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AGUS76 KRSA 031430
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
730 AM PDT Fri May 3 2024

...COOLER TEMPS WITH WIDESPREAD PRECIP OVER NRN CA/SRN OR THIS
AFTERNOON/TONIGHT BEFORE SPREADING ACROSS THE REGION SAT (ISOLATED T-
STORMS SAT OVER NRN CA) AND DIMINISHING INTO SUN AS A LOW MOVES
THROUGH...
...LIGHT SHOWERS OVER THE NRN PORTION OF THE REGION MON AS A
DISTURBANCE PASSES TO THE NORTH FOLLOWED BY DRYING CONDITIONS THE
REST OF THE PERIOD AS HIGH PRESSURE TAKES OVER...

.METEOROLOGICAL CONDITIONS (FRI AM - THU AM)...

A large surface and upper low reside over the Gulf of Alaska this
morning as a tropical moisture plume of 1-1.25" PW stretches from
just offshore of the PacNW to north of the Hawaiian Islands ahead of
the surface low`s cold front. Over land, some weak ridging is
building into the west coast as a trough moves through northern NV.
The low will quickly approach into this afternoon bringing precip to
nw CA as the moisture plume travels south along the northern CA
coast. The cold front will gradually move across CA from nw to se
tonight and through Saturday spreading precip inland across the
state and to the south into central CA along with the moisture
plume. Models have this moisture traversing the SF Bay Area tomorrow
morning before reaching Point Conception in the afternoon. Behind
the front/moisture, the upper low will begin to push inland around
the CA/OR border Saturday afternoon carrying precip further across
NV. The core of the low is then progged to head eastward across
northern CA with the core of the low stretched across CA/NV by the
evening. This may result in some isolated thunderstorms across nrn
CA on Saturday. As the low moves inland up north, the moisture plume
will continue to travel to the south passing coastal srn CA
overnight Saturday before exiting into Baja Sunday morning. Precip
will gradually diminish across the region Saturday evening into
Sunday morning with showers now more concentrated over NV as the low
moves through. The system will then exit the region Sunday evening.

The bulk of the precip for the next 6 days is expected with this
first system later today through Saturday. Models are in relatively
good agreement on the highest precip totals occurring along the
north coast and across the northern Sierra. QPF clusters for the 24
hrs ending Saturday afternoon are only split into 2 groupings with
80% of the ensembles in cluster 1. Cluster 2 contains none of the
ECMWF or CMC members but is comprised of 67% of the GFS ensembles.
The main differences are over the north coast and the
northern/central Sierra where over half of the GFS ensembles favor
heavier precip compared to the ECMWF and CMC. Cluster two has at
least an inch more across the north coast and 0.50-1" more over the
northern/central Sierra compared to cluster 1. After Saturday
afternoon, precip will become focused across the Sierra and nrn NV
with lingering showers for much of the rest of CA. Today`s QPF was a
blend of the morning WPC guidance and the latest NBM. The result was
about a 0.10-0.30" decrease for a good portion of northern CA/NV and
central CA for today through Sunday. QPF 12z Fri-12z Mon: 1.50-3"
north coast, 0.75-2.50" Sierra, 1-2" Shasta, 0.25-1" Greater Bay
Area, 0.30-1" rest of nrn CA and down the valleys (1-1.50"
foothills), and a few hundredths to 0.10" or so for coastal soCal
(0.10-0.50" in the hills).

This low will act to cool temperatures down to 10-20 deg F below
seasonal normals with over 20 deg below normal for nw CA starting
tomorrow and lasting through Sunday. Freezing levels will also lower
from nw to se starting at 7.5-12 kft today dropping to 3-7.5 kft
north of I-80 Saturday morning and spreading to 2.5-6.5 kft north of
Point Conception by Sunday morning. Freezing levels will then
rebound as the low exits the region throughout Sunday.

Some weak ridging will then move over CA/NV into Monday drying out
most of the region. Some light showers possible over the northern
CA/NV borders and into srn OR as another system approaches the
PacNW. Troughing will then dig into northern CA/NV Monday afternoon
continuing shower potential for those areas into early Tuesday.
Afterwards, high pressure will build into the eastern Pacific
keeping the region dry and bringing temperatures back up closer to
normal. QPF in the extended is all expected on Monday as that trough
digs into the northern portion of the region. Forecast was mainly
the latest NBM coming out to 0.25-0.60" over the Cascades, 0.10-
0.50" over the Smith Basin, and generally a few hundredths to a
tenth of an inch elsewhere along nrn CA/NV.

Freezing levels Monday onward about 3.5-8 kft north of I-80 and 8-14
kft to the south with higher freezing levels spreading across the
state mid week. Expecting most of CA to be above 8 kft by Tuesday
evening and up to 14 kft along the soCal coast with only ne CA
and nrn NV down to 5-7.5 kft.


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



AS

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