Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 13 2024 15:36:01 FOUS30 KWBC 131535 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1035 AM EST Fri Dec 13 2024 Day 1 Valid 16Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... ....16Z Update... The inherited Slight Risk along the Coastal Range of Northern=20 California remains on track with just a few minor adjustments to=20 the threat areas based on incoming 12Z CAMs. Guidance has continued to trend slightly more amplified with the approaching upper-level=20 shortwave trough tonight. NAEFS shows the IVT will exceed 750=20 kg/m/s upon approach, topping the 99th climatological percentile=20 near Eureka, CA for 00Z Saturday then southward into Sonoma and=20 Napa counties by 06Z tonight. The HREF suggests low-chance=20 neighborhood probabilities (10-30%) for localized amounts=20 surpassing 8" through 12Z Saturday. In addition, the ECMWF EFI=20 sports a >0.8 signal for significant QPF vs climatology for early-=20 mid December from the Coastal Range north of the Bay Area to the=20 Shasta/northern Sierra Nevada. The EFI signal indicates there is=20 the potential for an extreme event, but note it does not account=20 for other non-meteorological factors. Of note, this AR is likely=20 to be fairly progressive and soils have recovered some over the=20 past two weeks. For these reasons, the Slight Risk was maintained=20 for this forecast cycle as this should reduce the areal extent of=20 the flash flood threat. That said, some localized considerable=20 impacts cannot be ruled out, especially in areas where excessive=20 rainfall amounts above 8" potentially come to fruition tonight and into early Saturday morning.=20 Mullinax --Previous Discussion-- Maintained the Slight Risk area over portions of northwest California with only minor adjustments needed based on latest deterministic and ensemble guidance. Rainfall should be growing in both areal coverage and intensity today as a plume of moisture is directed from the eastern North Pacific into northwest California. Event total rainfall amounts through early Saturday morning are still forecast to be in the 3 to 5 inch range across the Slight Risk area. By late tonight/early Saturday morning the heavier rain will shift southward as a strong mid level shortwave approaches the region. This feature will act to increase large scale ascent, while also cooling the mid levels enough to allow for some weak instability to develop within the IVT axis. The 13/00Z HREF neighborhood probabilities of rainfall exceeding 0.5" in an hour are over 50% across most of the Slight risk...both across the most favored upslope areas and also within a narrow southward shifting axis of stronger convergence. This does not appear to be a high end event given the progressive nature of the system...although some creek and stream flooding and a few mudslides will be possible. Bann Day 2 Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... Rainfall associated with an atmospheric river will be on-going as the Day 2 period begins at 14/12Z...with the plume of deepest moisture continuing to be directed in to parts of northwest California although the axis of heaviest rainfall rates/amount will be gradually be easing eastward in time with the main shortwave while being shunted southward ahead of the surface cold front. southward with time. The higher rates should quickly diminish with the departure of the shortwave moves...with the risk of heavy rain over by 18z. Thus this Slight risk is really only for the first few hours of this day 2 period. Bann Day 3 Valid 12Z Sun Dec 15 2024 - 12Z Mon Dec 16 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent.| Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9joQ_vH6egz019m2c9p6OZZEKO4TqMleMXyrtkGXJ6Zw= PC6Qc398VA4aqoeehmJ0X4REX3tZqjGadnK-j7KxtdW_zCY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9joQ_vH6egz019m2c9p6OZZEKO4TqMleMXyrtkGXJ6Zw= PC6Qc398VA4aqoeehmJ0X4REX3tZqjGadnK-j7KxHTLIXP4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9joQ_vH6egz019m2c9p6OZZEKO4TqMleMXyrtkGXJ6Zw= PC6Qc398VA4aqoeehmJ0X4REX3tZqjGadnK-j7KxoeOdrIY$=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) .