Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7
000
AGUS76 KRSA 061631
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
830 AM PST Tue Dec 6 2016

...WEAK SYSTEM MOVING ACROSS NORTHERN AREAS TODAY...
...NEXT ROUND OF PRECIP ARRIVES WED LASTING INTO FRI...

.24 HOUR PRECIPITATION TOTALS (ENDING DEC 6 AT 400 AM PST)...

A cold front brought light precipitation to far nrn CA, srn OR, and
NW NV. Amounts were 0.1-0.4" in the Smith basin and generally less
than 0.1" for the remainder of the north coast down through the Eel
basin with dry conditions further south.  Lower elevations of the
upper Klamath basin reported up to 0.1" with 0.1-0.4" in the srn OR
Cascades. Generally a few hundredths of an inch or less fell in NE
CA and NW NV.


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

Broad upr trof resides across the western two-thirds of North
America this morning with a weak s/wv trof rotating through the
cyclonic flow near the southern OR coast. With an origin of this
system from the northwest...there is not a lot of available for
precip production. 06/12Z KMFR RAOB indicates the majority of the
moisture below 650-mb with a recorded PW of 0.37-inch. This is
actually below normal (86-percent) for this time of year. IPW
sensors along the coast near the CA/OR border all show similar
values right around 0.40-inch. Radar shows scattered precip moving
toward the region from the northwest...currently affecting areas
along the coast down to Cape Mendocino and inland over the upper
Klamath River basin. Amounts have been fairly light since 06/12Z
with automated gages indicated totals generally near or less than
0.10-inch. Through the rest of today...the s/wv trof will continue
its track toward the southeast and bring light precip to northeast
CA into northern NV from the I-80 and US-50 corridor northward.

Attention turns next to an impressive plume of moisture advecting
NNE from near the Hawaiian Islands...reaching 40N this morning.
Blended TPW imagery shows peak values within this plume approaching
and exceeding 2.00-inches between 20N and 30N. As a deepening upr
low builds across the western portion of the Gulf of Alaska and an
upr ridge resides to the east of Hawaii...expect the flow aloft to
channel the moisture plume toward the west coast...intersecting CA
near the SF Bay Area.

WAA/Isentropic lift type precip will be on the increase on Wednesday
afternoon through the evening hours into Thursday morning...mainly
from the I-80 corridor northward. Best lift coinciding with the
lowest condensation pressure deficits are generally between the 295-
and 300-K surfaces. Also noticeable on Wednesday will be the
increase in height of the snow levels as the warmer airmass moves in
from the southwest. Through the I-80 corridor of CA...look for
initial freezing levels of 3500- to 4500-feet (early Wednesday)
jumping to 9500-to 10500-feet (Thursday).

The majority of precip that falls on Thursday will be associated
with the plume of moisture aimed at the SF Bay Area inland toward
the central Sierra with an uptick in amounts as a s/wv trof moves
across the PACNW. There are still some discrepancies in amounts with
the GFS and NAM remaining the wetter of the solutions...the GEM next
in line...and then the less impressive EC. Also...there has been
some slight wavering where the heaviest precip amounts will fall.
But in general...the I-80 corridor looks to be a good bet at this
point.

Later Thursday into early Friday...look for precip to begin to drop
off in intensity as the moisture plume erodes a bit and sinks toward
the south. This is a bit of a change from the previous QPF
issuance...which kept precip going a bit heavier from the SF Bay
Area inland toward the central Sierra.

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

GFS guidance seemed to be a compromise amongst discrepant model
solutions, and WPC supported a solution weighted toward the GFS/GEM
and their ensemble means.  Since the GEM seemed a bit out of phase
earlier in the extended-range period, we chose to closely follow the
GFS solution for this time range.  Expect lingering precip across
cntrl CA and nrn NV, especially the Sierra, Fri through Sat evening
as a moisture plume continues to aim at the region.  It`s worth
noting that NAM and GEM solutions are much wetter in the Sierra Fri,
while SREF/GFS/ECMWF are all on the light side.  A shortwave passing
by to the north should cause a bit of an enhancement in precip rates
in the far north later Fri.  Precip should then wind down for Sat.
Another wave that is expected to remain mainly to the north could
bring light amounts to NW CA and srn OR by late Sun night.  For Fri,
expect freezing levels to start off around 5000-8000 ft in nrn CA
and far nrn NV and 8500-10000 ft across the Sierra, lowering to 3000-
4000 ft in the north and 5000-9000 ft in the Sierra and cntrl NV by
late Fri night.  Expect freezing levels 5000-8000 ft in NW CA Sun
night.


.HYDROLOGICAL CONDITIONS...

Light rain over the CA/OR border region has had minimal impact on
rivers. Low snow levels are also keeping most rivers near present
levels.

Rainfall forcast for later this week is expected to bring renewed
rises over the northern two thirds of CA.  Flows on the Cosumnes
River could approach monitor stage at Michigan Bar Thursday evening
into Friday.

All forecast points are expected to remain below monitor stage for
the next five days.


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

KL/DRK/KL/PF

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.