Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 071705
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
930 AM PST Wed Dec 7 2016

...PRECIP RETURNS TODAY WITH WET CONDITIONS INTO EARLY NEXT WEEK...

.24 HOUR PRECIPITATION TOTALS (ENDING DEC 7 AT 400 AM PST)...

A disturbance aloft brought light precipitation to the far nrn
portion of the region.  Totals were 0.25" or less on the north coast
from the Eel basin northward and just a few hundredths of an inch
for the remainder of far nrn CA as well as nrn NV.  Over in the
upper Klamath basin, totals were 0.25" or less in the lower
elevations and 0.2-0.6" in the srn Oregon Cascades.


.METEOROLOGICAL CONDITIONS (SHORT TERM: WED AM - SAT AM)...

Latest setup shows an upr trof residing over the western Gulf of
Alaska...while high pressure is situated to the east of the Hawaiian
Islands. In between...southwest to west flow exists with a decent
moisture plume stretching from near Hawaii to just off the CA coast.
Deeper moisture exists closer to the lower latitudes (PW approaching
2.00-inches)...while PW generally from 1.10- to 1.33-inches is
reaching between 35N and 40N. A rather weak SFC low is located along
40N just inside 140W with a warm front extending toward the
southeast. Radar is already showing echoes starting to stream toward
the coast ahead of this frontal boundary...but much of this moisture
is aloft with observations between Point Arena and the Monterey
Peninsula reporting clouds at or above 12000-feet. 07/12Z RAOBs
from around the region shows a dramatic dry layer generally between
750- and 500-mb that will need to be saturated before widespread
precip commences. This should occur after 07/18Z along the north
coast down to the SF Bay Area...but farther inland this won`t take
place until after 08/00Z.

WAA/isentropic lift along with lowered condensation pressure
deficits will ramp up this afternoon and evening...still most
pronounced from the 295- to 300-K SFC between the CA/OR border down
to the SF Bay Area inland to the central Sierra past the crest to
northern NV. Look for steady precip to fall into tomorrow afternoon
before precip begins to becomes lighter. Freezing levels will go
through quite a transformation by later Thursday. Currently from the
I-80 corridor northward...the freezing level is at the surface.
However...by tomorrow afternoon/evening they will be between 6000-
and 7000-feet near the CA/OR border inland to far northern NV. Down
near the I-80 corridor of CA into western NV they will be between
9000- and 10000-feet. So initial precip type of snow at lower
elevations will transition to all rain for a majority of the area
over the next 24 hours.

Late Thu into early Fri...the moisture plume will be focused on
central CA. Without much forcing...best amounts will be on west
facing slopes where orographics will generate mainly light precip.
Finally by the afternoon into the evening...a s/wv trof moving
toward the PACNW along with the next surge of moisture from the
southwest will bring an uptick in precip mainly for the north coast
from Cape Mendocino northward...the Shasta Lake drainage...and the
northern/central Sierra. Freezing levels will finally begin to drop
by a couple thousand feet late in the period from their peak late
Thursday.


.METEOROLOGICAL CONDITIONS (LONG TERM: SAT AM - TUE AM)...

An overall wet pattern is expected to continue for the nrn portion
of the region through much of the period.  The ECMWF solution
continues to look like an outlier and was discounted for this
morning`s forecast creation.  For the most part, a blend between the
00 UTC GFS and GEM solutions was used as a starting point.  For Sat,
expect decreasing showers as the air mass dries out in the wake of
the earlier moisture plume.  Models indicate the arrival of the next
moisture plume on the north coast Sat night.  Isentropic lift and
good onshore flow could lead to a period of at least moderate precip
rates on the north coast and in the nrn Sierra at times Sun through
Mon night.  The latest GFS favors a second wave Mon night with
heaviest amounts while the previous runs (and our forecast) favor
the Sun afternoon/evening period.  Hopefully details will come into
focus with future model runs.  Expect freezing levels 4000-6000 ft
in far nrn CA and 6000-10000 ft in the Sierra on Sat, gradually
rising to 7000-11000 ft across precipitation areas by late Mon.


.HYDROLOGICAL CONDITIONS...

Widespread precipitaiton over Northern and Central CA will result in
increased river stages throughout the region.  The steady
precipitation over the next few days will result in modest rises,
but all locations are expected to remain below monitor levels.


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

KL/DRK/KL/BW

$$



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