Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
845 PM PST Friday Dec 26 2014
Synopsis...a frontal system moving from the north will bring rain
to The Lowlands and heavy snow to the mountains late tonight through
Saturday night. Strong high pressure building over British Columbia
behind the frontal system will push modified Arctic air south across
the area Sunday night...bringing mostly sunny and cold weather to
the area for the first half of next week.
Short term...satellite imagery shows a weather system bearing down
on Washington from the north. This will spread rain into the area
after midnight. Snow will fall in the mountains and it is forecast
to be heavy at times through Saturday in the Cascades. Rain over The
Lowlands will become showery in the afternoon though the heavy snow
will continue in the mountains. Snow will taper Saturday night.
The 1.3 km resolution uw WRF-GFS has 24-hour amounts in the Cascades
of 16-40 inches...with the highest amounts on The Mount Rainier.
Stevens Pass appears to get around 20 inches with Snoqualmie Pass a
bit less. At any rate the Winter Storm Warning currently in effect
for the Cascades looks appropriate.
Cold air begins seeping into western Washington from the north on
Sunday. There will still be scattered showers Sunday and Sunday
night before moisture is finally pushed south on Monday. The snow
level will probably be 500-1000 feet so a little snow in that
elevation band is possible at times...especially Sunday night. Areas
that might see a little snow include the northern foothills of the
Olympics and the Black Hills southwest of Olympia.
Skies will clear from the north on Monday. Highs will be in the 40s
both Saturday and Sunday. After a cold Sunday night with temperatures
dipping into the low to middle 30s...Monday will be cool with highs
unlikely to get out of the 30s. Burke
Long term...previous discussion...GFS and European model (ecmwf) keep a strong upper
level ridge offshore and dry cold north-NE flow aloft over West Washington through
Wednesday. Sunny cold and dry will prevail with min temperatures in the
teens and 20s and maximum temperatures in the 30s to near 40. The upper level
ridge weakens enough for a weak upper level shortwave trough to move
southeast across the area Thursday night and/or Friday. Kam
Hydrology...flooding is not expected for the next 7 days.
Aviation...northwest flow aloft over the Pacific northwest with an
upper ridge extending northeast from the California offshore waters
across Washington. The ridge will move east tonight and a frontal
system...which is along the British Columbia coast this evening...
will move southeast across the forecast area on Saturday. The air
mass will remain moist and stable.
After partial clearing this afternoon...cloud layers at roughly 020
and 040 are increasing again. This trend will continue tonight...
with further deterioration late tonight and Saturday morning as
precipitation spreads southeast across western Washington.
Ksea...south to southeast wind 4-8 knots...becoming south to southwest
10-16 knots gusting 24 knots late Saturday morning. Clouds will gradually
lower tonight and rain will begin around 4 am. Mcdonnal
Marine...a frontal system will move southeast along the British
Columbia coast tonight...and then move through the coastal and
inland waters of western Washington Saturday. Strong onshore flow
will follow the front Saturday night. Small Craft Advisory winds are
likely all waters ahead of and in the wake of the front...and at
least brief marginal westerly gales are a good bet in the central
Strait of Juan Delaware fuca and East Entrance Strait Saturday afternoon
and evening. The gale watch will be upgraded to a Gale Warning with
the evening forecast.
Northerly offshore flow will develop Sunday and Monday as high
pressure moves south through British Columbia. Small Craft Advisory
northeast winds are likely for the coast and portions of the Strait
while gales are possible for at least the northern inland waters and
West Entrance Strait later Sunday night and Monday. Weaker offshore
flow will prevail Tuesday as the high moves into the northern Rocky
Some of the highest astronomical high tides of the year will occur
this weekend. However...current guidance remains on a downward trend
for western Washington locations. Will continue to monitor...but
there is increasing confidence that no further coastal flood
products will be needed in the short term. Mcdonnal
Washington...Winter Storm Warning late tonight through late Saturday night
for the Cascades.
Pz...Gale Warning Saturday afternoon and evening central Strait
of Juan Delaware fuca and East Entrance Strait.
You can see an illustrated version of this forecast discussion at