Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 12 2023 15:34:40 AWUS01 KWNH 121534 FFGMPD FLZ000-122130- Mesoscale Precipitation Discussion 0178 NWS Weather Prediction Center College Park MD 1133 AM EDT Wed Apr 12 2023 Areas affected...South Florida Concerning...Heavy rainfall...Flash flooding possible Valid 121530Z - 122130Z Summary...Widespread rainfall with embedded showers and thunderstorms will support localized rainfall rates of 1-2"/hr through much of the afternoon. Isolated instances of flash flooding are possible, mainly over the Miami metro area. Discussion...Stratiform rainfall coverage has been gradually increasing across south-central Florida this morning, and more recently embedded showers and thunderstorms are also increasing in coverage. The most impressive of these discrete cells have been ongoing near and north of the Florida Keys, and slow deviate motions have been observed towards the east-northeast with an established right-moving supercell north of the Lower Keys. Otherwise, embedded shower and thunderstorm motion has generally favored a more northwest to northerly track in line with the 0-2 km mean wind (on the order of 15-20 kts). All of this convection is situated along and north of a frontal zone, which appears to be slowly lifting north as a warm front as of this morning. The mesoscale environment along and ahead of this frontal zone is characterized by precipitable water values of 1.7-1.9 inches (near the max moving average per MFL/KEY sounding climatology), ML CAPE of 500-1500 J/kg, and 0-3 km bulk shear of 15-25 kts. Moderate moisture transport from the east is occurring in the lower levels of the troposphere (mostly focused at 925 mb), and this onshore flow along the Gold Coast may help to enhance convective activity in and around the Miami metro this afternoon. The CAM guidance with regard to QPF has been rather dispersed since the overnight 00z cycle, but the 12z HREF has come into some better agreement that has increased confidence a bit for the afternoon forecast. The HRRR has remained the largest consistent outlier overall, initially failing to meaningfully initiate the convection that has occurred in and around the FL Keys, but has since done a modestly better job assimilating and propagating this convection. Farther northeast along the Gold Coast, the HRRR has been the most persistent in developing deep convection with the depiction of rainfall rates as high as 2-3"+/hr (which has so far failed to materialize). There is still quite a good bit of skepticism concerning the development of deep convection with rates this high, but the 12z ARW and NAM-nest have hints of this potential in the vicinity of the Gold Coast through 21z. The environment could support these rates should deep convection develop, but it appears more likely that embedded showers and thunderstorms will remain capped closer to 10-12k feet (with a distinct thermal cap largely preventing updrafts from accelerating much above this level). Even so, wet bulb zero heights are near this 10-12k ft height which will support highly efficient warm rain processes with 1-2"/hr rates easily being realized. Since these rates may be concentrated along the Gold Coast where low-level frictional convergence will be maximized (with the potential for some repeating/training), any localized instances of flash flooding are most likely across the Miami metro area (where urbanized terrain is also most likely to result in excessive runoff). Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8bPMv8pVwqC0sW0qmRDwACnW0LNSRy2smfhZo4r9F1R_MmnZ0pjnyfQ1sUOu3OfOMT0G= InPh2R81-z3vUO6p24jRKqc$ for graphic product... ATTN...WFO...MFL... ATTN...RFC...SERFC...NWC... LAT...LON 26588048 26528002 26238000 25798008 25268038=20 25138081 25138114 25588136 25888172 26248176=20 26428121=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .