Area Forecast Discussion
Issued by NWS Blacksburg, VA

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644 FXUS61 KRNK 280553 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 153 AM EDT Sun May 28 2017 .SYNOPSIS... A frontal boundary will remain positioned across or near the area the remainder of the holiday weekend. A series of disturbances will move southeast along this boundary and impact our area with rounds of showers and storms. Drier weather returns briefly on Tuesday before yet another disturbance arrives on Wednesday. && .NEAR TERM /THROUGH TODAY/... As of 830 PM EDT Saturday... Westerly downslope flow over the mountains is resulting in the break between the storms over eastern VA vs. the storms over far western VA. HiRes models maintain this break, although certainly can`t rule our a shower during the overnight as the westerly flow weakens allowing passage for a few showers over the mountains. The overall severe threat should also wane as CAPE decreases post daytime heating. The severe reports so far this evening have mainly been from large hail. Excessive rainfall has also been a problem in far western VA where the storms have moved over the same area. After the precipitation threat wanes, skies will start to clear a bit, and winds light and variable. This will help yield some late night and early morning fog, especially in areas that received precipitation this past evening. On Sunday, our area will remain in a westerly flow that will continue to be conducive to upstream shortwave troughs moving into the area from the Ohio Valley. Guidance continue to offer a solution that advertises a system more potent than the one expected to impact the area this evening. Compared to today when we have been hindered by the cap aloft into the afternoon, any cap tomorrow will not linger as long, and convection is expected to initiate earlier. The Storm Prediction Center has the area slated under a slight risk tomorrow as well. Low temperatures will be in the lower 60s across the mountains with low to mid 60s across the Piedmont. On Sunday, high temperatures are forecast in the mid 70s to near 80 across the mountains and around 80 to lower 80s across the Piedmont. && .SHORT TERM /TONIGHT THROUGH TUESDAY/... As of 210 PM EDT Saturday... A continuation of the active, unsettled weather pattern appears likely for Sunday evening and potentially through the balance of the short-term period for our NC foothills/eastern VA counties. Rounds of strong to at times severe thunderstorms are possible mainly Sunday evening, with a more conditional risk Monday. In addition, potential for hydrologic/localized flash flooding may be possible at any point in slow-moving cells but confidence is low on timing and location. While not totally a washout, as mentioned previously, those with outdoor plans this holiday weekend are still strongly encouraged to keep close monitor of the weather forecast, as there remains a good deal of uncertainties specific to timing Sunday night/Monday and then on both convective timing and location for Monday. For Sunday Night: An elevated mixed layer associated with steep mid- level lapse rates, emanating from the southern Plains/Mississippi Valley in fast belt of zonal mid-level westerlies, is expected to maintain ample enough instability for the Sunday night period. This is true even well after sunset. Position of Sunday late-day convection will heavily influence location of PoPs/Wx. Poor run-to- run consistency among the guidance suite casts some level of uncertainty here. Feel that the higher coverage of PoPs should be in eastern locales, from the Blue ridge foothills eastward, and mainly before midnight. Unidirectional (westerly) deep-layer wind profile and sizable deep-layer speed shear suggest a probability of linear storm mode for the before-midnight period. There may tend to be a break in storm coverage for the overnight with relative/weak shortwave ridging and/or because the cold front will have shifted eastward. For Memorial Day: Exact location of the NE - > SW oriented surface cold front lends uncertainty to coverage and location of thunderstorms. The NAM is the fastest and would have already cleared the forecast area during the overnight/early-morning hours, while the GFS is about 6 hours slower. If the GFS is correct, there may be a conditional risk of a few stronger/psbl severe storms. While deep layer shear magnitudes still would support persistent updrafts, instability values are progged to be lower (around 1000-1500 J/kg MUCAPE) but may begin sooner as CIN is lower than recent days. Kept PoPs closer to Chance and highest in eastern and southern sections. Presently SPC`s Day 3 outlook maintains General to a slim area of 5% severe/Marginal Risk across our southeastern VA and northern NC counties. This seems reasonable for now, though given the uncertainty in frontal position, it`s possible the threat may extend slightly further northwest. Greater potential does exist further south. Front then slows and may stall into the evening of Memorial Day from southern DE southwest into the Deep South. This occurs ahead of a mid-level trough in the southern stream progged across central AL and in increasing broadly cyclonic flow aloft evolving over the northern tier of states. The slowing/stalling frontal zone may foster additional opportunities for showers and storms roughly from Boone east and northeast to south of Lynchburg. For Tuesday: Weak ridging and drier conditions should exist across northern and western counties, north of a stalled frontal zone that will continue to focus rainfall chances along southern portions of the forecast area. Front appears to drift southward by the afternoon, so I`ve reflected PoPs generally lowering into the afternoon in the NC foothills/Piedmont counties. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 210 PM EDT Saturday... Brief period of clearing/dry conditions Tuesday evening. By Wednesday, a northern-stream mid-level disturbance pivoting around the cyclonic flow aloft advances southward may generate another round of showers along its surface cold front. This appears to clear the forecast area late Wednesday night. Clearing and notably cooler and drier conditions still appear in the offing for Thursday, with 850 mb temperatures dropping to values from +8 to +10C, compared to values nearly twice those in the near-/short-term periods. Late week into the weekend appears to be more unsettled again. Due to potential active weather in the short term, stuck mostly with a blend of guidance for the extended. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/...
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As of 150 AM EDT Sunday... Remaining showers and isolated thunderstorms over southern Virginia and northern North Carolina will gradually weaken and move south through 09Z. IFR ceilings were beginning to develop north of this precipitation. Low confidence how extensive IFR clouds will be. Thunderstorms will develop again this afternoon. There is a slight risk for severe storms...containing large hail and potential for downburst winds. Extended Aviation Discussion... Deep convection becomes less organized Monday into Tuesday with only hit and miss sub-VFR conditions. Perhaps late night and early morning sub-VFR river and mountain valley fog. Better organization of convection Wednesday with the passage of a cold front. Thursday is expected to be VFR and dry.
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&& .HYDROLOGY... As of 115 PM EDT Saturday... We continue to have a river flood warning in effect for the Roanoke River at Randolph...the river level falling. Elsewhere, hydrographs at many small creek and larger main stem rivers continue to show a general receding trend. While the receding trend is expected to continue, any thunderstorm that moves through the area will result in sudden rises. Looking ahead into the Memorial Day weekend...there is the potential for multiple rounds of showers and thunderstorms. Heavy rainfall is possible from any thunderstorms, especially in storms that can be slow moving, affect urban catchments and/or anchor to the terrain. Flash flood guidance values are low (1.25-2.5" in 3 hours lowest in the mountains and highest in the southern Piedmont), and would precondition the region for flash flooding and rises on smaller creeks if rainfall amounts and rates prove high enough. Confidence is low on expected rain amounts and specific periods of time of greater risk. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...None. NC...None. WV...None. && $$ SYNOPSIS...DS NEAR TERM...DS/PM SHORT TERM...AL LONG TERM...AL AVIATION...AMS/PM HYDROLOGY...AL is the U.S. government's official web portal to all federal, state and local government web resources and services.