Winter Weather Advisory 16, NWS Eureka, CA

AREA FORECAST DISCUSSION
National Weather Service Eureka CA
402 AM PST Tue Dec 6 2016

.SYNOPSIS...The upper flow regime will waver between NWLY and
zonal throughout the short and extended forecast periods as
a series of disturbances move W to E across the NERN Pacific.
Near the surface, a recent cold air intrusion will persist
through Wednesday morning, and then be replaced by warmer/moist
onshore flow Wednesday night into Thursday.

&&

.DISCUSSION...An upper-level shortwave trough was located
immediately offshore of WA/OR early Tuesday morning. Numerous
showers were occurring near and downstream from this
feature...with the SRN extent of shower activity spreading east
across NWRN CA. Snow levels are estimated to reside from 2500-3000
feet over interior Humboldt and Del Norte Counties, which may be a
bit marginal for continuation of the inherited Winter Weather
Advisory...will continue to evaluate for early cancellation.
Otherwise, showers will dissipate across the entire region by
Tuesday afternoon as the previously mentioned shortwave trough
exits east.

Skies will subsequently begin to clear behind the departing
trough, which will favor enhanced radiational cooling supportive
of lows from 32-36 F Tuesday night for coastal Humboldt and Del
Norte Counties. Thus, a frost advisory has been issued. Surface
winds then become weakly offshore Wednesday morning and afternoon
in advance of a frontal wave moving E across the NERN Pacific. A
mid/high level cloud shield is expected to quickly progress E to
NRN CA ahead of the wave, with stratiform rainfall becoming
widespread by late Wednesday afternoon. In addition, a pre-
frontal 30-50 kt low-level SLY jet is forecast to advance E
toward the coast during Wednesday afternoon, which will aid in
strengthening ridge top winds to near advisory level.

&&

.EXTENDED DISCUSSION (THU-MON)...Forecast models beginning to
show some consistency now through the latter days of this week
and this weekend.  For Thursday and Friday...plenty of showers on
the back side of a rainy wednesday afternoon and evening...as an
upper trough continues to deliver upper disturbances moving through
a general westerly zonal flow while combining with plenty of
moisture at all levels. Saturday may be a little drier as a brief
break between weather systems occurs. Then the beginning of
another atmospheric river pattern starts to set up on Sunday
setting the stage for plenty of rain on Monday and
Tuesday....followed by even more rain Wed and Thursday. Heights
and thickness values during this very wet period look fairly high
so much of the precipitation will fall as rain...except for
perhaps the highest elevations (greater than 5000 ft) of the cwa
where plenty of snow will accumulate.

&&

.AVIATION...A weak trough has been producing scattered showers along
with occasional MVFR cigs and vsbys at the coastal terminals
overnight. An onshore flow will keep light shower activity going
through the morning hours. Building high pressure this afternoon
will weaken and reduce the shower activity. Expect clearing and
drying by this evening. Shallow misty air will form at or nearby
the the coastal airports by mid evening, but easterly drainage
winds will clear it out by midnight.

&&

.MARINE...NW swell has been fluctuating around 9 to 12 ft at 13
seconds overnight. The wave models continue to be too low and
combined seas have been increased about 1-2 feet above guidance.
Thus the small craft advisory has been extended through at least
this evening. Seas should finally drop below advisory level of 10
feet later tonight as the northerly flow becomes easterly overnight.
There will not be a long enough window to completely drop the small
craft advisory in the outer waters with southeasterly winds ramping
up through the day. Thus the advisory has been extended through the
morning for the outer waters 10-60nm offshore.

A vigorous frontal system will approach Northern California coastal
waters from the west on Wed. The GFS continues to be the fastest and
strongest with southeasterly winds ramping up to GALE around mid to
late Wed afternoon. The NAM and ARW were not too far behind and have
boundary layer winds approaching 45 kt near Cape Mendo in the late
afternoon. Confidence is high the outer waters will see GALE force
winds. The GALE WATCH has been upgraded to a GALE WARNING for the
outer waters. The wave generation with this storm will be short
period dominated. Large steep short period waves will take til Wed
night to build significantly. Even though winds will start to trend
down on Thu, short period southerly and southwesterly waves will
produce another round of hazardous seas. Elevated seas around 10-13
feet with short periods will probably linger into Friday before
laying down on Sat.

&&

.EKA WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory from midnight tonight to 10 AM PST Wednesday for
     CAZ101-103-104.

     Winter Weather Advisory until 10 AM PST this morning for CAZ102-
     105.

NORTHWEST CALIFORNIA COASTAL WATERS...Small Craft Advisory until 1 PM PST this afternoon for PZZ450-
     455-470-475.

     Gale Watch from Wednesday morning through late Wednesday night
     for PZZ470-475.

&&

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For forecast zone information
see the forecast zone map online:
http://www.weather.gov/eureka/zonemap.png

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