Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 19 2023 08:06:51 FOUS30 KWBC 190806 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 AM EDT Tue Sep 19 2023 Day 1 Valid 12Z Tue Sep 19 2023 - 12Z Wed Sep 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF THE SOUTHERN PLAINS AND MISSOURI VALLEY... ....Southern Plains/Missouri Valley... WV satellite imagery this evening shows our next disturbance traversing the southern Rockies with convection already in-of the TX Panhandle. This shortwave will become the primary focus for later this afternoon and evening as large scale ascent within a favorable thermodynamic environment will trigger a round of strong to severe thunderstorms across the Southern Plains, migrating eastward during an expected upscale growth convective mode for the back end of the D1 time frame. A sharp increase in regional moisture is already occurring thanks to a prominent 35-40 kt LLJ bisecting the plains as the area lies on the western flank of a sprawling surface ridge analyzed over the Tennessee Valley, and the eastern side of developing low pressure along the lee of the Rockies. PWATs running between 0.75-1" over the plains east of a line from SJT to DDC will see a rapid increase towards 1 standard deviation above climatological norm by the time we reach this afternoon. This will greatly increase regional instability over much of east TX up through OK and KS which will play a role in the convective pattern later today. 2000-2500 J/kg of MLCAPE is forecast to the east of the dryline centered over the western rolling plains up through southwest KS. As the mid-level perturbation exits the Rockies to the plains, this will garner the triggering mechanism to initiate the first round of thunderstorms across the southern plains along and to the east of the dryline (western edge of the theta-E gradient). Multi-cell and discrete supercells will be the initial modes of convection when the storms begin, but increasing LLJ pattern over central and eastern OK up into eastern KS will promote a shift to more organized cell clusters promoting heavy rainfall coverage over a larger area. This evening and overnight into Wednesday AM will be the primary time frame of interest with regards to flash flooding potential as the expected complex will provide the coverage and magnitude necessary to bring more widespread impacts to the areas above. Hi-res ensemble (HREF) and associated deterministic are all fairly aggressive with the totals across the aforementioned area of interest, especially over east OK up through southeast KS where HREF neighborhood probabilities of >2"/24 hrs have shot up near 40% with a signal for 1-2"/3 hrs between 00-06z Wednesday up to a large area of 20-40%. Deterministic totals are obviously very high with a multi-model consensus of a 4-6" maximum somewhere in the vicinity of the risk area. Bias-corrected ensemble means are also fairly robust with over 1.75" bulls-eyed over NE OK up to the KS line. This actually is in agreement with CIPS analogs placement of max QPF based on the top 5 analogs within the 00z run, which would place the QPF centroid right in the middle of the forecast Slight Risk. The northern and southern expansions are for multi-model consensus on secondary peaks within the QPF field based on convective development and propagation within the spread. There is some deviation on where exactly the complex, or remnant MCV goes by the end of the forecast period, so there could be some shifts in the overall SLGT and adjacent MRGL in subsequent updates. However, will maintain the current positioning based on HREF blended mean QPF and outline of the 20% probability of 2" or greater within the ensemble forecast.=20=20 ....South Florida... Another day with a stalled front nearby, another day for thunderstorms capable of producing excessive rainfall rates of 2-3"/hr. Area averaged guidance soundings over South Florida showed MLCAPE of 2,000 J/kg available, as well as PWs near 2" and averaged low-mid level RH values near 80%. 00z HREF probabilities are once again fairly robust, even for FL standards with signals for 3+" and 5+" totals within the D1 time frame (50-60% and 20-30% respectively). The enhancement of the front and sea breeze convergence creates a favorable scenario for localized flooding, especially in urbanized corridors along the coast(s). There is more of a signal today for the southwest coast being impacted by heavy rain today in due part to a progressive sea breeze propagation from the east and heavy storms forming in-of the Everglades creating subsequent outflows. This is the biggest difference from the last few days, but the impact potential still stands leading to a maintenance of the MRGL risk from previous forecast.=20=20 Kleebauer Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-L-s3mRESONW8fPDyyApql8xWkocss9oFFwDTOPQHaPI= -AmLnU3xTE0AL4cbW2zkm7A39cLFjAYPTKhAwC-U3Nao-6o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-L-s3mRESONW8fPDyyApql8xWkocss9oFFwDTOPQHaPI= -AmLnU3xTE0AL4cbW2zkm7A39cLFjAYPTKhAwC-UEYtvd8I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-L-s3mRESONW8fPDyyApql8xWkocss9oFFwDTOPQHaPI= -AmLnU3xTE0AL4cbW2zkm7A39cLFjAYPTKhAwC-UIt8JtmU$=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) .