Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
354 PM PST Wednesday Dec 17 2014
Synopsis...persisting rain showers will diminish overnight. Then
building high pressure will result in a mostly dry day on
Thursday. An approaching cold front will bring the next
opportunity for rain late Thursday night through Friday morning.
Short term...(today through thursday) a weak disturbance has prolonged
very light shower activity today for mostly areas north of Cape
Mendocino. The 23z radar imagery shows light to moderate rain
offshore making its way onshore. The approaching surface low will
bring one last shot of rain this evening as it slides past the northwest
California coast. Additional rainfall through tonight will be on
the lighter side amounting to around one quarter inch. Snowfall
with the lingering showers will not pose a major concern this
evening as precipitation will be light and snow levels are
forecast to gradually increase overnight.
500mb heights will gradually build overnight into Thursday as a weak
upper level ridge shifts over the West Coast of the United States.
This should result in mostly dry weather early in the day, however,
moisture streaming over the ridge could result some light sprinkles
through the day. Then, a boundary associated with an upper level low
located off of the coast of British Columbia will approach northwest
California Thursday evening. Recent model runs have sped up the
arrival of the rain to Thursday evening but still remain in decent
agreement with one another. Rainfall can be heavy at times with most
areas looking at 0.75 inches overnight and higher terrain across
favorable areas getting as much as 1.5 inches. Snowlevels will
remain around 5000 feet as most of the precipitation falls but
will decrease into the morning. Flurries may be observed at some
mountain passes however snowfall should mostly be restricted to
the Trinity Alps with the pass on Highway 3 south of Callahan
seeing potential impacts.
Longterm...(friday through wed) showers will diminish Friday resulting
in a mostly dry day however the wet pattern continues Saturday and
again Sunday as flow becomes more zonal. The longwave model hints
towards the development of a longwave ridge across the western
United States this weekend. I fear models may potentially be over
doing the precipitation for this weekend but made little to no
changes due to uncertainty. Stronger ridging will build over the
eastern Pacific early next week bringing warmer temperatures and
drier weather through the middle of the week. Both the GFS and European model (ecmwf)
hint at another trough for late next week but differ in the
strength. Again few changes were made to the long term and climatology
probability of precipitation were used for the tail end of the forecast. \Kml
Aviation...a weakening low pressure system offshore will provide
more periods of light to moderate rain as well as occasional MVFR
conditions tonight. Lingering moist air will make IFR ceilings and visibilities
possible at Ukiah and other valleys in the interior by early Thursday
morning. Conditions should improve to VFR through the day on Thursday.
Marine...winds have turned southerly in advance of a weakening low
approaching the waters. Ship reports showed winds around 15-20 knots
which seemed to match better with the GFS...hi-res-nmm and hrrr. The
sref and nam12 looked underdone. Both the hires-nmm and hrrr show a
ribbon of 20-25kt in the outer waters and around cape mendo through
this evening. Also seas have been running 2 to 3 feet above the model
guidance through the day. Most of this wave energy has been coming
from 280-290 degrees. Thus extended the Small Craft Advisory through
tonight. The models continue to show the westerly swell trending
downward on Thursday into Thursday night as a stronger frontal
system approaches the waters through the day. Southerly winds will
start to ramp back up to small craft levels of 20-25 knots by Thursday
evening. The models tend to be too slow with winds increasing...so
the advisory has been extended into Thursday and Thursday night. The front
will move inland by Friday morning leaving much lighter winds in its
A hurricane force low currently south of the Aleutian Islands will
produce a large swell which will propagate toward the norcal coast
and arrive on Friday. Ascat pass showed 50-55kt in the southern half
of the low. This agrees quite well with the wave watch iii winds.
Confidence is increasing a large westerly swell will hit the
northern cal coast on Friday. The exact magnitude of the swell will be
much more certain when the swell GOES by the offshore buoys; buoy #2
250nm west of blanco which is the only one left.
For now have stayed closed to the model guidance which has the swell
peaking around 17 to 19 feet at 18 seconds Friday night. Breakers
around 25 feet will be possible Friday night. Jason-2 altimetry
measurements thus far has captured only the sides of the wave maximum.
Hopefully we get a pass right over the center to confirm the wave
heights in the next 12 hours. A marine weather statement has been
issued to call attention to this large swell event which should
produce hazardous surf conditions.
In addition...forerunners will bring a potential for sneaker waves
during the day on Friday. Looking at the sea state spectrum there
will be another westerly wave group present...8-10 feet around 13
seconds on Friday. The shorter period southerly wave group should be
negligible. Confidence it not high that all of the variables will
come together here for this to be a high end threat for sneaker
The swell will subside over the weekend. A warm front will approach
the waters on Saturday into Saturday night bringing another period
of Small Craft Advisory southerly winds.
Coastal flood...persistent southerly winds continue to keep water
levels above the astronomical tides by 1 to 2 feet. The astronomical
tides are nearly 8 feet in Humboldt Bay on Saturday and 8.1 feet on
Sunday. Depending on how persistent the southerly winds are...this
could cause some coastal flooding in the low lying areas around
Humboldt Bay. In addition, with these high water levels the high
surf may push water up higher in exposed coastal areas. Mkk
Northwest California coastal waters...
Small Craft Advisory until 3 PM PST Thursday for pzz450-455.
Small Craft Advisory until 3 PM PST Friday for pzz470-475.
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