Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 07 2023 08:27:46 FOUS30 KWBC 070827 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 AM EST Thu Dec 07 2023 Day 1 Valid 12Z Thu Dec 07 2023 - 12Z Fri Dec 08 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z Fri Dec 08 2023 - 12Z Sat Dec 09 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 3 Valid 12Z Sat Dec 09 2023 - 12Z Sun Dec 10 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PACIFIC NORTHWEST... ....Northwest... An atmospheric river is expected to impact portions of the Pacific Northwest Saturday into Saturday night. This event is not as strong, nor will it last as long, as the recent significant event. Nonetheless IVT is forecast to peak over 500 with an overall duration approaching 24 hours. This should be enough to drop 2-5" of rain across the favored terrain of northwest OR and western WA. Given saturated ground conditions and rivers/streams still running well above average, would expect some additional flooding impacts with this event. There has been a northward trend in just about all the deterministic and ensemble guidance and so the ERO was shifted in this direction as well. Snow levels are lower with this event, so the majority of impacts should be along the coastal ranges of northwest OR and western WA into portions of the Olympics, and thus this is where the Slight risk was maintained. ....OH/TN Valley into the Southeast... A large scale trough and cold front will bring a heavy rainfall threat to a large swath of the OH/TN valley into portions of the Southeast Saturday into Saturday night. A broad Marginal risk was maintained stretching from LA to GA and north to OH, where 1-3" of rainfall is generally expected. The best instability is forecast across the southwest portion of this risk area, mainly from LA into eastern AR and much of MS...and thus would expect to see the highest rainfall rate potential across this corridor. Meanwhile the stronger and more persistent mid/upper forcing is located to the north and east of this area, generally across northern MS/AL and into portions of TN and KY. It is here where the models are indicating the highest QPF totals, with 2-3" areal averaged amounts a possibility. Although this area will have a bit longer duration of stronger forcing, the system as a whole is still looking pretty progressive off to the east...which should put a cap on the upper bound of rainfall magnitudes. There very well may be a targeted Slight risk somewhere within the broad Marginal, but for now not confident enough in the location to go with an upgrade at this point. Most of this area remains in drought conditions, with below normal soil saturation and streamflows...indicating it will likely take more rain than normal to start getting flood impacts. The northern axis from northern MS into KY has the best model QPF signal and forcing to work with...but questions regarding duration and the degree of instability suggest the flash flood risk is still marginal at this time. Areas further southeast over LA into MS should have a higher threat or more intense rainfall rates...but questions exist to the degree of more upscale convective organization, so think a Marginal should suffice here as well. At this point tend to think the most likely location where we may see overlap of stronger instability and organized/brief training convection may be central to northern MS...but would prefer to see more model agreement before upgrading to a Slight. Thus overall would characterize this event as having high confidence in producing a localized flash flood threat over a broad area...but lower confidence on if and where any more concentrated risk could evolve. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5LfV5HqCzI5q9QtVHWFcVlxNEo_yPQnCnVWGU0Q0t4xE= O-YI2i2YVebnAsJeagnjHWNYU_3sxHVKoud1KJNA8xkvTTM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5LfV5HqCzI5q9QtVHWFcVlxNEo_yPQnCnVWGU0Q0t4xE= O-YI2i2YVebnAsJeagnjHWNYU_3sxHVKoud1KJNAhbLTTSw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5LfV5HqCzI5q9QtVHWFcVlxNEo_yPQnCnVWGU0Q0t4xE= O-YI2i2YVebnAsJeagnjHWNYU_3sxHVKoud1KJNA-PhA8nE$=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) .