Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
940 PM PST Friday Feb 27 2015
Synopsis...northerly flow aloft and low level offshore flow have
developed and are ushering a much drier air mass into western
Washington tonight. As a result...showers have tapered off...and
skies will continue clearing overnight. A weak system will move
through Sunday night and Monday...with dry weather again Tuesday and
Short term...upper low over over northwest Oregon this evening move south to the
Bay area by late Sat...allowing a longwave ridge axis near 140w to
expand into the Pacific northwest. Surface high pressure has already settled into the
interior of b... the kbli-cywl gradient getting as strong as
-12 mb as of 03z. As a result...a short-lived Fraser outflow event
is underway. The air mass coming out of b.C. Is not terribly cold.
However...the air mass is dry...with the dewpoint at kbli now down
to 23f. Drier air and large-scale subsidence will spread across the
forecast area overnight and early Sat morning...causing the few remaining
sprinkles to dissipate with skies clearing from north to south.
High pressure over b.C. Will move out east of The Rockies on
Sunday...with pressure gradients over western Washington relaxing on Sat night.
Winds will become much lighter.
The flow aloft will back to northwest on Sunday as a weather system
approaches from the northwest. This weather system will drop down
from the northwest Sun night and Monday...bringing clouds. The European model (ecmwf) and NAM
show rain all areas with this feature but the GFS just has spotty
precipitation and the Gem is dry. The forecast is a blend of these
models for now which means mostly cloudy with a chance of rain.
Long term...from previous discussion...an upper ridge will be over
the northeast Pacific Tuesday and Wednesday...giving dry northerly
flow aloft to western Washington. There will be some breezy north
winds Tuesday as a surface high moves into southern British
Columbia...then less wind Wednesday as the high shifts southeast.
Models show the upper ridge weakening a bit Thursday with a warm
front moving over the top. This will bring some clouds to the area
and perhaps a chance of rain...with the best chances coast and far
north. A cold front will approach from the west on Friday. This day
could wind up being dry but a chance of rain cannot be completely
ruled out for now. Schneider
Hydrology...flooding is unlikely for the next 7 days.
Aviation...an upper level low over Oregon will continue tracking
south tonight. The flow aloft over western Washington is northerly. Light
showers are limited to the southern portion of the County Warning Area this
evening...mainly south of SeaTac as this system pulls farther south.
Meanwhile moderate north to NE surface winds will scour out the low
level clouds with skies becoming clear from north to south tonight.
Although kclm will see a ceiling with NE upslope winds. Patchy
shallow fog will be limited to kolm overnight. Expect VFR conditions
on Sat with offshore flow. 33
Ksea...north winds will prevail tonight to 10-15 knots. Skies clearing by
09z with VFR conditions expected. 33
Marine...north to northeast flow will continue across western Washington
tonight as a 1008 mb surface low tracks south along the southern
Oregon coast. Small craft advisories are in effect for all waters.
High pressure over interior b.C. Will maintain offshore on Saturday.
A weak surface trough will cross the waters on sun for brief onshore
winds. North to NE flow will develop Monday night through Tuesday as
another surface ridge builds over b.C. 33
Pz...Small Craft Advisory all waters.
You can see an illustrated version of the forecast discussion at