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AGUS76 KRSA 302144
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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
130 PM PST Thu Jan 30 2025

...WET PATTERN EXPECTED TONIGHT INTO TUESDAY MAINLY OVER SRN OR AND
NORTH AND INTO CENTRAL CA/NV WITH LOW CONFIDENCE IN PRECIP AMOUNTS
AND COVERAGE...

.METEOROLOGICAL CONDITIONS (SHORT TERM: THU PM - SUN AM)...

Weak ridging over the region this afternoon shifts east tonight as
an upper level trough and moisture plume approach the west coast.
This will bring periods of precip to Srn OR and Northern and Central
CA and NV as moisture aims at different parts of the region and
waves move through into Saturday as low moves south along B.C.
coast. An upper level shortwave trough and associated frontal system
move to the west coast tonight with the front moving south and east
across the area Friday. This taps into a moisture plume that takes
aim at the Nrn CA Coast with PWs around 1.15 inches tonight and
drops south with PWs around 1.3 inches around the Bay area and about
1.1 inches into the Central Valley Friday afternoon and evening then
stretching from around Cape Mendocino to around Point Conception
Friday night into Saturday morning. Precipitation amounts for 12Z
Fri-12Z Sat generally around 1-3 inches possible over Nrn CA coast
and into the Shasta Basin and into the Nrn Sierra and 0.25-1.25 inch
for the Sac Valley and  up to 1 inch for the Central CA coast and up
to 1.5 inches along Srn Sierra and up to half an inch for the Nrn
and Central SJ Valley. The models vary with the plume Saturday into
Saturday night for lower confidence in precip amounts and coverage,
especially over the northern end. The 12Z GFS focuses the plume
around the Bay Area Saturday afternoon and erodes some of it
Saturday night. The 12Z EC and 18Z NAM12 keep some moisture off the
Srn CA coast into Saturday afternoon/evening then the 12Z EC focuses
it around Cape Mendo to South Bay Area Saturday night but Nam12
keeps aimed at Srn CA. The 24hr QPF 50th Percentile clusters ending
12Z Sunday show amounts around 2-5 inches in the Sierra 1.5-3 inches
along Coast from Cape Mendo to Monterrey Bay for about 3/4 (77%) of
the members and the other cluster dominated by the GFS (40% GFS and
20% Ec members total of 22% members) showing lower amounts with only
1.5-3.5 inches for the Sierra and half an inch or less for Sac
Valley and half an inch to 1 inch for Nrn Ca Coast and 0.5-1.5 along
Central CA coast with locally amounts to around 2 inches around
Santa Cruz mtns). The clusters show a few hundredths to 1.5 inches
for the NW CA coast and near the ORCA border showing uncertainty on
the northern end. The forecast has around 0.5-2 inches Cape Mendo to
Santa Cruz Mtns and 0.5-1.25 inches for Big Sur Coast and around 2-
4.5 inches for the Nrn and Central Sierra.  Although there is low
confidence in these amounts depending on where moisture plume goes.

Freezing levels around 9000 to 11000 feet today before falling to
around 4500 feet to 5500feet near the ORCA border and 6000 to 8000
feet for the Nrn Sierra and Nrn NV above 8000-feet for
central/southern areas on Friday and around 3500-5000 ft near ORCA
border and 7000-9000 ft for the Nrn Sierra on Saturday and 6000-
8000ft for Nrn NV and remaining 8000 ft and above for Central and
Srn Ca/NV.


.METEOROLOGICAL CONDITIONS (LONG TERM: SUN AM - WED AM)...

A tropical moisture plume is still on track to bring widespread
precipitation to nrn/cntrl CA the rest of the weekend and through
early next week. There remains uncertainty in the forecast as was
the case this morning. Models do continue to trend towards the upper
edge of the moisture plume pulling away from the north coast. The
12z run of the det ECMWF reflects this shift now predicting about
0.25-0.75" in the 24 hrs ending Tuesday morning where the 00z run
had about 1-2.50+". The GFS in its 12z run has shifted northward for
this time frame with the center of plume around Point Arena as
opposed to the 06z run where it was centered over the Bay Area/I-80
corridor as it shows the moisture plume taking a steeper
orientation. This brings the two models in better agreement at least
for Monday. The ECMWF in general still prefers a more northerly
focus of the moisture on Sunday while the GFS prefers the Bay Area.
Once the moisture plume begins to travel south along the coast as it
is nudged by the upper low to the north, the models diverge again.
The GFS is still quicker at carrying the plume through the central
coast and into srn CA compared to the ECMWF. 00z Wednesday, the GFS
has the moisture plume centered over Point Conception while also
narrowing quite a bit entirely pulling south of Monterey. The ECMWF
has the northern edge of the moisture plume still around San Jose at
this time. The consequence is the ECMWF predicting more widespread 1-
2" for Tuesday across the central coast and over coastal Santa
Barbara/Ventura counties while the GFS is closer to 0.25-0.75" with
locally higher amounts. Ensembles are divided similarly as the 00z
runs with disagreement amongst the members as to where along the
coast the moisture plume will settle and how far south precip will
spread. All this to say the afternoon QPF remains highly uncertain
due to model differences and ensemble spread.

QPF has continued to trend downward over nw CA this time by 0.50-1"
and up over the northern Sierra by similar amounts. Changes on the
order of 0.10-0.25" elsewhere. Afternoon package was a blend of WPC
guidance, the 19z NBM, and the previous forecast. Low confidence,
focus of heavy rainfall will depend on the location of the moisture
plume and how much if at all it pulls back northward/how quickly it
progresses to the south. QPF 12z Sun-12z Weds: 5.50-12" northern
Sierra, 3.50-7" central Sierra, 4-7" Shasta, 3.50-6" Eel/Mad basins,
2-4.50" north coast/North Bay, 2-4" central coast mountains, 1-2.50"
rest of Bay Area/ne CA, 1-4" southern Sierra, 0.75-3.50" S to N down
the valleys, 0.50-1.25" coastal srn CA north of LA county and a few
hundredths to 0.25" along the coast to the south.

Freezing levels Sunday 3-9.5 kft from nw CA to I-80 and 8.5-12 kft
south of I-80 to the srn Sierra. Lower levels will gradually spread
south into early next week down to 2-7.5 kft north of I-80 Monday
evening and 7-11 kft from I-80 to the southern Sierra. Tuesday
evening 1.5-4.5 kft north of I-80 and 4-8 kft from I-80 to the
southern Sierra.




QPF graphics are available at www.cnrfc.noaa.gov/qpf.php



Osborne/AS

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