This very warm air will further fuel the potential severe storm environment Saturday afternoon. High temperatures Friday should be in the 70s west to the lower 80s east rising even further Saturday to the 80s across much of the forecast area, except 70s mountains. && .LONG TERM /SUNDAY THROUGH TUESDAY/... As of 430 AM EDT Wednesday... Continued unsettled and wet, unfortunately. The Midwest/Ohio Valley Upper Low will continue to slowly deepen and shift further east through the time frame, but remain upstream of the area. A potent short wave will move across the area Sunday. The baroclinic zone should be located near the NC/VA border as this occurs, so look for another good chance of widespread showers and thunderstorms once again. Instability will remain a factor and shear/dynamics will be even better than Saturday, so once again there is a continued threat for severe Sunday along with locally heavy rainfall. The axis of the upper low will remain west of the area through the period and beyond keeping an active weather pattern in place. Expect scattered to numerous showers and thunderstorms each day until potentially late week when the upper trough may shift far enough east to reduce the threat for such some. Temperatures will be seasonably warm with no wedges indicated and 850mb temps mostly +10C or better until after the trough moves east of the area late week. This will yield maximum temperatures mostly in the 70s west to 80s east with minimum temperatures mostly in the 50s mountains to the 60s Piedmont. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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As of 635 AM EDT Wednesday... Expect overall poor flying conditions to prevail at most 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 this afternoon as the ongoing wedge/stable layer shifts east with thunderstorms possible. At the moment will keep sites on the IFR side with some MVFR at times today. Next round of showers and possible storms to return by this afternoon from west to east with continued overall sub- VFR into tonight. Will keep VCTS mention across the south and west while edging east into KBCB although trends appear slower. Showers exit to the north after midnight with some lingering rainfall possible across northern/western sections late. However combination of developing stratus/fog once the rain tapers and upslope flow develops over the west to keep overall IFR in place until early Thursday. Extended Aviation Discussion... Rainfall should become a bit more showery by Thursday with sub-VFR mainly associated with the showers/thunderstorms 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.
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&& .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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