Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 101759

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
1000 AM PST Sat Dec 10 2016



A warm and very moist system moved into the region yesterday,
bringing a wave of heavy precip to much of Northern CA. Totals were
high over the Smith basin, which picked up 3-5" through the early
morning hours, with about 1.75-3" reported over the Eel R Basin with
locally higher totals over higher terrain. The Russian R Basin
clocked about 1-2.5" , with about 0.5-1.5" over the Napa R Basin.

Further inland, the Srn OR Cascades picked up 0.5-1" precip, with
similar amounts both on the W and E sides. The Shasta basin measured
2-3" precip, with up to 4" amounts over the east side. The Northern
Sierra received quite a bit overnight into the early morning hours,
with widespread 3-3.5" amounts and up to 4" over the Feather and
Yuba. The American saw closer to 2" through 12Z this morning, with
amounts tapering off to 0.5-1.5" just south of highway 50. This has
been a perfect set up for spillover into the East side of the
Sierra, with amounts around 0.5-1.5" east of the crest.


Ongoing significant precipitation event in the nrn Sierra this
morning as a moisture plume with PW values near or above 1.4" and H7
flow around 40-45 kts along with good isentropic lift.  Freezing
levels should remain in the 9000-10000 ft range this morning and
gradually fall to 8000-9000 ft this afternoon and 7000-7500 ft this
evening.  Overall, we raised amounts for portions of the Sierra from
near I-80 northward this morning while overall cutting amounts
further south, especially for the 12-18 UTC period today.  GEFS H85-
H7 moisture flux anomalies reach about +3 for nrn CA locations with
this system, but reach up to +5 further inland across nrn NV.

The moisture plume is expected to gradually work its way south, with
the focus of heavy precip shifting into the n-cntrl Sierra by
afternoon and cntrl Sierra this evening.  As mentioned previously,
overall amounts in the models are lower than yesterday`s runs
suggested for the cntrl Sierra, with perhaps the exception of the
GFS.  By late tonight expect a rapid diminishing of precip amounts.

Outside the Sierra, expect mainly light showers across nrn/cntrl
CA/NV with a few light showers making it into srn CA later tonight
and Sun morning as lower-level flow diminishes and in the absence of
other significant forcing.  For Sun into Mon morning, expect just a
few light showers across the north due to warm advection and
proximity to the upper jet.  Could see the beginnings of the next
system work their way into nrn CA at the end of the short-term
forecast window as isentropic lift begins again ahead of the next


By Tuesday morning, the next system queues up along the west coast,
with the warm front reaching shore early Tuesday morning. Another
pulse of the heavily moisture laden plume originating near Hawaii
takes aim at an area from the Santa Cruz Mtns north towards Cape
Mendocino. Expect moderate to locally heavy precip to affect much of
the North Coast from the Eel to the Napa Tuesday morning, then
moving inland to Shasta and the Northern Sierra by late morning and
afternoon. Expect heavy precip to continue over the Northern Sierra
for most of the day, as moisture is efficiently advected inland from
the plume offshore. Looks like another efficient spillover event for
the Reno area as well, mostly in the late afternoon and evening
hours. Overnight and into Wednesday, the band of heavy
precip lifts Northward ever so slightly from around the Santa Cruz
Mtns into the I-80 corridor and the northern Sierra from the Feather
down to around the Stanislaus, up to a position from around the
Russian/Napa, through the Sacramento Valley along and just north of
the I-80 corridor, and into the Northern Sierra from the Feather
into the American.

Later in the day Wednesday (while moderate to heavy precip continues
over the aforementioned areas), a S/WV trough finally deepens and
moves towards the Nrn CA coastline. An associated surface low moves
into position just off the Big Sur coastline, aiding in drawing
significant moisture into Central CA. The GFS is the most pronounced
model with this surface low by far, with the EC mostly focusing on
the frontal passage Thursday. At this time, it does look like a
significant wave of precip will move into the region on Thursday
associated with the cold front, though the exact placement and
intensity are not well discerned. The 10/00Z EC is weaker on
Thursday into early Friday, and mostly focused on Central and
Southern CA. The 10/06Z GFS is quite fond of the strong surface low,
and pulls additional orographically driven heavy precip into the
Central Coast, Srn CA, and the Sierra Thursday evening and night.
The recently arrived 10/12Z GFS has a shallower s/wv trough and
develops the surface low in a more elongated pattern further north,
resulting in more heavy precip for the North Coast Thursday and
drawing into Central and Srn CA Thursday night. The current forecast
reflects something more similar to the 10/00Z EC, which was more in
line with the solution presented by the 10/00Z GFS- light to
moderate precip over Central and Srn CA.

Freezing levels will be on the moderate to higher side during this
longer term portion of the forecast. On Tuesday, expect freezing
levels to start out around 4000 ft over the OR/CA border, and around
7000 ft over the Nrn Sierra. Expect them to climb during the day,
reaching around 5000-ft over the North and around 8500 feet over the
Sierra by Tuesday night and Wednesday morning. As the cold front
approaches on Thursday, freezing levels will again drop
significantly, to around 3000-ft over the North and down to around
6000-ft over the Sierra. By Friday morning, could even see Freezing
Levels as low as 5000-ft over the Northern and Central Sierra.


Heavy rainfall with high snow lines is producing sharp rises in the
Northern half of California this morning.

The Upper Sacramento River is expected to crest early this
afternoon. Overflow flow into the Sutter Bypass is expceted to begin
Sunday at Tisdale Weir.  Slight overflow at Colusa Weir is possible
as well.

Along the coast, the Eel River at Fernbridge is expected to exceed
Monitor stage later tonight.  The Russian River near Hopland is
forecast to rise to right near Monitor Stage around noon today.

The west slopes of the Sierra Nevada are getting high amounts of
runoff with the high snow levels.  The Cosumnes River at Michigan
Bar is expected to crest above Monitor Stage later today.

On the east side of the Sierra Nevada, high snow lines and high rain
rates are expected to bring the Truckee River to Flood Stage near
Truckee this afternoon.  The Truckee River near Reno is expected to
crest near Monitor stage later tonight.

Renewed rises are expected middle of next week as another round of
rainfall is forecast for Northern California.

All other points are expected to remain below Monitor Stage.

CNRFC is planning on remaining staffed until 10 PM PST Sat, Dec 10.
Normal staffing will then resume at 6 AM PST Sun, Dec 11.

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



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