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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.