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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
750 AM PDT Tue Apr 29 2025

...WEAK SYSTEM LATER TODAY THRU THURS WITH LIGHT PRECIP AND ISOLATED 
T-STORMS, MAINLY OVER THE SIERRA AND EASTWARD...
...ANOTHER LARGE COLD CORE SYSTEM EXPECTED FRI THROUGH THE WEEKEND...

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

A shortwave approaches the PacNW this morning as ridging sits over 
nrn CA/NV with an upper low over the Four Corners. The ridge 
overhead will keep conditions dry for much of today along with near 
to slightly above normal afternoon high temperatures. The 
approaching shortwave will reaching nrn CA late this afternoon into 
the evening generating showers over parts of nrn CA/NV. This system 
will continue to dig further to the south across CA through mid-week 
deepening as it does so. This will continue to produce scattered 
showers for Wednesday, mainly over the Sierra and eastward into NV. 
Additionally, models persist in showing instability as the trough 
deepens particularly for Wednesday afternoon/evening when 
thunderstorms are most likely. Best chances will be across the 
Sierra and eastward into NV. As this trough travels through CA, an 
upper ridge over the eastern Pacific will push inland behind it into 
the PacNW and nw CA raising temperatures to about +5 to +10 deg F 
above normal for Wednesday. Some time Thursday morning, the trough 
is expected close off into an upper low over srn CA. This will shift 
shower and thunderstorm chances a bit south over the central/srn 
Sierra and eastward. Some showers may stretch as far west as the srn 
CA coast, but accumulations should be minimal. QPF for the Tues-
Thurs system is forecast at 0.10-0.75" across the Sierra and 0.10-
0.25" or so over the NV mountains. Lower elevation areas can expect 
generally less than 0.10". Locally higher amounts in any developing 
thunderstorms. Above normal temperatures again for Thursday over nrn 
CA as the ridge drifts inland over WA/OR.

A larger system is still set to approach heading into the weekend. 
Models have an upper low over the Gulf of Alaska with an elongated 
trough and front moving towards the west coast. Showers expected on 
Friday between lingering precip from the previous system and initial 
showers ahead of the next. There are differences between the 
GFS/ECMWF on the structural and timing aspects of the next system, 
but it looks like the southern end of the front will reach the north 
coast by the evening with it and the eastern edge of the trough 
moving inland overnight. Precip should then spread into the Bay Area 
and across the Sierra Saturday morning as the core of the system 
approaches the coast. By Saturday afternoon, the trough should've 
closed off into an upper low somewhere along the nrn CA coast before 
traveling through the Bay Area while hugging the coastline 
overnight. Both the GFS and ECMWF have the core of the low somewhere 
near Point Conception by early Sunday (though the ECMWF is a bit 
further south). Things diverge a bit from there with the ECMWF taking 
the low inland across srn CA Sunday morning and beginning to exit se 
CA into AZ in the afternoon. The GFS keeps the low along the coast 
continuing to head south across coastal soCal through most of Sunday 
afternoon before taking its inland shift in the late afternoon/early 
evening. Either way, the core of the low looks to move out of the 
area by the end of the period with the back edge of the system still 
encompassing the entire region. Expect this system to generate 
scattered showers across CA/NV for most of this time period. 

Highest precip amounts are expected across the Sierra and eastward 
into nrn/cntrl NV with lesser amounts across CA. QPF 12z Fri-12z 
Mon: 0.75-1.50" across the Sierra/nrn NV mountains, 1-2.25" central 
NV mountains, 0.30-1" nrn/cntrl NV, 0.10-0.30" rest of NV, 0.25-
0.75" Shasta/ne CA (locally up to 1" mountains ne CA), 0.10-0.50" 
rest of nrn CA/srn CA mountains east of San Diego, and generally 
0.10" or less for the rest of CA. Again locally higher amounts for 
any thunderstorms. 

This will be a colder system as well dropping both afternoon high 
temperatures and freezing levels across the region. A few 
thunderstorms may be possible as well. By Saturday, most areas will 
see anomalies of -5 to -15 deg F with some locations reaching -20 
deg F. Sunday will be the coolest with anomalies at -10 to -20 deg 
F. Freezing levels will go from 9-11.5 kft today through Thursday 
down to 4-8.5 kft north of I-80 Saturday morning spreading to Point 
Conception by the late afternoon. Freezing levels will reach a 
minimum early Sunday down to 4-7.5 kft for most areas outside of far 
se CA and eastern NV before rebounding from nw to se into Monday as 
the system exits. Areas north of I-80 by then should be back up to 8-
9.5 kft over the Sierra and up to 11 kft along the north coast.