Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 01 2025 07:57:35 FOUS30 KWBC 010757 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 257 AM EST Sat Feb 1 2025 Day 1 Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20 NORTHERN CALIFORNIA... A nearly stationary positively tilted longwave trough will be=20 anchored across central British Columbia today. It will direct a=20 zonal but powerful 150+ kt jet streak into the Pacific Northwest.=20 The jet streak will in turn guide an atmospheric river (AR) moving=20 eastward after soaking Hawaii into northern California today. The=20 support of the zonal jet will allow the AR's moisture to extend well=20 inland from the coast, including into Nevada, Utah, Idaho, and=20 Wyoming. The areas getting the greatest rainfall will be the=20 northern Sierra Nevada mountains and secondarily the coastal ranges=20 around and north of the Bay Area. For just this Day 1 period, expect=20 4-6 inches of rain from west of Lake Tahoe north and west to about=20 Paradise, CA. 1-3 inches are expected for the northern Coastal=20 Ranges from the Bay Area through southern Trinity County. IVT values for this atmospheric river event peak at around 850 kg/ms=20 around 18Z/10am PST in the Bay Area this morning according to both=20 GFS and EC ensemble output. While this alone is notable, it's far=20 from extreme. It will be the duration of this event that will make=20 it stand out from most ARs. While IVT values will drop closer to 500=20 kg/ms in the following days, they will remain elevated near that=20 level for 3 consecutive days after today. Using IVT as a proxy for=20 heavy rain, this would suggest the heaviest rain of the event will=20 be this afternoon and evening, but smaller disturbances responsible=20 for brief upticks in precipitation rates become increasingly=20 difficult to predict at extended time ranges, so it's very possible=20 additional heavy rainfall will occur in the subsequent days. PWATs advecting into the coast with the AR will get as high as 1.25=20 inches today. NAEFS mean PWAT probabilities put this at the 98th=20 percentile as compared with a 30 year average. Additional flooding=20 concerns are also forecast around the 2024 Park Fire east of Red=20 Bluff and north of Chico. Here, hydrophobic soils and a lack of=20 vegetation will convert most, if not all of the rainfall to runoff,=20 resulting in locally worse flooding as compared with surrounding=20 areas. Changes from the previous forecast were minimal, with expansions of=20 the Marginal Risk drawn in around Lassen N.P. The highest elevations=20 will see snow, but with such high snow levels (6,000-7,500 ft), the=20 large majority of the expected precipitation in northern California=20 will be in the form of rain. NASA Sport soil moisture imagery shows=20 much of Northern California's soils are below to well below normal,=20 so much of the rain, especially in the populated valleys should=20 start out as beneficial...whereas the flooding threat will be the=20 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=20 NORTHERN CALIFORNIA... 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=20 drop, however the persistent IVT should support a prolonged=20 moderate to locally heavy rain. Snow levels are quite high, and=20 thus the Slight risk area goes into the northern Sierra, and even=20 a bit to the east of the crest given the strong inland IVT=20 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=20 Plumas counties) through 12z Monday. This portion of the Sierra=20 has some of the highest 48hr ARI thresholds in the country, so=20 this region is clearly used to heavy rainfall amounts. With that=20 said, these forecast totals are still exceeding the 2 to 5yr=20 thresholds. Seems like it'll be a cold rain so not sure how much=20 SWE loss we'll get in the higher elevations where there is=20 snowpack (rain might mostly get absorbed into the snowpack). Given=20 these totals and periodic rates around 0.5" per hour, at least some flood impacts appear likely, with higher end Slight risk=20 probabilities justified. The lack of convective rainfall rates,=20 combined with the fact this area can generally take quite a bit of=20 rain (as seen by the high ARI thresholds), suggests these higher=20 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=20 area, brining 48hr totals into the 3-6" range. A lack of strong=20 frontal convergence should keep rainfall rates lower here, but the prolonged nature of the rain should still allow for an uptick in=20 flood impacts as conditions continue to saturate. 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=20 NORTHERN CALIFORNIA... Not much change to the IVT axis is forecast into Monday, meaning=20 the prolonged moderate to heavy rainfall event will continue across northern CA. Snow levels should begin to drop by this time=20 resulting in less impacts over the higher terrain of the Sierra,=20 but a continued uptick in flood impacts over coastal areas into the foothills where conditions will continue to saturate. By Monday=20 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.=20 Model spread begins to increase by day 3 with the 00z NAM and GFS=20 a bit further north with the heavier rainfall axis than the 00z=20 ECMWF, UKMET and GEM. Interestingly the 18z ECMWF AIFS is closer=20 to the GFS/NAM camp. Big picture wise the differences do not end=20 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=20 updates. There is some potential for higher end Slight risk, or=20 maybe even MDT risk, probabilities along coastal areas given the=20 expected uptick in rainfall rates over what should be fairly=20 saturated ground conditions by this point. However a lack of=20 stronger dynamics and lack of instability interacting with the IVT=20 plume may prevent rates from getting high enough to cause more=20 widespread significant impacts. So will just maintain the Slight=20 risk for now, but either way anticipate at least some continued=20 flood impacts Monday and will continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_iPWbFb7QlmGUryZ8IkXSxd1l8BeN44jUiXHB-sIsPW1= qApMNQ4fN6KgJ6DMZaxLhKZcIM-3QUg0EKX3hLgAtIcpoyo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_iPWbFb7QlmGUryZ8IkXSxd1l8BeN44jUiXHB-sIsPW1= qApMNQ4fN6KgJ6DMZaxLhKZcIM-3QUg0EKX3hLgAwOHz6x8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_iPWbFb7QlmGUryZ8IkXSxd1l8BeN44jUiXHB-sIsPW1= qApMNQ4fN6KgJ6DMZaxLhKZcIM-3QUg0EKX3hLgAQ6ihzHo$=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) .