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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
750 AM PDT Thu Apr 22 2021

...LATE SEASON SYSTEM MOVES IN FOR THIS WEEKEND INTO MONDAY...
...BEST PRECIP AMOUNTS OVER SHASTA LAKE DRAINAGE AND ENTIRE SIERRA...

.METEOROLOGICAL CONDITIONS (THU AM - WED AM)...

Positively tilted upr trof axis extends from eastern Montana back 
southwest to off the southern CA coast. This brings cyclonic north 
to northwest flow across the region for today. Still looking at the 
potential for some scattered light precip to pop up over the 
southern Sierra under this flow regime. Also...with a deep marine 
layer continuing across coastal southern CA...there remains a 
possibility of some light precip generally between Los Angeles and 
the CA/MX border. KNKX 22/12Z RAOB indicates the depth of the moist 
layer up to almost 5000-feet.

The upr trof finally shifts to the east late Thursday into early 
Friday as an offshore fairly narrow wavelength upr ridge slides 
across the area for dry conditions much of Friday. Then...a deep upr 
low currently spinning over the Aleutian Islands will shift toward 
the northeast Pacific with its effects beginning to be felt across 
the region on Saturday with an initial s/wv trof rotating around the 
primary circulation toward northern CA. Cooler temperatures will 
begin to be ushered in across northern CA with precip potential best 
near the CA/OR border.

The core of the cooler airmass will really move overhead on Sunday 
into Monday with high temperatures dropping generally 10- to 25-degF 
below normal and precip spreading across much of the region. The 
overall system will dig southward just off the west coast and then 
swing toward the region. Onshore southwesterly low- and mid-level 
flow will advect the moisture plume of between 0.75- and 1.00-inch 
PW inland with the best amounts falling across the Shasta Lake 
drainage and down the length of the entire Sierra under efficient 
orographic lift. Amounts through the storm event will generally 
range from 1.00- to 2.00-inches. Freezing levels in this area will 
also drop to approx 4000- to 6000-feet...and potentially 500-feet 
lower as the coolest air associated with the core of the system 
moves across the area.

Look for the primary circulation to shift inland Sunday late 
afternoon or early evening near the CA/OR border. Then a couple 
disturbances on the backside of the overall system will drop toward 
central and southern CA with precip increasing in this area later 
Sunday into Monday. Back to the north...precip will become more 
scattered and light as the flow aloft turns to the northwest and 
available moisture drops off. 

Finally for mid-week...conditions dry out as an offshore upr ridge 
moves toward the west coast. Look for temperatures to rebound 
slightly above normal for the second half of the week.