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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
730 AM PDT Tue Sep 26 2023

...LIGHT PRECIP OVER NORCAL TODAY INTO WEDS AM DUE TO A COUPLE 
WEAKER FRONTS...
...AN UPPER LOW MOVING INTO THE REGION OVER THE WEEKEND WILL BRING 
ADDITIONAL SCATTERED SHOWERS AND BELOW NORMAL TEMPS BY 10-20 DEG F...


.METEOROLOGICAL CONDITIONS (TUE AM - MON AM)...

An upper low remains over the Gulf of Alaska this morning as 
yesterday's cold front now sits across central CA and nw NV. The 1"+ 
PW moisture plume can still be seen on TPW imagery around the front. 
Only a few scattered showers this morning have also been picked up 
by radar as not much dynamic support exists near the moisture plume. 
Observations over the past 24 hours report the highest precip 
amounts have been along the north coast at 1-3.5" with lesser totals 
away from the coast and to the south. The weakening front will 
continue to move through central CA the rest of today producing 
light scattered showers, mainly again over the north coast and 
through to Shasta/the northern Sierra.

This afternoon, an embedded shortwave will develop in the southern 
end of the main upper low just offshore of the PacNW. Models then 
have the shortwave closing off into a small upper low later this 
afternoon with the main now smaller low further northward along the 
BC coastline. The southernmost upper low along with a surface 
counterpart and renewed front will move into the PacNW starting 
tonight bringing another round of showers to southern OR and 
northern CA. Amounts are not forecast to be as high as the previous 
front with most of the precip closer to the core of the surface low 
to the north. Shower activity should diminish by Wednesday afternoon 
as the main upper low gradually makes its way down the BC coast 
towards the PacNW. In the meantime, dry conditions are expected the 
rest of Wednesday and Thursday while troughing deepens over the 
western U.S.

Since yesterday, the GFS/ECMWF have come into better agreement on 
the timing of the arrival of the main upper low at the end of the 
week. Both are now showing the low heading into WA by early Friday 
morning while troughing deepens into northern/central CA. This may 
result in some light showers along the CA/OR border Friday morning 
before precipitation is spread further into the region the rest of 
the day and into Saturday. There are still some differences in the 
timing and positioning of the upper low with the GFS further to the 
west compared to the ECMWF, but the gist is scattered showers on 
Saturday as the system expands to encompass the entire west coast. 
There may be a thunderstorm or two as well. The upper low is then 
forecast to move deeper into CA/NV the rest of the weekend while 
producing additional showers before shifting trajectory to the 
northeast into Monday. The majority of the precipitation from the 
weekend low is expected east of the Sierra with the highest amounts 
over the NV mountains (1-1.5"). Accumulations lessen across the 
Sierra and then become even more scattered for the rest of CA.

In addition to on and off precipitation, the overall troughing 
pattern will be conducive to continued below normal temperatures 
across the region. This cooling will peak over the weekend as the 
larger upper low deepens over CA/NV where afternoon high 
temperatures are forecast at 10-20 deg F below seasonal normals. 


Please Note: This product may NOT be routinely updated Friday
through Sunday between Jun 1 and Oct 1. Please refer to
www.cnrfc.noaa.gov/dailyBriefing.php for a graphical summary of
weather and hydrologic conditions.