Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 202346 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 746 PM EDT Thu Apr 20 2017 .SYNOPSIS... High pressure over the Southeast States will move east and weaken this evening into tonight. A cold front will approach from the west tonight and enter the mountains Friday morning. This front will stall near the NC/VA border Saturday, with a secondary low moving across the area Sunday. This will keep us in a wetter pattern through the weekend. && .NEAR TERM /THROUGH FRIDAY/... As of 745 PM EDT Thursday...Surface lee trough, which in conjunction with moderate instability facilitated development of scattered to numerous showers and thunderstorms producing periodic instances of sub-severe hail/wind gusts up to 40 mph in stronger cores, has now shifted east of the central Virginia Piedmont. Surface pressure rises/weak ridging are now taking place across much of the forecast area and will continue to prevail through the early overnight. Cooling temperatures should allow for modest remnant instability to be reduced as well. Per upstream observations and recent HRRR/18z NAM runs, it`s not until a pre- frontal trough now sparking thunderstorms in southeast Ohio through parts of east- central Kentucky nears our area well after midnight when the chances for showers re- blossoms. Generally indicated diminishing shower coverage in the next hour or two, blending in the most recent HRRR in to the prior forecast. This keeps any remnant/rogue showers to western parts of Mercer, Summers and along/west of the Greenbrier Valley through midnight. As the Piedmont and Southside saw at least some showers and with dewpoints in the upper 50s to mid 60s common in these areas, developing shallow inversion may lead to areas of patchy mist or ground fog. Not thinking dense fog at this point as there is enough of a southwest flow to keep visibilities from falling too far. I`ve made no changes to lows at this point which look on track, but did try to better account for current temperature/dewpoint obs and near-term trends through midnight. Previous near term discussion issued at 400 PM follows... Scattered showers and thunderstorms have develop this afternoon in the unstable warm airmass. Surface boundary with associated surface trough acting as a trigger for storms with best coverage in Southeast West Virginia. The combination of orographic lift and instability creating convection along the Blue ridge mountains. The Day One convective outlook places the threat for severe weather to our northwest with the better instability closer to the cold front. An isolated strong storm may be possible this afternoon into this evening. Shaped pops this afternoon into tonight towards a blend of the HRRR and NAM. With the loss of solar heating this evening, expect showers and thunderstorms to dissipate. A cold front in the Ohio Valley will move east tonight and reach the Central Appalachians and Mid-MS Valley by Friday morning. Ahead and along the cold front, scattered showers and isolated thunderstorms will move into the mountains Friday morning. Low temperature tonight will range from the mid 50s in the mountains to the lower 60s in the Piedmont. A wet Friday is expected for our region as a cold front pushes east with scattered showers and thunderstorms. The Day 2 Convective Outlook has place the entire forecast area in a marginal threat for severe weather Friday. There is a severe weather potential with possible thunderstorm activity along/ahead of the southward advancing surface cold front. The main threat is for damaging winds and hail. High temperatures Friday will vary from the mid 60s in the northwest mountains to the mid 80s in the Piedmont. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... As of 400 PM EDT Thursday... The need for an umbrella or a raincoat may be an understatement this weekend. Potential for 1 to 1.5 inches of rain is high, with moderate confidence for 2 to 2.5 inches of rain from Friday night through Sunday night inclusive. A stalled surface front is forecast to bisect the Mid-Atlantic Region through the weekend, extending from the North Carolina Coast Saturday into an area of low Pressure over western Tennessee. This area of low pressure is forecast to slowly move east along the stalled front providing a focus for occasional showers and thunderstorms which may morph into an all out period of rain or periods of rain at times through the weekend. Our forecast area is expected to be on the cooler, north side of the front, thus resulting in cooler than normal temperatures, winds primarily out of the north or northeast. The only exception may be Saturday when strong southwest wind flow aloft across the mountains battles with the impinging northeast low level flow coming in from northern WV/VA resulting in potentially warmer conditions for the higher elevations and into far southwestern VA and NC. The more dense cooler air will eventually win out by Saturday night with the entire CWA succumbing to the cooler temperatures by Sunday. The severe weather threat at the present time looks marginal per abundant cloud cover which is forecast to limit instability. Of greater concern will be the potential for runoff issues per increasing confidence for repetitive rainfall and long duration which could linger into Monday. Model Median forecasts for some of the Rivers, such as the Dan River, are indicating at least 50 percent chance for area rivers to experience a rise of half bank to three quarters bank full, and a 30 percent chance for near bankfull or minor flooding. This introduces the potential need for a Flood threat headline at some point for the weekend, and will continue to highlight this in the Hazardous Weather Outlook. Folks that have plans that involve area streams and rivers this weekend should pay close attention to the forecast and potential changes which may occur with respect to the river stage forecasts. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 420 PM EDT Thursday... Low pressure will linger along the Mid Atlantic coast Monday, the models trending slower with its departure. This suggests Monday may turn out to be a wetter day than currently forecast with potential for clouds to linger into Tuesday before any sort of drying trend becomes established. For now will trend pops upward for Monday and hang onto some cloud cover for Tuesday. With an east or northeast wind, temperatures will be slow to recover until the sun returns for mid Week. A dry forecast is currently being entertained for Wednesday and and Thursday. A surface front is forecast to move into the Ohio Valley from the Great Lakes, but think this front will stall north of the area as High pressure amplifies over the southeastern CONUS. Temperatures Monday will be cooler than normal, trending closer to normal Tuesday, and then above normal for the second half of the week. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... As of 745 PM EDT Thursday... Earlier scattered to numerous showers and storms have dissipated and/or shifted eastward into eastern Virginia. This leaves the TAFs in VFR conditions that should continue through much of the overnight. Will be watching for development of IFR visibility fog at Lynchburg and Danville given recent rain and that dewpoint/moisture values are currently highest. Winds generally southwest 4-6 kts, lightest with eastern extent. A pre-frontal trough, and later a cold front, will focus additional showers into southeast West Virginia by 08-10z. Expect a deterioration in ceilings toward VFR/MVFR from west to east with increasing coverage of showers. Development of scattered thunderstorms mainly during the afternoon, though a rough timing would be toward the mid to late Friday afternoon hours per the 3-km NAM. Kept either VCSH and/or unrestricted showers due to low confidence at this time range. Winds southwest around 6-10 kts, veering westerly slightly during the afternoon. Extended Aviation Discussion... VFR to MVFR cigs into Friday night with this front. As front moves south Saturday morning, will see more coverage and this is especially true Sunday. During this time, expect cigs/vsbys to be highly variable but most of the time should be sub-VFR. Front moves east Sunday night, with some lingering sub-VFR possible over the mountains, but mainly VFR Monday with high pressure nosing in from the northeast. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK NEAR TERM...AL/KK SHORT TERM...PM LONG TERM...PM AVIATION...AL/KK/WP is the U.S. government's official web portal to all federal, state and local government web resources and services.