Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 23 2024 19:42:09 FOUS30 KWBC 231941 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 241 PM EST Sat Nov 23 2024 Day 1 Valid 16Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SIERRA NEVADA... ....16Z update... While the IVT plume has weakened considerably over the past 24 hours and will continue to do so through today as it progresses southward into southern California, an isolated threat for runoff/flooding will continue along the foothills/lower elevations of the Sierra Nevada. Moisture and instability look to remain limited (0.50 to 0.75 inch PWATs and pockets of CAPE in excess of 500 J/kg...via recent RAP guidance) but W to SW unidirectional flow may support a few isolated low-topped showers capable of training with 0.5+ in/hr rates. This potential will maintain a low- end flash flood threat across very wet antecedent soils and area burn scars. The Marginal Risk was extended northward to include the Park burn scar in the northern Sacramento Valley. Otto ....previous discussion follows... 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... ....1930 UTC Update... No changes made to the Day 2 ERO based on=20 the 12Z guidance. This includes the array of HREF probabilistic=20 data, which continues to depict spotty elevated probabilities of=20 0.50+ in/hr rates during day 2 in the Marginal Risk area, and the=20 south- north axis of 250-500 kg/ms IVT aligns along the NorCal=20 coast. Hurley ....Previous discussion... Locally heavy rain will be possible Sunday into Sunday night over=20 northwest CA into far southwest OR. A front slowing near the coast=20 and a weak low to mid level wave riding along it will support a=20 modest uptick in IVT and plentiful low level convergence. Moisture=20 is a limiting factor for heavy rainfall with the better moisture=20 plume situated well south and not really getting drawn into this=20 system. Thus this is not expected to be a significant atmospheric=20 river, and normally would not expect a system of this nature to=20 pose a flood risk. However given the significant rainfall over the=20 past few days, this additional rainfall may be enough to cause some localized additional flooding as the area will be more=20 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=20 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.=20 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=20 for flooding with limited moisture and dropping IVT values...=20 however this area remains more hydrologically sensitive than normal following the significant atmospheric river event the past few=20 days. Additional rainfall over far northwest CA into southwest OR=20 is generally 1-3" in the guidance, with isolated totals of this=20 magnitude also possible over the western slopes of the northern=20 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://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6z9vjm7pjQIcLdQ478pgh90Aj50GhPAKMBW_CBl2raji= 6PHnC0OZNHKwbPuZD8U_q1tJSBQ_QC_D0SFT4Xl_U8LLCvg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6z9vjm7pjQIcLdQ478pgh90Aj50GhPAKMBW_CBl2raji= 6PHnC0OZNHKwbPuZD8U_q1tJSBQ_QC_D0SFT4Xl_ltnxbe0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6z9vjm7pjQIcLdQ478pgh90Aj50GhPAKMBW_CBl2raji= 6PHnC0OZNHKwbPuZD8U_q1tJSBQ_QC_D0SFT4Xl_xnmhDZw$=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) .