Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 09 2024 00:18:07 FOUS30 KWBC 090017 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 717 PM EST Fri Nov 8 2024 Day 1 Valid 01Z Sat Nov 09 2024 - 12Z Sat Nov 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... ....16z Update... Little change needed for the 01Z update...with the biggest changes resulting from adjustments to better fit early evening radar and satellite imagery and latest CAM trends. Much of the western part of the Slight Risk was removed as cooler and drier air sweeps eastward behind a steadily progressive cold front.=20 Early evening radar showed several lines of showers and=20 thunderstorms were still poised to cross the Slight Risk area this=20 evening and overnight...where a 25 to 35 kt low level jet continued to tap deeper moisture from the Gulf of Mexico and established an axis of precipitable water values exceeding 1.75 inches that extended across eastern Texas into portions of eastern Oklahoma and nearby Louisiana and Arkansas. In this area...the Slight Risk is=20 considered to be on the higher- end of the probability spectrum=20 (25%+), as this region has the best chance of realizing the highest rainfall rates (1-2"/hr) and resultant short- term totals (20-30%=20 odds of 5" exceedance, per 40-km neighborhood HREF probabilities=20 through 09/12Z). Drier antecedent conditions in this region should=20 limit the extent and intensity of flash flooding (with FFGs=20 generally ranging from 3-4", coverage should remain scattered and=20 below significant flash flood thresholds). Bann Day 2 Valid 12Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL LOUISIANA... 20z Update: MDT risk upgrade across portions of central LA with this update. Flash flooding is becoming increasingly likely across this corridor, with locally significant impacts possible. Favorable setup for heavy rainfall with moisture streaming north from Rafael into the stalling front and upper jet. Global models and HREF members are both showing a heavy rainfall threat in their QPF as well. There remain some differences in the location of heaviest rain, but generally seeing a consensus towards central LA. We've got 3-5" of rain in the forecast, but there's a reasonable possibility of localized 6-8" amounts. Some flash flooding is possible Saturday morning, but an uptick in the coverage and magnitude of impacts is expected during the afternoon and evening hours. Chenard ....Previous Discussion... An upper level low will move faster towards the northeast. As Rafael moves west over the central Gulf, a plume of moisture with precipitable water values of 1.5-2" gets drawn north ahead of the surface low's strong cold front. The front will likely be the dominant forcing for storms in this region. As on Friday, the southern areas of Louisiana and Mississippi will have the greater moisture. MU CAPE rises towards 1000 J/kg across parts of LA/MS/TN. Areas further north have soils which are more saturated/flash flood guidance is lower. The front should progress slow enough that training storms tracking north along the front could still cause flooding problems, especially over portions of LA, MS, and TN. Hourly rain totals to 2.5" and local amounts of around 6" appear likely. Lingering disagreement on exactly where the maxima are expected kept the risk level from being raised to Moderate, though an upgrade remains possible in future updates should this placement issue resolve itself and model QPF magnitudes don't back off. Roth Day 3 Valid 12Z Sun Nov 10 2024 - 12Z Mon Nov 11 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST, LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY... AND UPPER OHIO VALLEY... 20z Update: No changes to the inherited Marginal risk areas, as they still look in good shape. May eventually be able to trim back the northern extent of the Marginal over PA/OH/WV given the dry soil conditions and lack of instability...but will maintain continuity for now with PWs forecast 2-3 SD above average and decent forcing in place. Chenard ....Previous Discussion... Central Gulf Coast northeast towards southwest Pennsylvania... Moisture and instability from the Gulf gets advected from the vicinity of Raphael northeast across portions of the Central Gulf Coast through the Tennessee and Upper Ohio Valleys. The forward progress of the front across much of the East should keep any excessive rainfall concerns minimal -- some of the risk area from eastern AR to the east-northeast just south of the OH river accounts for recent rains/some soil saturation. The latest NCEP and CMC guidance suggests that Raphael gets much closer to the Gulf Coast than the NHC forecast, which led to avoiding use of the GFS, NAM, and Canadian guidance explicitly. Since the distance of the storm from the region at this time frame is unclear, it's also unclear how high precipitable water values get, though they should rise above 1.5", particularly in the southern portion of the area near the coast, which should be enough for heavy rain concerns. It appears that enough instability could be available along the immediate Gulf Coast for convection with heavy rainfall. Hourly rain totals to 2" with local totals to 5" appear achievable along or near the immediate Gulf coast. Whenever better agreement amongst the guidance on the future of Rafael as well as the placement any rainfall maximum near the coast occurs, a Slight Risk could prove useful at that later time. To the north, hourly totals up to ~1" with local totals to 2" appear to be the maximum potential. Pacific Northwest... Precipitable water values rise to 0.75-1" with a front moving into the area. Inflow from the Pacific rises to 50-60 kts from the south-southwest near the front. In the wake of the front, instability moves in due to colder air aloft. Hourly rain totals exceeding 0.5" appear likely here, which would be most problematic in burn scars. Left the Marginal Risk area intact though simplified the area's appearance. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_BmXW38OVE_2Fy8jgwwwbUplgRaVOsegIc6eSmO1bsfY= g-ai0IxCYLOwvBjkkickJgaBK1XBUpFxHSIKoLlC2AYWzqA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_BmXW38OVE_2Fy8jgwwwbUplgRaVOsegIc6eSmO1bsfY= g-ai0IxCYLOwvBjkkickJgaBK1XBUpFxHSIKoLlCUIgBNM0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_BmXW38OVE_2Fy8jgwwwbUplgRaVOsegIc6eSmO1bsfY= g-ai0IxCYLOwvBjkkickJgaBK1XBUpFxHSIKoLlCMeZSMwc$=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) .