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AGUS76 KRSA 221652
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
855 AM PST Fri Nov 22 2024

...COLD FRONT FINALLY SWEEPS ACROSS THE REGION TODAY...
...BEST PRECIP SHIFTS FROM NORCAL THIS MORNING TO NEAR/SOUTH OF I-80
THIS AFTERNOON AND EVENING...
...SYSTEM IMPACTS REGION MON/TUE BEFORE DRYING FOR END OF THE WEEK...

.METEOROLOGICAL CONDITIONS (SHORT TERM: FRI AM - MON AM)...

The overall pattern will begin to evolve today...after persistent
moderate to heavy precip affected northern CA for the past 48 hours
or so as a nearly stationary rich moisture plume took aim at the
west coast north of the Golden Gate. Amazing to look at precip
totals at stations just a reasonably short distance apart...like the
Santa Rosa Airport that has seen over 10.00-inches of rain while
approx 65-miles away at SFO there hasn/t even been 1.00-inch yet.
The current setup shows an area of low pressure currently maturing
west of the mouth of the Columbia River along 130W with a frontal
boundary extending along the coast OR and northern CA coast before
arching offshore near Point Arena. Over the northeast Pacific broad
troffing exists with a disturbance near the base of the cyclonic
flow crossing 135W with an associated jet streak beginning to nose
toward northern CA. This feature will help to enhance precip along
the cold front later this afternoon and evening near the I-80
corridor. Would not be surprised to see a NCFR develop in this area
with the 22/12Z HRRR hinting at this in its Composite Reflectivity
field...which would bring a brief period of intense precip as it
passes. Behind the frontal boundary...precip will finally begin to
taper off with intensity becoming lighter and coverage becoming more
scattered.

By early Saturday morning...the moisture plume will have dropped
south to the central CA coast with the best precip shifting across
the central/southern Sierra back toward the Big Sur coast. And then
as the moisture plume and associated cold front makes its way toward
southern CA...the feature will become disconnected from the upper
support and dissipate. Still expecting amounts to drop off from near
Point Conception (0.50-inch or so) to approx 0.25-inch or so around
the Los Angeles area and eventually a few hundredths of an inch in
the vicinity of San Diego. A few disturbances moving through the
southwest flow will cross northern CA with some upticks in shower
activity early Saturday...but with limited moisture and a quick
moving s/wv trof...amounts are expected to be mainly light.

Freezing levels are currently peaking or have peaked ahead of the
cold front with 8000- to 9000-feet along the CA/OR border this
morning...and 9500- to 11500-feet along I-80 (greatest northeast
NV). As the frontal boundary slides across the area...cooler air
will usher in from northwest to southeast bottoming out later
Saturday into early Sunday from 3500- to 4500-feet for far northern
areas...5000-feet or so along I-80...5500- to 7000-feet across
central CA...and a less dramatic drop across southern CA (8000- to
12000-feet).

For Sunday...the initial sfc low that moved up the Pacific Northwest
coast several days back will rotate back around and make its way
toward the north coast. Being much much weaker...around 1000-mb as
it crosses the CA/OR border area...look for an another quick uptick
in mainly light precip across northern CA ahead of the final
disturbance with this round of precip expected to arrive in the long
range.


.METEOROLOGICAL CONDITIONS (LONG TERM: MON AM - THU AM)...

Monday morning, models show an upper low off the Pac NW coast and a
moisture plume reaching the central CA coast.  Increasing moist
isentropic lift should lead to an uptick in precipitation rates
across northern/central CA Mon.  However, with low-level flow on the
weaker side and overall lack of strong forcing aloft, most guidance
keeps precip on the lighter side initially.  Later Mon afternoon
into Mon night as a cold front approaches and a vort max swings
closer to northern CA, low- and mid-level winds are expected to
increase, all leading to potentially heavier precip rates across the
central CA coast and especially the southern half of the Sierra.
There is considerable spread among ensemble members, with some
showing heavy precip and some light or almost none.  Some of the
ensemble members show potential for somewhat heavy precipitation
remaining nearly stationary for a few 6-hour periods.  Overall
consensus is for the moisture plume to continue to impact the area,
especially central portion of the region into Tue before the plume
pushes south and ridging moves over the west coast.  CNRFC QPF
favored a blend of WPC and NBM guidance. Freezing levels are
forecast to range from 4000-6000 ft in far northern CA/NV, 6000-9000
ft central (area of highest precip amounts), and over 10,000 ft
further south.


QPF graphics are available at www.cnrfc.noaa.gov/qpf.php



Kozlowski/Lerman

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