Area Forecast Discussion
Issued by NWS Blacksburg, VA

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000 FXUS61 KRNK 190904 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 504 AM 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 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 /09Z SATURDAY THROUGH WEDNESDAY/... As of 150 AM EDT Saturday... Poor flying conditions will continue today with low clouds, fog and convection. MVFR/IFR ceilings with pockets of LIFR will continue this morning. Visibility will depend on intensity of rain and the amount of fog. As the upper low over the Mid Mississippi Valley fills and lift northeast additional periods of showers and thunderstorms are expected today into tonight. As winds comes around to the southwest ceilings will improve back to MVFR this afternoon but low confidence with the timing. Showers should diminish this evening followed by areas of sub- VFR in fog/low clouds. Medium confidence on ceilings, visibilities and winds during the taf period. .Extended Aviation Discussion... Moisture will decrease a bit more on Sunday as the deep tropical moisture shifts 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. && .HYDROLOGY... As of 230 PM EDT Friday... A Flash Flood Watch is in effect for the entire area into early Saturday. The main concern through tonight will be for localized flash flooding, 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. With locations along the Blue Ridge having already seen several inches of rain since last night, added heavy rain will likely cause more flooding espcly around Roanoke and points north/east overnight 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. Thus the flash flood watch has been extended into early Saturday morning as given such wet antecedent conditions, it wont take much additional rain to cause at least minor flooding. Showers and storms are expected to become more scattered in nature on Saturday as the main axis of deeper tropical moisture shifts slightly east. However locally heavy downpours could still occur with heating and along another weak feature pushing in from the west. Current thinking is that coverage wont be widespread enough to cause widespread flooding but something to watch. Runoff from the heavy rain last night and today will finally lead to potential river flooding by Saturday espcly along the Roanoke and Dan Rivers where river flood warnings may be needed. && .RNK WATCHES/WARNINGS/ADVISORIES... VA...Flash Flood Watch until 8 AM EDT this morning for VAZ007- 009>020. Flash Flood Watch through this evening for VAZ022>024-032>035- 043>047-058-059. NC...Flash Flood Watch until 8 AM EDT this morning for NCZ001-002- 018. Flash Flood Watch through this evening for NCZ003>006-019-020. WV...Flash Flood Watch until 8 AM EDT this morning for WVZ042>044- 507-508. && $$ SYNOPSIS...KK NEAR TERM...KK SHORT TERM...DS/RAB LONG TERM...DS/RAB AVIATION...AMS/KK HYDROLOGY...JH

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