Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 081723

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
930 AM PST Thu Dec 8 2016



A warm front brought widespread precipitation to nrn and cntrl CA
and srn OR, with a few locations in nrn NV seeing light amounts as
well.  Overall totals on the north coast were around 0.5-1.3",
although lighter around 0.1-0.5" in the Smith and lower Klamath
basins.  Russian/Napa basins received around 0.4-0.7".  Around the
Bay Area, totals were generally 0.5" or less, locally up to 1".
Totals were around 1" in the Santa Cruz Mtns.  Precip on the cntrl
coast quickly tapered from around 0.1-0.5" near Monterey Bay to zero
at Pt Conception.  San Joaquin Valley locations received 0.25" or
less, heaviest in the north.  Sacramento Valley stations reported
0.2-0.6".  Around Trinity and Shasta Lakes, totals were 0.5-0.75".
The upper Klamath reported 0.1-0.3" with similar amounts in NE CA.
Totals in the Sierra were around 0.1-0.5" on the west slope with 0.1-
0.25" on the east side.  A few locations in nrn NV reported 0.1" or


Fetch of moisture continues to stream toward the region from near
the Hawaiian Islands toward the northeast...currently intersecting
the central coast. This plume is situated between a deep upr low
over the western half of the Gulf of Alaska and an upr ridge to the
east of Hawaii. Both the 08/12Z RAOBs at KOAK and KVBG show a PW
value right around 1.00-inch...which places central CA right around
200-percent of normal for this time of year. Also interesting is how
shallow the majority of the moisture is in the profile...generally
at or below 700-mb. As a result...locations farther from radars are
not representing the precip all that well. One in particular is the
Monterey county coast...where precip gages since 08/12Z are showing
amounts reaching or surpassing 1.00-inch while radar shows very few
weak echoes in the same area. With the best moisture streaming
inland from the central coast...precip gages since 08/12Z are
showing the highest values in the Sierra between the American River
basin south through Sequoia NP with totals generally near 0.25- to
0.33-inch. Also...as expected...freezing levels ramped up overnight
under the WAA pattern. The 08/12Z KOAK RAOB recorded a freezing
level of just over 9300-feet this morning. Down the coast at
KVBG...the freezing level is now close to 10200-feet. This warm air
is translating inland with the Sierra seeing rain mainly below 8000-
to 9000-feet (from north to south).

Through the rest of today into Friday morning...WAA/isentropic lift
will weaken and precip will taper off to lighter amounts with the
best precip over the west facing slopes of the higher terrain of
northern CA. This "lull" in the precip will be short-lived as the
trend through Friday will be to increase precip mainly across
northern CA and northern NV...areas near and north of I-80 corridor.
This will be the result of the next surge of moisture reaching the
coast and WAA/isentropic lift ramping up. This surge of moisture
looks to contain more available moisture with both the latest
(08/00Z) EC and GFS (08/12Z) indicating PW values right off the coast
just shy of 1.50-inches. Best isentropic lift coinciding with the
lowest condensation pressure deficits is generally between 295-K
and 305-K SFC.

The biggest overhaul of the morning issuance of the QPF was on
Saturday (Day 3). The previous forecast showed a drying trend across
the region...following closer to what the GFS/GEM were depicting
yesterday. However...with the GFS/GEM now on board with the EC
showing a wet day (especially over the northern half of the
area)...decided to trend in this direction. Timing is still not
ideal...but the overall wet scenario appears the way to go. In terms
of areal coverage of the heaviest precip...the models are in
relatively good agreement through Sunday morning with the focus
being the American River basin south to near the Merced River basin.
For now...totals in the latest forecast for this stretch of the west
slope of the Sierra are from 2.00- to 3.50-inches. Finally by later
in the period (later Sat into early Sun)...a s/wv trof moving toward
the PACNW down to the CA/OR border area and far northern NV will
push the moisture plume southward to the central coast with precip
finally beginning to taper off again. Freezing levels look to be in
the range of 8000- to 10000-feet from north to south across the
Sierra...which will result in precip type being rain across a
majority of these basins.


Expect diminishing showers over portions of cntrl CA and nrn NV Sun
morning as a weak cold front moves south and weakens further.  The
far north coast and upper Klamath basin should see continued precip
through Sun night, moving into nrn NV late Sun into Mon, however,
with an upper jet over the region and disturbances aloft.  Leaned
toward a GFS/ECMWF blend for this period.  Expect freezing levels
3000-5000 ft except 8000-10000 ft in the Sierra Sun morning.

Model agreement is not good beyond Mon.  Leaned toward the ECMWF
solution for late Mon into late Tue since the GEFS mean at H5 was
similar to the deterministic ECMWF and the deterministic ECMWF
looked similar to the ECMWF ensemble mean.  The 12 UTC GFS that has
recently arrived is similar in timing with the system for Tue
despite being shifted further south.  Expect precip to increase over
nrn/cntrl portions of CA/NV and srn OR by Tue as orographics and
isentropic lift increase.  Expect a moisture plume with PW values
up to 1.1-1.3" with sfc low pressure off the coast.


Widespread precipitaiton over Northern and Central CA will result in
increased river stages throughout the region.  The majority of the
forecast locations should stay below monitor levels.  No weir flow
is expected along the Sacramento River.  The Cosumnes River at
Michigan Bar is the only location forecasted to rise slightly above
monitor stage.

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



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