Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Oct 22 2023 23:26:52 FOUS30 KWBC 222326 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 724 PM EDT Sun Oct 22 2023 Day 1 Valid 01Z Mon Oct 23 2023 - 12Z Mon Oct 23 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth/Mullinax Day 2 Valid 12Z Mon Oct 23 2023 - 12Z Tue Oct 24 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN PARTS OF THE SOUTHERN PLAINS... ....Southern Plains... Monday kicks off what will be a stormy week throughout portions of the Nation's Heartland. The synoptic setup features a closed upper low over California Monday morning diving southward towards northern Baja California by Monday night. Farther east, a large dome of high pressure over the East Coast will provide an ideal setup for a strengthening low level jet (LLJ) Monday afternoon and into the overnight hours. 850mb winds by 00Z Tuesday will increase to as high as 35 knots in south-central Texas, which is above the 90th climatological percentile according to NAEFS. The LLJ will act to advect increased levels of 850mb moisture flux as far west as the TX/NM border, and as far north as the central Plains. The "x-factor" in this setup is Norma as its remnant 700mb moisture tracks from northern Mexico into the Southern Plains Monday afternoon and into the overnight hours. There is additional synoptic-scale support that comes in the form of a 200mb jet streak forming over the Central Rockies and Plains, which in turn is placing its divergent right-entrance region over Texas. PWs across West Texas will be considerably anomalous for late October. NAEFS shows values from the Big Bend on northeast to the Permian Basin topping 1.5", which would top the 99th climatological percentile. Sampled model soundings from the Rio Grande on north and east to as far as the Red River sport warm cloud layers up to 12,000ft deep and modest MUCAPE ranging between 500-1,000 J/kg. There is also supportive sfc-6km wind shear values as high as 30 knots that can supportive mesocyclones in some developing thunderstorms, thus making them potentially more efficient rainfall producers. The prolonged LLJ in tandem with a favorable synoptic scale setup could also result in training or back building convection Monday night and persisting into Tuesday. The 12Z HREF shows 3-6 hr QPF > 3-6hr FFG probabilities that are in some cases up to 20-25% Monday night. The variable limiting the flash flood threat initially is the area sorely needs rainfall and soils have been rather parched over the past couple weeks. It is worth noting, however, should >2"/hr rainfall rates occur, soils can be so dry that they are not able to soak in the more excessive rainfall rates quick enough. This in turn can lead to rapid runoff on surfaces and in poor drainage areas. Given these reasons, have hoisted a Marginal Risk from the Big Bend to the Red River where the best mix of anomalous PWs and instability support >2"/hr rainfall rates within potentially training cells is likely to occur. Mullinax Day 3 Valid 12Z Tue Oct 24 2023 - 12Z Wed Oct 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... 20Z Update... ....West Texas northeastward into the the Central Plains... Most deterministic and ensemble guidance has maintained an overall good consensus on the QPF footprint from the Rio Grande River on north and east to the Upper Great Lakes. By Tuesday morning, the remnant 700mb vorticity lobe that was associated with Norma is expected to be over West Texas and become fully absorbed into the upper trough to the west by Tuesday afternoon. By 00-06Z Wednesday, NAEFS shows PWs that are above the 99th climatological percentile from eastern Kansas to southern Iowa. Area averaged soundings also showed highly saturated profiles containing up to 500J/kg of MUCAPE as far north as eastern Kansas. ....Western Great Lakes... Made some minor adjustments based on latest QPF, but the overall reasoning provided from overnight still looks good. Warm cloud layers appear rather deep for late October and should yield efficient rainfall rates, but given the lack of instability, most rates should be manageable enough to keep any flooding concerns to highly localized/poor drainage areas. Mullinax ---Previous Discussion--- ....West Texas northeastward into the the Central Plains... Flow aloft over the portions of the lower Gulf Coast will back and become increasingly difluent in response to the approach of a mid- and upper-level low marching across northern Mexico during the period. This leaves the area from southern Texas across Oklahoma and into southeast Kansas in a region of moist and confluent flow. Precipitable water values should be in the 1.5 to 1.75 inch range at the start of the period and then increase to nearly 2 inches in parts of Texas by evening given the continued moisture transport off the Gulf of Mexico. This...combined with the divergence associated with an southerly upper level jet and the approach of the a surface cold front...should set the stage for thunderstorms capable of producing locally heavy rainfall rates and a some 2 to 4 inch rainfall accumulations by early Wednesday morning. By 25/00Z...the Precipitable Water plume will be 2 to 3 standardized anomalies greater than climatology for this time of year while the 1000 mb to 850 mb moisture flux values range from 2 to 4 standardized anomalies above climatology will keep replenishing moisture ahead of the system. The 22/00Z UKMET was an easterly outlier among the operational models...which seems like the least likely outcome given the placement of the upper level energy. ....Western Great Lakes... There is a Marginal Risk area over portions of the Upper Peninsula of Michigan into northwest Wisconsin as energy in the northern stream taps into moisture being drawn northward ahead of the Southern Plains system. A cold Tuesday afternoon...upper level divergence will be increasing as the axis of 120 kt to 135 kt upper jet and works in tandem with low level moisture flux convergence along and ahead of a surface cold front. With could result in locally intense rainfall rates...with the Canadian and UKMET both showing 24-hour rainfall amounts around 2.5 inches while the ECMWF and NAM produced roughly half of those amounts.=20 Given the relatively dry antecedent conditions over the past couple of weeks...only isolated problems are anticipated even if the heavier rainfall amounts verify. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9SGCmDl7vk4TGA01zyoGt598mCScGWoP6Mb64Ho7PYrJ= mORoHtvWiT8zmuIr34LJU-WP8kNLmtcYf1Y3HWc18Nz1BEM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9SGCmDl7vk4TGA01zyoGt598mCScGWoP6Mb64Ho7PYrJ= mORoHtvWiT8zmuIr34LJU-WP8kNLmtcYf1Y3HWc1t76y9Nk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9SGCmDl7vk4TGA01zyoGt598mCScGWoP6Mb64Ho7PYrJ= mORoHtvWiT8zmuIr34LJU-WP8kNLmtcYf1Y3HWc1o8FOaSw$=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) .