Subj : Heavy Rain/Flooding AL/MS To : All From : Mike Powell Date : Tue Apr 22 2025 08:11:00 AWUS01 KWNH 221159 FFGMPD TNZ000-ALZ000-MSZ000-221800- Mesoscale Precipitation Discussion 0157 NWS Weather Prediction Center College Park MD 758 AM EDT Tue Apr 22 2025 Areas affected...portions of north-central AL and northeast MS Concerning...Heavy rainfall...Flash flooding possible Valid 221200Z - 221800Z Summary...Additional scattered totals of 2-4" are likely and may overlap accumulations of 2-3" from early this morning/overnight. Isolated to scattered instances of flash flooding are possible. Discussion...Convection has been percolating over the past several hours over northwestern AL into far northeast MS, in the vicinity of a weak, stalling surface front and low pressure. While not overly impressive synoptically, a weak shortwave aloft is noted with an associated 50-60 kt (mini) jet streak, providing just enough divergence aloft to support sustained convection (with 0-6 km shear of only about ~20 kts). Meanwhile, the low-levels are becoming increasingly supportive of sustaining convection as well, as weak 925-850 mb moisture transport has prevented convection from becoming outflow dominate with 3-hr change in ML CAPE of 50-150 J/kg (resulting in current ML CAPE of 250-750 J/kg). Precipitable water of 1.5" (above the 90th percentile, per BMX sounding climatology) has supported occasional rainfall rates to 1.0-1.5"/hr (per MRMS estimates, as the peak rainfall rates have occurred in observation sparse areas). Going forward, there's a good chance that some of the recent trends of backbuilding convection will continue, as upwind propagation vectors in the vicinity of the convection are 5 kts or less (taking into account the mean storm flow and opposite flow of the 850 mb jet). In addition, storm motions are also relatively slow (10-15 kts), so the combination of backbuilding and appreciable individual cell residence time may occasionally support hourly rates/totals on the order of 2-3"/hr. Overall the 00z/06z CAMs are struggling with the depiction of convection in the region, though the 00z FV3 is an outlier in depicting 2-4" totals through 15-18z (though this may be displaced too far south). More recent HRRR runs (since 06z) are doing a better job depicting convection in the correct location, though still probably a bit too weak given the trends (only showing highly isolated totals of 2"+). Thinking additional scattered totals of 2-4" are likely through 18z, and much of this could occur over areas that have already seen 2-3" this morning. Given that the realization of any flash flooding is dependent on localized training of convection with overall weak forcing and relatively high uncertainty, isolated to scattered flash flooding is considered possible. Churchill ATTN...WFO...BMX...HUN...JAN...MEG... ATTN...RFC...LMRFC...SERFC...NWC... LAT...LON 35158589 34028607 33268695 32578762 32408843 32518937 33068950 33568943 34238915 34708760 $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .