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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
800 AM PST Sun Dec 5 2021



Satellite imagery and observations once again show patchy stratus 
and fog across CA this morning. This has produced a little drizzle 
again overnight. A broad upper ridge remains in place over the 
eastern Pacific steering moisture up and around the region as a 
weakening surface boundary approaches northern CA. Dry conditions 
will continue the rest of today and tonight as the upper ridge 
shifts westward ahead of an approaching shortwave trough. A surface 
low will move towards British Columbia into tomorrow with a cold 
front stretching south/southwestward along the west coast of the 
U.S. and out over the eastern Pacific. Light showers ahead of the 
front are expected along the north coast and CA/OR border late 
tonight into early Monday morning. Precip will remain possible from 
northwest to southeast into mid/late Tuesday morning as the front 
moves through the region. QPF amounts look minimal with a few 
hundredths to a tenth of an inch for most of the area and about 
0.10"-0.25" for the north coast and southern OR Cascades as well as 
over the southern Sierra. As the surface boundary moves through, a 
shortwave trough will move southward offshore of CA on Tuesday 
bringing possible light showers to SoCal late Tuesday morning and 
into the afternoon. Amounts, should this occur, would be fairly 
unimpressive. Freezing levels will lower some tonight and tomorrow 
with this system down to 7-10 kft from north to south before quickly 
rebounding from NW to SE back to 10-12 kft by the evening.

The upper ridge over the eastern Pacific will be gradually displaced 
further westward over the Pacific as a trough moves into western 
Canada on Wednesday. This trough will expand to engulf much of the 
western U.S. by late this week resulting in the start of a wet 
pattern for the forecast area. A large area of surface low pressure 
over Canada at this time will also have a long cold front stretching 
out over the Pacific Northwest and into the eastern Pacific. Models 
show some light showers as a result north of Cape Mendocino on 
Wednesday, though would likely only be a few hundredths and up to a 
tenth of an inch. This boundary looks to pass through northern CA 
Wednesday night into Thursday morning bringing a chance of 
widespread precip to the region. Models still have the highest QPF 
amounts over the Sierra as the system moves through. Differences 
remain regarding precip totals with the deterministic GFS once again 
higher than the ECMWF by a not insignificant amount. Ensembles from 
the EC are more promising with multiple members showing higher 
totals than the deterministic run for Thursday. Models also show a 
plume of 1+" PW moving along the CA coast into Thursday with timing 
fairly similar between the GFS and ECMWF. At the moment, expecting 
the highest precip amounts to occur before Thursday afternoon, but 
confidence on exact totals remains low.

For Friday, both the GFS and ECMWF show some embedded shortwave 
energy moving into the interior. This may result in some showers on 
Friday for the eastern portion of the region. The GFS has this 
energy further westward than the ECMWF pulling light precip into 
more of NV than the ECMWF. QPF amounts at this time look fairly 
light. This energy will have left the area by Friday night leaving 
dry conditions into early Saturday morning. Another frontal system 
then looks to approach the Pacific Northwest next weekend.

This aforementioned expanding area of low pressure will also usher in 
cooler air and lower freezing levels from NW to SE on Wednesday. 
Levels will drop to 4-6 kft by mid/late Wednesday afternoon for the 
north coast. This trend will continue throughout the work week with 
much of the area down to 2.5-6 kft by late Thursday morning and 
remaining lower through Friday. CA will start to see levels rise 
again Friday evening from SW to NE back to 8-11 kft while NE NV 
continues to lower below 2 kft.