Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 212320 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 720 PM EDT Sat Oct 21 2017 .SYNOPSIS... High pressure will control our weather tonight into Sunday. A strong cold front will approach Sunday night and then cross the region Monday through Tuesday, bringing good coverage of showers and some gusty winds. High pressure returns to the area by the latter half of the upcoming week. && .NEAR TERM /THROUGH SUNDAY/... As of 700 PM EDT Saturday... High pressure at the surface and aloft will slowly slide east tonight into Sunday, Light winds and low level moisture will create patchy mountain and river valley fog late tonight into Sunday morning. Low temperatures tonight will range from the lower 40s in the mountains to around 50 degrees in the piedmont. The upper ridge will push out into the Atlantic ocean Sunday. The upper flow will become increasingly southerly as a deep trough develops across the Tennessee Valley in response to the combination of a southern stream upper low moving northeast from the Mid-South and a northern stream upper trough moving southward. Clouds will increase Sunday as Gulf Moisture is advected northward with increasing PVA/WAA aloft. The 850 mb temperatures will be around plus 12 to plus 16 on Sunday. Afternoon high temperatures will vary from the mid 60s in the mountains to the upper 70s in the Piedmont. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/... As of 400 PM EDT Saturday... This period of the forecast will feature unsettled weather with a pronounced change from dry to wet, yielding upwards of an inch of rain across the entire CWA with isolated higher amounts of close to 2 inches near the Blue Ridge Parkway. Latest model runs suggest the bulk of the QPF will fall withing a 12 hour time frame from 3PM Monday through 3AM Tuesday. The upper flow will become increasingly southerly/meridional as a deep trough develops across the TN Valley in response to the combination of a southern stream upper low moving northeast from the Mid-South and a northern stream upper trough moving southward from the north central United States. This will bring a marked deterioration in the weather Sunday night into Monday with clouds and rain spreading across the forecast area from southwest to northeast. Rain will initially be light Sunday night and early Monday, then increase in intensity Monday afternoon as the southern stream low lifts northeast toward the area, and culminating as a period of moderate to heavy rain as the trough passes overhead Monday night. Occasional heavy rain showers are expected with PWATS around 1.6, well above normal for late October, and strong PVA/dynamics as the southern stream upper trough takes on a negative tilt and begins to phase with the northern stream upper trough. QPF should average in the 1.0 to 2.0 inch range for much of the CWA. Only brief, minor flooding (poor drainage from leaves clogging ditches and drain pipes)problems are expected given the recent long dry spell, and relatively fast movement of the system. With respect to the thunderstorm threat, there is a small window of opportunity for thunderstorms as the upper trough passes overhead Monday night, mainly southeast of a line from UKF- ROA-LYH. During this time frame the combination of a strong 50+kt 850mb LLJ, upper diffluence, weak instability on the order of 100-400 J/kg CAPE, and notable shear will spread across the area in association with the LLJ and trough phasing. Still cannot completely rule out the potential for a QLCS and potential brief spinups within the line across mainly Piedmont/Southside areas. We are not currently outlooked, but this remains something that will need to be watched closely. Otherwise, the bulk of the heavier precipitation will shift east-northeast by Tuesday morning leaving scattered showers in its wake as the northern stream upper low digs southward into the area. A dry slot (drying aloft) may limit overall coverage of showers Tuesday and will make the temperature forecast a bit tricky (potentially warmer than forecast) as the colder air from the northern stream trough will not arrive in earnest until Tuesday night. Until the arrival of the northern stream trough Tuesday night, temperatures will remain mild, supported by rising dewpoints associated with the southern stream system. Passage of the of the southern stream trough will leave the forecast area within an area of neutral advection early Tuesday with Tuesday`s temperatures more of a reflection of whether we develop any breaks in the cloud cover from the dry slot. A more bonafide push of colder air will take place late Tuesday and Tuesday night with the passage of the northern stream trough. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 415 PM EDT Saturday... On Wednesday, the upper trough will begin to shift east of the region, but keep cold advection and clouds across the region. This will result in some continued threat for showers across mainly the Alleghanys southward into the western mountains of NC. Possibly early in the morning, low-level temperatures and thickness values will support a mix of rain/snow in the higher terrain, but nothing supports any accumulation or much QPF at this time. With regard to the remainder of the period, all of the extended models have trended much more progressive with the upper trough, lifting it out Thursday, west to southwest flow aloft returning to the area and allow temperatures to moderate quickly. Wednesday looks like it will be the coolest day of the week, then moderating for Thursday and Friday. Models are now hinting at another upper level long wave trough for the weekend with the potential for showers and bonafide cold frontal passage. The GFS brings clouds/showers into the area as early as Saturday whereas the ECMWF is 24 hours slower holding off on any appreciable QPF until Sunday. Attm will introduce low chance threat for showers Saturday, but nothing more until models can resolve the timing. && .AVIATION /23Z SATURDAY THROUGH THURSDAY/... As of 710 PM EDT Saturday... High pressure will remain across the area through the TAF valid period yielding primarily VFR conditions. The exception will be a few hours Sunday morning of some mountain and river valley IFR/MVFR fog. LIFR fog is possible at KLWB Sunday morning. Any sub-VFR conditions will quickly improve to VFR a couple of hours after sunrise. High confidence in ceilings, visibilities and winds during the TAF valid period. Extended Aviation Discussion... A deep upper trough and associated strong cold front will move into the region from the west by Monday with sub-VFR conditions in rain/showers along with increasing wind. Colder, blustery, unsettled weather with perhaps largely MVFR conditions across the mountains will linger through the later half of next week. MVFR showers may return by early next week. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...KK NEAR TERM...KK/RCS SHORT TERM...PM/RAB LONG TERM...PM/RAB AVIATION...KK/RCS

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