Area Forecast Discussion
Issued by NWS Eureka, CA
FXUS66 KEKA 171321
AREA FORECAST DISCUSSION
National Weather Service Eureka CA
431 AM PST Tue Jan 17 2017
.SYNOPSIS...A series of storms will bring periods of rain, gusty
winds and mountain snow through the weekend.
.DISCUSSION...A significant storm poised offshore will bring
periods of rain and windy weather to Northwest California tonight
through the evening hours on Wednesday. Light rain will probably
start this afternoon over Del Norte county and then increase in
coverage tonight as a 1+ inch precipitable water plume sags south
and eastward. The primary concern for tonight is not so much the
rain but the wind. Both the GFS and NAM indicate surface gradients
tightening up with 925mb winds around 45-50 kt just offshore and
over the North coast. Stable conditions will mitigate the wind
threat for the coastal plain. All that momentum probably will not
mix down to mean sea level over land. The higher terrain and exposed
ridges will no doubt blow with gusts to 50 mph. We hoisted a wind
advisory starting tonight, primarily for the coastal headlands and
the upper elevations of Del Norte and Humboldt counties. The
strongest winds may hold off til early Wednesday morning when the
surface front arrives. The strong ribbon of wind will sag southward
through the day on Wednesday and a wind advisory may be necessary
for the Mendocino coast.
Rain rates tonight with the incoming storm should be light.
Moderate rates may develop early Wednesday morning across Del
Norte county where most of the rain will fall through the night.
Rain rates will ramp up everywhere Wed afternoon and evening as
the moist plumes progresses southward across the forecast area.
This will be the window for heavy rain and possible urban and
street flooding. An inch or two of rain will accumulate during
this time frame. Orographic lift will no doubt produce locally
higher amounts in southern Humboldt and northern Mendocino
counties. The rain will wind down late Wednesday night. This will
mitigate the threat for major flooding on main stem rivers. Rivers
will rise and advisories for flood prone areas or roads will need
to be considered once the rain commences. One location in
particular will be the Russian river at Hopland which is projected
to hit 15 feet. This will cause minor flooding of highway 175.
Snow levels with this first episode of rain will be fairly high
tonight into Wed. The exception will be in northeastern Trinity
county where BUFKIT profiles indicate the potential for snow as
low as 3500 feet on Wednesday. Both the NAM and GFS show a cold
wedge remaining in place all afternoon and evening on Wednesday
when the heaviest rain rates arrive. The NAM does have a cold bias
and the BUFKIT temperature profiles seem too cold with snow piling
up in Weaverville all day. Currently, we believe the levels will
be higher, above 3500 to 4000 feet, though heavy wet snow may be
observed as low as 3000 feet. For now will hoist a Winter Storm
Watch for 6-8 inches in 12 hours above 3500 feet. Granted with
snow levels that high, above 3500 feet, there are no major travel
impacts on route 299; Buckhorn summit and Oregon mountain summit.
Snow levels should remain above 4000 feet over South Fork Mountain
Summit through the brunt of the heavy rain. Levels will fall on
Thu, but precip will wind down and become scattered light showers.
The rain will quickly change over to scattered showers by Thu
morning as cooler air aloft and weak instability arrives behind
the moist plume. Low topped convection will be possible Wed night
into Thu morning over the waters. Ridging on Thu in advance of the
next storm should cap off or reduce the shower activity. The GFS
and ECMWF were in fine agreement with the timing of another
vigorous storm which will bring more heavy rain and strong winds
Thu night into Friday morning. The shower activity may ease up
some on Sat as another low winds up offshore. It is interesting to
note that the GFS has been consistent with rapid cyclogenesis
offshore over the weekend. It will be quite windy with this storm
if the GFS pans out with 50 kt of surface winds over the waters.
The track, position and rate of deepening of this last storm will
also be favorable for strong winds over land. We will have to wait
and see how this plays out over the next several days.
After this last kicker, the atmosphere should slowly transition
to a drier flow regime early to mid next week. The models have
been having difficulty on how quickly a ridge will build and
confidence is not high we will receive any rain early next week.
Thus precip chances near climatology have been invoked.
Last but not least, huge westerly swell will impact the beaches
Friday into Saturday. Swell on the order of 25 to 30 feet with
breakers in excess of 35 feet will be possible. The exact
magnitude and timing are still sketchy, however confidence
remains high this swell will pose a significant threat to
beachgoers and mariners. Standby for more information and surf
warnings or advisories.
.AVIATION...VFR to MVFR conditions and light winds will continue
at KCEC and KACV through the early morning. The lower conditions
will be due to reduced ceilings and/or visibilities due to stratus
and patchy fog. MVFR to IFR conditions will continue at KUKI
through the early morning with the lower conditions due to reduced
ceilings and lower visibilities due to stratus and patchy fog. VFR
conditions and light winds to calm conditions will prevail at KUKI
this afternoon. VFR conditions and moderate and gusty winds will
prevail at KCEC and KACV this afternoon. MVFR to IFR conditions
will occur at KCEC and KACV late this afternoon and tonight with
gusty winds and showers will reduce ceilings and visibilities.
.MARINE...A strong low pressure system off the southwestern
Canadian coast will begin to increase the pressure gradient
across the waters today. Therefore the winds and seas will
increase across the waters this afternoon and tonight. Elevated
winds and seas will persist across the waters through this weekend
as a series of cold fronts move through the region. A small craft
advisory will be in effect across the northern outer waters
through this afternoon. A small craft advisory will be in effect
across the southern outer waters through early Wednesday morning.
A gale warning will be in effect across the northern outer waters
this afternoon through Wednesday afternoon and across the southern
outer waters early Wednesday morning through Wednesday afternoon
due to increasing winds. A Small craft advisory will be in effect
across the northern near shore waters through early Wednesday
morning. A small craft advisory will be in effect across the
southern near shore waters through Wednesday morning. A gale
warning will be in effect across the northern near shore waters
early Wednesday morning through Wednesday afternoon and across the
southern near waters Wednesday morning through Wednesday afternoon
due to increasing winds. Seas will increase more across all the
waters on Friday.
CA...Wind Advisory from 7 PM this evening to 11 AM PST Wednesday for
Wind Advisory from 10 PM this evening to 11 AM PST Wednesday for
Wind Advisory from 10 PM this evening to 1 PM PST Wednesday for
Winter Storm Watch from Wednesday morning through late Wednesday
night for CAZ107.
NORTHWEST CALIFORNIA COASTAL WATERS...
Small Craft Advisory from 9 AM this morning to 9 AM PST Thursday
for PZZ450. Small Craft Advisory from 9 AM this morning to 3 PM
PST this afternoon for PZZ470. Gale Warning from 3 PM this
afternoon to 3 PM PST Wednesday for PZZ470. Small Craft Advisory
from 9 PM this evening to 9 AM PST Thursday for PZZ455. Small
Craft Advisory from 9 AM this morning to 3 AM PST Wednesday for
PZZ475. Gale Warning from 3 AM to 3 PM PST Wednesday for PZZ475.
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