Subj : Heavy Rain/Flooding TN/MS/AL/LA/AR To : All From : Mike Powell Date : Sun Mar 23 2025 19:09:05 AWUS01 KWNH 232206 FFGMPD TNZ000-ALZ000-MSZ000-ARZ000-240400- Mesoscale Precipitation Discussion 0079 NWS Weather Prediction Center College Park MD 605 PM EDT Sun Mar 23 2025 Areas affected...Western & Middle TN...Northern MS...Northwest AL... Concerning...Heavy rainfall...Flash flooding possible Valid 232205Z - 240400Z SUMMARY...Multiple rounds of intense showers/thunderstorms with rates of 1-1.5"/hr may allow for a few streets of repeating cells and 2-3" localized totals nearing/exceeding FFG values resulting in possible widely scattered incidents of low-end flash flooding, particularly near urban centers. DISCUSSION...GOES-E WV suite depicts a strong, compact short-wave across N IL/S WI moving quickly east with a lower-scale wave/mid-level trough accelerating eastward into the upper-Ohio River Valley. This is accompanied by a strong upper-level jet streak that has rounded the base of the main trof with solid right entrance ageostrophic response across the lower Ohio Valley providing strong divergence along and south into the Tennessee River Valley.=20 Surface and LLJ has responded with increasing Southwesterly flow fluxing higher theta-E air northward with Tds into the upper 50s and lower 60s. Filtered insolation and mid-level steepening lapse rates have made the area modestly unstable with MLCAPE nosing across W KY into W TN over 1000-1250 J/kg, along and ahead of solid cold front. Additionally, GOES-E Vis and sfc to boundary layer streamlines suggest a confluence axis between deeper Gulf moisture from the Western Gulf and return from the central/eastern Gulf becoming increasingly confluent across central MS to NE MS into Middle TN. With conditionally unstable air, this has been sufficient to break out weaker but slowly increasing convective activity across Middle TN... with some convection/flux convergence supporting rates of .5-.75", which is expected to increase over the next few hours. Forward propagation is likely to limit overall totals with any individual cell, but could be stage setting for later development. As the line expands, cells will have capability for back-building/flanking line development that is probable to align with strong/fast steering flow resulting in some accidental training. However, further upstream cold front is starting to advance with solid 90 degree convergence along the leading edge southwesterly LLJ/moisture plume. Cells will expand in coverage and width and should be capable of 1-1.5"/hr rates as they advance, with similar very fast (40+ kt) forward speed, likely limiting overall rainfall totals. Hence, any flash flooding potential will require these cells to traverse areas already affected by 1-2" totals with earlier rounds. If so, 2-4hr spot totals of 2-3" are possible. This is in range of the lower FFG values over the TN valley (1.5-2"/hr & 2-3"/3hr). As such, a few widely scattered instances of flash flooding are considered possible particularly after 00z and in/near urban settings. Gallina ATTN...WFO...BMX...HUN...MEG...OHX... ATTN...RFC...LMRFC...OHRFC...SERFC...NWC... LAT...LON 36488810 36478551 35158602 34288698 33908848=20 34009006 34279071 34979061 35638953 AWUS01 KWNH 232256 FFGMPD MSZ000-LAZ000-ARZ000-240430- Mesoscale Precipitation Discussion 0080 NWS Weather Prediction Center College Park MD 656 PM EDT Sun Mar 23 2025 Areas affected...Northern LA...Southern AR...Central MS... Concerning...Heavy rainfall...Flash flooding possible Valid 232300Z - 240430Z SUMMARY...Strong moisture flux convergence with initial convective development will allow for very quick moisture loading and strong downdrafts capable of quick 2"/hr rates (15-minute totals of 1-1.5" psbl). As cold front moves east, interaction with older warm sector convection may allow for localized repeating/training and spot totals of 2-3" in 1-2hrs resulting in possible flash flooding. DISCUSSION...GOES-E Visible/10.3um and RADAR mosaic denote scattered destabilization occurring across the warm sector mainly along the Lower Mississippi River Valley at the intersection of the nose of instability axis (2500-3000 J/kg) of MLCAPE that extends from the Heart of Texas into northeast LA and the nose of the deepest sfc to 850mb moisture core (LPW of .8-.9") across south-central LA into northeast LA. This boundary layer moisture is quickly advancing northward toward a southward pressing/well defined (steep isentropes) that is bisecting AR from NE to SW.=20 These cells are expected to be slower moving but with steering more northward toward the frontal zone before effective bulk shear increases due to stronger flow aloft across the Delta Region into the Bootheel of MO. The collision of the air masses is likely to result in rapid convective development along the front from NW MS across S AR into N LA and eastern TX mainly in the 01-02z time frame. There has been solid signal throughout the day toward this time frame, but trend in the HRRR is a bit earlier and more focused resulting in very strong moisture flux loading to the line. Stronger mid-level dry air will likely aid stronger downdrafts due to mixing but given loading is expected to have heavy rainfall totals in quick sub-hourly amounts.=20=20 Experimental HRRR 15-min estimates 1-1.25" at onset around 0130z become near 1.75" by 02-03z across N LA. Hourly totals of 2-2.5" seem plausible given weaker overall steering before cold front/outflow boundaries begin to organize and advance eastward.=20 While soil moisture is about 45-55% (in all but the braided portions of the MS River...65+% there), this is running in the 10-15th percentile, suggesting upper-soils may initially not allow much infiltration). Still, the spotty/smaller areal coverage may limit the overall magnitude of flash flooding suggesting highly focused/localized at least initially across S AR/N LA for spotty possible flash flooding. It is only later toward 04-06z with some organization and potential west to east training that localized totals may reach 2-4" through 06z. 18z HREF and recent HRRR runs support the best probability to be in proximity to the AR/LA/MS intersection and points east-northeastward. Confidence is increasing toward possible scattered incidents of flash flooding through this time period before cells become too scattered/progressive generally after 06z, but will continue to monitor trends closely. Gallina ATTN...WFO...JAN...LZK...MEG...SHV... ATTN...RFC...LMRFC...SERFC...WGRFC...NWC... LAT...LON 34389006 34318925 34248863 33868836 33378837=20 32958857 32398922 32039070 31829179 31709325=20 31989383 32779367 33569269 34099146=20 $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .