Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 16 2024 08:28:41 FOUS30 KWBC 160828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 327 AM EST Tue Jan 16 2024 Day 1 Valid 12Z Tue Jan 16 2024 - 12Z Wed Jan 17 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z Wed Jan 17 2024 - 12Z Thu Jan 18 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF FAR NORTHWEST CALIFORNIA INTO SOUTHWEST OREGON... We opted to introduce a small Marginal risk across portions of far northwest CA and southwest OR with this update. A strong mid level shortwave will move across western OR between 06z-12z Wednesday which will result in an uptick in rainfall intensity. After 12z onshore upslope flow will continue to be directed into this region to the south of a surface low expected to be positioned off the WA coast. This is not the strongest core of IVT, but it is persistent enough to result in up to a 12 hr period where 0.25"-0.5" rainfall will be possible within the favorable western facing terrain. The consensus of current guidance would suggest 09z-21z Wed as the most favored time for these heaver rainfall rates. The 18z experimental RRFS ensemble supports 20-50% chance of 0.5"/hr rainfall during that window, with the 00z ECMWF depicting 1"/3hr rainfall. There remains some model spread with regards to the axis and magnitude of heaviest rainfall amounts. The GFS remains a bit lighter, but the 00z ECMWF/NAM/UKMET/HRRR all support some heavier totals into this region. There are several pieces of guidance that suggest 2 day rainfall amounts upwards of 4-7" over this area, which may begin to cause some mainly minor runoff concerns. Chenard Day 3 Valid 12Z Thu Jan 18 2024 - 12Z Fri Jan 19 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS NORTHWEST OREGON INTO WESTERN WASHINGTON... We will maintain a Marginal risk across portions of northwest OR into western WA with this update. A well defined Atmospheric River will be positioned offshore Thursday and the main question becomes how much of the elevated IVT values make it onshore. The 00z GEFS and 00z ECENS generally suggest the main core of IVT stays just offshore, which would likely limit flood impacts over the area. However it is close, and model guidance has been bouncing around with the exact positioning of this IVT and heavier rain axis. The 00z NAM and UKMET are more aggressive in bringing heavier rain onshore and would bring a higher threat of 3"+ rainfall totals. The latest NBM probabilities of exceeding 2" within the 24hr period are 30-70% (highest in the Olympics), with 3"+ probabilities of 20-40% over the Olympics. Overall this appears like a lower end Marginal risk at the moment, and some chance it is eventually removed altogether. However with it just being introduced on the previous shift, and the possibility of the IVT plume shifting a bit east and focusing 3"+ rain into the region...we think it is best to maintain the Marginal risk for now. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6bs663k4B2g3xHcppZLNBQRhsNQmELW51T-eNKggSV9A= YSevbl0zXqGSYYP4vTIfgnYwVOF4Kg-ZJZufR09K4a7SnHc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6bs663k4B2g3xHcppZLNBQRhsNQmELW51T-eNKggSV9A= YSevbl0zXqGSYYP4vTIfgnYwVOF4Kg-ZJZufR09KckHOCL0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6bs663k4B2g3xHcppZLNBQRhsNQmELW51T-eNKggSV9A= YSevbl0zXqGSYYP4vTIfgnYwVOF4Kg-ZJZufR09KYoGKI_k$=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) .