Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 01 2024 00:39:12 FOUS30 KWBC 010038 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 838 PM EDT Thu Oct 31 2024 Day 1 Valid 01Z Fri Nov 01 2024 - 12Z Fri Nov 01 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF LOUISIANA AND MISSISSIPPI... A Marginal risk remains from portions of central LA into southern and central MS. Localized cell mergers and training may result in a few instances of flash flooding over portions of these areas. PWs around 2" and MLCAPE of 1000 j/kg are supportive of efficient rainfall with a few instances if 2-3" per hour rainfall expected where cells briefly train. Some cell merging and training is expected over portions of southwest to south central MS through=20 the evening hours where a slowing line of convection is=20 intersecting cells moving northward within an axis of southerly=20 flow. Guidance indicates that the axis of enhanced convergence that is driving this threat should weaken within a few hours, with=20 convective intensity also decreasing. However the MCV that is=20 evident over central LA may help sustain this convergence a bit=20 longer than the HRRR indicates...and do think isolated 2-4" amounts within a couple hours are possible. Instability is a tad higher over LA, and convergence near the remnant MCV may linger longer here. While differing in location from run to run, recent HRRR runs indicate the potential for a few slow moving convective cells/clusters over central LA tonight. In the HRRR these cells are anchoring near the instability gradient/boundary and are able to persist long enough to produce localized amounts in excess of 6". Confidence in only average at=20 best, but the HRRR scenario is plausible given the setup and=20 ingredients in place. The main question will be whether the MCV=20 stays pronounced enough to support continued low level moisture=20 transport/convergence into the instability gradient. Dry antecedent conditions have resulted in high FFG values over=20 the region. Thus flash flood issues should generally stay very=20 localized in nature and focused within any more sensitive urban=20 areas where excess runoff from these heavy rates is more likely.=20 However if the HRRR scenario of 6"+ amounts over portions of LA=20 pans out overnight, then a more robust flash flood threat could=20 evolve, so will need to monitor trends through the overnight hours. Chenard Day 2 Valid 12Z Fri Nov 01 2024 - 12Z Sat Nov 02 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND ADJACENT SOUTHERN ROCKIES... 2030Z Update... Introduced a targeted Slight Risk area in southeast New Mexico where an uptick in model QPF and model-derived ingredients overlap with an area that was soaked less than 2 weeks ago and remains hydrologically sensitive...with the expectation that the risk is best late in the period and continuing into the Day 3 period. The maximum Areal average rainfall amount looks to be around an inch...which normally wouldn't warrant a Slight (especially with the given the high flash flood guidance being so high). But with the trends of increasing precipitable water, thetae and model QPF over over and near the site of the flash flood...felt a Slight was appropriate here while a Marginal remained more appropriate elsewhere from the plains of southeast New Mexico to western Oklahoma. Bann 0830Z Excessive Rainfall Discussion... Another shortwave trough will move inland across the West on Friday as ridging stays in place over much of the Southeastern US. Warm Gulf moisture will be advecting northward into the southern High Plains ahead of the trough. Meanwhile, the strongest instability (1000+ J/kg of SB CAPE) will likely be in place over West Texas and eastern New Mexico. Areal coverage of thunderstorms is forecast to increase during the overnight hours (after 00z) into portions of southeastern NM and the TX Panhandle, propagating eastward into portions of western OK Oklahoma and far southern Kansas. Maintained the Marginal Risk from prior cycles, given PWs increasing to 1.0"+ (above the 90th percentile, per SPC sounding climatology). While most CAMs still do not reach out to the timeframe of interest, the ones that do (the FV3 and CMCreg) suggest the potential for localized 3"+ totals, as well as support from the downscaled ECMWF. Decided to maintain the Marginal for this cycle, given considerable uncertainty and dry antecedent soil conditions. Churchill/Campbell Day 3 Valid 12Z Sat Nov 02 2024 - 12Z Sun Nov 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... 2030Z Update... Few changes were needed given fairly decent run to run agreement with the scenario of a much more potent longwave trough digging into the Southwest US that results in more organized convection capable of producing heavy rainfall. Opted to not to jump all the way to the full extent suggested by the ML ERO guidance...but felt an expanded areal coverage was warranted based on the placement suggested by spaghetti plots of QPF from the ensembles as well as the global models combined with the idea that the rainfall on Day 3 will function to prime the soils for more runoff/flooding potential on Day 4 and beyond. Will continue to watch how much overlap there is and upgrade if necessary. Until then...thinking is that the risk from Saturday into early Sunday is a high-end Slight Risk from parts of Oklahoma into far north Texas. Bann 0830Z Excessive Rainfall Discussion... A much more potent longwave trough is progged to dig into the Southwest US on Day 2, as leading shortwave impulses are expected to result in much more organized convection over much of the Southern Plains. Continued southerly low-level flow ahead of the impulses should result in PWs pushing 2" (near record levels, per SPC sounding climatology) with ample forcing (left exit region of 100+ kt subtropical jet streak) for organizing convection. The WPC QPF calls for areal average totals of 2-3", which is still well below the 90th percentile of the PQPF (~4-5"). Given strong ensemble support, the inherited Slight Risk was expanded a bit (with the expectation of verification on the higher-end of the Slight Risk probability spectrum from Northwest TX/Big Country into south-central OK). Given very dry antecedent conditions, a Slight Risk was deemed sufficient (but will need to be reevaluated for upgrade potential with future updates). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5_UoLTmQYR0fuAD228hQet4eWbUzO9WiC5fyKCKwxcHw= fxofNhFm06fWd8KK9UIOapAq20XbRJXjHNI_w7wmcIEChU0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5_UoLTmQYR0fuAD228hQet4eWbUzO9WiC5fyKCKwxcHw= fxofNhFm06fWd8KK9UIOapAq20XbRJXjHNI_w7wm6vDvbpI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5_UoLTmQYR0fuAD228hQet4eWbUzO9WiC5fyKCKwxcHw= fxofNhFm06fWd8KK9UIOapAq20XbRJXjHNI_w7wmFbu04ek$=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) .