Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 12 2023 08:10:23 FOUS30 KWBC 120810 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 AM EDT Fri May 12 2023 Day 1 Valid 12Z Fri May 12 2023 - 12Z Sat May 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS... ....Texas... A significant flash flood event could evolve Friday night across portions of south central TX. Forcing, moisture and instability all look to be in place to support an excessive rainfall event. Forcing is being driven by a mid level shortwave moving out of Mexico and broadly diffluent flow aloft in between the subtropical jet to the south and a jet streak to the north of the area. This mid/upper forcing persists through the period, and does not show much eastward progression with time. This type of persistent forcing evolution often favors training/backbuilding. Strong southeasterly low level flow off the Gulf of Mexico and a mid/upper level connection to the Pacific will support anomalous PWs and efficient rainfall. Instability should be on the order of 3000-4000 J/kg, with plenty of upstream instability to maintain convection. The main question is the last remaining ingredient for flash flooding...heavy rainfall rate duration. The aforementioned slow moving forcing supports prolonged duration, and deep layer southerly flow, combined with southeasterly low level flow, supports training/backbuilding. As convection organizes it should develop a cold pool and propagate eastward into the low level flow and downstream instability pool. Exactly how quickly this happens is the big question. Initial convection should develop along the Edwards Plateau as forcing and upslope southeasterly flow increases. Think we will see a period of backbuilding/training during this convective phase, before cold pool generation results in an east or southeast progression of activity. At this point leaning towards the slower HREF solutions, as some of the members have a tendency to become cold pool dominant too quickly. Thus would not be surprised if we end up on the higher end of the HREF output for QPF amounts. Thus still thinking this is an event where swaths of 6"+ rainfall are a possibility, with 2-3" an hour rates. Scattered to numerous instances of flash flooding, some of which could be higher end in nature seem possible within/near the Moderate risk area. Convection is also likely along the dryline over west central TX during the afternoon hours. This activity will initially be slow moving and should pose an isolated to scattered flash flood risk. Locally significant rainfall and flash flooding is possible with this activity as well, but coverage should be a bit lower than with the Hill Country activity, and thus a Slight risk should cover the threat. Although guidance has been trending up over this area, and so will need to continue to closely monitor. Overnight this dryline activity should survive eastward into central TX...but the exact magnitude and organization of this activity remains unclear. However a broad Slight risk seems warranted as a flash flood risk could continue into central TX overnight. ....Northern Plains... Isolated to scattered flash flooding is possible over a large portion of the Northern Plains today into tonight. A Slight risk stretches from NE into much of SD and into west central ND. A closed deep layer low moving slowly northward will provide plentiful forcing for convection. A stationary front over the western Dakotas will continue to be one focus for persistent convective activity. Instability is a bit more meager here, however strong low level easterly flow and persistent convergence will still support periodic rainfall totals over 1" in an hour. The 00z HREF supports additional widespread 1" rainfall totals here, with isolated to scattered 3"+ totals likely. The 2nd area of concern is closer to the deep layered low and associated warm front from NE into southeast SD and northwest IA. This activity will have more instability to work with, and thus 2"+ an hour rainfall is a possibility. This activity should be a bit more progressive, but do expect we will at least see some localized swaths of backbuilding/training. HREF probabilities also support isolated to scattered 3"+ rainfall totals here as well. ....MS Valley into the OH/TN valley and Southeast... A broad Marginal risk will cover a lot of this region today/tonight. Would have liked to try to narrow this area down, but found it hard to do, hence the very large Marginal. Above average moisture and instability will cover this entire region, so any convection will be capable of heavy rainfall rates, and where convection persists, could result in localized flash flooding anywhere within this risk area. The actual coverage of FFG exceedance will probably be pretty low over this area, but localized instances could occur just about anywhere within the risk based on 00z HREF and other QPF guidance. Given this HREF output and the favorable moisture/instability parameters...thought going with a large Marginal was the best course of action. Chenard 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!5oLJjkT76R6ASTojpoh1eHQBfxkuUJxn7ThH1airIWot= kkA0jjPExYto7wbS4tFjpbpaIGZe4qSYHpLo0rixc5n_NYQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5oLJjkT76R6ASTojpoh1eHQBfxkuUJxn7ThH1airIWot= kkA0jjPExYto7wbS4tFjpbpaIGZe4qSYHpLo0rixODv2eA8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5oLJjkT76R6ASTojpoh1eHQBfxkuUJxn7ThH1airIWot= kkA0jjPExYto7wbS4tFjpbpaIGZe4qSYHpLo0rix2dpYIa8$=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) .