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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.