Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 232336 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 736 PM EDT Sun Apr 23 2017 .SYNOPSIS... Expect wet weather at least through Monday night, with surface low in the Tennessee Valley slowly moving eastwards to the South Carolina coast by Tuesday morning, while upper low slowly traverse the same general area. && .NEAR TERM /THROUGH MONDAY/... As of 635 PM EDT Sunday... The forecast update for early this evening will reflect mainly one notable change. A gap of a few hours in the widespread rainfall, to more of a scattered coverage, is expected across the southeast portion of the area. This gap will translate northward from near the NC/SC border before additional precipitation fills the gap back in later in the evening. Have also made minor adjustments to the hourly temperatures based upon the latest observations and expected trends through the evening. Some rain-cooled areas across the Mountain Empire region of southwest Virginia are already cooler than the overnight forecast low. Have adjusted the forecast low accordingly. As of 327 PM EDT Sunday... Flood Watch continues for most of the region through Monday evening. Positively titled upper level trough will deepen as it pivots to the southeast with a closed mid level low eventually traveling across the Southeast States. This pivoting will result in increasing difluent mid and upper level flow and associated divergence to the northeast of the low. A dual upper level jet structure may enhance upper level divergence over western North Carolina this evening into Monday morning. Rain, heavy at times will spread northeast across the region this evening into tonight and continue into Monday. Moderate to heavy rain is expected in the flood watch area, with 1.5 to 3 inches likely through Monday night. Locally higher amounts likely near the Blue Ridge, south of Roanoke. Initially the northern edge of the rain shield is sharp and utilized the 16z HRRR which captured the rain placement on the WSR-88d the best, then used a blend of GFS and NAM. The Day 1 excessive rainfall outlook placed an moderate risk across southwest portions of forecast area. This is supported by the better lift, jet dynamics, upper difluence and terrain enhancement. Models are hinting at higher qpf to our south across North Carolina and South Carolina. This may be suggesting potential for elevated convection to our south. Not expecting any thunderstorms or deeper convection for our area with wedge in place, and this should keep hourly rainfall rates limited. However, long duration rains will create a flood threat especially for creeks/streams and rivers. The flash flood threat will be isolated but cannot rule out some pockets across the southern Blue Ridge this evening into tonight. In contrast, for late tonight, there is some potential for dry slot to work toward the Mountain Empire before the easterly fetch off the Atlantic pulls rain back to the west Monday. Complex forecast tonight, but models agree in general weather of cloudy and rain. Low temperatures tonight will range from around 40 degrees in the mountains to near 50 degrees in the Piedmont. Increased pops on Monday in the west as vertically stacked cyclone slowly pinwheel southeast. Timing looked a tad slower on the movement. Good moisture transport continues Monday with PWAT around 1.25" along/east of the Blue Ridge and into the Piedmont. Stable thermodynamic profiles owing to residual wedge should hinder any thunderstorms. Northeast to north winds especially in the east will increase in speeds Monday afternoon into Monday night. This is a result of the strengthening pressure gradient between high pressure over the Northeast and low center over the Carolinas. Played high temperatures a little cooler with rich cloud cover and rain with readings from the mid 40s in the mountains to the mid 50-s in the Piedmont. && .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/... As of 320 PM EDT Sunday... Wedge is gone on Monday night as surface and low level winds respond to approaching low. Bufkit guidance showed easterly flow and a saturated sounding 6 to 9 thousand feet deep. This strong upslope and Atlantic inflow will prolong widespread moderate to locally heavy rain. Axis of 850MB jet crosses northern North Carolina and southern Virginia Monday night and will be pointing toward Maryland and northern Virginia on Tuesday morning. Little variation in GFS/NAM forecast track and timing of surface and upper low. Upper low moves off the southeast coast Monday night then tracks northeast, very close to the coast, passing over the Outer Banks on Tuesday night. On Wednesday the 500 mb low is far enough away from Virginia that moisture wrapping around the west side of the system will be retreating from the Virginia, West Virginia and North Carolina. Will be slowing down the clearing on Tuesday. Similar to current forecast will have a low probability of showers in the Virginia piedmont on Tuesday night and a dry forecast for Wednesday. Stayed on the cooler side of guidance for maximum temperatures on Tuesday since more cloud cover is expected. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 300 PM EDT Sunday... 500MB heights rise Thursday and Friday over the southeast United States but with an overall deepening upper trough over the central United States. A deep closed low develops over the Rockies by the end of the week which will keep much of the east downstream in broad southwest flow. At the surface a low pressure system tracks into the Great Lakes. The front with this system stalls in the Ohio and Tennessee Valleys on Thursday and Friday. 850MB temperatures increase to as high as +18 on the ECMWF on Friday over southern Virginia and northern North Carolina. Will have Saturday and Sunday with continued above normal temperatures. && .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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As of 725 PM EDT Sunday... Only the area generally along and north of a KLWB-KLYH line remains VFR this evening. The remainder of the area is experiencing IFR/MVFR ceilings with some pockets of LIFR. Visibilities have been variable between VFR and MVFR. Through the overnight hours, light to moderate rain will be across the region along with primarily LIFR/IFR ceilings and VFR/MVFR visibilities thanks to the rain and light fog. Little change in prevailing conditions is expected through the day on Monday. Winds will generally be from the northeast across the central and eastern sections of the region at 5 to 10 kts, with western sections having more of an easterly wind and similar speeds. Higher elevations will have slightly higher prevailing winds with some gusts in the 20 to 25 kt range. Extended Aviation Discussion... Poor aviation conditions expected to continue through Monday night, and at times into Tuesday, thanks to the slow moving upper low and attendant sfc system. Look for gradual improvement sometime Tuesday from west to east with better flying wx for by midweek, though another front may bring scattered MVFR showers/storms toward late Thursday in the mountains. Drier weather will return for Friday.
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&& .HYDROLOGY... As of 300 PM EDT Sunday... No changes to the flood watch, with still some model differences in placement of highest rainfall threat. High-res models and analogs along with pattern recognition favor the southern Blue Ridge from Roanoke to Boone, east to the piedmont of VA/NC as those areas which appear to be in line to receive 2-3" through early Tuesday. This amount of rain combined with the 1 to 2+ inches that has already fallen since Friday should lead to hydro issues on rivers, as well as smaller creeks and streams. Looking at the analogs this pattern favors April 5th, 1993, and March 27th, 1993 which produced some minor to moderate flooding along portions of the Dan and Roanoke River mainly downstream of Danville and Altavista. Still will have to see how this sets up as we had been dry prior to this event. WPC has kept the moderate risk of FFG exceeded over the southern Blue Ridge. As mentioned in the near term, deep convection seems limited, thereby will have to rely on upslope component to enhance rainfall rates, so isolate flash flooding is possible, but more longer term >6hr flooding is more likely. Saturated ground may also result in downed trees and rock or mud slides. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flood Watch through Monday evening for VAZ007-009>018-022-023- 032>034-043>046-058-059. NC...Flood Watch through Monday evening for NCZ001>006-018>020. WV...Flood Watch through Monday evening for WVZ042. && $$ SYNOPSIS...KK/WP NEAR TERM...DS/KK SHORT TERM...AMS LONG TERM...AMS AVIATION...DS/KK/WP HYDROLOGY...KK/MBS/WP is the U.S. government's official web portal to all federal, state and local government web resources and services.