Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 180004 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 804 PM EDT Thu Aug 17 2017 .SYNOPSIS... Warm, muggy conditions to continue tonight and into part of Friday ahead of a cold front. Scattered to numerous showers with scattered thunderstorms continue late this afternoon into early this evening. A few storms may produce localized downpours along with gusty winds. Another opportunity for scattered showers and thunderstorms, a couple of which may be strong east of the Blue Ridge. Still rather warm into the weekend, but with slightly less humidity. && .NEAR TERM /THROUGH FRIDAY/... As of 331 PM EDT Thursday... Very warm and humid out there this afternoon with feed of dewpoints in the upper 60s to mid 70s in place across the Appalachians and VA/NC Piedmont. With mostly sunny skies, atmosphere has destabilized with surface-based CAPEs anywhere from 2000 - 3000 J/kg. Regional radar shows widely scattered showers and thunderstorms from the Blue Ridge foothills to the Mtn Empire into southeast WV. Storms become more numerous in association with leading edge of 500 mb height falls and focused by a pre-frontal trough which extends from southeast OH through central WV to just northwest of Knoxville. Rest of this aftn/tonight: Shower and thunderstorm coverage should be primarily focused from Wilkesboro to Rocky Mount to Lynchburg westward, due to southeasterly orographic lift along the Blue Ridge and the pre-frontal trough. Isolated thunderstorms can`t be ruled out in the Southside and the upper NC Piedmont, where air mass is more unstable, mainly from activity drifting off the southern Blue Ridge. With precipitable water values from near 2" to 2.3", primary concern is for localized heavy downpours and potential short-fused hydro concerns in slow-moving cells. Dual-pol radar rainfall mosaic estimates small footprints of near 2" rainfall (1-1.5" hourly rate) from slow convection over eastern Watauga County. Not everywhere will see rates this high but the potential is there. Severe threat is minimal given poor vertical wind profile/shear and think mainly sub-severe/SPSable cells with gusty downdrafts should predominate. Given pool of better instability (MUCAPEs as much as 3200 J/kg) from Patrick-Pittsylvania counties southward to the NC Triad, that could be one area to watch for possible marginal severe. By 7-9 pm, guidance shows a general west-east decrease in shower coverage with waning instability and passage of pre-frontal trough. Leftover showers and storms could linger a couple hours after that point into the NC Piedmont/Southside VA. Should see brief clearing from the Blue Ridge west, and given background humid air mass that could lead to a good coverage of patchy fog. Eastern and southern counties may struggle to fully shake free of clouds overnight with patchy fog mainly in cloud breaks here, while enough southwest wind flow across the Mtn Empire and southeast WV to limit fog coverage. Warm, muggy lows from the upper 60s to mid 70s. Friday: Should have a good amount of early-day cloud cover in place east of the Blue Ridge, ahead of the primary cold front which 12z NAM/GFS progs to be near our western doorstep by sunrise. This is maybe a little sooner than recent guidance had indicated. Question for Friday is on convective coverage and strength given the earlier frontal timing and potential for morning cloudiness to slow destabilization in the east. Should have more limited chances for thunderstorms west of I-81 perhaps as soon as mid to late morning, but with an increasing trend as the front makes its eastward advance. The better chance for strong to localized severe storms is to the northeast in northern VA where deeper shear and strongest lift is located, but a corridor of stronger storms could exist from Amherst County to Surry County eastward. Heavy downpours certainly possible along with localized gusty winds in strongest cells. Front should be clearing the forecast area by mid to late afternoon, with notably drier air filtering in on northwest flow. Until then, expect another warm and muggy day in the Piedmont (heat index values 95- 98). Earlier timing of frontal passage leads to lower highs in the upper 70s/lower 80s from I-81 westward, with highs mid/upper 80s east, though a few potential low 90s in the central VA/upper NC Piedmont. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/... As of 331 PM EDT Thursday... Cold front will cross east of the area Friday night taking most showers out of the region by late evening if not sooner as evidenced by progged lowering 850 mb moisture/theta-e early on. Thus only leaving in some low chance pops east of the Blue Ridge during the evening before clearing overnight. Also may see a few upslope induced showers far west behind the front but should be quite isolated with only a brief mention needed. Lows mostly 60s except lingering muggy low 70s southeast. Boundary will briefly stall just to the southeast Saturday ahead of a rather strong trailing upstream shortwave trough that will pivot across Saturday afternoon. This may cause some backing of the flow including a bit of a northward jog in moisture before better subsidence arrives behind this passing wave Saturday night. Given rather dry forecast soundings and weak northwest flow, appears can leave out pop mention for now, with mainly just scattered cumulus around. Upper flow will then flatten under slowly building heights on Sunday with surface high pressure building in from the northwest. Forecast profiles again indicate quite a bit of dry air aloft that should again limit shower chances. However with moisture nearby to the south and the low level flow turning more southeast in the afternoon, cant totally rule out isolated convection across the far south or southwest where guidance shows some return instability into Sunday evening. Otherwise partly to mostly sunny at this point. Will remain quite warm/hot through the weekend given little change in temps aloft behind the front and weak downslope flow until late Sunday. Should be a little less humid Saturday before becoming muggy again later Sunday with highs both days in the 80s mountains to lower 90s piedmont. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 331 PM EDT Thursday... Flat upper ridging will remain in place to start next week as surface high pressure shifts from the Mid Atlantic on Monday to off the southeast coast by midweek. This along with warming aloft within a west to southwest trajectory should keep things quite warm and increasingly humid from later Monday into Wednesday. Convective chances early on appear iffy with only faint forcing later Monday when higher PWATS begin to advect back north, followed by gradual height falls on Tuesday when deeper moisture will be somewhat back in place. Most guidance showing only isolated showers/storms mainly south/east late Monday with perhaps a scattering of storms Tuesday espcly where aided by orographics. Highs likely around 90 east Monday/Tuesday and 80s elsewhere except possibly a little lower Tuesday far west pending clouds/showers. 500 mb pattern will once again become more amplified as another upper low crossing north of the Great Lakes helps dig out an even deeper eastern trough by the end of the period. This feature will also propel a strong cold front toward the area Wednesday with the boundary likely just south of the region during Thursday. Lead pre-frontal trough axis likely to spark better coverage of showers/storms Wednesday afternoon/evening, followed by cooler/drier air during Thursday. Therefore will include higher chance to low likely pops Wednesday and then mostly residual showers southern sections Thursday pending timing of the frontal exodus by then. Should see some cooling due to clouds including more showers Wednesday, then cool advection behind the front resulting in highs only 70s mountains to mid 80s east Thursday. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... As of 800 PM EDT Thursday... Somewhat complicated forecast through the period due to pre- frontal trough and associated convection moving through the forecast area now, and a new frontal system moving into the area from the west Friday. Would expect scattered convection to continue through about 02Z before generally dissipating. However, may not see all activity diminish due to very moist and warm/unstable air mass in place. Dissipating convection may reach western areas toward daybreak. Mostly VFR ceilings through the evening, outside MVFR ceilings in TSRA/SHRA activity. Overnight, with moist ground and late day rainfall in many spots, look for patchy dense fog, mainly impacting BCB and LWB, with patchy MVFR fog at other locations throughout the late night and early morning hours. A period of IFR-LIFR conditions are possible at BCB and LWB in particular, with MVFR elsewhere. Expect convection to redevelop and increase along the new frontal boundary as it moves through the CWA Friday. Best chance for widespread stronger convection will be across the Piedmont. Widespread MVFR ceilings are expected west of the Blue Ridge during the morning, with mostly VFR ceilings elsewhere outside of SHRA/TSRA activity. Winds will be mostly light and variable overnight, becoming WSW-W 4-8kts after daybreak Friday, with low end gusts at BLF during the afternoon. Medium confidence in ceilings throughout the TAF valid period. Medium confidence in visibilities throughout the TAF valid period. Low confidence in thunderstorm potential throughout the TAF valid period. Medium to high confidence in wind speed and direction throughout the TAF valid period. Extended Aviation Discussion... Weak high pressure will build into the area Saturday. Dry advection and light to moderate west to northwest winds behind the front Friday night could keep overnight fog coverage limited, but a potential sub- VFR ceiling is possible at Lewisburg and Bluefield. Mainly VFR conditions for the weekend under weak high pressure. Scattered thunderstorms and periods of MVFR conditions return next week, with late night/early morning fog possible almost any day. && .EQUIPMENT... As of 755 PM EDT Saturday Aug 12th... KFCX doppler radar will continue to be down for the rest of the month due to a failing bull gear. It will only be operational during this time frame for brief periods if an imminent significant widespread severe weather and/or hydrologic event occurs. Operating the system for any amount of time until the bull gear is replaced risks a catastrophic failure that would further extend system downtime. From the 17th through the 27th the radar will most likely be down completely as the repairs are expected to be made during this period. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AL NEAR TERM...AL SHORT TERM...JH LONG TERM...JH AVIATION...AL/RAB EQUIPMENT...RAB/WERT

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