Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 170006 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 806 PM EDT Fri Mar 16 2018 .SYNOPSIS... High pressure across the region this evening will weaken and give way to a warm front working into the area from the southwest toward morning. A low pressure system will move along this front Saturday into Saturday night, with the front sinking south into the South Carolina and Georgia by Sunday morning. Another storm system works in from the west early next week. && .NEAR TERM /THROUGH SATURDAY/... As of 715 PM EDT Friday... Our updated forecast will reflect little change through the evening hours as as cirrus slowly expand in coverage, and eventually start to thicken. The biggest change in the forecast overnight will be the period from late tonight into early Saturday morning. Given how dry the low level are across the region, and the latest guidance from both the 18z GFS/NAM and the latest runs of the RAP13 and HRRR, have delayed the onset of precipitation by roughly three hours across western sections of the area. This will result in a reduction of any wintry weather across the Alleghany Highlands as temperatures will have climbed slightly above the critical 32 F mark by the the time any measurable precipitation arrives. Brief trace amounts cannot be ruled out, but their timing and scope look to be extremely limited. As of 230 PM EDT Friday... Weak high pressure across the region will maintain mostly clear skies this evening with some increase in mid/high clouds through midnight. Moisture should then begin to ramp up as the flow aloft backs more southwest ahead of low pressure heading east out of the central states and north of an approaching warm front. However any precipitation associated with this lead swath of lift will encounter very dry air that remains in place from this afternoon. Latest guidance also shows much of this precipitation slower to reach the region with the majority holding off arrival until around 12z Saturday. This timing critical to possible light icing across the far northern valleys where cold air likely to be trapped from rapid cooling this evening. Current forecast soundings also suggest a quick jump in temps once clouds thicken, and if the rain is slower then little threat of icing as expect all to slide above freezing by mid morning Saturday if not sooner. Therefore since the window for mainly elevated icing appears small, wont hoist any headlines at this point, while maintaining some light ice mention around daybreak from the Greenbrier Valley east across Bath/Rockbridge counties. Otherwise mainly chance pops far west late with increasing clouds elsewhere including lows in the 20s/low 30s north to low/mid 30s south before all rise late. Warm front ahead of a trailing strong upper disturbance will lift northeast across parts of the south and west Saturday before getting shunted back south by late in the day. Guidance showing a rather disorganized area of rain working through the region into the afternoon with best coverage west and less east per increasing westerly flow aloft. This suggests high pops west and mainly chance east for a tenth to a quarter inch QPF. Models attempt to break southwest sections into the warm sector late in the day ahead of the main upper impulse with some instability along the nose of a strong 85h jet to the west. However the degree of leftover clouds and stability key to just how much at least elevated convection could develop with best chances of stronger storms along the southwest periphery where closer to the better upper support. Thus left in some afternoon thunder mention southwest third while trimming a bit southeast where should be more stable from earlier rainfall. Could have a large range in highs pending where the warm front ends up with readings running from 40s extreme north to low/mid 60s south. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... As of 330 PM EDT Friday... Shallow deamplifying ridge over the central part of the country will slowly slide eastward over the region through the weekend. This will push the cold front through the region Saturday night along with any associated shower/thunderstorm activity. As mentioned, convective parameters are marginal but any convection that does manage to form along the boundary Saturday afternoon will likely ride the boundary through the region early Saturday night with a continued threat for large hail/wind. Once the boundary is finally clear of the region high pressure will build over the mid Atlantic region with generally fair weather for Sunday, though weak southeast flow may be able to generate some sprinkles in the mountains. A short wave will undercut the ridge Sunday night with a surface low brushing by to our south, possibly close enough to bring some showers to the NC/VA border by daybreak. As the upper ridge axis crosses the region a warm front and short wave energy ahead of an approaching closed low will provide good forcing over a developing wedge to help spread the chance of showers further north through the region on Monday. By Monday night a full latitude trof will begin pushing in from the west with a good wedge signature at the surface. This will generate abundant isentropic lift over the wedge as a low moves through the Ohio valley, creating widespread rainfall across the Appalachians and central mid Atlantic. Temperatures will start the period near seasonal normals, but start trending cooler as wedge conditions become established. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 330 PM EDT Friday... A closed low will develop in a broad full latitude trof and move through the eastern US through late next week. This will develop another strong low along the mid Atlantic coast and keep temperatures well below normal through Friday. Tuesday looks to be rather wet especially in the east with good dynamic forcing over the wedge, and there may be an opportunity for some thunder outside the western periphery of the wedge on Tuesday afternoon but at this point confidence in convection is low so will not mention in the grids. Things get more interesting Tuesday night into Wednesday as the primary area of low pressure strengthens along the coast and pulls colder air down into the region. While confidence is not high this far out in the forecast, the scenario currently advertised by guidance brings the potential for a transition to wet snow Tuesday night through Wednesday with accumulations possible. As the low moves up the New England coast we will transition to upslope snow showers west of the Blue Ridge with additional snow accumulations possible into Friday morning. Definitely worth keeping an eye on later model runs to see how solutions evolve over the next few days. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... As of 745 PM EDT Friday... Clouds will gradually thicken and lower as the night progresses, with patchy light rain arriving across far western sections towards daybreak with the approach of a warm front. However, VFR conditions are expected into at least the mid-morning on Saturday. By mid- day, many area in the west will be experiencing a generous coverage of light rain with MVFR ceilings. Precipitation east of the mountains will still be limited with VFR conditions still prevailing. By the time we get to the afternoon, the precipitation will take on more of a convective nature, with showers likely across the mountains with scattered to isolated showers east of the crest of the Blue Ridge. Ceilings across the mountains will be IFR/MVFR with MVFR likely east of the mountains. Isolated to scattered thunderstorms will be possible by the mid to late afternoon mainly within an area south of a KBLF-KBCB line, and west of a KBCB-KMWK line. Extended Aviation Discussion... Should see an improvement Sunday as the front shifts south. A more organized low pressure system will track into the area from the central and southern Plains for Sunday night and Monday. This is expected to bring sub-VFR ceilings and visibilities and widespread rain. MVFR conditions are expected Monday night into Tuesday. As the low deepens near the coast, wrap around precipitation including mountain upslope snow showers will likely keep sub-VFR in place for much of Wednesday including strong north to northwest winds. && .EQUIPMENT... As of 940 AM EDT Friday... The Mount Jefferson NWR remains off the air and will likely be sometime next week before it is operational again. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH/WP NEAR TERM...DS/JH SHORT TERM...MBS LONG TERM...MBS AVIATION...DS/JH/WP EQUIPMENT...AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.