Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 28 2023 01:02:48 FOUS30 KWBC 280102 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 902 PM EDT Fri Oct 27 2023 Day 1 Valid 01Z Sat Oct 28 2023 - 12Z Sat Oct 28 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL TO NORTHEAST TEXAS... A stalled frontal boundary will pose a flash flood risk through the overnight hours across portions of central to northeast TX. We have introduced a targeted Moderate risk upgrade with this update across portions of the area. A few flash flood warnings have already been issued, and the overall pattern should generally get more favorable for heavy rainfall as the night progresses. Thus areas of additional flash flooding, some of which could be significant in nature, appear likely. Deep layered moisture will continue to push northward into the stalled boundary, with PWs over 2" across the region. As the night goes on lower level convergence along the front is expected to intensify. Meanwhile, mid and upper level forcing is also forecast to increase, with a subtle mid level shortwave and increased divergence aloft. Thus forcing from the lower to upper levels should generally become more favorable overnight into early Saturday. The main uncertainty deals with instability. CAPE is probably highest now, which is helping drive some of the intense rainfall rates we are seeing this evening near the stalling front. As the night progresses this instability should gradually decrease, which does raise some questions with regards to the intensity/organization of subsequent convection. At this time think enough instability will persist that the aforementioned stronger forcing will be able to overcome the decreased instability and still support organized intense convection. But this is not for certain, and instability trends will need to be monitored. Assuming we do maintain some degree of instability, convection should tend to get more organized as the forcing increases overnight...and with the stalled front and propagation vectors aligned generally parallel to this front...training/backbuilding convection will be a concern. To summarize...while there is some uncertainty with regards to the magnitude of the flash flood risk overnight...the continued favorable setup suggests at a minimum some areas of additional flash flooding are likely. This is especially true given the lowered FFG due to recent heavy rainfall across the area. Worst case scenario is that instability persists, convection continues to grow upscale, and we end up with a relatively narrow corridor of persistent training heavy rainfall rates. If this evolves then a significant flash flood threat could develop. Given the high confidence of some additional flash flooding, and the conditional high end flash flood risk, we think a MDT risk upgrade is warranted. This is surrounded by a broader Slight risk, where some flash flooding is possible, but the coverage/organization is expected to be less. Chenard Day 2 Valid 12Z Sat Oct 28 2023 - 12Z Sun Oct 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX INTO THE LOWER OHIO VALLEY...=20 ....20Z Update... Shortwave energy is expected to eject northeast into the Arklatex and eventually the Ohio Valley by later in the period allowing for a continued convective threat over the course of Saturday. Initial surge of energy will induce the best convective threat with the heaviest rainfall rate potential over the course of the period. 12z HREF probabilities for 1"/hr are running between 40-50% across southeastern OK into southwest AR with 30-40% spread across the Ozarks down to the I-30 corridor. This is likely in conjunction with the best ascent focused over the expected stalled frontal boundary over the southern portion of the Ozarks as forecast totals between 2-3" are increasingly likely given the convective scheme and potential for training along the boundary. Local maximum is still around 5" with CIPS analogs indicating multiple events of similar synoptic evolution yielding those types of totals, as well as the 90th percentile NBM indicating higher threshold around the 3-4" range. Showers will continue through the period with some thunderstorm activity anticipated later in the D2 time frame as a secondary shortwave ejects northeast out of TX into southeast OK and AR once again which will bleed into the D3 period. The biggest change will be the convective pattern shifting to a better elevated thunderstorm risk as weak capping in the lower boundary layer is forecast across deterministic, but a skinny CAPE profile above the inversion demonstrates the localized convective threat leading to low probability heavy rains (~1"/hr) for the latter part of the period. This will add to the previous heavy rainfall, creating another flash flood potential despite the rates not being as stout as the previous round. Kleebauer =20 ....Northeast TX into the Lower Ohio Valley...=20 Precipitable water values of 1.5-2.2" are expected near a slow-moving front draped across the region ahead of a strengthening positively tilted upper level trough. The degree of low-level inflow and effective bulk shear available should organize convection into training bands as the flow is fairly uniformly out of the west-southwest with height. MU CAPE of 500+ J/kg should be sufficient to achieve hourly rain totals to 2.5" at times, which would be a problem across area terrain. Local amounts in the 3-5" range are indicated by the available guidance overall. Despite recent dryness, the Slight Risk area from continuity looked within reason which was shifted minimally based upon the most recent model guidance. Since guidance is converging slowly on the expected axis of heavy rainfall, chances of flash flood exceedance are growing. A Moderate Risk cannot be ruled out at some point, especially if available instability increases within model guidance. =20 Roth Day 3 Valid 12Z Sun Oct 29 2023 - 12Z Mon Oct 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST TEXAS INTO THE LOWER OHIO VALLEY... ....20Z Update... Little change necessary to the overall risk areas for the D3 time frame. Stalled front extending from north Texas through the Lower Ohio Valley will be the focal point for continued moderate to locally heavy rainfall on Sunday. Persistent southwesterly flow running parallel to the frontal boundary will induce convective training potential within a very moist environment parlayed across the aforementioned area. Isentropic ascent pattern will also occur across west TX with the frontal boundary slowly pressing south into a modestly unstable environment. This will be another area of interest for the locally heavy rainfall threat within an area of lower FFG's given the recent rainfall, and added urban corridors along and south of I-20. With that in mind, decided to expand the MRGL threat a bit further west into the central and eastern Permian Basin. The heaviest rainfall is forecast across the Arklatex into the Ozarks and lower Ohio Valley as the best combined ascent from a shortwave ejecting overhead of the stalled front located in-of the above area. With the compounding heavy rain events in the Sat-Sun period, the flood threat will linger over very similar areas from D2, thus continuing the SLGT risk located over the areas expected to see the multiday threat. Kleebauer Southern Plains into the Ohio Valley... Precipitable water values will be in slow decline this period, maximizing above 1.75" early on before falling into the 1.25-1.5" range by Monday morning. A positively tilted upper level trough will be approaching the region from the west and north causing some forward progression to the front across Texas and the Lower Mississippi Valley with time. The degree of low-level inflow and effective bulk shear available should organize convection into training bands during the day Sunday as the flow is fairly uniformly out of the west-southwest with height, but should be fading somewhat Sunday night into Monday morning. MU CAPE of 250-500 J/kg should be sufficient to achieve hourly rain totals of 1-2" at times early on, which would be a problem across area terrain and places that rained heavily the preceding day, but the 00z NAM indicates the instability fades with time on Sunday and Sunday night. Local amounts in the 2-4" range are indicated by the available guidance overall. There was some temptation to raising the threat level to a Moderate Risk, particularly early in the period when the parameters for heavy rainfall look best which would account equally for heavy rainfall continuing into Sunday morning and how much rain fell the preceding day, but the minimal instability forecast was the biggest drawback. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4JoSHCiCRdzNN0y_x7GN6kuHEmmEqBIpgMwgTKBqIMjj= 0cRfJctAizgefSbgitInj_lTH7lFURgtJNHtzsxj4vMre6U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4JoSHCiCRdzNN0y_x7GN6kuHEmmEqBIpgMwgTKBqIMjj= 0cRfJctAizgefSbgitInj_lTH7lFURgtJNHtzsxjlojbEBU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4JoSHCiCRdzNN0y_x7GN6kuHEmmEqBIpgMwgTKBqIMjj= 0cRfJctAizgefSbgitInj_lTH7lFURgtJNHtzsxjs8OmfnY$=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) .