Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 142325 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 725 PM EDT Sat Apr 14 2018 .SYNOPSIS... A deep low pressure system was over the Mid Mississippi Valley this afternoon with a warm front extending into Pennsylvania and a cold front trailing into Texas. This system will move east tonight and Sunday, pushing a strong cold front, and possible a leading squall line, through the Mid Atlantic region on Sunday. By Monday the low will be over the Great Lakes with colder air and gusty winds over our region. && .NEAR TERM /THROUGH SUNDAY/...
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As of 710 PM EDT Saturday... No major changes needed to the forecast for this warm Saturday evening. Will see clouds increase overnight with showers/rain breaking out along the southern Blue Ridge. Previous discussion from mid afternoon... Dry and breezy today will be the "calm before the storm" Sunday. Expect increased clouds tonight into tomorrow as a potent frontal system approaches from the west. A very deep upper level low will begin an approach toward the region with an increasing negative tilt. Ample moisture will be flowing into the region on an increasing south to southeast flow. Precipitable water values and integrated water vapor transport are exceptionally high. Strong, but below advisory level, southeast winds will be possible as the pressure gradient strengthens ahead of the surface low. A few stray showers early Sunday will be possible, mainly confined at and near the northwest mountains of NC. Guidance is beginning to come in to agreement that a potentially dangerous squall line will be entering the western boundary of the forecast area around 18Z/2PM Sunday ahead of the cold front. Lapse rates and CAPEs look to be rather weak ahead of this front, however a copious amount of shear will more than compensate. This line of storms will cross through the entirety of the forecast area, exiting through the eastern side shortly after 00Z. Damaging thunderstorm winds appear to be the main threat, however a couple of isolated tornados will also be possible. Behind the initial batch of storms a wide band of precip with additional embedded thunder will precede the main cold front adding an additional threat of flooding. Rainfall amounts during this time frame in augustness of 1-2" will be commonplace with possible much greater amounts if any of the storms begin training over a specific location and along the eastern side of the Blue Ridge where the upslope component will aide in enhancing rainfall amounts.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/... As of 310 PM EDT Saturday... The prefrontal squall line will be exiting the region to the east Sunday evening, but the actual cold front will just be entering the western portion of the region around 7PM, and will not move off to our east until around Midnight. The greatest severe threat will likely accompany the squall line ahead of the front, but we will not be entirely clear of the wind threat until the actual front passes, and heavy rain and hydro concerns will also accompany the actual frontal passage. Strong northwest flow will usher in much colder air behind the front with rain changing to snow at the higher elevation west of the Blue Ridge late Sunday night with no significant accumulation expected. The vertically stacked low will move slowly through New England before gradually pulling out and allowing for some ridging on Tuesday. This will allow strong gusty northwest winds to continue the flow of cold and moist air into the region, and keep upslope rain showers going west of the Blue Ridge through Monday and Monday night before gradually tapering off early Tuesday. The showers will be a mix of rain/snow at the higher elevations and become predominately wet snow Monday night as the cold air deepens, but any accumulations are expected to be under an inch. Conditions will gradually improve west of the Ridge through Tuesday as high pressure builds in. Temperatures will be well below normal through the first part of next week, and strong gusty winds will make it feel even colder. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 310 PM EDT Saturday Progressive and energetic pattern will be in place through the latter portion of the week. Low pressure moving through the Great Lakes will push a warm front through the Appalachians and central mid Atlantic region on Wednesday and bring a notable warm up in temperatures. However, this same low will then push a cold front through the region from the northwest and bring a chance of showers back to the forecast Wednesday night into Thursday. Guidance differs on the midweek evolution of surface features with hints that a wave may develop over the southeast and slow progression of the front with precipitation lingering much longer into Thursday night with some upslope mixed rain/snow showers at the higher elevations west of the Blue Ridge. For now will try to split the difference and bleed precipitation chances over a bit longer to cover the possibility. High pressure will then build in for the start of the weekend with fair weather and the expectation that any precipitation associated with the next low moving out of the mid Mississippi valley will hold off until at least Saturday night. With all of the energy driving the progressive regime, expect windy conditions to be prevalent most of the week before relaxing as we head into Friday. && .AVIATION /23Z SATURDAY THROUGH THURSDAY/...
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As of 715 PM EDT Saturday... Unsettled weather coming Sunday and major impacts are expected over the area with line of shower/storms moving in. Until then, expect VFR conditions this evening, then models showing low level moisture increasing overnight with MVFR cigs anticipated over most of the area by 12z Sunday, with possible IFR at times around BLF/BCB. Should be a mix of MVFR to VFR cigs after 14z Sunday before leading edge of showers moves into BLF/LWB in the 19-20z time frame. BCB/ROA 20-21z, and LYH/DAN 21-23z. Thunder is possible but not expected to be widespread so kept it VCTS at all sites. Gusty winds with the leading edge of showers/embedded storms may have a quick shift from south- southeast to west-southwest for less than an hour. Moderate to heavy showers expected as well, so mainly looking at Visibilities around 1-3sm. Medium to high confidence in wind forecast. Extended Aviation Discussion... Main area of heavier showers exits after 6z Monday in the east. Late Sunday night-Early Monday, 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. Next front arrives Wed night-Thu with chance of showers, with gusty winds and sub-VFR cigs Thursday in the mountains.
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&& .HYDROLOGY... As of 1015 AM EDT Saturday... Hydrologic ensembles from GEFS and North American (CMC-GFS) models continue to advertise rapid rises on nearly all the major rivers in southeast West Virginia, southwest Virginia and northwest North Carolina 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 forecast rainfall amounts driving these forecasts but most are clustering around 2 inches storm total with the usual terrain enhancement effects which impacts the upper Dan River basin (VA Blue Ridge) most effectively. There also will be run-to-run variation in forecast river levels. Forecasts through Sunday morning will be using forecast rainfall amounts, then the forecast Monday morning will be largely based on observed rainfall amounts. Forecasts from Saturday morning for the New and Greenbrier are showing rises to just above action stage on the Greenbrier at Alderson. The James River with 48-hours of rainfall showed a sharp rise within-bank rise Sunday night and Monday, and takes Buchanan just above action stage. On the Roanoke and Dan Rivers sharp rises are forecast Sunday night and Monday, with Roanoke reaching action stage Monday. Danville and Paces on the Dan river will rise close to flood stage Monday night and South Boston is forecast to rise above flood stage Monday night, cresting on Tuesday. Then there is the National Water Model (NWM) which uses the single run QPF solutions from the GFS, the latest available from 06Z/2AM Saturday 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 GFS. It remains to be seen how well the model performs with the actual rainfall which continues to be problematic with potential effects of upstream thunderstorms 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 forecast from early Saturday morning was showing a slight risk for excessive rainfall (exceeding flash flood guidance) on Sunday across much of the Appalachians. 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 widespread flash flooding. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...AMS NEAR TERM...AMS/JR/WP SHORT TERM...MBS LONG TERM...MBS AVIATION...JH/JR/WP HYDROLOGY...AMS is the U.S. government's official web portal to all federal, state and local government web resources and services.