Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
303 PM PST sun Dec 21 2014
Synopsis...strong moist west flow aloft will continue this evening
for showers mainly along the coast and in the mountains. An upper
level ridge will build over the region Monday for drier weather. A
frontal system will move over western Washington late Monday night
and Tuesday followed by an upper level trough on Wednesday for
periods of rain. Drier and cooler northwest flow aloft will develop
Short term...winter solstice at 303 PM PST today.
Strong westerly winds aloft will continue to bring moisture to
western Washington this afternoon and evening. Showers are mainly along the
coast and in the mountains. Seattle is in the rain shadow with
little to no precipitation. An upper level ridge will start to build over
the Pacific northwest tonight with showers tapering off across western Washington. This
ridge will remain in place on Monday for mainly dry weather. The ridge
will then flatten on Tuesday while a cold front slides S/southeast from b.C.
Into western Washington. This system will bring rain to mainly the olympic
peninsula and north Cascades Tuesday afternoon. Rain will spread south
through the remainder of the County Warning Area Tuesday night as the baroclinic band
continues to slowly shift south. Rain will continue through
Wednesday as an upper level trough digs offshore then moves inland.
This cool upper trough will knock snow levels down to 2500 feet with
snow possible at the mountain passes. 33
Long term...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
Hydrology...the Flood Watch has ended for Pierce...Thurston...and
Lewis counties. The Cowlitz river at Randle will crest several feet
below flood stage early this evening.
The only river with a Flood Warning still in effect is the skokomish
in Mason County. The skokomish will fall below flood stage tonight.
Flooding is unlikely Monday through Saturday. Burke
Aviation...moderately strong westerly flow aloft...becoming lighter northwest
flow aloft later tonight and Monday. A ridge of high pressure will
build Monday and Monday night. Air mass moist and slightly
unstable...becoming stable overnight. A Puget Sound sound convergence zone
could develop at times...affecting mainly the western Snohomish and far
north King area north of ksea. Ksea/kbfi wind directions are not expected to
be affected but the radar trend should be watched closely. Kpae will
possibly be affecting with periods of lower MVFR ceilings in any heavier
showers. A mix of MVFR/VFR ceilings through early this evening...then
MVFR ceilings more widespread later this evening and overnight.
Ksea...south-southwest wind 8-10 knots. Dtm
Marine...onshore flow will ease tonight. There will be enough flow
this evening for small craft winds to continue over most
waters...but marginal for the north interior and S/central Puget Sound sound.
Westerlies in the Strait will be stronger with northwest winds to 25 knots
funneling down through Admiralty Inlet during the evening. Higher
pressure develops Monday with winds generally under 20 knots most
areas. A frontal system will arrive around Tuesday with some small
craft winds again possible. Another low will move inland just S of
the waters Wednesday and Wednesday night. Dtm
Washington...Flood Warning for the Skokomish River.
Pz...Small Craft Advisory all waters.
Small Craft Advisory for rough to severe conditions Grays
You can see an illustrated version of this forecast discussion at