Hydrometeorological Discussion National Weather Service / California-Nevada RFC / Sacramento CA 730 AM PDT Thu Aug 27 2026 ...COMBINATION OF OFFSHORE TROPICAL MOISTURE AND MONSOON MOISTURE TO BRING PERIODS OF SHOWERS AND THUNDERSTORMS TO THE REGION... ...WIDESPREAD COOLING STARTING TOMORROW WITH BELOW NORMAL TEMPERATURES EXPECTED THROUGH MID NEXT WEEK... .METEOROLOGICAL CONDITIONS (THU AM - WED AM)... Remnant tropical moisture from Iselle approaches the coast, drawn in between troughing offshore of the PacNW/nrn CA and a strong area of high pressure over the Four Corners/TX. Current radar and lightning detection show some returns approaching the srn CA coast with several flashes/strikes. Unclear how much if any of this precip will actually reach the surface. Although TPW imagery shows 1" PW of moisture along the entire southern/central CA coasts, looking at the morning 12z soundings shows a pretty decent dry layer from the surface up to about 700 mb at VBG and NKX. Official forecast for this morning has some scattered few hundredths. Models do show some showers potentially developing over the southern Sierra starting this afternoon. The bulk of the moisture is expected to move through central CA and NV tomorrow into Saturday. This should result in showers and possibly thunderstorms over the Sierra and eastward into nrn/cntrl NV. A few showers may make it into the valley, but nothing substantial. The vast majority of the precip should be over the Sierra and eastward into NV. Models then have the back end of the moisture pulling out of CA overnight Friday with lingering showers over NV on Saturday. In addition to this approaching tropical moisture, continued monsoon moisture rotating into se CA and srn/ern NV is expected to bring scattered showers and thunderstorms this afternoon/evening and continuing each day through Saturday before drier air begins to move in on Sunday. The current pattern should produce a little cooling compared to yesterday with near normal temperatures outside of southern CA and ern/nrn NV. Expect more notable cooling tomorrow and over the weekend as an upper low descends into the PacNW from the BC coast and expands troughing further into CA. Once that low moves out, another is expected to quickly take its place keeping below normal temperatures around through at least mid next week. Another round of tropical moisture is then on tap early next week being brought in this time to southern CA. There is a good deal of uncertainty at this point on how much precip this may generate. The official forecast right now for Tuesday into Wednesday shows up to 0.10-0.25" over coastal srn CA and lesser amounts further inland. That drier air moving in Sunday is expected to prevent the tropical moisture from getting too far northward, but how far it does get is another point of contention amongst the ensembles. The "det" NBM is much more widespread with the precip some of which is above even the 90th percentile while the 50th remains bone dry. More than half of the ensemble members of both the GEFS and ECMWF are dry over LA while the wetter members show over 1.50" in a 24 hr period. Looking at the 24 hr QPF clusters ending 00z Thursday, there is a range of QPF of 0" to 3" over parts of coastal srn CA. The wettest cluster is only about 4% of the ECMWF and 15% of the CMC with none of the GEFS members going that high. There is also disagreement on where the precip goes. The wetter cluster 3 stops at southern Monterey County with showers down to San Diego while cluster 2 is shifted north centered over Monterey County with nothing south of LA. Cluster 1, which contains over 75% of the overall ensembles is dry. Bottom line, a lot of uncertainty in the forecast for day 6. Went with a blend of WPC and the det NBM while trimming back the northern extent of precip based on looking at the other det models and the predicted location of that tropical moisture. The result is generally less than 0.10" with 0.10-0.25" over LA county and showers cut off around Point Conception. To summarize, a combination of tropical moisture from the eastern Pacific and monsoonal moisture from the desert southwest will result in periods of showers and thunderstorms over parts of the region through mid next week. Continued lows dropping in from the northwest will bring widespread below normal temperatures starting tomorrow. The QPF in the first 24 hrs was a blend of the NBM/WPC and the west WRF and beyond that was generally WPC and the ECMWF. QPF for the next 6 days: a few hundredths to 0.25" nrn Sierra, 0.25-1" central/southern Sierra, 0.10-0.75" central/nrn NV, a few hundredths to 0.25" coastal srn CA and 0.10" or less for interior srn CA. Freezing levels 13-16.5 kft through the weekend with some fluctuation lowering early next week to 10-15 kft. 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. |