Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 140732 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 332 AM EDT Sat Apr 14 2018 .SYNOPSIS...
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High pressure offshore will continue to extend across the region today into tonight. A cold front will push east into the area by later Sunday possibly bringing strong storms with heavy rainfall to the region Sunday into Sunday night. Cooler than normal temperatures under gusty northwest winds are expected much of next week after cold frontal passage.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 300 AM EDT Saturday... Upper ridging including surface high pressure offshore will remain dominant today with continued warm southwest flow in place. Some low level moisture should begin to advect into southwest sections early this morning before mixing out with mainly high/mid clouds expected to prevail for much of the day. Cant totally rule out an isolated shower popping up across the far west but too iffy to include much pop so basically keeping things dry/warm through the afternoon. Given such a warm start with many locations still in the 60s, and expected sunshine within the warm advection regime, appears will see highs on the warm end of guidance so bumped up temps a few degrees today. This should allow values to reach around 80 east and 70s west outside of the higher elevations. Front aloft will approach from the west overnight allowing a better veering profile to take shape with inclusion of a bit more low level southeast trajectory late ahead of this feature. Models again show moistening but mostly in the low levels where guidance has been overdone with moisture depth lately. Think best chances of showers will come after midnight but mostly southern Blue Ridge westward similar to a 00z GFS/ECMWF blend. Thus reduced pops some overnight as well with only isolated pops east and mid range chances along/west of the I-77 corridor. Otherwise should see increasing clouds, but still quite mild per mixing/clouds with lows mostly upper 50s to mid 60s.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 150 PM EDT Friday... Active weather expected for the latter portion of the weekend. A closed low will move from the mid Mississippi valley into the Great Lakes region through the first part of next week. This will drive a strong cold front through the Appalachians and central mid Atlantic region late Sunday and Sunday night. This is a very dynamic system and will bring a threat for both severe weather and flooding. Southerly flow starts to amplify ahead of the approaching front and strong gusty winds will begin to affect the region from the mountains of NC into the higher elevations of southeast WV, and especially the Mountain Empire of VA Saturday night into Sunday. Precipitation will also be on the increase mainly in upslope flow along the Blue Ridge ahead of the front Saturday night into early Sunday, along with some rogue showers out across the piedmont. Guidance continues to advertise excellent deep layer moisture transport with GFS synthetic water vapor imagery showing a plume extending from deep in the Gulf and Caribbean and NAEFS precipitable water anomalies remaining close to 3 standard deviations above normal. Widespread rainfall around 1.5 inches seems reasonable with higher amounts of 2 to 3 inches possible along the Blue Ridge due to orographic effects even though the more southerly fetch will be less than perpendicular to the Ridge. Additionally, Hysplit backward trajectories continue to indicate that any convection along the front to our south will not be very efficient at inhibiting moisture transport, and the slower propagation of N-S oriented convection along/ahead of the front does not fit the conceptual research model favoring moisture cutoff so still have confidence in the beefy QPF numbers. Despite dry antecedent conditions, the expected QPF and potential for enhanced rainfall rates due to embedded convective elements and orographic effects may be enough to produce some localized flooding, and ensemble forecasts are now showing a couple of river forecast points rising into minor flood stage within the envelope of uncertainty. Will continue to highlight the hydro threat in the Hazardous Weather Outlook and wait to see if any refinement in the timing/location of expected heavy rainfall can be made before issuing flash/flood watches. Further details can be found in the hydrology section. The severe threat will be arriving in a high shear/low CAPE environment along and just ahead of the front. Expect activity to enter the region from the west Sunday evening, then be moving of of the region further east into the piedmont later Sunday night. Hodographs continue to look long and loopy with generous low level helicity values, and CAPE values have increased a bit. Expect the mode of convection will favor a wavy QLCS moving across the region ahead of the front with the potential for damaging wind and possibly brief spin-ups. While the best instability will generally reside east of the Blue Ridge, expect strong kinematic environment will allow for severe threat further west/northwest which will encompass locations west of the Ridge the as well. Unsure if any discrete cells will be able to form ahead of the main activity where there would be access to more instability. Will be watching this situation closely for any changes in timing or location of features that would impact the region. Behind the front cold air will pour in on strengthening northwest winds and expect a ptype transition to a mix of rain/snow for the higher elevations west of the Blue Ridge. Accumulations are expected to be an inch or less for the hilltops. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 150 PM EDT Friday... The upper pattern looks progressive through the end of the week. This should yield generally fair, albeit quite windy weather through Friday. Lingering upslope rain/snow showers will end Tuesday morning as high pressure briefly builds over the southeastern US. A fast moving low will move into the Great Lakes region on Wednesday, pushing a cold front into the mountains Wednesday with a chance for showers Wednesday afternoon into Wednesday night. Upslope rain/snow showers will linger across the high elevations west of the Blue Ridge through Thursday before high pressure returns to the eastern US on Friday. Temperatures will generally be a bit below normal, though a rebound to above normal is expected Wednesday ahead of the cold front. The very gusty conditions will make it feel notably cooler. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... As of 110 AM EDT Saturday... Overall VFR conditions will continue overnight into Saturday under mainly high clouds. Some low stratocu may move north to the NC-VA border in the early morning hours but this remains iffy and mainly supported off the latest Nam. Clouds look to be scattered out enough to continue forecasting for VFR, however some uncertainty does exist and occasional conditions for MVFR BKN cigs could be possible espcly east early this morning. Daytime heating should lift any cloud cover to above 3kft tomorrow. These clouds may also delay mixing, which will postpone gusty winds into the afternoon. Does appear that winds could again gust to 20-25 kts Saturday afternoon espcly over the mountains. Extended Aviation Discussion... Saturday night into Sunday, sub-VFR conditions will become more common with the approach and passage of a potent cold front. Isolated to scattered thunderstorms will be possible on Sunday. Late Sunday night, winds become gusty from the northwest in the wake of the front. Flight conditions trend to VFR east of the Blue Ridge with lingering sub-VFR across the mountains into Monday. Gusty to very gusty northwest winds will continue into Monday night through Tuesday. Expect overall VFR on Wednesday with a gusty southwest wind likely ahead of yet another approaching cold front. && .FIRE WEATHER...
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As of 330 AM EDT Saturday... Still no wetting until Sunday, which may come in the form of downpours of heavy rain associated with showers/thunderstorms from a strong cold front. Dewpoints will be significantly higher from this afternoon into Sunday giving the air a more summery feel with higher humidity. However could still see areas across the northwest where humidity levels remain below 35 percent and coincide with gusty winds to produce a low end enhanced fire weather threat. Thus will headline within the Fire Weather Forecast but run without an Enhanced fire danger statement for now. Winds will also remain gusty on Sunday, with some strong gusts with the frontal passage. Ventilation and dispersion will generally be good to excellent limiting smoke management issues.
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&& .HYDROLOGY... As of 150 PM EDT Friday... Hydrologic ensembles from GEFS and North American (CMC-GFS) models continue to advertise rapid rises on nearly all the major rivers in the CWA in response to the Sunday rain event. Most of the rises are within-bank to near Action stage with possible Minor flood stage on a few of the more susceptible locations, mainly the Dan River and lower Roanoke. As usual there is considerable run-to-run variation in the QPF driving these forecasts but most are clustering around 2 inches storm total with the usual terrain enhancement effects which hits the upper Dan River basin (VA Blue Ridge) most effectively. SREF ensembles are not yet capturing the event with QPF just beginning to appear in the model runs. The RFC deterministic forecasts available on the AHPS web pages are showing no rises in the Yadkin, Roanoke or Dan basins as the QPF falls outside the 48- hour window for SERFC. OHRFC forecasts (which are using QPF from the entire event) for the New and Greenbrier are showing rises to near flood stage on the New at Radford and just over flood stage on the Greenbrier at Alderson. The James River with only 48-hours of QPF shows a sharp rise within-banks right at the end of the forecast period. Later forecasts should clarify the effects of the forecast rainfall. Then there is the National Water Model (NWM) which uses the single run QPF solutions from the GFS, the latest available from 06z run which focuses most of the heavier rainfall east of the Blue Ridge. These simulations suggest the potential for flood stage to be exceeded along the Dan River and lower Roanoke (below Smith Mountain Lake) with rises to below bankfull along the James, New and upper Roanoke rivers. The response of some rivers in the NWM appears to be highly sensitive to QPF variations as the 00z GFS input drove stages along the Dan to levels which are very unlikely to be realized. It remains to be seen how well the model performs with the actual rainfall which continues to be problematic with potential CSTAR effects and local convection likely playing a role. In addition, the flash flood threat cannot be discounted as high rainfall rates are likely with convective elements but with a fairly progressive system it will take some persistent training to cause flash flooding. WPC is currently showing a slight risk for excessive rainfall (exceeding flash flood guidance) on Sunday across much of the CWA. The 3-hour flash flood guidance currently ranges from about 1.75 inches in the wetter mountain areas up to 3 inches or more in parts of the drier VA piedmont where moderate drought conditions continue to exist. Given these fairly high numbers, some advisory level flooding such as ponding of water and minor runoff issues seems likely with this event, but not flash flooding. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...MBS LONG TERM...MBS AVIATION...JH/RCS FIRE WEATHER...JH/PM/WP HYDROLOGY...PC

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