Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
925 PM PST sun Dec 21 2014
Corrected the first paragraph of the short term segment.
Synopsis...high pressure aloft will build over the region
overnight and then prevail over the area on Monday for a drying
trend. A low pressure system emanating out of the Gulf of Alaska
will begin impacting the region on Tuesday. Expect a cold front to
sweep across the area on Wednesday and usher in a colder air mass.
scattered showers continued over the County Warning Area late this afternoon...and
there was a lone lightning strike detected over the olympic range
earlier this evening. Also...it appeared that a Puget Sound sound
convergence zone /pscz/ was developing over the central Puget Sound
sound at this time. Overall...anticipate the shower activity to
wind down overnight. By Monday morning...any showers should be
confined to the Cascades and within the pscz.
An upper ridge will build over the Pacific northwest overnight.
The axis of this ridge will be over western Washington Monday evening. The
upper ridge will gradually weaken Monday night through Tuesday...and this
will allow an upper low emanating out of the Gulf of Alaska to
approach from the northwest. A cold or occluded front will slowly
move into the County Warning Area from the northwest during the day Tuesday. The best
chance of precipitation will be over the northwest half of the County Warning Area.
It looks like the front will stall over the County Warning Area Tuesday night and then
become re-energized by an upper trough digging off the coast. The
latest GFS solution indicated that this system will split...with
much of the energy diving toward northern California...leaving western Washington
in the weaker northern branch. Either way...this trough will be
colder than the recent storms. Thus...expect snow levels to
plummet to near 2500 feet Wednesday afternoon behind the cold front...
per the latest GFS. 05
Concerning tides...some of the higher astronomical high tides of
the year will be occurring this week along with slight tidal
anomalies due to passing weather systems. It looks like minor
tidal overflow is possible for some locations around Puget Sound sound
early Tuesday morning with a better chance early Wednesday
Long term from the previous discussion...
north/northwesterly flow will prevail across western Washington on
Christmas day with isolated to scattered showers mainly in the
Cascades. Temperatures are close to normal with highs in the lower to middle
40s. Weak upper level ridging will continue on Friday. The weekend
forecast is less certain as the GFS and European model (ecmwf) differ on the timing
of the next system. The GFS is very progressive with this next
system which may push in as early as Friday night. The European model (ecmwf) however
stalls this system offshore until Sat night. Will keep the
forecast nudged toward climatology for now. This trough may linger into
sun for more showers...then drier weather is in store early next
week as another upper level ridge develops over the NE Pacific. 33
the Skokomish River continued to recede and will fall below the
flood stage of 16.5 feet by midnight tonight.
Flooding is not expected Monday through this upcoming weekend.
Aviation...northwest flow aloft with an upper ridge building over
the northeast Pacific tonight and Monday. At the surface...weak high
pressure over the offshore waters will move over western Washington
on Monday. Air mass moist and slightly unstable...becoming stable on
Patchy low clouds this evening should fill in tonight under weak
onshore flow. The air mass will on Monday and low clouds will
probably break up for at least some area by afternoon.
Ksea...mostly high clouds this evening but low clouds are expected
to develop tonight...probably burning off Monday afternoon. South
wind 4-8 knots. Schneider
Marine...onshore flow will ease as weak high pressure over the
eastern Pacific moves over western Washington tonight and Monday.
West swell 10-14 feet will continue over the coastal waters.
A frontal system will move through the waters Monday night and
Tuesday...bringing Small Craft Advisory winds to most areas. A
trailing trough or surface low will affect the waters Wednesday
although models are not in good agreement with this. Schneider
Pz...Small Craft Advisory for hazardous seas in effect until 4 am
Tuesday for the coastal waters and West Entrance to the
Strait of Juan Delaware fuca.
Small Craft Advisory in effect until 6 am Monday for the
central and eastern Strait of Juan Delaware fuca.
Small Craft Advisory for rough bar in effect until 7 PM Monday
for the Grays Harbor bar.
You can see an illustrated version of this forecast discussion at