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CNRFC Hydrometeorological Discussion

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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
215 PM PST Tue Jan 26 2021

...HEAVY PRECIP EXPECTED TO SPREAD ACROSS CA LATER TODAY THROUGH 
EARLY FRI...
...WEAKER SYSTEMS EXPECTED IN THE NORTH THIS WEEKEND...

.METEOROLOGICAL CONDITIONS (SHORT TERM: TUE PM - THU NIGHT)...

IR imagery shows an impressive frontal band of the coast with plenty 
of cold, unstable air behind the front.  Light precip is beginning 
to fall across parts of NW CA, with heavier amounts and a small band 
of thunderstorms off the north coast.  Heavy precip is still 
forecast to arrive from around the Russian basin into the upper Eel 
basin and Shasta drainage this evening and across the central coast 
and much of the Sierra by tonight.  Latest guidance indicate a bit 
more forward progress of the system by late tonight versus the 
morning forecast.  There is still a fair amount of uncertainty 
regarding the location of heavier precip rates as the front stalls 
along the central coast into the southern half of the Sierra Wed and 
possibly lifts back to the north a little Wed night.  WPC/NBM 
guidance was largely used, with a bit of HRRR blended in for the 06-
12 UTC Wed period on the central coast to bring in some higher 
amounts that the HRRR tends to perform well on.  The exact location/ 
extent of the heavier amounts is still highly uncertain with plenty 
of model spread on the details.  By Thu into Thu night, the system 
is forecast to move east and spread heavy precip across southern CA 
higher terrain as well as the southern Sierra.  The amounts in 
southern CA were increase a bit in line with the latest WPC 
guidance.  Freezing levels remained mainly unchanged from the 
morning.


.METEOROLOGICAL CONDITIONS (LONG TERM: FRI AM - MON AM)...

First area of focus on Friday morning will be with the axis of the 
upr trof initially near the west coast traversing the area into the 
later evening hours. Precip will be best near the base of the 
cyclonic flow aloft...which ends up across coastal southern CA 
between Los Angeles and San Diego counties. Some slight timing 
differences between the 26/12Z models allows for just a tad more 
precip in the EC as compared to the GFS. Overall...WPC QPF looked 
reasonably close to the going forecast. Only a slight bump up in 
amounts over San Diego county were made during the 12-18Z period on 
Friday.

Next...a weak s/wv trof will traverse northern CA and northern NV 
rather quickly in general westerly flow aloft. This will bring 
mainly light to locally moderate precip to areas primarily near and 
north of I-80. Quickly on the heels of this system is a deeper upr 
low that moves toward the northeast Pacific and the Pacific 
Northwest. The southern extent of the system will affect northern CA 
on Day 6. Currently WPC QPF is a bit too close to the typically wet 
NBM in the extended. Followed closer to the previous forecast along 
with the GFS and EC.