Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 202354 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 654 PM EST Fri Jan 20 2017 .SYNOPSIS... Weak surface high pressure will lead to a brief period of dry weather tonight. A warm frontal boundary situated across the Gulf Coast states starts to increase the moisture and rainfall again by Saturday night into Sunday. Periods of steady moderate to at times heavy rain are possible at various points this weekend, resulting in gradual within-bank river rises. The low with this system will move to Virginia by Monday. High pressure then builds into the region, with more seasonal temperatures and a period of gusty northwest winds Monday night into Tuesday. && .NEAR TERM /THROUGH SATURDAY/... As of 620 PM EST Friday... Main problem tonight will be forecasting where areas of fog develop or do they just remain as low clouds. Current cloud deck east of the Blue Ridge is around 500 feet. Locations under 500 feet may see some patchy fog, while others above 500 feet will be stuck with low visibilities and patchy drizzle. Skies were mostly clear before sunset across the mountains. Some high clouds will invade this evening, but think radiational cooling should allow valleys to reach cross-over temperatures with patchy dense fog developing during the pre-dawn hours. Previous forecast... For Tonight: In general, I think what we see is what we`ll get for tonight...that being continued cloudy skies, with patchy areas of mist/light fog scattered about. Most guidance maintains a saturated, shallow inversion around/below 900 mb per BUFKIT profiles that probably stands to get reinforced as southwest flow aloft increases temperatures in the low to mid- levels. Inversion should at least maintain broken to overcast cloudiness and result in only a small fall in temperatures from current values tonight. Given light wind conditions, there probably will be areas of patchy fog, and while some of the 4-km NAM/SREF visibility progs would argue for dense fog I`m not convinced that visibilities will drop all that low for a prolonged period. Prospect of fog is probably more limited along the Mountain Empire up into southwest West Virginia given that winds should be sufficiently strong enough to keep visibility from dropping much should any fog develop there. Kept temperatures mainly in the mid to upper 40s for tonight. For Saturday: Aforementioned inversion should hang tough until at least mid-morning, though when we can break free of the inversion is somewhat of a question mark. Once that happens, with 850 mb temps of around +9 to +11C, temperatures should rise more quickly into the 50s to near 60. In mid-levels though, we`ll start to see increasing 500 mb height falls starting by the afternoon. A returning warm front begins to increase precipitation chances from south to north during the afternoon hours. However the trend in most model guidance today has been to slow the speed at which the front and its QPF shield lifts north. So I`ve essentially produced a gradient in PoPs for the afternoon, with slight chance along a line from Bluefield to Roanoke to Appomattox and high chance to low Likely across the southside of Virginia into the North Carolina foothills and Piedmont. Modest elevated instability advects into our North Carolina counties late Saturday afternoon (SI`s nearing -1 and roughly 75 to 150 J/kg elevated CAPE); I couldn`t discount the idea of a rumble of thunder but it may be the exception vs the rule and areas in central/southern NC probably stands a better chance of seeing any thunder though early Saturday evening. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... As of 245 PM EST Friday... Complex low pressure system to evolve during the weekend into early next week as a very strong upper low across the southern Plains Saturday night slowly works east into the region on Monday. Should initially start out in a weak wedge Saturday night, in between a warm front to the south including a weak area of low pressure over the Carolinas, and weak ridging to the north. This has trended to slow down the onset of more widespread rainfall working up from the south on latest models with best insentropic lift more confined to possible convection along/south of the warm front. However as shortwave energy lifting north around the upstream upper system heads north, expecting an axis of light to moderate rainfall to work north into southern sections early Sunday and across the region during the day. Could even see some heavier rain southwest sections where a period of strong upslope may combine with the passage of the weak surface low to result in even some elevated thunder. Latest guidance even showing some weak instability and strong shear as the wedge erodes so included isolated thunder across much of the south/west Sunday. Appears a good inch or more of QPF possible from late Saturday night through Sunday with rainfall likely tapering from south to north by late Sunday. Otherwise mainly chance to low likely pops south Saturday night, then categorical Sunday all sections. Highs very trick on Sunday with potential to zoom well into the 60s south/west, while the north could linger closer to 50. Stacked upper system with associated surface reflection will likely slide into western sections Sunday night and then perhaps linger before shunting slowly east/northeast on Monday. However lots of uncertainty about how far north the system will get and where bands of heavier rainfall might set up by then. Does appear that another round of showers and even shallow convection will take shape by later Sunday night given strong progged upper diffluence, with focus now appearing over the west per latest EC/GFS mix. Therefore pushed higher pops farther west into Monday morning with another 1 to 2 inches possible before things taper off to only far western mountain showers by later Monday. This may be enough to result in localized flooding issues espcly Blue Ridge but given possible rounds of heavier rain vs a prolonged period, appears overall threat may be a bit less. However will still need to watch given wet soil conditions and possible primer heavy rainfall on Sunday. Highs Monday under the evolving cool pool mainly 45-50 west to mid 50s east at this point. && .LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 215 PM EST Friday... Very wet period to start the week will be winding down Monday night as the upper low begins to lift off to the northeast making for mainly wrap around/upslope driven precip into Tuesday morning. Colder air aloft will be trailing the main axis of showers overnight with any below freezing temps aloft not arriving until very early Tuesday morning. However the boundary layer still looks rather warm so thinking mostly liquid outside of the higher northwest elevations. Otherwise likely pops to initialize northwest sections Monday evening with mostly chance elsewhere, lowering to low pops western slopes by midday Tuesday and dry all other sections. Strong northwest jet in the wake of this system likely to push gusts to near advisory levels west overnight into early Tuesday across the higher elevations so may mention in the HWO. Highs Tuesday still above normal with 40s mountains to 50s east. Shortwave ridging develops later Tuesday and continues into Wednesday night with weak surface high pressure extending north into the region. This should result in a period of dry/mild weather for midweek with highs well above normal and likely 60s east of the mountains. This will begin to mark the end of the period of above normal temps and bring the reality of winter back as a longwave trough pushes southeast and across the region by the end of the week. Cold front preceding this upper trough will cross the region on Thursday which will be the transition day to colder air that arrives on Friday. Weak impulse sliding up ahead/along the boundary could produce a band of rain/showers by Thursday morning before things switch to a mainly mountain upslope snow shower regime under cold northwest flow within the cyclonic trajectory by Friday. Expect highs by then to only range from 30s west to low/mid 40s east under blustery northwest breezes. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... As of 645 PM EST Friday... This evening, shallow saturated subsidence inversion develops and stays in place through Saturday morning. This lends itself to relatively high confidence in prolonged degraded flight conditions through the overnight in prevailing IFR to VLIFR conditions for most of the TAFs (except mainly VFR at BLF) from stratus and fog/mist. Subsidence inversion begins to erode late Saturday morning which should allow for a slow improvement in flight categories through 18z Saturday. Rain begins to approach from the south very late in the TAF period associated with next frontal system, but unlikely to affect any of the TAFs through 18z Saturday. Extended Aviation Discussion... A slow-evolving but potent frontal system affecting a good part of the southeast CONUS will lead to a potentially long period of sub-VFR conditions this weekend due to low clouds/stratus and rain. Rain may at times be steady to heavy and may include a rumble or two of thunder, particularly Saturday night into Sunday. Precipitation begins to trend more showery/intermittent on Monday before trending VFR/MVFR for Tuesday. && .CLIMATE... As of 305 PM EST Thursday... Record warm Mins for January 21 Blacksburg....42 in 1954 Bluefield.....47 in 1999 Danville......56 in 1954 Lynchburg.....51 in 1927 Roanoke.......51 in 1959 Record highs for January 21 Blacksburg....59 in 1954 Bluefield.....60 in 1999 Danville......68 in 1959 Lynchburg.....72 in 1932 Roanoke.......74 in 1932 && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL/RCS SHORT TERM...JH LONG TERM...JH AVIATION...AL/RCS CLIMATE...AMS

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