Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 04 2023 07:53:52 FOUS30 KWBC 040753 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 252 AM EST Mon Dec 04 2023 Day 1 Valid 12Z Mon Dec 04 2023 - 12Z Tue Dec 05 2023 ....THERE IS MODERATE RISK OF EXCESSIVE RAINFALL FOR THE WEST/SOUTHWEST FACING SLOPES OF THE OLYMPIC MOUNTAINS IN WESTERN WASHINGTON STATE... A strong cold front associated with a 954 mb low in the Gulf of Alaska will move into the coast of WA and northern OR, and stall out there by later tonight. Moisture riding along and ahead of the front will move northeastward into the coast, allowing heavy rain to impact the Willamette Valley and Puget Sound regions between the coastal/Olympic ranges and the Cascades. Many of the meteorological elements continue to remain on track/in place as prolonged (but strong) southwesterly warm advection (which is now underway) across much of the Pacific Northwest, in particular across western Washington into northwest Oregon. This strong WAA will expand south and eastward with time throughout the day. Given the strong flow (over 35-50kts at 850mb and 50+ in 700-500mb layers) over much of the northeastern Pacific with 1.25 to 1.5+ Total PWat values, IVT values will be increasing from 500 kg/m/s nearing 1000 kg/m/s by 03z into the Olympic Range within the core of the greatest moisture flux convergence along/ahead of synoptic height-falls that are slow to slightly below average in eastward progression throughout the day. Given this moist flow is generally orthogonal to the ridge lines of the Olympics, this should support enhanced rain-rates which will start to increase at or above 0.50"/hr with occasional embedded areas in favorable orographic ticking up to 0.75"/hr. This should result in the most impressive rainfall totals on the west facing slopes of the Olympics, where there's good agreement that rainfall totals of 5 to 7 inches (with local amounts up to 9 inches) expected. The new Hi-Res CAM guidance continues to provide higher confidence in localized higher values of 7"+ totals (and 60-80% chance of neighborhood exceedance of 8" through 12z in the Olympic Range). The HREF does denote that rates may not rise to meteorological rates necessary for flash/rapid inundation flooding, given 2"/3-hr probability only reach 50% and less than 10% for 3"/3-hrs; however, given prolonged warm advection over snow pack in the Olympic Range (which is warming through depth), rain on snow will contribute to runoff problems.=20 In coordination with the National Water Center (National Water Model), there are hints of 2-2.5" of snow water equivalent to be utilized about the same time, likely resulting in up to .25"/hr further rates added to the streams and rivers in the area to further enhanced the considerable potential flooding impacts noted in their Flood Hazard Outlook (FHO). So, these combination of factors and agreement from Seattle/Tacoma local forecast office and NWC, a Moderate Risk of Excessive Rainfall was maintained over much of the Olympic Range in support of hydrologically significant (rivers to Moderate/Major flooding) event to unfold today into Tuesday. The Slight and Marginal Risk areas were maintained as well, surrounding the aforementioned westward facing slopes of the coastal/Olympic ranges and Cascades across WA and into northwestern OR. Churchill/Gallina/Wegman Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-s0PY14MQn6NT7rv0XQleUBg3eZ3cYfMlf0yc0evwu7m= BAHVjOrsHTYfTKse9Xc2-yIXl5Rptg5YtZe4c-LF52CH6Og$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-s0PY14MQn6NT7rv0XQleUBg3eZ3cYfMlf0yc0evwu7m= BAHVjOrsHTYfTKse9Xc2-yIXl5Rptg5YtZe4c-LFD42bpIE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-s0PY14MQn6NT7rv0XQleUBg3eZ3cYfMlf0yc0evwu7m= BAHVjOrsHTYfTKse9Xc2-yIXl5Rptg5YtZe4c-LFsgaPuEw$=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) .