Hydrometeorological Discussion National Weather Service / California-Nevada RFC / Sacramento CA 725 AM PDT Wed Oct 21 2026 ...COOLER TEMPS ALONG WITH SHOWERS AND THUNDERSTORMS TODAY/TOMORROW AS A LOW MOVES THROUGH... ...LINGERING SHOWERS OVER THE SIERRA INTO NV ON FRIDAY WITH DRY CONDITIONS THEN EXPECTED FOR MOST AREAS OVER THE WEEKEND... .METEOROLOGICAL CONDITIONS (WED AM - TUE AM)... An upper low currently sits offshore of the central coast rotating showers and thunderstorms into CA. Lightning detection shows activity north of I-80 and along I-5 with numerous additional strikes over the Desert Southwest and offshore near the core of the low. Accumulations overnight have been relatively minimal down the CA valleys at 0.10" or less with up to 0.25" observed for parts of NV. Current radar shows a band of precip headed for srn CA. Expect precip and thunderstorms to continue to rotate inland throughout the day as the low inches closer to the coast. Heavier rainfall rates are anticipated for this afternoon along coastal srn CA as 1-1.3" PW of moisture favorably flows into the Transverse. HREF exceedance probabilities show a 10-50% chance of 0.50"+/hr rain rates over the western Transverse and portions of coastal soCal with a 10% chance of 2"/hr should any cells train over an area. 0.50-1"/hr rates remain possible for parts of the central and srn CA coasts through tonight as the core of the low begins to push onshore. Models continue to have this occurring around Point Conception moving through srn CA tomorrow. The low will then open into a trough as it heads into NV by the evening while continuing to produce scattered showers and thunderstorms across the region. The core of the system will lift into nrn NV Friday morning with troughing extending out offshore of Baja. The main focus of precip at this point will be over the Sierra and eastward into NV, and in lesser amounts than expected for today/tomorrow. Both the GFS and ECMWF now show a secondary low forming in the base of the trough axis Friday afternoon over srn CA. This may keep a few light showers lingering overnight. Uncertainty remains in the forecast as ensemble spread continues to show a wider range of possibilities. Spread over parts of coastal srn CA is still at about 0.50-2" in a 24 hr period for the current low. An upper ridge will build in behind the exiting system over the eastern Pacific late Friday drifting overhead on Saturday. This will keep Saturday dry while kicking off some warming bringing afternoon high temperatures closer to seasonal normals. Then another upper low approaches from the Gulf of Alaska towards the PacNW on Sunday. There is some disagreement between models as to the trajectory and timing of this next low. The ECMWF brings some showers into the north coast Sunday night through Monday as the system pushes through OR/WA. The GFS is a bit slower on the approach and more northerly with its path keeping precip out of the area entirely. Looking at the ensembles, some show a front from this system (to varying degrees of strength) moving through the north coast some time between Saturday afternoon and Monday afternoon. Spread for 24 hr QPF at Arcata is 0 to 1" for Sunday and Monday. Current forecast shows just a handful of scattered showers producing only a few hundredths of an inch over far nrn CA, but this may change in the coming days. QPF was again mainly a blend of WPC/NBM, but blended in the west WRF over the Transverse and the Sierra for some time frames. Overall, QPF increased from yesterday by 0.10-0.75" north of LA and went down similar amounts to the south. Majority of the precip is still expected for today/tomorrow diminishing on Friday. QPF through 12z Sat: 0.50-1.75" coastal srn CA, 1.50-2.50" Transverse Range, 0.75-2" Sierra, 0.25-1" across most of NV, 0.10-0.75" for the rest of nrn/central CA, and 0.10" or less for se CA. Freezing levels lowering from west to east today down to 9-12 kft across CA by the late afternoon and 8.5-10.5 kft mid Thursday morning. Levels will rebound west to east later Friday as the system pulls out of CA while lowering over ne CA/nw NV down to 7-8 kft. The region should be back up to 11-13 kft or so Sunday afternoon. Please Note: This product may NOT be routinely updated. Please refer to the following product issued by the CNRFC www.cnrfc.noaa.gov/Daily-Briefing for a graphical summary of weather and hydrologic conditions. |