Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 200807 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 407 AM EDT Thu Apr 20 2017 .SYNOPSIS...
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High pressure weakens and shifts south of us today. A cold front will enter the mountains Friday morning, stalling 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.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 320 AM EDT Thursday... Fog and low clouds across most of the area this morning, more fog where it cleared Wednesday afternoon, which prompted a dense fog advisory. Visibilities are mainly in the 1 to 3 mile range but some areas near the Blue Ridge and I-81 corridor between Christiansburg and Marion have dense fog. Will let the advisory run til 8am. Patchy dense fog possible lee of the Blue Ridge but thicker low stratus is prevent fog from becoming as dense. Weakening high pressure and CAD erosion is taking place later this morning. Some low level moisture convergence in place will still result in a few showers early. As we head through the day will see clearing skies for most with buildup of Cumulus in the afternoon. Models hinting over the Blue Ridge more for development of showers and storms, as opposed to elsewhere, and given southwest flow appears will have scattered coverage most places with higher threat around the southern Blue Ridge from Boone to Floyd. Enough instability to throw in a few storms, but not expecting any severe threat. Will be a much warmer day with dewpoints in the 50s leading to a little more humid feel than recently. Will see highs range from the lower to mid 70s in the higher ridges, to upper 70s in the valleys west of the Blue Ridge, with lower to mid 80s east. Lack of forcing this evening will lead to dissipation of showers and storms. Cold front expected to stretch from western Ohio southwest to Oklahoma this evening then shift to the Central Appalachians and mid-MS Valley by Friday morning. Should see increasing chances of showers by dawn mainly west of a Tazewell to Lewisburg line, while a the rest stay fairly dry through most of the night. A mild night with lows in the mid 50s to around 60.
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&& .SHORT TERM /FRIDAY THROUGH SUNDAY NIGHT/...
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As of 330 AM EDT Thursday... A wet weekend is in store for the Appalachians and central mid Atlantic region. A sprawling cold front extending from low pressure moving through the Great Lakes region will move through the region on Friday. Dynamic forcing associated with the front is not great, but guidance shows some instability pooling ahead of the front and there is some shear present so expect showers and thunderstorms to develop with a few strong to possibly severe storms mainly south of I64. The front will then stall out and play a prominent role in our weather through Sunday as waves ripple along the front and it meanders back and forth across the area and interacts with a developing high pressure wedge. The first wave will push the front back to the north with strong isentropic lift as it runs into the developing wedge. Guidance shows a good amount of instability just outside of the wedge mainly west of Route 220 and south of I81. This will support deeper convection and enhanced shear in the vicinity of the frontal boundary looks to provide better support for the development of strong to possibly severe storms mainly south of the VA state line Saturday afternoon into Saturday evening. For Saturday night and Sunday, a vigorous upper trof will be digging into the southeast US and drive development of a low pressure system that will slide by to our south. The interaction of this low with the in-situ wedge will create strong isentropic lift with deep moisture to work with. This will develop a very soaking rain on top of precipitation that falls on Friday and Saturday, so hydro issues will become a concern as 3 day QPF values will be around 2 inches and embedded convective elements may produce locally higher amounts. As the low approaches the Atlantic coast it will finally take the frontal boundary with it and allow precipitation to taper off late Sunday into Sunday night. Temperatures ahead of the front on Friday will be well above normal with low to mid 80s east of the Blue Ridge, low to mid 70s west. However, with the developing wedge and low passing by to our south on Sunday, temperatures will cool considerably. By Sunday highs will only be in the mid to upper 60s east of the Ridge with mid 50s/around 60 degrees to the west.
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&& .LONG TERM /MONDAY THROUGH WEDNESDAY/...
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As of 250 PM EDT Wednesday... Not until Sunday night into early Monday morning does the axis of this upper trough/low move east of our area and allow for drier high pressure to enter our forecast picture. A dry forecast is predicted for Tuesday and Wednesday, along with a warming trend. Temperatures Monday will be around normal to slightly below normal, with readings Tuesday and Wednesday around normal to slightly above normal.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/... As of 127 AM EDT Thursday... Where it cleared Wednesday evening fog formed along with some low stratus. Further east in the wedge, clouds never cleared, but deeper moisture moved out enough to leave behind a low deck IFR/LIFR cigs from Roanoke to LYH/DAN Question will be how long fog lasts in the west, and if development occurs in LWB which is as of 05z, VFR. With some sct/bkn AC/SC/CU moving into WV/VA think will see some periods of MVFR tempo LIFR, but then BCB/LWB should stay IFR or worse in vsbys through 12-14z. Though vsbys will not be as low further east will be hard pressed to see ceilings rise above LIFR/IFR until after 12-14z. BLF will be the only site that looks to meander between VFR to IFR, mainly vsbys with some mixing occurring keeping low clouds and thicker fog from forming. Will see wedged erode by late Thursday morning with all sites VFR. Winds shift more toward the southwest to west and isolated to scattered showers are possible mainly along/west of the Blue Ridge. Since coverage will be widely scattered or less, no mention in the tafs at this point, but looks like BCB/ROA may have the best shot of seeing showers in vicinity. Extended Aviation Discussion... As cold front approaches Friday will see an uptick in shower and possibly thunderstorm coverage mainly in the mountains. Will keep VFR to MVFR cigs 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...Dense Fog Advisory until 8 AM EDT this morning for VAZ010>020. NC...Dense Fog Advisory until 8 AM EDT this morning for NCZ001-002- 018. WV...Dense Fog Advisory until 8 AM EDT this morning for WVZ042-044- 507. && $$ SYNOPSIS...WP NEAR TERM...WP SHORT TERM...MBS LONG TERM...DS AVIATION...AL/RCS/WP

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