Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
458 am PST Wednesday Dec 17 2014
Synopsis...an upper level trough moving over the area will
continue to bring showers to the area today. Thursday a ridge will
keep the area mainly dry although some sprinkles are possible.
Another cold front is expected Thursday night and Friday morning.
Short term...an upper level trough moving over the area is
bringing widespread showers to the area this morning. Snow levels
are a bit higher than previously expected coming in between 4000
and 4500 feet this morning. Additional snow accumulations of 1 to 3
inches are expected this morning. Locally higher amounts are
possible...but amounts should be high enough in elevation to
remain above most highways and to allow the Winter Weather
Advisory to expire at 7am. Showers will start to diminish this
afternoon as the trough starts to push eastward. There may be a
slight intensification of the showers again late this afternoon as
a shortwave moving down the backside of the trough moves over the
These showers will diminish overnight as a the ridge builds into
the area on Thursday. This ridge looks a bit more dirty than in
previous model runs so have increased probability of precipitation and added sprinkles to
the forecast. Current model runs show only very light quantitative precipitation forecast amounts
here and there over the area. This will need to be watched as it
gets closer. The next front moves into the area late Thursday
evening into early Friday. This looks to bring some gusty
southerly winds to 30 miles per hour and periods of locally heavy rain to
the area. This front has sped up a bit from previous runs and now
Friday afternoon looks to be drier than previously expected.
Saturday a warm front looks to move across the area bringing
fairly significant rain to areas north of Cape Mendocino. This is
a fairly big change from previous forecasts...but the GFS and
European model (ecmwf) are in good agreement on this so increased probability of precipitation across the
area for Saturday and Saturday night. Sunday the rain starts to
taper off and lift to the north as the ridge starts to strengthen.
Long term...models are in good agreement for building high
pressure into the region starting late Sunday after the warm
front brings a chance of rain to del Norte County on Sunday. High
pressure remains through middle week. This will likely allow valley
fog to form overnight. Weak offshore flow will likely keep the
coast fairly clear allowing temperatures to drop and opening the
possibility for some frost. Trn/mkk
Aviation...a low pressure system has been bringing showers to the
region overnight and will continue to spread showers over northwest
California through the morning hours. After a very brief lull shower
activity will re-enhance this afternoon and overnight as another
weak system moves through the area. Most locations will remain MVFR
today though some periods of VFR will occur at times. Rpa
Marine...low pressure continue to fill as it drifts east and north
into Oregon. Some westerly winds with gusts to 30 knots are still
moving through the waters but should decrease and turn southerly as
the low weakens and the next weak system approaches. This will allow
the shorter period westerly waves to subside significantly today
into tonight...likely below small craft criteria for a small period
tonight into early Thursday. On Thursday another stronger front will
approach allowing the southerly winds to ramp back up into small
craft into Thursday night. The front will move inland by Friday
morning leaving very light winds in its wake.
A hurricane force low currently over the central Pacific will
produce a large swell which will propagate toward the West Coast
arriving on Friday. Both ascat and rapidscat indicate that the low
is producing hurricane force winds and these winds are aimed toward
the northwest California coast on a great Circle Route. Thus
confidence in the large swell building into the waters is high...but
the exact magnitude of the swell is still uncertain. There is the
possibility of dynamic fetch as the storm moves east along the great
Circle Route which may result in an underestimate of the final swell
height in our waters on Friday into Friday night by the enp model.
For now have kept in line with the model data indicating a peak 17-
19 feet swell at 18 seconds Friday night. Swell forerunners will
bring a potential for sneaker waves on Friday followed by very
large surf with breakers possibly around 25 feet Friday night.
Using Jason-2 altimetry data...hopefully it will be possible to
verify the magnitude of this swell over the next 24 hours. Have
issued a marine weather statement to highlight this high
confidence large swell.
The swell will subside over the weekend. A warm front will approach
the waters on Saturday into Saturday night bringing another period
of possible small craft southerly winds. Rpa
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
Winter Weather Advisory until 7 am PST this morning for caz004.
Northwest California coastal waters...
Small Craft Advisory until 9 PM PST this evening for pzz450-455.
Small Craft Advisory until 9 PM PST this evening
Hazardous seas warning until 7 am PST this morning for pzz475.
Hazardous seas warning until 7 am PST this morning for pzz455.
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