Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 240843 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 443 AM EDT Wed May 24 2017 .SYNOPSIS... Wedge of cool air will remain in place overnight as a weak wave of low pressure slides east and offshore along a frontal boundary just south of the area. A lull in the rain will occur early Wednesday, before another area of low pressure arrives Wednesday afternoon with more showers which may persist at times through Thursday. Weak high pressure will follow this system for Friday. && .NEAR TERM /THROUGH TONIGHT/... As of 200 AM EDT Wednesday... Quick update to let the going flood watch expire given exodus of more widespread rainfall. May still see a few bands produce a quick shot of moderate rain over the region overnight but not enough to cause flooding. Remainder unchanged for now. Previous discussion as of 1130 PM EDT Tuesday... Had some decent rainfall the past 6-12 hours but overall the flooding threat is still confined to the Dan River, as the rain rates have not been that high. Will keep the flood watch til 2am, as we are watching another area of convection over the Smokys that may sneak into SW VA/NW NC by 1am. However, all high-res models show this area dissipating to light showers or drizzle after 2am, so thinking the expiration of the watch at 2am looks reasonable. Otherwise, overnight we will see less moderate rain but enough low level moisture convergence with southeast flow to keep chancey pops around. Setup favors drizzle at times as well as some fog, especially over the ridges. Previous discussion from early evening... Forecast still looks rainy though some breaks developing in the piedmont though still drizzly/light rain. Shower more focused over the mountains this evening, with high-res models shifting this area northeast through midnight. Should see a drying trend from southwest to northeast by late evening/midnight though not completely dry, given depth of moisture. Will also have to watch upstream convection over Al/northern GA as it moves toward the NC mountains/foothills/piedmont after midnight. The high-res models weaken it, but not so sure this will occur. No changes to the flood watch, with minor flooding still possible though focus looks to shift more toward the mountains. Previous discussion from this afternoon... Radar imagery shows that the shield of precipitation associated with wave of low pressure sliding northward along the front lingering in the east has spread across the piedmont, with a secondary area of precipitation to the west advancing up the mountains. Guidance seems to be handling the wave in the piedmont fairly well, but not so much for the precipitation in the west. Given very wet antecedent conditions in the mountains, have expanded the flood watch to the western border. As dynamic forcing pulls off to the northeast overnight we will see a decrease in precipitation, though fully expect some degree of shower activity to be around through the night. The next wave will move out of the southern Appalachians as a large closed low digs southward into the lower Mississippi valley, with a trailing cold front not pushing through until Wednesday night. These systems will be running into a solid wedge east of the Appalachians, setting up another period of rain with good isentropic lift over the wedge. However, there are good dynamics in place and the wedge will be eroding, allowing instability to creep in from the west and south in s a highly sheared environment, opening a window for embedded thunderstorms late tomorrow afternoon with possibly linear organization to embedded convection heading into Wednesday night. The current day 2 SPC outlook for a marginal risk of severe across the south and west portions of our area looks right on target and the situation will be watched closely. Additionally, the expected precipitation tomorrow may warrant another flood watch if later guidance continues to indicate sufficient rainfall. Expect temperatures to fall slowly tonight and remain well below normal through Wednesday. && .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 /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... As of 1130 PM EDT Tuesday... 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. Appears threat of heavy rain will be over through the overnight, and convection upstream is progged to weaken as it moves into the stable airmass. Still the Dan River is in minor flood at South Boston, and is expected to reach minor flood stage at Danville and Paces. Next round of convection/heavier rains will occur Wednesday 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 south of Roanoke, east to southside VA into the NC piedmont. Flood or Flash Flood watches may be needed for these areas if models stay consistent. This will be decided overnight or early Wednesday morning. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...RAB LONG TERM...DS/RAB AVIATION...JH/MBS/WP HYDROLOGY...JH/MBS is the U.S. government's official web portal to all federal, state and local government web resources and services.