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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
725 AM PDT Mon Sep 25 2023

...PRECIP OVER NORCAL TODAY/TOMORROW AS A COLD FRONT MOVES THROUGH 
THEN AGAIN INTO WEDS DUE TO ANOTHER WEAKER FRONT...
...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 (MON AM - SUN AM)...

A large surface/upper low is currently situated in the Gulf of 
Alaska while the system's cold front has arrived along the coast of 
the PacNW and northern CA. TPW satellite shows a plume of moisture 
at 1-1.50" PW surrounding the cold front and offshore into the 
eastern Pacific. Radar imagery has showers over the OR coast and 
moving into coastal norCal. Precipitation will continue over the 
north coast the rest of the morning before spreading inland this 
afternoon as the front moves onshore. Heaviest precipitation today 
expected around the Smith Basin and Cape Mendocino.

Precipitation will lighten and become more scattered tomorrow as the 
front weakens and mostly dissipates in the afternoon. Meanwhile, the 
upper low in the Gulf of Alaska will split off into two by Tuesday 
afternoon. One smaller low further to the north offshore of BC and a 
trough closer to WA. Beneath this shortwave, a small surface low 
will develop just west of the WA/OR border along with another front. 
This will bring another round of precipitation into the north coast 
overnight through Wednesday morning, though the majority of the 
precip is expected north of the CA border. The upper shortwave will 
follow into the PacNW on Wednesday as the upper low offshore of BC 
makes its way towards the west coast. Most of Wednesday and Thursday 
are forecast to be dry as the west transitions between these two 
features.

Beyond this, there is some disagreement between models on when the 
upper system reaches the PacNW. The ECMWF is much faster, bringing 
the center of the low into WA Thursday afternoon/evening while the 
GFS keeps the system offshore along the coast. Both show the system 
expanding to cover much of the west coast by Friday evening 
spreading some showers into northern CA. The quicker onshore 
movement of the ECMWF is resulting in the model spreading showers 
further to the south across the region into the weekend more quickly 
than the GFS. By Saturday afternoon, both models have a now large 
upper low centered between CA/NV and expanded to cover the entire 
west coast. This will result in cooler temperatures at 10-20 deg F 
below normal for the weekend along with scattered showers.

Most of the precip for days 1-3 is forecast along the north coast at 
1-3.50". Highest amounts over the Smith Basin and around Cape 
Mendocino with lesser amounts as you get away from the coast. For 
days 4-6, most of the precip is forecast east of the Sierra over the 
NV mountains at 0.50-1" due to the upper low. 


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.