FOUS30 KWBC 230748 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 248 AM EST Sat Nov 23 2024 Day 1 Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SIERRA NEVADA... A cold front and the remnants of the long duration, multi-day atmospheric river that has been impacting the West Coast will be dropping south across the central Sierra Nevada early this morning. As the deeper layer moisture flux parameters/IVT magnitudes continue to weaken and thin out, the rainfall rates will be continuing to drop off. However, there still may be a sufficient level of upslope flow/orographic ascent in the foothills of the central Sierra Nevada for a few more hours early this morning of spotty 0.25" to 0.50"/hour rainfall rates. Some weak post-frontal instability is also still forecast which could allow for some scattered pockets of heavy shower activity going through midday to impact some of the western slopes of the central Sierra Nevada. This could result in some isolated quick- hitting 0.25" to 0.50" rainfall amounts, but these convective cells are forecast to be progressive. Overall, the additional storm total amounts based off the 00Z HREF guidance may reach as high as 1 to 2 inches. Given the very wet antecedent conditions, the additional rains may foster some additional localized runoff/flooding concerns. Thus, the Marginal Risk area over portions of the Sierra Nevada will be maintained. Orrison Day 2 Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON... Locally heavy rain will be possible Sunday into Sunday night over northwest CA into far southwest OR. A front slowing near the coast and a weak low to mid level wave riding along it will support a modest uptick in IVT and plentiful low level convergence. Moisture is a limiting factor for heavy rainfall with the better moisture plume situated well south and not really getting drawn into this system. Thus this is not expected to be a significant atmospheric river, and normally would not expect a system of this nature to pose a flood risk. However given the significant rainfall over the past few days, this additional rainfall may be enough to cause some localized additional flooding as the area will be more hydrologically sensitive than normal. We are still seeing some spread in model QPF output over this period. If some of the more aggressive solutions pan out then we could see 1-3" of rainfall is spots with hourly totals peaking around 0.3-0.4" in the typically more favored upslope areas. Chenard Day 3 Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON... Showery conditions will likely continue across portions of northern CA and southwest OR Monday into Monday night as a deep layered low and shortwave energy moves inland. Certainly not a classic setup for flooding with limited moisture and dropping IVT values...however this area remains more hydrologically sensitive than normal following the significant atmospheric river event the past few days. Additional rainfall over far northwest CA into southwest OR is generally 1-3" in the guidance, with isolated totals of this magnitude also possible over the western slopes of the northern Sierra Nevada. A more significant plume of moisture/IVT will actually move into portions of southern/central CA...generally from portions of San Luis Obispo and Monterey counties into the western slopes of the southern Sierra Nevada. QPF has been trending up with this system, although still seeing some model spread. The latest model consensus suggests a possible 2-3" of rain along the coastal ranges and the southern Sierra below snow levels. Not seeing any instability with this system and large scale forcing is only modest, so hourly rainfall rates are generally not expected to exceed 0.5". Given this, and the fact that most of this area has not seen significant rainfall recently, will continue with no areas in the excessive rainfall outlook for now. Chenard Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt