Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Nov 21 2024 00:46:08 FOUS30 KWBC 210045 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 745 PM EST Wed Nov 20 2024 Day 1 Valid 01Z Thu Nov 21 2024 - 12Z Thu Nov 21 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... 01Z Update:=20 Latest MRMS QPE and observed rainfall reports are starting to indicate more .50+ in/hr rates along the NorCal coastal ranges, especially northwest of Santa Rosa. Expect these areas of heavier rainfall rates to expand northward overnight, including the King Range south of Eureka, especially after 06Z per the latest (18Z) HREF probabilities. Given the latest trends, there is no need to change the configuration of the current Moderate, Slight, and Marginal Risk areas. Hurley ----Prior Discussion---- The latest model guidance remains consistent with the overall evolution of the strong multi day atmospheric river event into northwest California. The low that deepened rapidly over the northeast Pacific Tuesday, will continue to anchor deep layered west southwest flow on the south side of this system, impacting northwest California day 1 with IVT values of 500-800 km/m-1 s-2. and 850-700 mb moisture flux anomalies of 2-4+ standard deviations above the mean. There is fairly good model agreement with heavy precip totals of 3-5" primarily over northwest California and the northern Sierra, with isolated totals of 6-8" possible. HREF neighborhood probabilities are very high for 5"+ amounts across northern California, with most areas showing 80-90%+, while 8"+ probabilities are 40-80% along the northwest CA coast and 40-60%+ over the northern Sierra. Hourly rainfall amounts of .25-.50"+ likely through the day 1 time period across northwest CA with increasing flooding risks with time as soils become saturated and streams and river levels rise. The HREF 1 hour probabilities for ..50"+ totals remain high in the max inflow band across northwest CA for all of day 1. This band does continue to sink south for the first few hours of day 1, but then become nearly stationary for the remainder of the day 1 time period. No significant change to the previous outlook. The slight risk was extended approximately 75 miles farther south in the lee of the northern Sierra where 2-4" rainfall totals are possible. Oravec Day 2 Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 22 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... Life-threatening flooding across coastal Northwest California is expected due to the very strong and long duration atmospheric river Thursday into Thursday night. Dangerous flooding, rock slides, and debris flows are likely. The strong Atmospheric river event into northwest California will continue into day 2 as deep layered southwesterly flow continues to the south of the northeast Pacific upper vortex. The anomalous deep layered southwesterly flow will support 850 to 700 mb moisture flux anomalies of 2 to 4+ standard deviations above the mean and IVT values in the 500 to 800 km/m-1 s-2 range across northwest CA. During the second half of Day 2, a strong vort max rotating around the base of the northeast Pacific vortex will support another rapidly deepening low moving northeast approximately 200 miles off the Pacific Northwest coast. While this second rapid cyclogenesis event will not be as strong as the Tuesday event, it will help to begin to push the best onshore flow axis back to the north during the second half of day 2, returning heavy precip into southwest Oregon. There is a likelihood of very heavy rains again falling Day 2 over similar regions from Wednesday. Hourly rates of 0.50"+ are likely at times supporting additional 24 hour rainfall amounts of 3-6" and isolated additional 24 hour totals of 6-8" across northwest CA into southwest OR. Storm total amounts of 12-16"+ possible across portions of northwest California. Given the 2 day totals in the same region, no significant changes were made to the previous High Risk area. Significant flood risks will continue to increase Day 2 along with rock and land slides as soils become saturated and streams and rivers continue to rise and overflow. Ahead of the next deepening low, rising heights over northern California will keep much of the precipitation in the form of rain. This will increase the runoff threats for areas in the lee of the northern Sierra. Taylor Day 3 Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN CALIFORNIA... The long duration atmospheric river does begin to wane throughout this period but at least for the first 6-12 hours, heavier precipitation with the final push of higher moisture is expected across coastal Northern California into southwest Oregon as well as portions of the northern Sierra Nevada. Warmer temperatures will keep snow levels pretty high such that even into the terrain areas, most of the precipitation is expected to fall as rain (aside from the highest peaks). This factor along with the rainfall totals expected over the course of the next 3 days will heighten the flooding risk, particularly for sensitive areas including burn scars. The Moderate Risk remains generally unchanged given the consistent model guidance and lack of significant changes in the QPF. In the northern Sierra Nevada, max amounts of 2-4" are expected - the latest PQPF shows high chances for at least 2" (above 80%) and moderate values for 4" (30-40%) in the terrain. For the lower elevation areas and the coastal ranges, additional amounts of 1-2" are expected. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9tqPnJf6fP8XjuixRzUbZucTU1CymzPLO01FpIyqhcaL= NCAt2ba6bIJx7RovwwGUlG0bE8LuoN3RePARGmi9XGd5W3o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9tqPnJf6fP8XjuixRzUbZucTU1CymzPLO01FpIyqhcaL= NCAt2ba6bIJx7RovwwGUlG0bE8LuoN3RePARGmi9njq2KQ0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9tqPnJf6fP8XjuixRzUbZucTU1CymzPLO01FpIyqhcaL= NCAt2ba6bIJx7RovwwGUlG0bE8LuoN3RePARGmi9JqzmG3U$=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) .