Area Forecast Discussion
Issued by NWS Blacksburg, VA

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365 FXUS61 KRNK 190021 AFDRNK Area Forecast Discussion National Weather Service Blacksburg VA 821 PM EDT Fri May 18 2018 .SYNOPSIS... A frontal boundary across the region will remain in place tonight before lifting north as a warm front on Saturday. Several waves of low pressure will ride north along the front acting to enhance showers and storms into tonight. Warm and humid conditions will continue into the weekend with showers and thunderstorms becoming more scattered over the weekend. && .NEAR TERM /THROUGH SATURDAY/... As of 805 PM EDT Friday... Southern edge of axis of moderate to heavy rain associated with a mesoscale vortex extended from Roanoke to South Boston Virginia. This band of showers will move northeast this evening. Another band of showers with the upper low will slide in from the southwest overnight. Since so saturated in spots and potential for evening coverage east, as well as late night redevelopment as seen along the residual front the last couple of nights, keeping high pops in place. This includes extending the flash flood watch across the entire region until early Saturday morning for now. With dewpoints so moist should only see lows drop off a few degrees, moreso across the north where the wedge looks to linger. Otherwise lows mostly low/mid 60s with a few upper 50s valleys. Weak subtropical low to the west will finally lift northeast into the Great Lakes on Saturday allowing the leftover front across the region to lift back north during the day. This in conjunction with the flow turning more southwest underneath this upper system should finally bump the axis of deep tropical moisture feeding inland from the Gulf Stream a little farther east by Saturday morning per latest Nam. However see added spokes of energy spilling east under the exiting system to the north which likely to combine with better instability Saturday afternoon to result in at least another band of convection far west and scattered coverage elsewhere. Think for now not enough to keep the watch going another period but may need to be considered later on. Otherwise will have lingering high pops Piedmont before a brief lull to chance pops prior to ramping back up espcly mountains later in the day. Flow slowly veering south/southwest within the warm sector should combine with perhaps periods of insolation to boost highs well into the muggy 70s for highs. && .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... As of 145 PM EDT Friday... The last of the precipitation associated with the slowly moving upper level trough will be on the decrease Saturday night as the system ejects eastward within the northern stream jet. Its associated cold front will shift southeast of the region and stall over the Carolinas on Sunday. On Sunday, precipitation will have the least amount of coverage across the area then it has had in many days. Isolated to scattered showers are expected across the far southeast thanks to the position of the departing front. Coverage will be scattered across the far northwest as well thanks to our next approaching system and daytime heating. 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. Temperatures during this portion of the forecast will average five to ten degrees above normal. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 1145 AM EDT Friday... This portion of the forecast will continue offer conditions that will a forecast with daily chances of showers and a few storms. 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. Our wet weather pattern is expected thanks to a deep, southerly fetch of moisture advecting into our region. 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 during the afternoon and early evening each day, especially on Tuesday and Wednesday. Temperatures during this section of the forecast will average five to ten degrees above normal. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... As of 815 PM EDT Friday... High confidence of widespread IFR to LIFR ceilings through 12Z/8AM. Medium confidence on MVFR visibility overnight. 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 on Saturday. As winds comes around to the southwest ceilings will improve back to MVFR on Saturday but low confidence with the timing. .Extended Aviation Discussion... Showers should diminish Saturday evening followed by areas of sub-VFR in fog/low clouds. 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 Saturday for VAZ007-009>020- 022>024-032>035-043>047-058-059. NC...Flash Flood Watch until 8 AM EDT Saturday for NCZ001>006- 018>020. WV...Flash Flood Watch until 8 AM EDT Saturday for WVZ042>044-507- 508. && $$ SYNOPSIS...JH NEAR TERM...JH SHORT TERM...DS LONG TERM...DS AVIATION...AMS/JH HYDROLOGY...JH

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