Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 211918
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
930 AM PST Sun Jan 21 2018

...PRECIP MOVING INTO NORTHERN CA TODAY AND AGAIN MIDWEEK...

.METEOROLOGICAL CONDITIONS (SUN AM - SAT AM)...

Light precip is beginning to move into the Smith basin this morning,
but low level SE flow is keeping the heaviest bands offshore still.
Expect winds to shift as the cold front approaches, and allow precip
intensity to increase through the day. While the moisture plume is
currently hanging offshore, amounts of around 0.5" or so are
currently hitting the coast, while being able to tap into amounts
closer to 0.9" later in the day.

Some changes in the models overnight have led to a lowered QPF
forecast for today and early Monday. All but the NAM currently keep
the bulk of the dynamics and moisture along the coast as the system
travels southward, making for a much drier inland forecast than
previously indicated. Expect moderate to locally heavy precip to
stretch down the North Coast through the day, continuing overnight
into early Monday morning. While some precip is expected to advect
inland overnight, expect amounts to remain at or below 0.5" from the
Sacramento valley into the Northern Sierra. Precip should quickly
wrap up during the morning and afternoon hours on Monday, leaving a
weak ridge over the west coast.

By Tuesday morning, a warm front is expected to move into the Smith
basin ahead of a stronger upper low dropping out of the Gulf of
Alaska. The main cold front is expected to approach coastal areas
late Wednesday afternoon, driven by a 100-kt upper jet and a 0.75"
pwat plume. Though moisture is only moderate, strong dynamics and
favorable onshore flow should make good use of what is available,
especially across the coastal slopes from the SF bay northward.
Precip is expected to move inland during the overnight hours into
Thurs morning, with heaviest totals over Shasta and the Sierra from
the Feather down to the Tuolumne.

Expect the trough axis to slide onshore during the late morning
hours Thursday, dragging moderate precip into the Southern Sierra
and N slopes of the Srn CA mtns. Scattered showers should continue
over the rest of the state through the rest of the day Thursday in
the unstable cold airmass behind the trough axis, slowly winding
down overnight into Friday morning.


.HYDROLOGICAL CONDITIONS...

The small river rises expected Sun/Mon have been reduced quite a bit
in today`s forecast due to reductions in QPF.  Larger river rises
are expected for the Tues/Wed storm with the largest rises focused
along the northern coast of CA. No rivers are expected to come close
to monitor levels for either of these two systems.


More information on the CNRFC website at http://www.cnrfc.noaa.gov

JM/BW

$$



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