Area Forecast Discussion
Issued by NWS Blacksburg, VA

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168 FXUS61 KRNK 300116 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 916 PM EDT Mon May 29 2017 .SYNOPSIS...
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A cold front will continue to push southeast of the area overnight, followed by another front entering from the northwest Tuesday afternoon. Yet another front passes across Wednesday, before high pressure builds overhead Thursday.
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&& .NEAR TERM /THROUGH TUESDAY/...
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As of 855 PM EDT Monday... Shallow convection continues to clip far southeast sections while northward advance has been limited by mid level dry air as seen off evening soundings and lower surface dewpoints per MSAS. Instability has also been rather weak over southern sections so appears given loss of heating that most added deeper convection will remain to the south ahead of the weak front now across southeast sections. Latest HRRR a bit more aggressive in holding showers in well after midnight espcly far southern third as the sheared wave aloft seen off the vapor loop crosses the region overnight. This so far has been overdone so leaning a bit more toward the drier Rap solution in keeping in some low pops southeast a couple hours after midnight as showers may tend to string out along the slow boundary. Otherwise not a lot of change with mainly cloudy skies per extensive mid deck, and lows 50s mountains to low/mid 60s east. Previous valid discussion as of 300 PM EDT Monday... Pops will remain on the low end early this afternoon, with meager convergence, however, a few showers are popping up and/or moving in from far SW VA, with some cu development over the Blue Ridge north of the James River. High-res models still content on keeping focus over NC and south today with storms becoming more robust by late afternoon along/south of I-40, but some should sneak into our southeast CWA by 23-01z, so higher pops to 40-60 look good from Yadkinville to South Boston. No big chances in the severe threat with best instability and shear over the piedmont of NC and east of Danville in VA, to points east and south. Drier air should filter into the low levels overnight west of the Blue Ridge, but expect slower removal of higher dewpoints to the east, as frontal boundary stalls from the Smokys/North Georgia to the coastal plains of NC. Seems we will keep some mid/high clouds across the piedmont overnight with a little clearing to the west. Lows tonight expected to be similar to this mornings, with mid to upper 50s west, to lower to mid 60s. Tuesday, not much change in the airmass, though slightly lower dewpoints are possible in the higher ridges. Southwest flow aloft and some weak vorts tracking along it may spark isolated convection but overall threat expected to be 20/30 pops at most in the afternoon. Highs will range from the mid to upper 70s mountains, to mid 80s east.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... As of 145 PM EDT Monday... On Tuesday evening, the passage of a weak surface trough out of the forecast area should allow for a brief period of dry weather for the overnight hours. With westerly low-level flow, the mid- Atlantic/central Appalachians region should remain removed from higher dewpoints over the central/eastern Carolinas. Temperatures should be fairly seasonable with lowering humidity levels into the 50s to near 60. Wednesday looks to be the most active day in the mid-week period. Low-level winds back slightly toward the southwest/south-southwest ahead of a potent mid-level shortwave and associated surface cold front over the Ohio Valley. These two features approach the central Appalachians late Wednesday into Wednesday evening. Should start Wednesday with mostly sunny skies but with increasing cumulus clouds due to steepening low-level lapse rates and modest moist advection as winds become more southwesterly. Frontal timing is a little slower than noted yesterday, moving across the region later Wednesday afternoon into the mid-evening hours. Relatively strong 500 mb jet of 60 kts contributes to unidirectional though strong speed shear with effective shear magnitudes of 50 kts. GFS and NAM are at odds on the degree of instability due mainly to potential daytime mixing of drier dewpoints. CAPEs range from 1500-1700 J/kg as modeled by 12z NAM using the most unstable parcel while the drier GFS comes in with CAPE values about half that much. Potential would exist for an organized scattered strong to severe thunderstorm risk if the NAM`s shear/instability parameter space proves correct, less so with the GFS. It is something that will need to carefully monitored. Current Day-3 severe convective outlook depicts a 5% severe/Marginal Risk from the Blue Ridge foothills westward, with general thunder further east. This seems appropriate given uncertainties as described, though the threat may be slightly higher if greater instability is realized. Behind the front, a strong shot of cooler and drier air then advects into the region late Wednesday night into Thursday. Surface ridge builds in from the Midwest allowing for the first real fully dry day experienced recently. This should knock temperatures down to highs in the mid 70s to lower 80s except cooler in the mountains but with notably drier conditions, and lows Thursday night in the 50s to near 60. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 100 PM EDT Monday... Forecast area then becomes embedded in a zonal, confluent mid-level flow for a large part of late week into the weekend. This is modulated by two primary 500 mb features: a persistent mid/upper- level low and cyclonic flow over the Northeast/Great Lakes, and increasing troughing over the Four Corners leading to greater southwest flow and warmer/more humid conditions from the Plains into the Tennessee Valley. Warmer and increasing humidity levels can be expected through the upcoming weekend, along with a return to a wet weather pattern in a general sense. Timing weak shortwave troughs/vorticity maxima in the confluent flow is difficult; and as such, I`ve kept chances for PoPs lingering through the upcoming weekend into early next week. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... As of 645 PM EDT Monday... Weak front over the area along with the arrival of a wave aloft from the southwest will bring periods of mid deck along with sct/bkn cumulus across southern sections into late this evening. This should make for mainly VFR cigs with potential MVFR around KDAN if any convection to the south lifts farther north. Will keep in a VCTS/tempo mix for shra/tsra at KDAN through at least midnight as this point. Otherwise rest of terminals should stay rainfree, though a light shower/sprinkle cannot be ruled out, but nothing to obscure vsbys or lower cigs below VFR. Latest guidance shifts most convection to the southeast well after midnight with lingering mid deck over the east and some high clouds over the west. Fog may become an issue mainly over the east/southeast where dewpoints remain higher and espcly around KDAN if some rainfall occurs. Also some late night valley fog possible over the west. Thus appears a period of MVFR/IFR in fog/stratus will be possible espcly at KLWB/KBCB/KDAN, but confidence remains low to medium of seeing much coverage. Any fog lifts around 13z/9am to VFR and some sct/bkn mid clouds Tuesday morning followed by sct/bkn cumulus in the afternoon. Another front expected by Tuesday afternoon appears to bring scattered to isolated threat of showers/storms to the area, but will be VFR overall. Extended Aviation Discussion... Overall VFR expected for mid and late week before showers and storms increase later Friday into Saturday. Exceptions will be any late night river fog, or shower/wet ground induced fog. Next threat of shower/storms Wednesday with yet another front and decent upper support, so will have to watch for stronger convection but appears any sub-VFR will be brief and mainly over the mountains. Thursday looks dry then more potential for scattered afternoon and evening storms Friday through the weekend with the best coverage on Saturday when more in the way of sub-VFR will be possible. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS/JH/WP NEAR TERM...JH/WP SHORT TERM...AL LONG TERM...AL AVIATION...AMS/JH/WP

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