Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 191639

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
940 AM PDT Mon Mar 19 2018



Pattern is beginning to set up for a moderate to heavy precip
event...especially along the central CA coast down through Ventura
county and then inland over the Sierra (primarily the southern half
of the range). WV/IR imagery shows a nearly vertically stacked
system spinning west of the CA coast approx near 35N/140W. Blended
TPW imagery indicates moisture beginning to stream northeast from
the lower latitudes at a source in the vicinity of 20N/140W...east
of the Hawaiian Islands. Already some PW values over 1.50-inches
entrained into the plume...which is being handled reasonably well by
the models.

Monday will be a dry day across the region as a s/wv ridge slides
overhead before being eventually displaced eastward over the
interior by mid-week. Light precip will reach coastal sections on
Tuesday morning right around 12Z as the leading edge of the moisture
plume reaches the central CA coast and WAA/isentropic lift begins to
ramp up. For Tuesday...best precip will fall along the coast between
Big Sur and the Santa Barbara/Ventura county line...with slightly
lesser amounts inland over the length of the west slope of the
Sierra. Freezing levels will begin to slowly ramp up with the Sierra
seeing 6000- to 8500-feet (north-to-south) early on Tuesday
increasing to 7000- to 10000-feet by early Wednesday. This is
indicative of the origin of the system and the tropical airmass
arriving from the southwest.

Wednesday into Thursday will see the upr low west of the CA coast
open up and phase with a s/wv trof dropping out of the Gulf of
Alaska. The 19/00Z EC and GFS are showing very slight variations on
the placement of the moisture plume through the day...but this far
out are in decent agreement. At this time...it should be said that
shifts in the orientation and placement of the moisture plume can
cause differences on where the best precip will fall. With the
arrival of the s/wv trof within 130W and moist flow from the south
and southwest will bring heavy precip to areas near Point Conception
and inland over the southern Sierra with 2.00- to 4.00-inches in the
forecast. This includes the Santa Barbara and Ventura county areas
(and Thomas Fire burn scar). The two heaviest 6-hour periods look to
be between 22/00Z and 22/12Z (Wednesday evening and overnight into
early Thursday morning). Also...with the phasing of the southern
s/wv trof with one arriving from the north...a round of moderate
precip is expected across the northern half of the area...just not
as heavy given the focus of the best PW air to the south. Freezing
levels across the Sierra will start near 7000- to 10000-feet (north-
to-south) and generally hover at these levels throughout the day.


Thursday will be a wet day will good moisture and one shortwave
brushing the northern portion of CA and another one approaching
along with onshore flow to central and Southern CA.  Southern CA and
the Southern Sierra should see some areas of heavy precipitation on
Thursday with good PW plume and onshore flow and short wave trough
approaching. Although there are some differences in the placement of
the PW plume which will determine the areas of heaviest precip.
Generally increased precip amounts from Bay area to So Cal and over
the Central and Southern Sierra for Thursday. The southern shortwave
trough will move inland Thursday night into Friday for possible
lingering showers over So Cal especially over the mountains. Another
disturbance moves into Northern CA Friday bringing areas of moderate
precip possible to the Northern CA. Upper level trough moves through
on Saturday for possible light precip mainly over the Northern
portion of the area.

Freezing levels Thursday morning around 3000-5000 feet over the far
north and 6500-9000 ft over the Sierra and 9000 ft or higher over So
Cal and will drop to around 2500-3500 ft over the far north and 4000-
6500 ft over the Sierra and around 6500-9000 ft over So Cal Friday


Rivers will continue to recede today before another system begins to
impact the area on Tuesday. Many basins across the area will receive
several inches of rainfall over the next 5 days, so expect rises on
rivers throughout CA.

There are several monitor stages currently forecast throughout the
forecast area. These include Michigan Bar on the Cosumnes, Bear
Creek at McKee Road in the San Joaquin, and Portola on the middle
fork of the Feather River. Several locations along the southern
central coast and socal are also approaching monitor stage. Runoff
in some areas across socal will be enhanced from recent burn scars.

The forecast on the Sacramento River has come down a bit since
yesterday, but still seeing the possibility of some weir flow at
Tisdale Weir by this weekend. Current forecast is for only a few
hundred cfs at the weir.

There will be rises on streams in the Sierra as well, but current
thinking is that runoff will be somewhat limited by the fresh snow
received last week. However, it`s difficult to forecast these rain
on snow situations, thus there is a more uncertainty in the runoff
forecast than usual. Flows are not expected to approach monitor
stages in the east Sierra or on the Merced at this time.

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



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