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Hydrometeorological Discussion
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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
735 AM PDT Sun Apr 21 2024

...GENERALLY DRY CONDITIONS WITH ABOVE NORMAL TEMPERATURES THRU
MONDAY BEFORE GRADUAL COOLING BEGINS TUESDAY...
...PRECIP CHANCES RETURN MID TO LATE WEEK, BEST CHANCES OF MORE
WIDESPREAD PRECIP THURS/FRI...

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

Dry conditions and above normal temperatures will continue through
Monday as high pressure builds over the eastern Pacific. Afternoon
high temperatures are expected to reach 5-15+ deg F above seasonal
normals across much of the region. Gradual cooling will then take
place starting Tuesday as an upper low approaches CA from the west.
Temperatures will ease from west to east throughout the week
starting at the coast Tuesday. Additionally, a weak surface low
setting up off the nrn CA coast may result in some scattered showers
over nrn CA/NV throughout the day with chances of thunderstorms in
the afternoon/evening. Regarding the mid-week low, models have come
into better agreement on the timing of this system. The GFS has
slowed down the arrival of the low which is much closer to the ECMWF
than it has been the past several days. Both models have the low
shifting to the south into Wednesday approaching coastal soCal, but
the GFS is still slightly faster on the core of the system reaching
land compared to the ECMWF (Weds aftn vs Weds eve). Either way,
expecting the low to bring some showers to parts of the
central/southern Sierra and eastward through central/ne NV Weds/Weds
night. Some light showers may also develop near the core of the low
as it passes through soCal. By early Thursday, the low is expected
to begin exiting CA into western AZ. Precip amounts through 12z
Thurs are not expected to be significant, with the highest amounts
about 0.25-0.50" on Wednesday from the central/southern Sierra into
ne NV. Only forecasting about 0.10" or less for the rest of nrn
CA/nw NV in the near term.

Later Thursday, another system will drop in from the Gulf of Alaska
as a surface/upper low. There are differences between the models on
this system as well. The GFS has the surface low further to the
north mid Thursday morning just offshore of BC while the ECMWF has
it just west of the WA/OR border. This is resulting in the ECMWF
predicting showers further across CA into the Bay Area and central
Sierra while the GFS confines precip mostly to northern CA/NV. This
also means the positions of the upper level system are different in
similar respects. The ECMWF has an upper trough stretched across the
PacNW and through much of CA/NV Thursday evening while the GFS shows
a shallower trough. The GFS is also further westward with the trough
axis on Friday with the core of the system still offshore while the
ECMWF has the system fully enveloping the region. Models come closer
together by Friday evening both showing an upper low and elongated
trough encompassing the entire western U.S. spreading precip to much
of the region. To illustrate what these differences mean in terms of
QPF, the ECMWF is predicting 1-1.50" along the north coast 12z Thurs-
12z Fri and 0.75-1.25" over the northern Sierra while the GFS has
0.25-0.50" and <0.10" respectively.

To summarize, dry conditions with above normal temperatures through
Monday before gradual cooling and a transition to a wetter pattern
begin the rest of the week. Still some uncertainty on the details
due to model differences. The majority of the QPF for this week is
expected on Thursday/Friday with that second and larger system.
Highest precip amounts over nw CA, the srn OR Cascades, and over the
nrn/cntrl Sierra. QPF 12z Thurs-12z Sat: 0.50-1" north coast (up to
1.50" Smith Basin), 0.25-0.80" srn OR Cascades/nrn/cntrl Sierra, 1-
2" eastern NV mountains, 0.10-0.50" rest of nrn CA/NV and a few
hundredths to 0.30" down the valleys and around the Bay Area.

Freezing levels 10-13 kft from N to S the rest of the weekend and
early next week. Freezing levels to then lower mid week down to 8-
10.5 kft then 6.5-10.5 kft by Thursday reaching 4.5-6.5 kft north of
I-80 and 6.5-11.5 kft to the south Friday morning. Lower freezing
levels will continue to spread across the region into the weekend
down to 4.5-6.5 kft north of Point Conception across CA and 6-7.5
kft generally elsewhere.


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



AS

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National Weather Service Weather Forecast Office
