Area Forecast Discussion
Issued by NWS Blacksburg, VA

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948 FXUS61 KRNK 171123 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 723 AM EDT Sat Mar 17 2018 .SYNOPSIS... A warm front will shift into the area from the southwest today, followed by a low pressure system moving across this evening. This system will move south into the South Carolina and Georgia by Sunday morning. Another complex storm system works in from the west next week, with wintry weather possible. && .NEAR TERM /THROUGH TONIGHT/...
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As of 700 AM EDT Saturday... HRRR has a good handle of area of showers moving into the Mountain Empire this morning, and should see activity spread northeast into the NRV and Alleghanys through 9-10am, but should also see dry air eat into somewhat, but at the same time this precip will moisten the low levels. Still some freezing rain possible early in the Highlands of VA into Greenbrier WV but overall no threat of any road issues. Still looks like this initial wave will shift off to the northeast while another pocket of energy with showers fades across the NC mountains, then will see stronger shortwave this afternoon which will bring better threat of showers and some thunderstorms for far southwest VA into NW NC. Previous discussion... Upper trough/low over Missouri this morning, will track east and become sheared out by late tonight over eastern NC, but the vort remains fairly strong. This morning we remain on the dry and north side of a warm front, with dewpoints in the teens. The regional radar shows light returns moving across the far southwest forecast area, but this is mainly mid level clouds. The main activity is further west from Indiana, south to near Clarksville TN. The 00z models overinitialized precip south of this convection, and with drier air in place and slower movement, curtailed pops to later in the morning, and keeping it dry in the southeast until late in the day. As far as ptype issues, could still see some light freezing rain or sleet in the Greenbrier Valley to Alleghanys later this morning, but slower onset after sunup looks to limit amounts even more, so no headlines or even a special weather statement needed for a 1-3 hour mixed light event, which should not impact travel. Forecast for the afternoon becomes a little more complicated with how far north the warm front tracks. With less precip to enhance in wedge, think the warmer air will surge a little further north this afternoon, putting temps into the 60s from the foothills along/south of the VA/NC border east into the piedmont, and as far north as possibly the Roanoke/Staunton River. Shear will be increasing as the shortwave enters the mountains this late this afternoon/evening, and along with some weak instability may present some gusty winds/hail scenario from the NC mountains, possibly as far north as the VA/WV border, southeast into the NC piedmont. Leaning toward a little more sunshine in the NC foothills today, before clouds increase with the convection. SPC still shows a marginal risk across our southwest 1/3 of the forecast area, as far north as a Lewisburg to Roanoke line, but feel storm threats will be slightly south of this line. Cloud cover and higher pops in the northwest today will keep highs in the mid 40s to lower 50s, roughly along/north of I-64, the north of Highway 60 east of the Blue Ridge. As better lift with shortwave arrives this evening increased pops to high chance to likely for much of the forecast area, then shifting the higher pops east of the forecast area after midnight as surface low moves to the coast. Drier air works in from the north late, but residual upper support and northerly low level flow may lock a few showers in place along the foothills/Blue ridge of Virginia, so kept a low chance here. Colder air swings back in late in the northwest, but most will have low temps in the mid 30s to around 40.
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&& .SHORT TERM /SUNDAY THROUGH TUESDAY/... As of 410 AM EDT Saturday... After some residual early-day showers across the southern third of the forecast area shift southward, overall Sunday is the nicer of the three days in the short-term period. Beyond Sunday night however, a wide range of potential outcomes remain on the table in what projects to be an active pattern. There is little consistency or consensus noted in the 00z NWP and ensemble suite on even synoptic-scale details. This leads to low forecast confidence, especially for the Monday night into Tuesday timeframe. Mostly sunny skies for Sunday after early showers end should help push high temperatures into the 50s in spite of light northerly flow and 850 mb temperatures between 0 and +2C. Into Sunday night, increasing isentropic ascent ahead of a warm front spreads a shield of cloudiness late in the evening. Until then, temperatures likely to fall areawide under some radiational cooling before rising into the early Monday morning hours. Looks like best chance for showers is south of Roanoke and more into the southern Blue Ridge/mountains of NW NC. Lows range from the low 30s in the Greenbrier Valley/Alleghany Highlands and upper 30s/low 40s elsewhere. Into Monday is where predictability starts going down the tubes, which linger into Tuesday as well. A mid-level shortwave of differing strength (NAM strongest, ECMWF is a middle ground while GFS is weakest/more open at 500 mb) and associated sub-990 surface low in the central Plains should be ongoing and progressing generally east. For Monday, I`ve sided toward the global model depictions which would spread warm advection-driven rain eastward into a lingering wedge. The operational NAM would introduce little rain this far north during the day on Monday until later Monday night when strong diffluence aloft and strong frontal convergence would force a weakening convective line coming out of the TN Valley. I view this as an outlier solution, but will show PoPs increasing into the Likely range Monday and high Likely to Categorical Monday night following the global models. What this means for Monday is a wet and chilly Monday with overcast skies and rain falling into a wedge, which supports high temperatures only in the 40s to low 50s. Lows in the mid 30s to mid 40s. The big question for late Monday night into Tuesday is the track of the progged surface low. While presently quite uncertain, this leads to differences in thermal profiles and precipitation type/amounts. The GFS tracks its surface low furthest north (from north-central TN across the VA/NC border), which supports to a milder/more rain solution until late Tuesday night. On the other hand the ECMWF is further south from across the Smokies into upstate SC and off the Carolina coast as a Miller B type system. This would lead to colder thermal profiles supporting a potential at least higher-elevation wet snow event for the southern Shenandoah/northern Blue Ridge into the Alleghany Highlands/Greenbrier mountains into Tuesday. Thicknesses are all above 540 mb so in a colder/more wintry solution this is not a situation where snow to liquid ratio would be high or even close to climatology. With temperatures being some 10 degrees different between the GFS and ECMWF, official forecast is essentially a middle ground but did defer somewhat to the colder ECMWF. This shows mostly rain except in the aforementioned higher elevation areas where rain/snow mix is offered. Opted for highs in the mid/upper 30s to the mid 40s. Colder temperature profiles may then result late Tuesday/Tuesday night as a second shortwave darting across the Gulf Coast from the southern Plains induces secondary low pressure development near the SC coast. There is plenty of room for significant change and best thing that can be said at this point is to stay tuned as a significant even small change would impact sensible weather in a big way. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... As of 410 AM EDT Saturday... 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 /11Z SATURDAY THROUGH WEDNESDAY/...
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As of 710 AM EDT Saturday... Will see a few showers impact the mountains this morning with fzra at LWB for an hour or two, though cigs/vsbys overall should stay VFR, but have tempo IFR vsbys/MVFR cigs at BLF due to heavier bands moving in. Otherwise, should see one area of rain/showers move across the west this morning, then somewhat of a break before better lift from low pressure arrives in the afternoon. Going with sub VFR conditions at BLF and LWB through the day, though BLF could rise above 3kft ahead of the approaching wave, before possible convection moves in after 20z. Once showers and low move overhead and into the piedmont this evening, suspect low level moisture is going to linger into the night with a light north to northeast flow, and models having low level moisture stuck in place. So, for most will see MVFR cigs/vsbys with potential IFR or lower with fog in the mountains. Confidence is medium to high on all parameters during the day valid time period but lower on vsbys after 00z Sun. 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.
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&& .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...WP SHORT TERM...AL LONG TERM...AL/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.