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AGUS76 KRSA 281532
HMDRSA

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

...AREAS OF SHOWERS AND POSSIBLE ISOLATED THUNDERSTORMS MAINLY ALONG
THE COAST AND HIGHER TERRAIN IN THE NORTH TODAY AS A LOW PRESSURE
SYSTEM/TROUGH IMPACTS THE WEST COAST THEN A FEW LIGHT LINGERING
SHOWERS POSSIBLE SATURDAY BETWEEN SYSTEMS...
...PERIODS OF PRECIP SUNDAY INTO MID/LATE NEXT WEEK, BUT UNCERTAINTY
IN THE DETAILS AT THIS TIME DUE TO MODEL DIFFERENCES AND SPREAD
AMONG THE ENSEMBLE MEMBERS...

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

A closed low remains offshore of BC and the PacNW this morning
generating scattered showers across nrn CA. Radar shows some returns
in far se CA just behind a weakening front over AZ. Showers will
persist across nrn CA/srn OR today as shortwave energy is rotated
into the area around the offshore low. Instability may result in
thunderstorms over those areas as well this afternoon. Troughing
will pass through the region overnight continuing to generate
scattered showers over nrn CA/NV and srn OR. Some weak ridging will
move overhead on Saturday as a surface low approaches CA from the
west. NV will see some lingering shower activity Saturday, as well
as possible afternoon thunderstorms over E NV, from the upper trough
as CA temporarily dries out between systems.

Differences in the models persist regarding the surface low for
Sunday. The GFS is slower on the arrival of the low compared to the
ECMWF by a good 6+ hrs, it also continues to mostly weaken the system
offshore before it reaches land while the ECMWF has it in tact long
enough to spread showers across much of the state Sunday morning.
Another larger surface and upper low will also be making its way
towards BC and the PacNW sending additional showers into nrn CA
Sunday afternoon/evening with moderate activity along the north
coast overnight. Differences in the handling of the surface low
result in the ECMWF predicting much higher totals across nrn CA on
Sunday compared to the GFS (0.75-1.50+" vs 0.10-0.75"). Ensemble
spread at Arcata for Sunday is about 0.50" to 2". A much higher
percentage of the ECMWF ensemble members favor higher totals
compared to the GFS as well. Bottom line, still a good amount of
uncertainty in the QPF for even a couple days out.

The morning forecast was a combination of the latest NBM and WPC
guidance. QPF 12z Fri-12z Mon: 0.75-1.75" north coast (up to 2.50+"
Smith Basin), 1-2" Shasta/nrn Sierra, 0.50-1.25" srn OR
Cascades/cntrl Sierra, 0.25-0.75" rest of nrn CA, 0.10-0.50" Bay
Area/central coast mountains, and a few hundredths to 0.25" for
central CA and the valleys.

Freezing levels this morning 4-5.5 kft north of I-80 and 4.4-8.5 kft
from I-80 to the southern Sierra lowering overnight down to 3-4.5
kft north of I-80 and 4.5-7.5 kft from I-80 to the southern Sierra.
Freezing levels will rebound the rest of Saturday due to some brief
ridging up to 5-7 kft north of I-80 and 6.5-9.5 kft from I-80 to the
southern Sierra in the afternoon. Levels will re-lower into Sunday
down to 4.5-6 kft north of I-80 and 5.5-8 kft from I-80 to the
southern Sierra in the early morning.


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

Upper/lower level low approaching the Pac NW coast on Monday
bringing some showers to Srn OR and Nrn CA and portions of Central
CA to around Point Conception and over Nrn NV, mostly along the
coast and over the higher terrain. The 06Z GFS has about a 1.4 inch
moisture plume aimed at the Srn CA coast Monday with PWs around an
inch along the Srn CA coast (which could bring more precip to Srn
CA) but the 00Z EC keeps it offshore. The forecast mainly used a mix
of the latest NBM and WPC and the previous forecast.  Forecast
precipitation amounts for Monday into Monday night are generally 1-
2.5 inches over the NW CA coast (highest in the Smith) and 1-2
inches for the Nrn and Central Sierra and 0.5-1.5 inches for the
Shasta Basin and Eel Basin and 0.25-0.75 for the Russian Basin and a
third of an inch or less for the Sac Valley and Nrn and Central SJ
Valley along the Central CA coast  and over Nrn NV (except up to
around half an inch over the higher terrain).

The models diverge with the track of the low on Tuesday into
Wednesday with the GFS progressing it inland through the Pac NW and
the EC digging a trough over the region and another low dropping
down off the Pac NW coast late Wednesday. The 06Z GFS brings
moisture plume to the Central and Srn CA coast late Wednesday while
the 00Z EC holds it offshore. Ensemble members also have various
solutions for lower confidence in the forecast for mid-week. The 24
hr QPF 50th Percentile QPF Clusters ending 00Z Thursday show the
majority of the ensemble members (78%- Canadian 80%, GFS 50%, and EC
94%) have half an inch or less of precip mainly along the Nrn CA
coast and over the Srn OR OR Cascades and Nrn/Central Sierra.
Another cluster with 11% of the ensemble members ( Canadian 15%, GFS
27% and EC 0%) still show pretty wet conditions for most of CA and
Nrn NV with around 1.5-4 inches along the Nrn CA coast from Cape
Mendocino to the Bay Area and over the Shasta Basin and along the
Sierra and 1-2 inches for the transverse range and 0.5-1.5 inches
for the the Sac Valley and 0.25-0.75 inches for the SJ Valley and
around a half an inch or less over Nrn NV. Another cluster with 9%
(Canadian 5%, GFS 17%, EC 6%) of the Ensemble members keeps
precipitation over the north but wet with amounts 1-3 inches along
the Nrn CA coast and into the Shasta Basin and 0.75-2 inches for the
Nrn Sierra and less than half an inch for Nrn NV. More of the
ensemble members and deterministic are trending drier for mid-week
but still a chance of higher precipitation amounts. Decreased precip
amounts for Tuesday in the current forecast. The current forecast
for Tuesday is a mix of the latest NBM and WPC with the previous
forecast and for Wednesday about 2/3 NBM and 1/3 WPC (wetter than
NBM). In the current forecast, precip over Srn OR and Nrn and
Central CA and Nrn NV with amounts around 0.5-1.5 over the Smith
Basin and King Range and 0.5-1 inch for the Nrn Sierra and Shasta
Basin around 0.25-0.75 inches for the Russian Basin and Central
Sierra and half an inch or less along the Central CA coast and Srn
Sierra and third of an inch or less for Sac Valley and around a
tenth of an inch or less for the SJ Valley and over Nrn NV. Precip
amounts for Wednesday 0.5-1.5 inches for NW CA coast and Shasta
Basin and along the Sierra and 0.25-0.75 inches for the Russian
Basin and along the Central CA coast and 0.1-0.5 inches for the
transverse range and Sac Valley and around a third of an inch or
less for the SJ valley and Nrn NV. These amounts will change has it
gets closer and hopefully the models are in better agreement.

Freezing levels generally  around 3000-5000 ft over Nrn CA and and
5000-8000 ft Central CA and 9000-12,000 ft over Srn CA and 5000-
1000ft over NV (NW-SE) Monday morning then drop to around 2500-4000
ft over Srn OR and Nrn CA and 3500-6500 ft over Central CA and 7000-
12,000 ft over Srn CA and around 3500-8000 ft over NV (N-S) Monday
night/Tuesday morning and around 3000-5500 ft over Nrn and Central
CA and 6000-9000ft over Srn CA and around 3500-6500 ft over NV
Wednesday morning. Uncertainty in the freezing levels with the
varying synoptic solutions for mid-week.



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



AS/Osborne

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