Area Forecast Discussion
Issued by NWS Blacksburg, VA

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308 FXUS61 KRNK 200640 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 240 AM EDT Tue Mar 20 2018 .SYNOPSIS... Low pressure over the central Plains will cross east over the Tennessee Valley and central Appalachians on Tuesday. The low will then deepen off the coast and track northeast Wednesday and Thursday. High pressure will cover much of the eastern United States on Friday. && .NEAR TERM /THROUGH TODAY/... As of 940 PM EST Monday... Shield of rain/convection edging closer to the forecast area, and starting to see light rain as far east as Wytheville, while deeper convection remains over Tennessee into Alabama and northeast Georgia. Latest HRRR and RAP have good handle on this and look for area of rain to work from southwest to northeast through the forecast from late this evening into dawn Tuesday, with average rainfall amounts of a quarter to half inch. Stable layer seems to hold in place, but cannot rule out a rumble of thunder making into to the mountains of NC into far SW VA by midnight. Overall, think temps will stay elevated, but could drop toward the lower to mid 30s in the Alleghanys late, with some sleet possible, but nothing of note winter wise until Tuesday night. Previous early evening discussion... Made some changes to the precip forecast this evening as radar coverage and mesoanalysis showing lack of precip, and forcing, as area of subsidence passes by per upper vort exiting to the northeast. Should start to see a ramping up in rainfall after 10-11pm from the southwest as models bring shield of rain/convection from mid TN into Southwest VA, with better upper support pushing into the mountains as well. Previous discussion from early afternoon... Vorticity advection, isentropic lift and upslope forcing will all contribute to widespread rain across southwest Virginia, southeast West Virginia and northwest North Carolina tonight. Water vapor loop shows a short wave over the Tennessee Valley which will cross the central Appalachians tonight. Will be leaning toward the heavier side of the guidance for rainfall amounts. Clouds and warm air advection will limit temperature drop tonight. Will use a non diurnal temperature trend for tonight. Models continue to indicate the beginning of cooler temperatures advancing down the eastern slopes of the appalachians late Tuesday. Otherwise there may a be sleet or snow mixed in with the rain north of I-64 Tuesday. BUFKIT forecast soundings keep Hot Springs rain until afternoon with a slow transition to snow in the afternoon. Surface temperatures may not drop below freezing during the day so any accumulation before dark will be minimal. && .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/... As of 330 PM EDT Monday... Guidance seems to have come into general agreement with the complex synoptic evolution of our midweek weather system. Several surges of short wave energy will carve out a broad, deep trof which culminates in a closed low moving across the Appalachians and central mid Atlantic region on Wednesday. The short wave energy rotating around the trof will drive development of low pressure that will move up the Atlantic coast Tuesday night and Wednesday as a surface reflection of the upper low lingers back to the west over the mountains, all while a wedge of high pressure lingers east of the mountains. As the initial wave Monday night departs to the east early Tuesday, expect some light lingering precipitation through early Tuesday afternoon. By Tuesday afternoon expect convection to develop on the western/southern periphery of the wedge as the upper trof begins to push over the region from the west. Convective elements will then ride up over the wedge as the complex structure of surface low pressure begins to develop. During this time also expect synoptically induced dynamic cooling to aid thermodynamic cooling and allow for a transition from rain to snow from north to south and from high to low elevations, with the possible exception of locations deeper into the NC piedmont. Snow will then continue through Wednesday with primary forcing from the closed upper low. Ptype will become complicated by diurnal effects of heating especially east of the Blue Ridge and at lower elevations and wet snow conditions may not be very efficient at accumulating. The event will gradually taper off as the upper low slowly pulls off to the east Wednesday afternoon/night and lingering upslope snow showers diminish by Thursday morning. The end result will be several inches of wet snow across the region Tuesday night through Wednesday with greater amounts west of the Blue Ridge and at higher elevations, and lower amounts further to the south and east. Currently leaning toward thermal profiles which would more favor sleet as a mixed ptype than freezing rain during the transition Tuesday night mainly north of Interstate 64 but this is a fine line and will be monitored closely. While confidence remains low in specifics, there is enough certainty in potentially impactful winter weather to warrant the issuance of a winter storm watch for locations along and west of the Blue Ridge from Tuesday night through Wednesday. This watch may be adjusted in timing and/or coverage, and some locations may eventually transition to a warning or advisory. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 330 PM EDT Monday... A large ridge being undercut by southern stream energy and moisture will move across the eastern US into the first part of next week. This will keep our weather active through Monday. After a brief break of fair weather on Friday, a warm front extending from low pressure in the midwest will begin to push precipitation back into the region Saturday. Temperature profiles point to the possibility of wintry precipitation especially from the Blue Ridge west through Saturday night. Solutions then start to diverge with the possibility of the frontal boundary and wedge lingering over the region with precipitation through the weekend, or possibly a brief break on Sunday before the next wave brings precipitation back for Monday. Will blend solutions to find a compromise, but the main point is to expect unsettled conditions into the first part of next week with temperatures generally below normal. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/... As of 200 AM EDT Tuesday... Rain will end by mid morning...then redevelop this evening and change to snow after midnight tonight. This is the result of an upper level low pressure area over the lower OH/TN valleys that will approach the mountains from the west today, then redevelop as a coastal storm tonight. The break in the precipitation today will allow visibilities to improve, but think cigs will continue to teeter with the IFR/MVFR category threshold. Change of p-type will occur around midnight tonight as colder air arrives from the north associated with the northeast wind and cold air advection on the back side of the developing coastal low. Heavy wet snow is likely later tonight from the Roanoke valley north and west into the mountains bordering WV/VA. Flight conditions will deteriorate rapidly tonight once the rain changes to snow with potential for LIFR after midnight and through the morning push Wednesday. Confidence is medium through the forecast period. Greatest uncertainty will be with forecast snow amounts tonight... the warm ground aiding in the melting process. Extended Aviation Discussion... As a low pressure system heads offshore later Wednesday winds will increase from the northwest and the snow will taper to flurries. Drier air should finally result in improving conditions Thursday as high pressure builds from the northwest. Another low pressure system will bring rain and the associated MVFR or lower conditions for Friday and Saturday. && .EQUIPMENT... As of 335 PM EDT Monday... The Mount Jefferson NWR remains off the air. The radio unit needed to repair the system is expected to arrive on Wednesday. The earliest this system will again be operational will be Wednesday afternoon. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Winter Storm Watch from late tonight through Wednesday evening for VAZ007-009>020-022>024. NC...Winter Storm Watch from late tonight through Wednesday evening for NCZ001-002-018. WV...Winter Storm Watch from late tonight through Wednesday evening for WVZ042>044-507-508. && $$ SYNOPSIS...AMS NEAR TERM...AMS/WP SHORT TERM...MBS LONG TERM...MBS AVIATION...PM EQUIPMENT...AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.