Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 181545
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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
750 AM PST Mon Dec 18 2017

...SYSTEM MOVES IN TUE INTO WED...
...LIGHT PRECIP NE NV FRI-SAT...

.METEOROLOGICAL CONDITIONS (SHORT TERM: MON AM - THU AM)...

Upr ridge is still situated over the far eastern Pacific as it seems
for the majority of this month...but will be disrupted by a s/wv
trof currently moving toward 150W. The latest models from overnight
have come into good alignment in terms of timing and strength of the
system. The slightly faster models were the way to go with precip
spreading along the north coast up toward the crest of the southern
OR Cascades during the afternoon on Tuesday. Also...the slightly
deeper and west track is looking to be the scenario that will pan
out...as well. Look for the s/wv trof axis to reach the coast early
Wednesday with the system diving toward the southeast as it moves
over the area. Precip will spread across northern CA just south of
the I-80 corridor from Tuesday evening into early Wednesday
morning...and then shift across central CA and the majority of the
state of NV during the morning and afternoon hours on Wednesday. By
the evening...the system will begin to track east of the area with a
dramatic cutoff to the precip as dry northerly flow sets up with the
upr ridge rebuilding over the far eastern Pacific. Best precip
amounts look to be over the north coast from Cape Mendocino
northward and crest of southern OR Cascades with 0.50- to 1.00-inch
(local to 1.50-inches). For the northern Sierra expect amounts to be
in the 0.25- to 0.75-inch range with amounts tapering off as one
goes south over the central/southern Sierra. Freezing levels will
start near 7500-feet near the CA/OR border and 10000-feet along the
I-80 corridor...and close to 11000-feet for central CA. By the time
the cold front traverses the area...the cold air will plunge across
CA and NV with freezing levels over areas near and north of I-80
from 2500- to 4000-feet. For central CA and southern NV look for
4000- to 5000-feet. Southern CA most likely will not see the drop in
freezing levels as the cold front erodes as it moves south and to
the east...seeing the warming from the already building offshore upr
ridge.


.METEOROLOGICAL CONDITIONS (LONG TERM: THU AM - SUN AM)...

Expect to see a few lingering showers over NE NV Thu morning in NNW
flow as an upper trough moves across the Great Basin.  00 UTC runs
of the ECMWF/GFS consistent with bringing a weak shortwave into the
Great Basin, with light precip affecting NE NV late Fri afternoon
into Sat morning.  Accumulations should be light, and precip is
expected to be frozen.


.HYDROLOGICAL CONDITIONS...

Rivers are steady near baseflow conditions across the area. A weak
system moving across northern CA on Tuesday night into Wednesday
will bring very minor rises on a few rivers along the coast.
Otherwise, expect little to no change on the rivers.


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

DRK/KL/MI

$$



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