Area Forecast Discussion
Issued by NWS Blacksburg, VA

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927 FXUS61 KRNK 192358 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 758 PM EDT Sat May 19 2018 .SYNOPSIS... A frontal boundary across the region will slowly lift north as a warm front today. Several waves of low pressure will ride north along the front acting to enhance showers and thunderstorms. Warm and humid conditions will continue this weekend. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... As of 945 AM EDT Saturday... A flood warning has been issued for the Roanoke River at Brookneal, and the flood warning for the Dan River at South Boston has been upgraded to moderate flood stage. Trimmed back PoPs to better match what is currently on the radar. Further, adjusted sky to better match the clearing over/near the Mountain Empire seen on satellite. Also adjusted temps to match current observations, but forecast highs still appear to be on track. .NEAR TERM /THROUGH SUNDAY/... As of 315 PM EDT Saturday... Flash flood watch in the east will continue through midnight. A frontal boundary will continue to lift north of the region this afternoon. For eastern portions of the forecast area, a main axis of gulf moisture is bringing ongoing isolated showers and plenty of clouds. Low pressure to the west will shift the flow more to the southwest allowing for moisture to continue to entrain into the region. This combined with a shortwave passing is the impetus for the showers and thunderstorms currently moving through the region which will continue into the nightime hours. Considering the ambient conditions from the copious amounts of rain we have seen lately, main concerns with any thunderstorm would come with any quick downpours or potentially any training which may quickly lead to flooding issues. During the pre-dawn hours, far removed from any diurnal support, most of the remaining showers will fizzle out. GFS guidance even suggests some scattering out of the cloud deck at this time and many locations getting some sunshine Sunday morning. This is evident of some weak high pressure building into the region as the main axis of gulf moisture continues toward the coast. This will set the stage for very warm and humid conditions leading toward chances for diurnally driven showers and thunderstorms during the afternoon. As would normally be the case in these summerlike conditions, without any larger scale forcing, these showers and thunderstorms will be more likely in the mountains, with some possibly forming over the Blue Ridge and drifting east over the lower elevations. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/... As of 315 AM EDT Saturday... Though important mesoscale details remain rather elusive in the period, which limits forecaster confidence on specific details to no better than moderate, large-scale regime favors a continuation of the wet weather pattern we`ve been stuck in of late. It`s unclear we`ll see the high rain rates as over the last couple days. Given the state of the soil moisture conditions, rivers, and south-to- north rain training on the eastern end of another subtropical low coming out of the eastern Gulf suggest that a renewed flood threat may again be a focal point in this period, at least in a localized sense. Overall synoptic regime will remain generally little changed, as a continued influx of subtropical moisture looks to resume again as another upper low moves up from the eastern Gulf. Meanwhile early in the short-term period (Sunday night into Monday), the westerlies which have been largely to our north will begin to buckle somewhat more to the south into the mountains of eastern WV into the northern/central Blue Ridge, with ill-defined/subtle mid-level vort maxes in this mid-level westerly regime serving as foci for additional showers and thunderstorms. Sunday night and early Monday, there`s some indication in the 12z guidance of a weakened convective complex as the stronger westerlies build south. Storms may again fire on remnant outflow from this overnight activity on Monday afternoon. Shear isn`t particularly strong suggesting disorganized pulse multicells are most probable if convection develops, but there is enough progged modest to moderate instability amid filtered sun to partly cloudy skies to offer a window of isolated to scattered strong storms north of Route 460. SPC does show a Marginal Risk/5% severe probability for this general area Monday. Any threat likely wanes with the diurnal cycle. Monday night through Tuesday night look to feature continued chances for showers and embedded thunder, with SREF showing PWATs of 1.25-1.5" advecting north/north-northeast against the Blue Ridge. Convective parameters are lower for Tuesday given abundant cloud cover, PoPs are toward the lower Likely range. From Sunday night through Tuesday, shown areal-average rain amounts between a half to an inch of rain, with localized potential for more in more persistent downpours that may unfold. Given partly to mostly cloudy skies governing much of the period, kept temperatures on the lower end of the guidance envelope on the highs, and milder side of guidance for the lows. Generally highs mid 70s to mid 80s and lows in the 60s, with a continued somewhat humid feel (dewpoints in the 60s). && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 315 AM EDT Saturday... Global models point to Wednesday and Thursday as days with a relative minimum in rain chances, as a high pressure area over New England wedges down the Appalachians. This will likely shunt the subtropical plume of moisture further to the south and allow for a couple of somewhat drier days. Temperatures begin to trend somewhat more seasonal for the midweek period. An early look at Friday and the Memorial Day weekend looks unsettled with a potential return the chances for rain showers and thunderstorms back into our area. This is indicated by most global models as a cold front slowly sags south from the Great Lakes/mid- Mississippi Valley, with another potential surge of moisture of subtropical origin returns from the south. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/...
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As of 755 PM EDT Saturday... Visible satellite pictures showed stratus banked up against the east side of the Blue Ridge late this afternoon. KROA was just on the edge of the low cloud and will oscillate between IFR and VFR through midnight. West of the mountains outside of the showers and thunderstorms ceilings were VFR. Two main bands of showers and isolated thunderstorms were tracking northeast across Kentucky, West Virginia, and Virginia this evening. Showers and thunderstorms were more isolates east of the Blue Ridge but a thunderstorm was just upstream of KLYH. Medium confidence of any improvement in the ceilings east of the mountains overnight and low confidence with the formation of any fog. The temperature at KBLF dropped down to the dew point as the first storm went through so at this point that is the most likely airport to get MVFR or lower visibility due to fog. As weak high pressure builds over the Mid Atlantic region on Sunday, moisture will decrease. Southwest Virginia, northwest North Carolina and southeast West Virginia will become VFR by late morning, with medium confidence in this timing. .Extended Aviation Discussion... Diurnally driven isolated showers and thunderstorms will develop Sunday afternoon. Another weak cold front will bring increasing coverage of showers and thunderstorms again on Monday. Scattered MVFR showers and thunderstorms are possible Monday night into Tuesday with another round likely Wednesday. Conditions are expected to improve later in the week.
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&& .HYDROLOGY... Update as of 1000 AM EDT Saturday... A flood warning has been issued for the Roanoke River at Brookneal, and the flood warning for the Dan River at South Boston has been upgraded to moderate flood stage. Previous discussion... A Flash Flood Watch is in effect portions of forecast area until Midnight for the potential for heavy rains. The Flood Warning continues for the following rivers in North Carolina and Virginia... Dan River near Danville Dan River at Paces affecting Halifax County Dan River at South Boston affecting Halifax County The Flood Warning continues for the following rivers in Virginia.. Roanoke (Staunton) River at Randolph affecting Charlotte and Halifax Counties Several days of heavy rainfall with amounts ranging from 2 to as much as 6 inches across parts of the Roanoke and Dan River basins will cause flooding along the river in parts of Rockingham, Pittsylvania, the City of Danville and downstream in Halifax County. Flash Flooding is a main concern until Midnight, as well as flooding on small creeks, streams, and smaller rivers as a result of bands of tropical rain showers and thunderstorms that are expected across the area. Added heavy rain will likely cause more flooding especially along and east of the Blue Ridge this afternoon into this evening as bands tend to train across the region. These bands will be very efficient rainfall producers with rates of 2 inches or more per hour possible. This could result in another 1 to 3 inches of rain in spots. Given such wet antecedent conditions, it wont take much additional rain to cause at least minor flooding. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flash Flood Watch until midnight EDT tonight for VAZ022>024- 032>035-043>047-058-059. NC...Flash Flood Watch until midnight EDT tonight for NCZ003>006- 019-020. WV...None. && $$ SYNOPSIS...JR NEAR TERM...JR SHORT TERM...AL LONG TERM...AL AVIATION...AMS/JR HYDROLOGY...JH/KK

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