Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 252303 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 703 PM EDT Tue Jul 25 2017 .SYNOPSIS... A weak front across northern North Carolina will shift south of the area tonight into Wednesday before dissipating. High pressure north of the region should continue to ridge south across the area through Wednesday night keeping overall dry weather in place. Another cold front approaches from the northwest Thursday into Friday exiting to our southeast by Saturday morning. && .NEAR TERM /THROUGH WEDNESDAY/... As of 702 PM EDT Tuesday...No major changes needed to the going forecast. Did make a few minor adjustments though. First was to remove isolated/slight chance PoPs for northern NC tonight as it appears that in surface METARs that the inverted trough axis has shifted southward. Visible satellite imagery trends imply that deeper cumulus development has been few and far between in this area where residual instability is the greatest (though is not significant). Also increased sky cover some for the after- midnight period east of the Blue Ridge into the foothills in VA/NC. Recent RAP and 18Z NAM/GFS guidance continue to show increased low-level RH/stratus as 2-m AGL to 850 mb flow becomes light easterly with passage of backdoor front to our northeast. Coverage may be somewhat greater in the southern Blue Ridge versus along the southern Shenandoah Valley. Outside of that though, forecast appears to be in good shape with counties west of the Blue Ridge likely to see sharply falling temps with sunset and areas of patchy fog in the New, Holston and Greenbrier River Valleys. Lows in the upper 50s to middle 60s appear on track, though if stratus comes in sooner than presently anticipated, possible that lows may need to be adjusted up some in late-evening update. Previous near-term discussion issued at 215 PM follows... Surface front has drifted farther south this afternoon with dry advection having pushed into northern North Carolina per latest dewpoints. This has basically shifted instability to the south of the CWA with only the far southwest currently holding onto a bit more of a cumulus field. Appears best chance at seeing any isolated showers will be across the North Carolina mountains where a weak inverted trough lingers and expect a little more convergence as winds start to turn more easterly by early evening. However not more than a 20 pop at best with clear skies elsewhere this evening. High pressure to the north shifts east overnight allowing a little deeper easterly trajectory to develop by morning. Some guidance again trying to bring low level moisture back into the Blue Ridge espcly far south where dewpoints should be a little higher. Think too dry elsewhere as models remain too moist, so running with mostly clear to partly cloudy overnight with patchy fog/stratus around. Should be quite comfortable overnight with low dewpoints and better radiational cooling under the high allowing temps to fall well into the 60s most spots, with even some 50s possible in the valleys. Will start to see low level moisture start to slowly rebound on Wednesday as the high to the north shifts farther east resulting in a gradual veering of the flow to a more southerly component later in the afternoon. This seen via model jump in dewpoints and northward shift in progged PWATs from south to north during the day. However instability remains lacking as well as upper forcing with any convection likely due to weak southeasterly flow convergence and orographics. This again mainly over the southwest with ridging holding across the north and east so trimmed pops back to only isolated/low chances mainly southern Blue Ridge. Otherwise expect any low clouds to mix out to more sunshine by afternoon with highs cooler, mostly upper 70s to mid 80s, within a lingering east- southeast flow until late. && .SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/... As of 205 PM EDT Tuesday... Surface high pressure will weaken Wednesday night into Thursday as a cold front approaches from the west. This front is stuck in zone flow and will slowly move from the upper Midwest Wednesday night to the Ohio Valley Thursday. This front will pick up speed, moving over the mountains early Friday morning, then east of the Blue Ridge Friday afternoon as an upper level low tracks across the Great Lakes. Diurnal heating and orographical lift should generate isolated to scattered showers and thunderstorms across the mountains early Thursday afternoon. These storms may slowly drift over the foothills towards evening. As the front approaches the mountains early Friday morning, the chance for wide spread showers will increase. Despite these showers coming in during non-diurnal heating, linger instabilities and a 30-40 knot low level jet could allow some strong storms to pass over the area overnight. The chances for showers and thunderstorms will move east of the Blue Ridge Friday morning, exiting the piedmont by sunset. The best dynamics with this front will remain north over Pennsylvania and Maryland Thursday night then along the New England coast Friday. For the forecast area, the severe weather outlook for Friday is marginal with scattered strong storms to possibly a few severe cells across the foothills and piedmont counties. If the front is slower by 6 or more hours, the severe threat could increase to slight as this would place the frontal passage during peaking heating Friday afternoon. Even if the timing is slower, the best dynamics will remain to the north. Temperatures will be slightly warmer than normal Thursday with low to mid 80s west of the Blue Ridge and upper 80s to lower 90s east. Friday will be a lot cooler across the mountains with morning showers and a frontal passage. High temperatures will range from the mid 70s to near 80F across the mountains. Temperatures will warm to near normal east of the Blue Ridge with afternoon temperatures in the mid 80s. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... As of 240 PM EDT Tuesday... An upper level low will track from the Great Lakes Friday, then south over Virginia Saturday. Heating under this cold pool will bring scattered showers to the region Saturday afternoon. Temperatures will actually be cooler than normal for the first time since late June. Surface high pressure will build over the region starting Sunday. This ridge of high pressure will keep the area cool and dry into early next week. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... As of 702 PM EDT Tuesday... VFR through at least 06z Wednesday. Thereafter, potential for some degraded flight categories in two general areas: (1) In developing easterly stratus along and east of the Blue Ridge, affecting Lynchburg and the southern Blue Ridge after 06z. (2) MVFR to LIFR patchy river valley fog in the New and Greenbrier Valleys, affecting Lewisburg and Blacksburg between 09-12z. Confidence is moderate at best on stratus ceilings; expect at least BKN/OVC VFR which at times may drop to MVFR category. Indicated potential for lower ceilings as a SCT025 group. Confidence is higher on fog development and end time. If stratus ceilings develop as anticipated, they may linger until mid-morning. Light north winds become northeast 3-6 kts in the Piedmont and foothills, though become light and variable west of the Blue Ridge. For Wednesday, again some question on timing stratus scattering out but by 15z most areas should be VFR. Should see renewed VFR fair weather cumulus for the afternoon; recent guidance continues to show spotty showers later in the TAF period but feel these are overdone with large mid-level dry air in place. Thus will not mention in the TAF at this point. Winds mainly light east to southeast through Wednesday. Aviation Extended Discussion... Should start to see a front approaching with better threat of showers/thunderstorms Thursday into Friday. Outside of storms expect VFR conditions, with possible late night fog by Friday morning. Saturday looks drier with northwest flow, but could see lingering low end VFR/high end MVFR ceilings in SE WV along with a few added shra/tsra mainly southern sections Saturday afternoon. Expect widespread VFR to return Sunday under high pressure following the front. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...AL/JH SHORT TERM...RCS LONG TERM...RCS AVIATION...AL/JH

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