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Hydrometeorological Discussion
Issued by NWS California-Nevada RFC
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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
800 AM PDT Mon Apr 22 2024

...MOSTLY DRY CONDITIONS TODAY ASIDE FROM SLIGHT CHANCE OF ISOLATED
SHOWERS/TSTORMS OVER THE CNTRL SIERRA AND CNTRL NV
...ABOVE NORMAL TEMPERATURES AGAIN TODAY BEFORE COOLING KICKS OFF
THE REST OF THE WEEK ALONG WITH A RETURN OF PRECIP CHANCES...

.METEOROLOGICAL CONDITIONS (MON AM - SUN AM)...

An upper ridge remains over the eastern Pacific as a weak surface
low develops offshore of nrn CA. Ridging will keep the majority of
the region dry today with continued above normal temperatures at 5-
15 deg F. The exception will be slight chances of isolated
showers/thunderstorms over the central Sierra and eastward through
central NV this afternoon/evening. The surface low will continue to
form offshore overnight and throughout Tuesday potentially resulting
in some scattered showers across nrn CA, the nrn/cntrl Sierra, and
parts of n/w NV along with isolated thunderstorms in the
afternoon/evening. The next system will then approach as an upper
low coming from the west. Later Tuesday, the low will take a shift
to the south headed towards soCal. There are still differences
between the models on the timing of this next system`s arrival. The
GFS remains ahead of the ECMWF bringing the core of the system just
offshore of Point Conception Weds afternoon while the ECMWF waits
until the evening. The ECMWF also has the core of the system further
to the south off the coast of Baja as troughing pushes inland across
central/southern CA. In either case, this is expected to produce
some precip Weds over the central Sierra and through central to ne
NV along with isolated thunderstorms. Amounts expected to be about
0.25-0.75" near the CA/NV border down to 0.10-0.30" across NV. Light
scattered showers across other parts of nrn CA will also be possible
as another system approaches from the nw.

Thursday morning, the upper low will move through srn CA exiting
into AZ throughout the morning and into the afternoon. Models
disagree on the exact timing. Meanwhile up north, a surface low will
approach the PacNW with an upper trough overhead. The GFS is a bit
quicker at the low arriving along the coast than the ECMWF and is
also faster at the upper trough expanding across the west coast.
Both models do show precip spreading across northern CA/NV in the
afternoon and evening. The upper trough is then expected to dig
further into the west spreading showers across CA/NV before shifting
its trajectory eastward later Friday. It is at this point that
models diverge again. 00z Saturday ensembles disagree on both the
depth of the trough and its east/west position. The ECMWF this time
is the quicker model with the axis of the trough along the eastern
CA border while the GFS is more over CA/NV. By Saturday morning, the
ECMWF has the back edge of the system almost fully out of the region
while the GFS still has troughing covering most of CA. The main
result is the ECMWF showing another system arriving along the north
coast mid Saturday morning while the GFS does not. Precip forecast
for the 24 hrs 12z Sat to 12z Sun out of the GFS is dry across nrn
CA while the ECMWF shows as much as 0.50-1.25" along the north
coast. Many of the ECMWF ensembles are on the drier side, leading to
lower QPF out of the NBM compared to the det run. Went with a blend
of the NBM and det guidance to show some limited showers over nrn CA
for Saturday. All this to say uncertainty in the forecast for most
of the week as models continue to disagree and have not been as
consistent as one would hope run to run.

The majority of the precip is still expected in the extended on
Thursday/Friday. Highest amounts along the north coast, over the srn
OR Cascades, and the mountains of ne NV. QPF 12z Thurs-12z Sun: 0.40-
1" north coast/srn OR Cascades (up to 1.50" Smith Basin), 1-2" ne NV
mountains, 0.10-0.50" rest of nrn CA/NV, and a few hundredths to a
tenth of an inch for cntrl CA/w NV.

Additionally, this change in pattern starting tomorrow will act to
cool conditions from west to east gradually throughout the week. By
Friday, afternoon high temperatures are forecast to be 5-15 deg F
below normal. Higher freezing levels will continue today at 10.5-
12.5 kft from n to s. Freezing levels to then lower mid week down to
8-11.5 kft then 6-10.5 kft by Thursday reaching 4.5-6.5 kft north of
I-80 and 6.5-11.5 kft to the south Friday morning. The quicker exit
of the upper trough will allow levels to rebound over the weekend up
to 6.5-9 kft north of I-80 and 8.5-10.5 kft to the south Saturday
morning.


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



AS

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National Weather Service Weather Forecast Office
