Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
845 am PST Thursday Dec 12 2013
Synopsis...a Pacific cold front will spread rain inland across
western Washington today. An upper level trough will follow the
front tonight with showers diminishing after midnight. Moist
northwest flow aloft will bring mostly cloudy skies and a chance of
showers on Friday. A weak cold front will move across the area
Saturday afternoon or night.
Short term...fog has dissipated over inland areas as clouds
increase. Radar shows rain just off the coast and rain is likely to
begin shortly on the coast. Rain will spread inland in the afternoon
and probability of precipitation are categorical. Rain is likeliest in Seattle 21z-03z.
Model quantitative precipitation forecast amounts have converged on 0.25-0.50 inch in the mountains
which translates to 3-6 inches of snow in the passes. That is below
advisory criteria. Could be a bit more on the volcanoes.
Another upper level ridge building over the NE Pacific will maintain
somewhat moist northwest flow aloft over the area Friday and Friday
evening. Showers will be light and mainly over the mountains.
Models still show a much weaker front embedded in the northwest flow aloft
reaching the area sometime Saturday. The models have been trending a
little slower with the timing...with the latest solutions bringing
rain to the coast Saturday afternoon and over the interior Saturday
evening. With the front being pushed across the top of a flat upper
level ridge by moderate west flow aloft...the models are probably on
target with the strength of the system...but possibly a little on
the slow side. With all the weather coming off the NE Pacific
through this weekend..temperature ranges will be close to seasonal
Long term...previous discussion...GFS and European model (ecmwf) agree on keeping
the area under moderate west flow aloft Sunday...then shifting it
slightly to the northwest Monday and Tuesday. Both models have an upper
level trough reaching the area around middle week but details are not
being resolved very well. Sunday through Tuesday will be an
uneventful period with a low chance of showers over the area...
mainly over the mountains. Precipitation may ramp up Wednesday if the
trough shows up on time. Kam
Hydrology...river flooding is not expected for the next 7 days.
Aviation...rain will increase across western Washington today as a cold
front moves inland. Klgx shows rain along the coast which will
spread into the interior by 20z. Ceilings may lower to MVFR range in
light rain. Expect showers tonight as the trailing upper level
trough moves overhead. The air mass will remain moist with MVFR
Ksea...light rain will move into the Puget Sound sound area by 20z with
ceilings lowering to MVFR range. Expect showers in the vicinity tonight.
Surface winds will remain southerly up to 12kt. 33
Marine...southerly flow will increase across western Washington
today as a cold front crosses the area. Small craft advisories are
in effect for most waters. Westerly swells will also increase to
10-12 feet. Winds will ease after midnight tonight. Expect light
onshore flow on Friday with generally light winds Friday night.
Southerly winds will increase again on Sat as another front moves
inland. High pressure offshore will maintain onshore flow across western
Washington Sunday through early next week. 33
Pz...Small Craft Advisory all waters except central Strait of Juan
You can see an illustrated version of the forecast discussion at