Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
105 FOUS30 KWBC 240051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 751 PM EST Sat Nov 23 2024 Day 1 Valid 01Z Sun Nov 24 2024 - 12Z Sun Nov 24 2024 The probability of rainfall exceeding flash flood guidance is less than five percent. Pereira 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... ...1930 UTC Update... No changes made to the Day 2 ERO based on the 12Z guidance. This includes the array of HREF probabilistic data, which continues to depict spotty elevated probabilities of 0.50+ in/hr rates during day 2 in the Marginal Risk area, and the south- north axis of 250-500 kg/ms IVT aligns along the NorCal coast. Hurley ...Previous discussion... 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 ACROSS MUCH OF THE CALIFORNIA CENTRAL VALLEY AND WESTERN SLOPES OF THE SIERRA, ALONG WITH PORTIONS OF NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON... ...1930 UTC Update... Have made a fairly sizable southward expansion to the Marginal Risk area on Day 3, to now encompass the CA Central Valley and adjacent Sierra foothills. This was based on the latest guidance trends, which are fairly consistent in showing a developing modest, W-SW to E-NE oriented AR coming ashore south of the Bay area Mon and Mon night. IVTs of 400-600 Kg/ms are expected to push ashore per both the GFS and ECMWF, with a corresponding TPW anomaly of 2.5-3 standard deviations above normal. Global QPFs, along with the NAM and GEM regional guidance, have come up, especially across the western Sierra slopes given the strengthening upslope enhancement. 24 hour rainfall totals of 2-5+ are noted from the models, again with the heaviest totals across far eastern portions of the Central Valley into the western slopes of the Sierra. Hurley ...Previous discussion... 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