Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 24 2025 14:01:12 AWUS01 KWNH 241401 FFGMPD LAZ000-241800- Mesoscale Precipitation Discussion 0169 NWS Weather Prediction Center College Park MD 1000 AM EDT Thu Apr 24 2025 Areas affected...southeast LA Concerning...Heavy rainfall...Flash flooding likely Valid 241400Z - 241800Z Summary...Localized flash flooding likely to continue and possibly expand to localities farther east (including the New Orleans metro) with additional isolated totals of 3-5" possible. Discussion...A persistent cluster of thunderstorms from overnight has grown into a mini MCS this morning, though GOES-East infrared imagery indicates only sporadic cold cloud tops with a relatively unimpressive appearance. Radar imagery tells a much different story, as shallow updrafts have been incredibly persistent with relatively slow storm motions (KHDC VWP indicating 10-20 kt low-level flow), backbuilding along the southern and southwest flanks, and upwind propagation towards the southeast (into the low-level flow). With SPC's SFCOA analysis indicating PWATs of 1.5-1.7 inches (near the 90th percentile, per LIX sounding climatology) and a reservoir of 500-1000 J/kg of ML CAPE directly to the south and southeast, there are concerns that this small convective complex will persist through the mid-morning to mid-day. Crucially, the influence of the left exit region of a subtropical jet streak (near 75 kts at 250 mb centered over South TX) is providing both 20-25 kts of effective bulk shear and enhanced upper-level difluence and lift. The persistence of the convection (with the aforementioned occasional cold cloud tops breaking through) suggests that the shear will continue to at least irregularly support deep convection, and this feedback loop allows for more rising motion and increased divergence aloft to support further new updrafts and initiation. Localized areas with the most efficient backbuilding and repeating of 1-3"/hr rainfall rates has resulted in estimated hourly totals of as much as 2-4" per MRMS (with KLFT recording back to back hours of 2.34" and 2.41" of rainfall earlier this morning). Past 6-hour MRMS estimates indicate scattered totals of 3-6" (with some of the heaviest and largest areas in the vicinity of the Lafayette and Baton Rouge metro areas). Hi-res CAM suites from 00z failed to capture virtually any of this activity, though more recent hourly runs of the HRRR and RRFS have done a much better job initializing and persisting convection (with the RRFS notably catching on to this trend much earlier than the HRRR). Analyzing these most recent model runs (10z onward) going forward, the output QPF suggests the potential for additional 3-5" localized amounts. As detailed above, the short-term trends and current mesoscale environment supports these amounts, and the heaviest amounts should continue to shift towards the east with upwind propagation (potentially putting the New Orleans metro area into greater threat over the next several hours). Ongoing flash flooding (some significant and life threatening) is likely to continue, possibly spreading eastward to areas that have seen little to no rainfall thus far. Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!5N9wt-OBb_mHc8SOlveQ8_orqihjzyBNmMnvotN0YMlp3nwwm_S4HY4wVzVzxOFhjss1= eIM2-KxOG8zkn_QiXAtouDw$ for graphic product... ATTN...WFO...LCH...LIX... ATTN...RFC...LMRFC...NWC... LAT...LON 30989140 30879083 30709028 30388974 29808987=20 29449035 29639090 29799159 29979225 30789197=20 =20=20=20=20=20=20=20=20=20=20=20=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .