Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 02 2025 00:52:32 FOUS30 KWBC 020052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 752 PM EST Sat Feb 1 2025 Day 1 Valid 01Z Sun Feb 02 2025 - 12Z Sun Feb 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... ....01Z update... The large scale setup described below largely remains valid for the remainder of the Day 1 period that ends at 02/12Z. The 00Z=20 sounding from OAK showed that the precipitable water value at OAK had increased to 1.47 inches between 01/12Z and 02/00Z. The northward shift in the axis of greatest moisture transport noted in the 12Z numerical guidance persisted in the 18Z run of the NAM but was reversed by the 18Z run of the GFS..although roughly 800=20 kg/m/s of moisture transport integrated over a deeper layer looks to be on track for the Coastal Ranges near or just north of San=20 Francisco by early evening before slowly translating north into=20 northern California for the remainder of the outlook period. Adjustments made to the outlook area still seem on track to cover the shifts in the guidance. The expectation is that rainfall rates should increase later tonight once some instability develops...resulting in rainfall amounts through 12Z Sunday=20 morning of roughly 1 to 3 inches along the Coastal Ranges and 3 to 5 inches for the northern Sierra Nevada ranges (with locally higher amounts favored terrain)...roughly from Amador to Butte counties=20 given the placement of a 35 to 50 kt west southwesterly flow axis at 850 mb. Bann ....previous discussion follows... A nearly stationary positively tilted longwave trough will be anchored across central British Columbia today. It will direct a zonal but powerful 150+ kt jet streak into the Pacific Northwest. The jet streak will in turn guide an atmospheric river (AR) moving eastward after soaking Hawaii into northern California today. The support of the zonal jet will allow the AR's moisture to extend well inland from the coast, including into Nevada, Utah, Idaho, and Wyoming. The areas getting the greatest rainfall will be the northern Sierra Nevada mountains and secondarily the coastal ranges around and north of the Bay Area. For just this Day 1 period, expect 4-6 inches of rain from west of Lake Tahoe north and west to about Paradise, CA. 1-3 inches are expected for the northern Coastal Ranges from the Bay Area through southern Trinity County. IVT values for this atmospheric river event peak at around 850 kg/ms around 18Z/10am PST in the Bay Area this morning according to both GFS and EC ensemble output. While this alone is notable, it's far from extreme. It will be the duration of this event that will make it stand out from most ARs. While IVT values will drop closer to 500 kg/ms in the following days, they will remain elevated near that level for 3 consecutive days after today. Using IVT as a proxy for heavy rain, this would suggest the heaviest rain of the event will be this afternoon and evening, but smaller disturbances responsible for brief upticks in precipitation rates become increasingly difficult to predict at extended time ranges, so it's very possible additional heavy rainfall will occur in the subsequent days. PWATs advecting into the coast with the AR will get as high as 1.25 inches today. NAEFS mean PWAT probabilities put this at the 98th percentile as compared with a 30 year average. Additional flooding concerns are also forecast around the 2024 Park Fire east of Red Bluff and north of Chico. Here, hydrophobic soils and a lack of vegetation will convert most, if not all of the rainfall to runoff, resulting in locally worse flooding as compared with surrounding areas. Changes from the previous forecast were minimal, with expansions of the Marginal Risk drawn in around Lassen N.P. The highest elevations will see snow, but with such high snow levels (6,000-7,500 ft), the large majority of the expected precipitation in northern California will be in the form of rain. NASA Sport soil moisture imagery shows much of Northern California's soils are below to well below normal, so much of the rain, especially in the populated valleys should start out as beneficial...whereas the flooding threat will be the most impactful in the foothills of the Sierras. Wegman Day 2 Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... Daytime update... Based on the 12Z guidance, made only a small adjustment north to the previous outlook areas. Overall, models have remained consistent showing a strong signal for heavy amounts -- especially within the Slight Risk area. The heaviest totals are expected to fall east of the northern Sacramento Valley into the Sierra Nevada foothills. Neighborhood probabilities from the 12Z HREF indicate that localized amounts of 8 inches or more -- falling mostly as rain -- are likely within that area. Previous Discussion... The slow moving atmospheric river will continue to bring a heavy rainfall and flood threat to portions of northern CA Sunday into Sunday night. The IVT core and max rainfall axis should shift a bit north compared to day 1 (Saturday). PW anomalies will gradually drop, however the persistent IVT should support a prolonged moderate to locally heavy rain. Snow levels are quite high, and thus the Slight risk area goes into the northern Sierra, and even a bit to the east of the crest given the strong inland IVT anomalies. The highest rainfall rates will likely be across portions of the northern Sierra where more anomalous IVT will focus into the terrain. 48hr rainfall amounts of 8-12" are forecast over portions of the northern Sierra (generally from Placer up to Butte and Plumas counties) through 12z Monday. This portion of the Sierra has some of the highest 48hr ARI thresholds in the country, so this region is clearly used to heavy rainfall amounts. With that said, these forecast totals are still exceeding the 2 to 5yr thresholds. Seems like it'll be a cold rain so not sure how much SWE loss we'll get in the higher elevations where there is snowpack (rain might mostly get absorbed into the snowpack). Given these totals and periodic rates around 0.5" per hour, at least some flood impacts appear likely, with higher end Slight risk probabilities justified. The lack of convective rainfall rates, combined with the fact this area can generally take quite a bit of rain (as seen by the high ARI thresholds), suggests these higher end Slight probabilities should suffice for now. A prolonged steady rain along coastal to valley areas will result in a widespread 1-3" of additional rainfall over the Slight risk area, brining 48hr totals into the 3-6" range. A lack of strong frontal convergence should keep rainfall rates lower here, but the prolonged nature of the rain should still allow for an uptick in flood impacts as conditions continue to saturate. Pereira/Chenard Day 3 Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... Daytime update... No significant changes were made to the previous outlook areas. Previous Discussion... Not much change to the IVT axis is forecast into Monday, meaning the prolonged moderate to heavy rainfall event will continue across northern CA. Snow levels should begin to drop by this time resulting in less impacts over the higher terrain of the Sierra, but a continued uptick in flood impacts over coastal areas into the foothills where conditions will continue to saturate. By Monday afternoon model guidance indicates we should see a strengthening of frontal convergence across northern CA, which should help drive an uptick in rainfall rates along the coast. Model spread begins to increase by day 3 with the 00z NAM and GFS a bit further north with the heavier rainfall axis than the 00z ECMWF, UKMET and GEM. Interestingly the 18z ECMWF AIFS is closer to the GFS/NAM camp. Big picture wise the differences do not end up having a major impact on QPF or the ERO (differences get bigger on day 4), but if the further north GFS/NAM end up being right the Slight risk area could end up shifting a bit north with future updates. There is some potential for higher end Slight risk, or maybe even MDT risk, probabilities along coastal areas given the expected uptick in rainfall rates over what should be fairly saturated ground conditions by this point. However a lack of stronger dynamics and lack of instability interacting with the IVT plume may prevent rates from getting high enough to cause more widespread significant impacts. So will just maintain the Slight risk for now, but either way anticipate at least some continued flood impacts Monday and will continue to monitor trends. Pereira/Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Yag52idsEQv2VgxpmBESMcj9mCuDCvrVcImrTAEnqLj= Jva6fdK9FjPyW0tKwu7YYef97QpeaPe5dC2EahgHEsEK4a4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Yag52idsEQv2VgxpmBESMcj9mCuDCvrVcImrTAEnqLj= Jva6fdK9FjPyW0tKwu7YYef97QpeaPe5dC2EahgHl6moq0w$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Yag52idsEQv2VgxpmBESMcj9mCuDCvrVcImrTAEnqLj= Jva6fdK9FjPyW0tKwu7YYef97QpeaPe5dC2EahgHGM17k5I$=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) .