Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 06 2024 20:00:31 FOUS30 KWBC 062000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 300 PM EST Wed Nov 6 2024 Day 1 Valid 16Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... 1600Z Update... A Marginal Risk area has been added to portions of central to northeast LA and also a small part of southwest MS to account for the ongoing slow-moving areas of heavy showers and thunderstorms across the region that are focusing along and out ahead of a quasi-stationary front. Please consult MPD #1131 for more details on this. Convective trends across this region will continue to be closely monitored through the afternoon hours for additional convective development. Elsewhere, the 12Z HREF guidance and recent HRRR guidance continues to support the development and expansion of broken areas of heavy showers and thunderstorms across the interior of the Southeast beginning later this afternoon and heading into the overnight hours. However, there has been a trend to concentrate the heaviest convective rainfall threat a bit farther south and east and a bit deeper into the coastal plain of southern and eastern GA and the SC Lowcountry. Additionally, there are multiple HREF models that are quite heavy with the rainfall potential across parts of the FL Panhandle. Regionally, the potential exists for as much as 4 to 8 inches of rain, with some isolated 10+ inch totals being supported by 12Z/Thursday. The 12Z HREF guidance shows 50 to 80 percent probabilities of seeing 5+ inches of rain from eastern parts of the FL Panhandle northeastward into the SC Lowcountry, and in a focused manner across parts of east-central GA and along the SC border, there are 60 to 80 percent probabilities of seeing 8+ inch rainfall totals. The heavy rains are again expected to be the result of increasing moisture transport poleward of Hurricane Rafael and interaction with an inverted surface trough, and with right-entrance region upper-level jet dynamics providing deeper layer ascent. Given the setup, which will favor rainfall rates of 2 to 3+ inches/hour and these impressive totals, there will be a strong concern for areas of high-impact flash flooding. These latest trends have necessitated the need to adjust the Slight and Moderate Risk areas farther south and east. Related to Hurricane Rafael, there may be some potential for outer bands of convection to impact portions of the lower FL Keys going through tonight, and as a result the Marginal Risk area has been maintained for this area. Orrison Previous discussion... In coordination with CAE/Columbia, SC and FFC/Peachtree City, GA forecast offices, a Moderate Risk upgrade was introduced with this update. A plume of deep tropical moisture, indirectly associated with Hurricane Rafael will continue overspreading portions of the Southeast today. The moisture plume will run into a developing front over Georgia and South Carolina. An upper level jet streak will add lift to the atmosphere. Late today into this evening, the nocturnal low level jet's strengthening will kick the moisture advection into high gear. Frontogenesis occurring over Georgia and South Carolina will act to focus the heavy rain over the narrow Moderate Risk corridor. Expect training convection to develop during the evening which will persist through the overnight and while slowly weakening in the pre-dawn hours, is likely to continue into western South Carolina into Day 2/Thursday. HREF probabilities show a 20 to 25 percent chance of a 100 year rainfall event in the Moderate Risk area. FFGs are very high in the area due to recent dry conditions with only very light rain. This has likely made some of the clay soils quite hydrophobic. The hydrophobic soils will likely increase the runoff from the heavy rain, which will likely quickly exceed FFGs as the heavy rain likely persists for multiple hours. Wegman Day 2 Valid 12Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AS WELL AS PORTIONS OF TEXAS PANHANDLE AND WESTERN=20 OKLAHOMA... 20z Update: Slight risk reasoning still holds over GA and SC. Only adjustment was to expand the risk towards the coast following=20 model trends. Convection Thursday evening into the overnight hours should drive=20 at least some flash flood risk over portions of TX and OK. Plenty=20 of large scale forcing and moisture in place to drive a widespread=20 convective risk...with the main uncertainty more relating to the=20 degree of instability present. The inherited Slight risk was=20 shifted southward with this update into more of TX. This is=20 supported by the 12z HREF probabilities and the 12z GEFS based=20 machine learning ERO guidance. The area encompassed by the Slight=20 risk should see slightly better instability which will allow for=20 the potential of higher rainfall rates. Chenard ....Previous Discussion... ....Southeast... Heavy rain will be ongoing across Georgia and especially South Carolina at the start of the period Thursday morning. While the rainfall will be diminishing in intensity, it's expected after the much heavier rains of Wednesday night, that the lighter totals forecast for Thursday will still be influential since antecedent conditions will be radically different by then as compared to where they are now. The Slight Risk threat largely is for the Thursday morning period with lingering rainfall from Wednesday night. Expect the rain to continue diminishing through the day as the moisture plume shifts westward with Hurricane Rafael. ....Texas Panhandle and Western Oklahoma... A positively tilted longwave trough and associated upper level energy in the form of jet streaks and shortwave energy will support the development of a strong low over the southern Plains. To the west, a significant amount of the low's moisture will fall in the form of snow. Where the precipitation will remain as rain, expect steady heavy rain across the Texas Panhandle and western Oklahoma. While rates are not generally expected to be notably heavy, the flood threat will be primarily focused on the long duration of steady rain, as well as the fact that much of this area was hit very hard with heavy rain a few days ago, so soils and rivers remain elevated in this region. Thus, low FFGs support the Slight Risk upgrade. Wegman Day 3 Valid 12Z Fri Nov 08 2024 - 12Z Sat Nov 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... 20z Update: The Slight risk over the Plains was adjusted a bit southward with this update. This aligns well with the forecast overlap of instability and moisture convergence. We continue to carry higher end Slight risk probabilities across portions of TX into western OK. Given the above average soil saturation already=20 in place, and the forecast overlap of rainfall Thursday and Friday=20 over this area, some instances of flash flooding are becoming=20 increasingly likely. Still some uncertainty with exactly where the=20 best overlap of convection and thus highest 2 day rainfall totals=20 will be...with the favored axis currently from north central TX=20 into southwest OK, where 3-5" is forecast. If confidence in this=20 area increases and/or totals trend up over 5" then a MDT risk=20 upgrade may eventually be needed. Chenard ....Previous Discussion... The upper level low over the Southwest will eject into the Plains on Friday. There will be significant upper level energy associated with the low, which combined with strong frontogenesis, plentiful Gulf moisture advecting up the Plains on the low level jet, and a strengthening surface low...expect heavier rainfall amounts across Oklahoma and Kansas on Friday as compared with Wednesday. While north Kansas didn't see as much rain a few days back compared with areas further south...much of the rest of the Slight risk area does overlap with both the heavy rain from a few days ago but continued rainfall from Day 2/Thursday. Thus, added an internal higher-end Slight for western Oklahoma into south Kansas. Depending on how much rain the area gets on Day 2, there is some potential for a Moderate Risk upgrade in this area with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-kcT4Kv2phf6IWgBnDtkWv3_-b850FTZ9A_mmVI7hbPV= J6L_vi4Dvy3_IZTS93tHIFjNSm9Xk2rCHJ6PX-Rcp2EqBNA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-kcT4Kv2phf6IWgBnDtkWv3_-b850FTZ9A_mmVI7hbPV= J6L_vi4Dvy3_IZTS93tHIFjNSm9Xk2rCHJ6PX-Rc-ZphcGE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-kcT4Kv2phf6IWgBnDtkWv3_-b850FTZ9A_mmVI7hbPV= J6L_vi4Dvy3_IZTS93tHIFjNSm9Xk2rCHJ6PX-Rcjiz1sso$=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) .