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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
800 AM PDT Mon Apr 28 2025

...WEAK SYSTEM LATER TUES THRU THURS WITH LIGHT PRECIP MAINLY OVER 
THE SIERRA AND EASTWARD...
...ANOTHER LARGE COLD CORE SYSTEM POSSIBLE FRI INTO THE WEEKEND...

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

An upper low moved through the region yesterday dropping showers 
across CA/NV. Observations over the past 24 hrs show about 0.10-
0.75" across the Sierra/nrn NV and scattered hundredths to a couple 
tenths across the rest of the area. Radar imagery this morning shows 
some showers still streaming into nrn NV near I-80 as the core of 
the upper low continues to exit the region. Troughing remains 
overhead keeping some isolated showers in the forecast over srn CA 
and parts of NV into this afternoon. Behind the trough lies a large 
upper ridge over the eastern Pacific that will build its way into 
the west coast later today into tonight. This will bring some brief 
drying and rise temperatures back up to near normal for Tuesday. 
Models then have a shortwave moving through the PacNW on Tuesday 
digging into nrn CA in the afternoon/evening before deepening 
overhead on Wednesday. This system is expected to produce additional 
showers on Tuesday and Wednesday first over nrn CA/NV before shifting 
into the Sierra and central NV on Wednesday. The ridge offshore will 
shift into the west coast to the north of this trough into Thursday 
as the system continues to head southward now focused over srn CA/NV 
for the rest of the day. This will again produce scattered showers, 
though mainly over the central/southern Sierra and eastward. A 
thunderstorm or two during this time will also be possible. QPF 
through Thursday will be lingering showers from the exiting system 
today and the trough Tuesday to Thursday. Best amounts across the 
Sierra at 0.10-0.50", 0.10-0.30" over the NV mountains, and a few 
hundredths to a tenth of an inch elsewhere.

Towards the end of the week a larger system will approach as a low 
moves through the Gulf of Alaska sending an elongated trough and 
front towards the west coast. Initial showers out ahead of the 
system expected Friday morning before the front reaches the north 
coast in the evening. Troughing will begin to move inland overnight 
and Saturday morning. The GFS has a closed low forming over the Bay 
Area/central coast Saturday evening while the ECMWF delays this 
another 6 or so hours forming the low instead over the srn Sierra. 
Either way precip will continue across the region during this time 
as the trough moves southeastward through CA. This will also lower 
freezing levels starting at 10-11.5 kft on Friday morning down to 6-
9.5 kft north of I-80 in the evening and 4-8 kft north of Point 
Conception Saturday afternoon. By the end of the period most of CA 
will be below 5.5 kft.

Highest QPF for Friday/Saturday across the Sierra and eastward into 
NV. Forecast was a blend of morning WPC guidance and the 13z NBM. 
QPF 12z Fri-12z Sun: 0.50-1" Sierra/Shasta, 0.25-0.75" nrn CA north 
of Cape Mendocino, 0.75-1.50" central/nrn NV mountains, 0.10-0.75" 
rest of nrn/central NV, and a few hundredths to under 0.25" for the 
rest of nrn/central CA. Maybe up to 0.10" for parts of coastal srn 
CA.