Small Craft Advisory 309, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
909 PM PST Thu Dec 21 2017

.SYNOPSIS...A weak upper level disturbance will move down out of
the north tonight and Friday bringing a chance of light rain or snow
to the area as snow levels will fall to below 500 feet. An upper
level ridge will build off the Coast Friday night and Saturday
resulting in dry low level offshore flow. A system will move into
the area later Sunday into Christmas Day bringing a chance of rain
or snow to the area and as a series of systems follow...the chance
for rain or snow will persist through next week.

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.SHORT TERM...Running a little bit warmer this evening when compared
to 24 hours ago with no obs reporting sub-freezing temps. Most
locations generally ranging from the mid 30s to around 40...although
locations in Skagit and Whatcom counties likely seeing temps just a
hair above 32. Turning attention to the radar...not much in the way
of any activity aside from some pretty weak echoes along the coast.

Moving on to the forecast...models in some degree of agreement on
large scale features...but remain at odds in regards to smaller
details which is currently making for confounding forecast dilemmas.
With current weak frontal passage...pattern holds as ECMWF tending
wetter than the GFS. 1000-850mb thicknesses looking similar...so the
possibility of some flakes cannot be ruled out. As to how quickly
precip moves on...again there is a discrepancy...with GFS pushing
things out by Saturday morning while ECMWF allows precip to linger
through Saturday afternoon. During this time frame...surface
conditions look to set up for some Fraser outflow...possibly
resulting in windy conditions for Western Whatcom county Friday and
Friday night.

Interestingly enough...depending on how the 00z ECMWF comes
in...models perhaps starting to come in line for timing of the next
system...as previous runs showed about a 12 hour difference in its
arrival. Now...solutions seem to be agreeing on a late Sunday
morning time frame for next round of precip to start along the coast
then move eastward. Unfortunately...the similarities end there...as
the GFS continues to advertise conditions being a bit warmer and
thus allowing for precip to fall as rain whereas the ECMWF has
conditions cool enough for snowfall. As has been present for several
runs now...the GFS tends to be a bit more progressive with this
second round of precip with conditions starting to dry out overnight
Sunday night and into Monday while the ECMWF continues to drag its
feet...with drying starting late Monday afternoon.

Given the continued inconsistency in models...hard to improve on
forecast that is already out there...so no updates needed.  SMR

.LONG TERM...From Previous Discussion...Long wave ridging will
remain offshore. Energetic short wave features are forecast by the
long range models to undercut the ridge and affect the west coast
Christmas Day onward. The GFS is generally farther south with the
short wave energy (and therefore drier) than the ECMWF. Both
solutions keep conditions cool through the period which results in
rather low snow levels and temperatures 5-8 degrees below average.
With all of the run to run and model to model inconsistencies
(mainly on the track and strength of the all-important short wave
features) the forecast will be broad-brushed through the period with
clouds and a chance of precipitation with low snow levels. Albrecht

&&

.AVIATION...Northwest flow aloft over Western Washington this
evening will continue tonight and Friday. A weak front will move
south across the area late this evening through Friday morning, with
precipitation locally falling as snow and rain/snow mix.
Surface high pressure moving south through British Columbia behind
the front will produce increasing northeast flow across Western
Washington on Friday. The air mass will be moist tonight and Friday,
mainly below 10,000 feet.

Skies were generally overcast across Western Washington at 8 pm with
ceilings OVC0015-025; locally higher VFR ceilings OVC030-050
continue in the north interior. Conditions will lower somewhat as
the front moves into the area, with local visibility 2-4SM
-SN/-RA BR. Marginal improvement is likely late Friday afternoon.

KSEA...Southerly wind 3-8 kt, becoming north to northeast 5-10 kt
early Friday morning. The weak front could bring snow flurries to
KSEA with perhaps a light dusting of snow; the main period of threat
is roughly 2 am through 10 am. McDonnal

&&

.MARINE...A weak front will move through the region tonight, keeping
winds light southerly over the interior. Strong high pressure will
build over British Columbia behind the front Friday and Friday
night, producing strong northeast pressure gradients and Fraser
outflow winds. Small craft advisory east to northeast winds are
likely Friday afternoon through Saturday night over the northern
inland waters, the Strait of Juan de Fuca, and the coastal waters.
Gales are possible over the northern inland waters and near the west
entrance Strait of Juan de Fuca Friday night through Saturday
morning. A gale watch remains in effect for this possibility.

The high will shift southeast into western Montana this weekend,
giving moderate to strong offshore flow Saturday. A front will reach
the area around Sunday night, turning winds briefly southerly.
Offshore flow will resume early next week. McDonnal

&&

.HYDROLOGY...River flooding is not expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Friday evening through Saturday morning for
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Friday afternoon through Saturday morning for
     Northern Inland Waters Including The San Juan Islands.
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