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AGUS76 KRSA 212107
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
110 PM PST Thu Nov 21 2024

...MODERATE TO HEAVY PRECIP CONTINUES OVER NORTHERN CA...
...COLD FRONT SWEEPS ACROSS THE AREA FRI INTO SAT...
...ANOTHER COOLER MODEST SYSTEM APPROACHING FOR MON INTO TUE...

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

Not much change in thinking from the morning to the afternoon
forecast issuance as this event continues to impact areas near and
north of I-80 with widespread moderate to heavy precip as the deep
moisture plume continues to take aim at the northern CA coast under
southwest onshore flow in the warm sector of the storm. Still
expecting a sfc low to continue to spin up from its currently
location near 40N/135W on its way to the north-northeast just off
the Pacific Northwest coast...which will pivot the main precip band
from the north-northeast to south-southwest ahead of the cold front
poised to move across the region on Friday into early Saturday.
Heaviest amounts are still expected over the higher terrain of
northern CA with 4.00- to 8.00-inches with localized higher amounts
from the Eel to Russian River basins along the coast and inland
between the Shasta Lake drainage and Feather River basin.

As the cold front sweeps across the region...precip will slowly
begin to taper off for northern areas during the afternoon and
evening hours...while precip finally will start to spread across
central CA. Intensities will pick up with the precip...but the
duration of the precip with the frontal boundary will limit totals
between 0.50- and 1.50-inches along coastal areas (up to 2.00- to
3.50-inches in the Santa Cruz Mountains)...and 1.00- to 3.00-inches
inland over the central/southern Sierra.

Not much precip will make its way toward southern CA as the moisture
plume is quickly shunted south across the region and the cold front
dissipates. From Los Angeles to San Diego...totals should remain
less than 0.25-inch.

Freezing levels will peak later this afternoon or evening with 9000-
to 11000-feet near and north of I-80...dropping tomorrow into
Saturday as the cold front finally ushers in the cooler airmass. By
late Saturday into early Sunday...these will range from 4000- to
5000-feet along and north of I-80 with 5500- to 7500-feet across
central CA and central NV...and 8000- to 12000-feet across southern
CA and southern NV.


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

Generally a decrease in precip amounts over the Sierra on Sunday
into Sunday night and increased amounts (up to a quarter of an inch)
along Central CA coast and into L.A. County and along the Central
and Srn Sierra Monday into Monday night and decreased amounts a
little over the north for Tuesday.

Low pressure system off the B.C./Pac NW coast on Sunday and weakens
as it drops southeast to off OR and Nrn CA coast on Monday and
trough moves onshore on Tuesday.  Moisture plume along the Central
and Srn CA coast and inland on Sunday then shifts south to Big Sur
Coast and around Point Conception and Srn CA coast and inland on
Monday and shifts farther south along Srn CA coast Tuesday and into
Baja Tuesday night. This will bring a chance of precip over most of
the region (except far SE CA) Sunday into Tuesday with the best
chances along the Coast (from ORCA border to around Point
Conception) and in the Shasta Basin and along the Sierra and
Transverse mountains.

The current forecast is mainly a blend of latest NBM mixed with some
WPC and previous forecast. Highest QPF amounts expected along the CA
Coast to around Point Conception and over the Sierra and Shasta
Basin with around 0.5-1 inch on Sunday. 0.75-2 inches possible along
the Sierra and NW CA coast and Shasta Basin and 0.5-1 inch along the
Central CA coast and transverse range and 0.25-0.75 in Central
Valley and over Ruby Mtns on Monday. Highest amounts 0.75-1.5 inches
along the Sierra and 0.25-0.75 along the coast (to around LA county
and over the transverse mtns and higher elevations of Nevada on
Tuesday.  Although, there is low confidence in the details and
precip amounts could get higher at times, especially along the coast
and over the Sierra and transverse mountains. There is still quite a
bit of disagreement in the models and ensemble members with
differences in timing and track and strength of the low/trough
leading to uncertainty in timing and amounts and locations of
precipitation for Sunday into Tuesday. The 24 hr QPF 50th Percentile
Clusters ending 12Z Monday show variable amounts and locations of
precip with about 40% of the Ensemble members (GFS-27% and EC 64%)
showing 0.75-2 inches over the Sierra and 0.5-1.75 along Central CA
coast and into Santa Barbara and Ventura County but the rest show
nothing to 0.75 inches inch over Srn Sierra. Almost 3/4 (74%) of
ensemble members show 0.75-2 inches possible along the Central CA
coast and Srn S Sierra on the 24 hr QPF 50th Percentile Clusters
ending 12Z Tuesday. The majority (65%) of total ensemble members -
(Canadian 50% and GFS 90% and EC 56%) show a half an inch or less
over Nrn Ca except up to an inch over the Nrn Sierra on the 24 hr
QPF 50th Percentile ending 12Z Wednesday.

Freezing levels fluctuate around 3500-5000 ft near ORCA border and
Shasta Basin and 4500-6000 ft for the Nrn Sierra and 5000-7000 ft
for the Central Sierra and 6000-8000 ft for the Srn Sierra and 8000
ft and above over Srn CA and 5000-8000ft over NV Sunday through
Tuesday then dropping to 4000-6000ft along the Sierra and over NV
and around 7000 ft and higher for the Srn CA mountains Tuesday
night.


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



Kozlowski/Osborne

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