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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
740 AM PDT Tue Oct 15 2024

...A PAIR OF SHORTWAVES TO BRING CHANCES OF PRECIPITATION TO NRN 
CA/NV TONIGHT INTO THURS AND THURS INTO FRI ALONG WITH ISOLATED T-
STORM CHANCES OVER NE CA AND NV...
...BELOW NORMAL TEMPERATURES WEDS INTO SAT THEN NEAR NORMAL...

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

Some ridging over CA/NV this morning as a weak upper low enters the 
Four Corners. This will keep dry conditions for most of today across 
the region along with afternoon high temperatures about 5 to 15 deg 
F above normal away from the coast. A large upper low is also making 
its way through the Gulf of Alaska sending a trough towards the west 
coast. This trough will begin to move onshore later today pushing 
the ridge out of the area and ushering in cooler temps. Chances of 
precip are also expected, though differences in the GFS/ECMWF remain 
present. The ECMWF continues to bring in showers sooner than the 
GFS, early this evening vs overnight tonight, and is showing higher 
QPF amounts that are a bit more widespread than the GFS. Looking at 
the QPF clusters, these differences are also present between the 
ensembles with the ECMWF showing higher QPF over the northern Sierra 
compared to the GFS. The det 24 hr totals ending 00z Thursday are 
around 0.10" for the GFS and closer to 0.25-0.75" for the ECMWF. QPF 
clusters 1 and 4 which comprise about 54% of the total ensembles 
between the GFS/ECMWF/CMC have as much as 1.50" over the northern 
Sierra while clusters 2 and 3 (46% of total ensembles) have roughly 
0.10-0.50". These drier clusters contain 90% of the GFS ensemble 
members. All this to say, there is still much uncertainty in the QPF 
for this next system. Both models do have the trough moving through 
the west coast tonight and tomorrow as the core of the low pushes 
into BC. The trough is expected to drop showers across northern CA 
and NV into early Thursday as it moves through along with slight 
chances of thunderstorms over parts of nrn CA and over most of NV.

Another trough will then spin up off the back side of the main low 
on Thursday dropping into the PacNW and nrn CA by the afternoon. 
This will bring afternoon highs down to 5 to 15 deg F below normal 
for Thursday/Friday with some areas as much as 20 deg below normal 
on Friday. It will also bring another round of showers, and possible 
isolated thunderstorms, to northern CA and NV late Thursday morning 
through Friday morning. Model differences are not quite as drastic 
with this system, coming into better alignment than yesterday on the 
majority of the precip falling north of the OR border. All the QPF 
clusters are showing less than 0.50" at the far nw corner of the 
Smith Basin with generally light amounts elsewhere across nrn CA and 
NV. The det runs are a little wetter over nrn CA bringing showers a 
bit further south into nrn CA than the ensembles, but amounts are 
around 0.10-0.25" or so.

QPF was a combination of morning WPC guidance, the NBM, and the 
ECMWF. Majority of the precip expected in the near term will be over 
srn OR, the northern Sierra/ne CA, and over nrn NV. QPF through 12z 
Friday: 0.30-0.80" nrn NV, 0.25-0.50" northern Sierra/ne CA/srn OR 
Cascades, 0.10-0.25" the rest of the CA/OR border and the central 
Sierra, and generally a tenth of an inch or less for the rest of nrn 
CA. Some light showers over far southern coastal CA possible with 
that second system later Thursday as it digs deeper into the region.

That Thursday trough will deepen into a closed low by early Friday 
morning as it heads into the Four Corners. The back edge of the low 
may result in some lingering showers over eastern NV and se CA. 
Behind the low, a ridge will build back into the eastern Pacific 
drying conditions out the rest of Friday and through at least mid 
Sunday morning while also raising temperatures back to near/above 
normal from nw to se. Just on the heels of the ridge is another 
trough that will move into the PacNW over the weekend. This may 
bring additional showers to the northern regional boarder late 
Sunday into Monday, but there are some timing differences between 
the GFS/ECMWF as well as disagreement on the structure of the 
trough. Kept amounts light for now blending WPC/NBM with the drier 
det GFS/ECMWF. This amounted to a few hundredths to just over 0.10" 
across the Smith Basin and a few hundredths to a tenth for the rest 
of the north coast. Some of the ensemble clusters are a bit wetter 
along the north coast with about half of the ECMWF ensemble members 
showing potential for over an inch. Will keep an eye on this over 
the next several days.

Freezing levels to gradually drop over the next couple of days from 
nw to se down to 8 to 10.5 kft north of I-80 mid day Weds and 
hitting a minimum of 5-8.5 kft Thursday afternoon. Lower levels will 
expand across nrn CA into NV throughout the rest of Thursday and 
into Friday before rebounding W to E by the afternoon (8.5-12.5 kft 
from the Sierra to the coast).