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Hydrometeorological Discussion
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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
730 AM PDT Sat Apr 20 2024

...WARM AND GENERALLY DRY CONDITIONS THROUGH MON ASIDE FROM A FEW
STRAY SHOWERS OVER NRN CA AND THE SIERRA...
...GRADUAL COOLING AND THE RETURN OF PRECIP CHANCES MID TO LATE WEEK
AS A LOW APPROACHES THE CA COAST...

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

High pressure is currently situated over the west coast bringing
mostly dry conditions this morning. An elongated upper low is
stretched across the Gulf of Alaska with a surface low offshore of
BC and a cold front outstretched to the west. Troughing will move
into WA this afternoon after the cold front arrives along the PacNW
coast. This may result a few light showers over parts of nrn CA. The
upper trough will pass through BC the rest of today and overnight as
high pressure builds over the eastern Pacific. This will act to warm
temperatures across the region through Monday with afternoon highs
rising to 5-15 deg F above normal away from the coast. The next
system then approaches from the west as an upper low.

Uncertainty remains in the forecast for the rest of the week as
models continue to disagree on the fate of the low. The ECMWF and
CMC are still much slower on the arrival of the system compared to
the GFS. Both models have a weak surface low setting up just nw of
the Bay Area Tuesday that may result in some light scattered showers
across nrn CA. Beyond that, the GFS speeds up the movement of the
upper low opening it into a trough offshore of srn CA mid Wednesday
morning while the ECMWF still has the system over 130W. The GFS then
takes the system through srn CA Weds afternoon before exiting into
AZ overnight into Thursday morning. The ECMWF only then has the
trough beginning to move inland across soCal Thursday morning. This
is also creating differences in the arrival, amounts, and extent of
precipitation across CA Wednesday into Thursday. The ensembles are
divided along similar lines with disagreements on timing based on
the parent model. Bottom line, showers are expected across nrn CA
and NV mid to late this week due to an upper low moving through srn
CA.

After the aforementioned system exits to the east, models show a
surface low dropping in from the Gulf of Alaska with a cold front
offshore of the PacNW and nrn CA. The GFS is ahead with the
progression of this system compared to the ECMWF and shows a more
organized surface low with higher precip amounts. From 18z Thurs to
00z Fri the GFS has just under 0.50" over the Smith Basin while the
ECMWF still has precip offshore and only at a few hundredths. These
differences carry over into early Friday as well. By the end of the
extended, the GFS has over an inch of precip over the Smith Basin
for the 24 hrs ending 12z Friday while the ECMWF has only a few
hundredths and the CMC a quarter of an inch.

All this to say uncertainty in the forecast for mid to late next
week. Expecting mainly dry conditions and above normal temperatures
through Monday before gradually cooling from west to east and a
return of precip chances for mid to late week. The QPF was a blend
of the morning WPC guidance and the latest NBM. QPF is highest over
nw CA and the srn OR Cascades at 0.25-0.50" or so with lesser
amounts elsewhere across northern CA/NV and down the Sierra.
Freezing levels 9-13 kft N to S this morning with higher freezing
levels spreading northward throughout the rest of the weekend.
Freezing levels to then lower mid week down to 8-10.5 kft then 6.5-
10.5 kft by Thursday depending again on the timing of the next low.
Freezing levels to reach a min Friday morning of 5-7 kft north of I-
80.


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



AS

$$
National Weather Service Weather Forecast Office
