613
AGUS76 KRSA 281545
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
830 AM PST Fri Feb 28 2025

...MAINLY LIGHT SHOWERS AND POSSIBLE ISOLATED THUNDERSTORMS TONIGHT
INTO SATURDAY OVER SRN CA AS A LOW MOVES THROUGH...
...MORE WIDESPREAD PRECIP POTENTIAL AND COOLER TEMPERATURES SATURDAY
NIGHT INTO MONDAY AS A LOW/TROUGH MOVES THROUGH THE REGION...
...LIGHT PRECIP POSSIBLE ON TUESDAY OVER PORTIONS OF THE NORTH AND
ALONG THE CENTRAL CA COAST THEN MORE WIDESPREAD PRECIP POSSIBLE
WEDNESDAY AS ANOTHER TROUGH/LOW MOVES THROUGH...

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

Warm and dry during the day today with high pressure over the region
and an upper level low offshore of Point Conception approaching the
Srn CA coast. Max temperatures today generally above normal by up to
20 degrees except near normal along the Big Sur and Srn CA coast
with onshore flow.  The upper level low moves off the Srn CA coast
Friday night and inland near the CA/ Mexico border Saturday morning
and off to the east into AZ in the afternoon.  This will bring a
chance of mainly light showers and possible isolated thunderstorms
Friday night into Saturday over the Srn CA coast and into the mtns
and possibly over the Srn Sierra and . Precip amounts will be
variable with convective nature but are generally expected to be
around a tenth of an inch or less.

As the upper low shifts to the east, a trough approaches the west
coast Saturday (around 130W 00Z Sunday) with a closed low developing
off the OR/Nrn CA coast Saturday night and low/trough moving over
the region Sunday into Monday.  This will bring showers and possible
isolated thunderstorms along with cooler temperatures Saturday night
into Sunday night. Max temperatures drop to below normal up to 15
degrees over most of CA (except SE) and generally near normal to 10
degrees above normal (warmest NE NV) over Nevada. Showers may start
as early as Saturday afternoon along the Nrn CA coast and spread
south and east over Srn OR and Nrn CA and most of Central CA
Saturday night and over most of the region (except SE CA and far Srn
NV) during the day Sunday. The models still differ in some of the
details so there is some uncertainty in the precip amounts and
coverage, especially on southern extent. Precip amounts will also be
variable due to the convective nature. Precipitation amounts for
Saturday night into Sunday night are generally around 0.5-1 inch for
the NW CA coast and Shasta Basin and along the Sierra and Central CA
and Santa Cruz and San Lucia Mtns and generally a third of an inch
or less elsewhere except up to 0.75 locally over the higher terrain
of northern and central NV and little to precip expected over SE CA
and far Srn NV.

Freezing levels around 9000-11,000 ft this morning then drop to
around 6500 to 8500 ft over Srn CA tonight into Saturday morning
with the low. Freezing levels drop to around 4000-6000 ft over Srn
OR and Nrn CA and around 5000-7500 ft over Central CA and Srn Sierra
with the approaching trough and associated cold front moving through
and 7000-9000ft over NV (W-E) and 7000-10000 ft over Srn CA Saturday
night and around 3500-5500ft over most of the region by Sunday night
with the low/trough over the region.


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

Monday morning an upper low will continue to traverse the region
dropping showers across CA and NV. The system will exit into the
Four Corners throughout the rest of the day with lingering showers
over the area. A transitional ridge is then forecast to build in off
the west coast behind the low as a trough approaches the PacNW/BC.
Models begin to diverge in the pattern after Monday night. There is
agreement on a ridge passing through the west coast on Tuesday, but
the GFS has the shortwave swinging northeastward through the PacNW
as well dragging a front with it and sending some additional showers
into the nrn CA coast. The ECMWF takes that shortwave further
northward into BC. As was mentioned yesterday, both models continue
to predict a developing surface low and trough west of central/srn
CA into Wednesday but continue to differ on the details. The ECMWF
brings showers into the coast Tuesday night while the GFS is now not
even taking this system to CA but instead dropping in a different
upper low from the northwest into nrn CA Wednesday afternoon. At
this time the ECMWF starts to move a broad trough through the
region.

The ensembles continue to be all over the place with 24 hr QPF
spread of around 1-1.50" for the nrn CA coast. The majority of
members (71%) show generally lighter showers for days 5/6 at about
0.50-1" across the Sierra and the central/southern CA coastal
mountains. The other two clusters are much wetter. Cluster 2 (20%
CMC, 27% GFS, 21% ECMWF) has 1.50-3+" over the central
coast/Transverse mountains and the Sierra while cluster 3 (15% CMC,
3% GFS, 6% ECMWF) focuses even heavier precip over the north
coast/Shasta and the northern/central Sierra. The point is, there is
a wide variety of possibilities of the timing/structure/coverage of
the second system expected mid week making for a highly uncertain
forecast.

To summarize, a pair of systems will bring showers to CA/NV in the
extended one on Monday and one mid week. Low confidence in the QPF
due to model differences. Forecast was a combination of morning WPC
guidance and the NBM. Trimmed back coverage of the mid week system
given the uncertainty in the pattern. QPF 12z Mon-12z Thurs: 0.50-1"
across the Sierra/central coast mountains/ne NV mountains (locally
higher Big Sur), 0.25-0.75" rest of CA coast/Shasta, and a about
0.10-0.25" for the rest of CA/NV. Little to no precip for se CA.

Freezing levels Monday morning down to 3.5-6 kft region wide rising
w to e throughout the day up to 5-8.5 kft in the evening. Mid
Tuesday morning as ridging moves in 8.5-10.5 kft from the southern
Sierra to coastal srn CA and 4-7.5 kft north of I-80. Freezing
levels re-lower on Wednesday down to 3.5-5.5 kft north of I-80 and 5-
7.5 kft from I-80 through the southern Sierra mid morning. By the
evening areas north of Point Conception 3.5-5.5 kft.


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



Osborne/AS

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