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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
750 AM PST Mon Dec 6 2021



Radar imagery showing some light returns over northern CA and 
southern OR this morning as a weak surface low and associated cold 
front approach British Columbia. Observations have recorded a few 
hundredths of precip over the past 6 hrs. Yesterday's upper ridge 
has shifted slightly south and westward allowing moisture to stream 
over region as can be seen on current water vapor imagery. Models 
also show a decent moisture plume of 1"+ PW that will move through 
the region through Tuesday morning. This will result in scattered 
precip moving from NW to SE throughout the day and into Tuesday 
morning. QPF amounts are forecast to be generally less than a tenth 
of an inch for most areas and 0.10"-0.30" for the wettest spots 
along the north coast north of Cape Mendocino, the southern OR 
Cascades, and the Central Sierra. Simultaneously, an upper low will 
move southward just off the CA coast through Tuesday. This has 
allowed for some improved precip chances for the Central Coast over 
the next 24 hours, however, models keep the bulk of the precip from 
this system offshore. Just a nudge eastward may result in higher 
precip totals than forecast for that area. 

Behind this upper low, ridging will temporarily move into the region 
keeping most locations dry. A large surface/and upper low over 
western Canada and associated cold front stretching southwestward 
off the coast of the PacNW will bring a slight chance of some light 
showers along the coast of CA north of Cape Mendocino and the 
southern OR Cascades, but likely not more than a few hundredths. 
This broad area of low pressure will expand on Wednesday to cover 
most of the western U.S. and potentially bring additional light 
precip to the northern half of the region. This expansion will 
continue into Thursday with the front also moving through northern 
CA overnight Wednesday resulting in more widespread precip for 
CA/NV. Highest totals Wednesday night look to be over the Sierra. 
Uncertainty remains on QPF amounts as the GFS continues to show 
higher totals than the ECMWF along with a wide range of 
possibilities amongst both model's ensemble members.

Freezing levels to trend downward today to about 7-10 kft north of 
SoCal by this evening before rising again temporarily from W to E 
later Tuesday into Wednesday morning back to 10+ kft. As low 
pressure expands, colder air will be ushered in resulting in 
dropping freezing levels starting Wednesday. Levels will lower to 3-
6 kft north of Cape Mendocino and over far northern NV by the 


With the upr ridge aligned well offshore along 150W...the next s/wv 
trof moving from west-northwest to east-southeast across the region 
will bring widespread light to moderate precip on Thursday. The cold 
front will already be traversing northern CA during the morning 
hours with the best precip focused on the length of the Sierra and 
points eastward across the southern two-thirds of NV (mainly south 
of I-80). Precip is also expected to reach down across southern CA 
with the best amounts focused on southwest and west facing slopes. 
Highest totals will generally be in the range of 0.25- to 0.75-inch. 
In terms of freezing levels...look for these to fall to approx 2500- 
to 4500-feet from I-80 northward...5000-to 6500-feet for central CA 
and central NV...and about 7000- to 10000-feet across southern CA 
and southern NV during the bulk of the precip.  

This s/wv trof will slide east of the region on Friday with 
lingering precip tapering off during the morning hours. Flow will 
turn northwesterly as an upr ridge axis moves toward the west coast 
and eventually cross the region early on Saturday. The next moisture 
plume will take aim at the Pacific Northwest on Saturday with the 
southern extent of this feature clipping northwest CA and southwest 
OR...which will bring light precip into early Sunday.

Beyond the forecast upr trof will shift across the 
northeast Pacific and dig off the west coast. This feature looks to 
bring unsettled conditions across the west coast into early next least. This is well represented in the CPC 8-to-14 Day 
Precip Outlook...which shows a higher likelihood of above normal 
precipitation across the west coast from Dec 13 through 19.