Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PST Wednesday Dec 11 2013
Synopsis...an upper level ridge will move over western Washington
today giving a dry day with partly sunny skies. A Pacific cold front
and the upper level trough following it will bring rain Thursday and
Thursday night with snow in the mountains. A weaker frontal system
will reach the area this weekend.
Short term...the morning satellite image shows a patch of fog and
stratus over the southwest...some high clouds coast and north...and
otherwise partly sunny skies over western Washington. Highs today
will be around 40. A Flat Ridge of high pressure will move across
the area today and this evening. Clouds will increase tonight as a
Pacific weather system approaches the coast.
Rain should begin on the coast Thursday morning and spread inland
during the afternoon...a pattern that has been consistent in the
models for days. Highs will rise a few degrees into the middle 40s.
Snow will fall in the mountains...probably in advisory amounts.
There is some potential for freezing rain in the Highway passes.
Will look at this for the afternoon forecasts.
Rain will change to showers late Thursday night and taper Friday.
Highs Friday will be in the middle 40s again. Burke
Long term...previous discussion...the GFS/European model (ecmwf) bring the offshore
ridge axis to the coast early Saturday morning...with another cold
front following behind the ridge. This system still looks weaker
than the Thursday cold front. Model timing is still showing some
minor variability... with the latest model solutions bringing the
cold front to the coast Saturday night. Precipitation amounts do not even
appear to reach advisory levels for the Cascades. Moderate to strong
west flow aloft is expected behind the front...which should keep a
chance of showers going. GFS/European model (ecmwf) diverge a little more from next
Tuesday Onward...so confidence kind of evaporates. Kam
Hydrology...river flooding is not expected for the next 7 days.
Aviation...an upper level ridge is over western Washington today
with light offshore flow at the surface. The winds aloft are
westerly. There is patchy low clouds and fog around Puget Sound sound and
SW interior this morning...with mainly scattered-broken high clouds
elsewhere. Expect VFR conditions this afternoon and evening with
increasing middle/high level clouds. The upper level ridge will shift
east tonight with a cold front moving inland Thursday morning. 33
Ksea...expect VFR conditions today...with increasing middle/high level
clouds tonight. Surface winds are light and variable...becoming southerly
Marine...expect light offshore flow across western Washington today. Winds
are light/variable across Puget Sound sound. Offshore flow will increase
tonight through Thursday morning as a cold front approaches. Will likely
see Small Craft Advisory winds for the coastal waters and West
Entrance of Juan Delaware fuca. This front will cross the coastal waters
by 18z Thursday...then shift inland Thursday afternoon. Light onshore flow is
forecast Thursday night and Friday. High pressure will shift inland Friday night
for generally light surface winds. Offshore flow will increase on
Sat as our next frontal system arrives. 33
You can see an illustrated version of the forecast discussion at