Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 181400 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 1000 AM EDT Sun Mar 18 2018 .SYNOPSIS... High pressure over the region today will drift east as a complex storm system approaches our region from southwest. This system will initially bring rain to the region late tonight and Monday, but wintry precipitation is possible by Tuesday night and Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... As of 955 AM EDT Sunday... Visible satellite pics show the back edge of the low clouds now along Route 460 and heading south. Expect this trend with the aid of low level dry air advection per lower dewpoints to the north to continue to allow more sunshine from north to south this afternoon. However clouds may get hung up over the NC mountains given a weak leftover surface inverted trough, so will hold clouds longer over the far southwest into this afternoon. Otherwise expecting any other leftover clouds trapped beneath the observed inversion off morning soundings to fade as brief subsidence strengthens under weak high pressure. 85h temps along with increasing sunshine and light mixing support highs 50s to lower 60s espcly given heating of drier air by mid to late afternoon. Previous discussion as of 315 AM EDT Sunday... A robust upper level shortwave seen on GOES-E water vapor is just entering our forecast area early this morning. This wave and its associated UVM has helped generate scattered showers and even some isolated thunderstorms from Roanoke and points east. LAPS data and SPC mesoscale analysis indicated weak positive CAPE and DCAPE values near this region on north side of a narrow band of higher dew points. Kept a small chance for thunder in the south through 10Z. As this wave zips past our area this morning, subsidence and drier air arriving from the northeast should erode clouds from north to south during the day. This southward push of clearing near the Ohio River seen on satellite at 07Z matches well with most of the model guidance, which projects clearing into our area by midday into early afternoon. With limited mixing today and lower 1000-850mb thickness values forecast, leaned toward the cooler side of guidance for highs today. With a progressive upper level flow in place, low level moisture quickly returns tonight as low level winds back ahead of the next system. Good isentropic lift develops and the nose of an 850mb theta- e ridge arrives late in the southwest which will bring a return of stratus to the southern Blue Ridge and even the chance of light rain. Temps may fall quickly after sunset, then become steady overnight as clouds thicken. && .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... As of 415 AM EDT Sunday... Quite active weather pattern indicated in the 00z suite of deterministic and ensemble guidance for the Monday through Wednesday time period. Pattern aloft begins roughly zonal but becomes a highly- amplified trough and cold pocket of air aloft toward the middle of the week. The main highlights in this period include a rather wet warm frontal passage Monday into Monday night, and then a prolonged cold rain to accumulating wet snow event Tuesday night into Wednesday. Specific to the latter, guidance differences still exist on thermal structure, with a resulting great deal of uncertainty as it pertains to snow accumulations. Elevation changes, marginal temperatures and time-of-day are among other considerations in the snow or rain setup for around midweek. Initially potent, though deamplifying 500 mb shortwave and associated surface warm frontal feature progresses across the forecast area from the TN Valley Monday into Monday night. PoPs steadily increase to Categorical levels by late Monday areawide in association with the frontal system. Global models are steadfast in showing rather significant QPF into the Monday night timeframe, perhaps enhanced by frontogenetic forcing which skirts across northern North Carolina into southern Virginia. QPF values range from three quarters to an inch of rain, with the heaviest amounts from the foothills of the Blue Ridge into the Piedmont. Though 850 mb temperatures would argue for milder daytime temperatures than the official forecast, with lots of clouds and rain I sided temperatures on the colder side for highs and somewhat milder on the lows for Monday. Highs should top in the mid 40s to low 50s, with lows upper 30s to mid 40s. Tuesday should generally feature a lull in precipitation, in the wake of the first frontal system affecting the region from Monday. ECMWF is slowest with moving this shield of precip out, while the NAM and GFS show clearing early Tuesday. Did show a general decrease in PoPs first half of Tuesday, but majority of guidance shows in this lull develops a strong wedge with northerly/northeasterly flow drawing colder 850 mb temps southward. 850 mb temperatures should trend somewhere around -1 to +3C areawide, coolest further north in the southern Shenandoah/Highlands areas. Exactly how cold low-level temperatures become in this period then become critical to how the Tuesday night-Wednesday period play out. Kept highs only in the upper 30s to mid-40s on Tuesday under cloudy wedge conditions. Tuesday night into Wednesday is about when model guidance indicates a mean trough and cool pocket of air aloft develops over the region. Before this process occurs, another strong mid-level shortwave trough induces surface low development in the southern Carolinas. Nearly all guidance show an inverted trough and/or mid-level deformation band focusing a precipitation shield north/northwestward across the Blacksburg forecast area, which begins Tuesday night and lingers into Wednesday. PoPs though Tuesday night - Wednesday become Categorical again. Dynamic columnar cooling should be taking place throughout this period as well, which suggests a changeover from rain to wet snow at least in the Blue Ridge/Greenbrier mountains Tuesday night. As the column keeps cooling into Wednesday, rain may mix over to wet snow even in lower elevations pending surface temperatures. Have the higher confidence in elevation-based significant wet snow accumulations, but given accumulations fall over a long duration it`s not clear if warning criteria would be met in a 12- or 24-hour period of time. Could at least envision Advisory type snowfall across the higher elevations. Feel the best chance for at least some limited wet snow accumulation in the lower valleys and into the Piedmont would be Wednesday with 850 mb temperatures between -4 to -8C and isothermal temperature profiles just off the ground. Will therefore mention accumulating wet snow potential in the HWO. For temperatures, kept lows Tuesday evening in the upper 20s to low 30s, with Wednesday highs ranging from 31-37 degrees. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 415 AM EDT Sunday... As the closed upper low moves across the region synoptic forcing lingers along with the decreasing possibility for wet snow through early Thursday. Northwest flow will also be on the increase and will support upslope snow showers at the higher elevations west of the Blue Ridge into Thursday night. The upper trof axis finally pushes to our east and allows for a break of fair weather on Friday before a warm front starts to push precipitation back in our direction from the west late Friday night. Temperatures will start the period well below normal, then moderate a bit but remain on the cool side through the end of the week. $$ .AVIATION /08Z SUNDAY THROUGH THURSDAY/... As of 0730 AM EDT Sunday... MVFR to IFR ceilings at all TAF sites except LWB at 1130Z (0730 AM EDT) will improve to VFR conditions by midday as subsidence behind a departing upper level system continues to spread over the region. Wind will remain light through the day. VFR conditions will be fairly short-lived, however, as moisture ahead of an approaching storm system from the southwest will result in MVFR ceilings returning late tonight/early Monday morning for most of the mountain TAF sites. Extended Aviation Discussion... This approaching system should push eastward from the Plains on Monday. The warm front associated with this system could bring IFR/MVFR conditions due to ceilings and showers for Monday into Tuesday. As the low pressure system heads offshore on Tuesday night, precipitation wrapping along the western periphery of this system could bring a period of snow showers, MVFR/IFR conditions, and gusty north to northwest winds on Wednesday. Drier air should finally bring conditions back to VFR by Thursday as high pressure builds from the northwest. && .EQUIPMENT... As of 340 AM EDT Sunday... The Mount Jefferson NWR remains off the air and will likely be sometime next week before it is operational again. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...PH NEAR TERM...JH/PH SHORT TERM...AL LONG TERM...AL/MBS AVIATION...PH EQUIPMENT...JH is the U.S. government's official web portal to all federal, state and local government web resources and services.