Subj : MESO: Heavy Rain/Flooding To : All From : Mike Powell Date : Tue Dec 26 2023 13:19:00 AWUS01 KWNH 261744 FFGMPD VAZ000-NCZ000-SCZ000-TNZ000-262342- Mesoscale Precipitation Discussion 1223 NWS Weather Prediction Center College Park MD 1243 PM EST Tue Dec 26 2023 Areas affected...in and near western Carolinas Concerning...Heavy rainfall...Flash flooding likely Valid 261742Z - 262342Z Summary...An uptick in heavy rainfall intensity is expected this afternoon as an upper level jet streak approaches the region. Hourly rain totals up to 1.5" with local amounts up to 3" are possible over saturating soils. Discussion...As the guidance hinted at early this morning, rainfall intensity has been on the wane towards midday. However, some increase in the MU CAPE pool upstream/to the south has been noted recently, with values of 250-500 J/kg indicated. This instability is using the southwesterly 850 hPa wind in the warm sector to advect northeast, which makes sense when using SPC mesoanalysis instability trends. Water vapor imagery appears to show a jet streak entering northwest AL at the present time which should approach the western Carolinas this afternoon. Moderate to heavy rainfall has appeared to cause the cold air dam to spread eastward in the western Carolinas when compared to earlier this morning and a new wave appears to be forming along it in the Piedmont of SC. Precipitable water values remain at or above 1.25". Convergent 850 hPa inflow of up to 40 kts from the southeast into the cold air dam is seen ahead of a synoptic scale warm front and northeast of the main surface low in GA. An area of 30 kts of effective bulk shear is approaching the area from the west. The 12z HREF indicates that the lull in rainfall intensity should last until 1 pm or so before ramping back up, presumably due to the upper level jet streak's approach to the region. MU CAPE may also rise back above 500 J/kg, if recent RAP runs are to be believed. This should allow hourly rain totals to rise back into the 1-1.5" an hour range, with local amounts of 2-3" possible. The region has seen moderate to heavy rainfall this morning, which is saturating area soils. If 2-3" of rain falls quickly enough, it would eclipse area three-hourly flash flood guidance values. It's possible that the area will be straddling the line between longer duration flooding and flash flooding today if MU CAPE remains below the RAP's expectations; uncertainty is greater than usual in this area. While HREF guidance provided the initial template to the area in which to depict, the slight eastward movement of the edge of the cold air dam caused an eastward nudge to the area of concern, as a precaution. Roth ATTN...WFO...CAE...GSP...MRX...RAH...RNK... ATTN...RFC...LMRFC...OHRFC...SERFC...NWC... LAT...LON 36838094 36538030 36058004 35688016 34708097 34238178 34818254 35828246 36418178 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .