Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 191818 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 218 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 /THROUGH TONIGHT/... 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. As of 442 AM EDT Saturday... Extended Flash Flood Watch east of the Mountains until Midnight and kept in the west this morning. Low pressure to the west will travel northeast and slowly lift the frontal boundary across the region north today into tonight. This in conjunction with the flow turning more southwest underneath this upper system should push the axis of deep tropical moisture feeding inland from the Gulf Stream a little farther east. Eventually, the axis should push to the Atlantic coast. Upper level shortwaves will combine with better instability this afternoon to result in scattered showers and a few thunderstorms. With a tropical airmass in placed with pwats around 1.5 inches and a saturated ground from recent rains, there remains the potential for flash flooding. FFG remains very low in many locations. Additional rainfall of one to three inches may be possible in some locations. The Day One Convective Outlook places most of the forecast area in general thunderstorms with a marginal risk to our west from Ohio valley southward into Kentucky, where a few bowing segments capable of damaging wind gusts are possible. The day one excessive rainfall indicated a potential of marginal to slight risk. High temperatures this afternoon will range from the upper 60s to the upper 70s in the piedmont. Convection associated with the slowly moving upper level trough will be on the decrease tonight as the system ejects eastward within the northern stream jet. Its associated cold front will shift southeast and stall over the Carolinas on Sunday. Low temperatures tonight will vary from around 60 degrees in the mountains to the upper 60s in the piedmont. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 500 AM EDT Saturday... On Sunday, precipitation will have the least amount of coverage across the area then it has had in many days as the boundary/baroclinic zone drifts further north into PA/OH. Weak upper ridging will be present across the area. The air mass will remain unseasonably warm and humid. Thus, with diurnal heating isolated to scattered showers/thunderstorms will remain possible, but coverage will be limited. Western areas have the best chance of seeing convective activity during the afternoon. Sunday night through Monday night, another frontal boundary will approach from the northwest and low level flow returns from the south. The combination should yield increased coverage of showers and some storms across the area, especially during the peak heating of the day. As with the last front, this one too will likely stall across the area as the upper ridge to our south prevents much more progress southward. Therefore, we may be setting up for another period of numerous showers/thunderstorms and excessive rainfall into the first half of next week. Temperatures during this portion of the forecast will average five to ten degrees above normal with muggy lows in the 60s and highs in the 70s and lower to mid 80s. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 500 AM EDT Saturday... An upper low will slowly make headway east from over CA/NV on Tuesday to an open wave over NM/CO by Friday. High pressure in the western Atlantic, east of GA/FL, will weaken and move east during this period. A series of shortwave troughs will progress southeast from the James Bay region of Canada to, and across, New England. A stationary baroclinic zone will extend from roughly NE/SD to PA/MD/WV/VA. A wet weather pattern is expected to continue across our region thanks to a deep, southerly tropical fetch of moisture. The proximity of the baroclinic zone, especially for northern parts of our region, along with the periodic shortwave troughs passing just to our north, and daytime heating, will be the sources of the lift that we will need to promote shower and thunderstorm development. The greatest potential will be during the afternoon and early evening each day, especially on Tuesday and Wednesday. Convective activity is expected to decrease later in the week as the frontal boundary weakens. Temperatures during this section of the forecast will continue to average five to ten degrees above normal. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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As of 200 PM EDT Saturday... Highly variable ceilings will be occurring through the forecast period. Many locations, particularly in the west, will begin the period VFR, however convection developing in the mountains will bring sub-VFR conditions if they move over station. Threat for showers continues into the evening and dissipates after loss of daytime heating. MVFR conditions will likely hang around through the evening hours due to lingering clouds left behind by the convection. In the east, sub-VFR to begin the forecast period as the axis of gulf moisture is still having a substantial impact on sky conditions and even bringing an occasional shower. Expect this axis to migrate east over the TAF period, slowly helping to improve conditions. All locations are expected to go VFR shortly after sunrise Sunday morning. GFS in particular scatters out even earlier through mainly the midsection of the forecast area, which is evident in the BCB/ROA TAFs. LAMP guidance suggests some light fog at all TAF sites overnight, but considering this guidance does not want to take visibilities much lower than 5SM, and the uncertainty that we will be able to get enough clearing overnight, have opted not to include them in the TAFs at this time. Medium confidence on ceilings, visibilities and winds during the taf period. .Extended Aviation Discussion... Moisture will continues to decrease Sunday afternoon/evening as it shifts further east allowing weak high pressure to make for more diurnally based isolated showers/storms Sunday afternoon. Another weak cold front will bring increasing coverage of showers and thunderstorms again on Monday. Monday night into Tuesday, a surface low is expected to have reached KY/OH/WV with the stationary front over PA sinking south as a backdoor cold front. Scattered MVFR showers and thunderstorms are possible Monday night into Tuesday with another round Wednesday likely result in periodic sub-VFR into the middle of next week. Convective activity is expected to decrease later in the week as the frontal boundary weakens.
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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...
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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.
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&& $$ SYNOPSIS...JR/KK NEAR TERM...JR/KK SHORT TERM...DS/RAB LONG TERM...DS/RAB AVIATION...JR/KK HYDROLOGY...JH/KK

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