Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
320 am PDT Monday Aug 31 2015
Synopsis...a frontal system will bring more rain to western
Washington today through Tuesday...especially the coast and north
part. Cool showery weather will continue Wednesday and Thursday
as an upper trough drops southeast into the area. The upper
trough will probably move east late this week...for drier and
warmer weather Friday and especially over the weekend.
Short term...a large deep upper trough remains over the region...
with its 5270 meter low centered over the northwest corner of
British Columbia early this morning. The leading nose of the next
frontal system is moving into the region...with its weak warm
front extending from Vancouver Island into Washington and the cold
front trailing west-southwest from Vancouver Island across the
offshore waters. This feature is bringing some light rain to the
northwest part of the forecast area...with breezy conditions from
around Seattle northward.
The warm front will move east of the Cascades later today...and
the cold front will move slowly through the forecast area later tonight
and Tuesday. Most of the precipitation today and tonight will be
along the north coast and in the Olympics...northwest interior...
and north Cascades with a half inch to an inch in many places. The
Puget Sound sound region and southwest interior will mostly receive a
quarter inch or less. Precipitation with the cold front on Tuesday
will be more widespread. The front will bring breezy conditions to
most of the forecast area.
The upper trough itself will dig southeast along the coast of
British Columbia today through Tuesday. It will swing through
western Washington Tuesday night and Wednesday...bringing showers
to the area without much or any discernible break behind the cold
front. Highs today through Wednesday will be mainly in the middle
60s to lower 70s...which is several degrees below normal.
The snow level will be near 9000 feet today but fall to 5000 to
6000 feet by Wednesday. Continuing moist westerly flow of roughly
25 knots between 5000 and 10000 feet will bring a foot or more of snow
to some of the high terrain of the Cascades and Olympics over the
next three days. Terrain above 8000 feet on Montana Baker will probably
get 2 to 3 feet of snow between today and Wednesday. Mcdonnal
Long term...models all show another shortwave digging into the
base of the upper trough...which will keep it over southern
British Columbia and the Pacific northwest through at least
Thursday. Model solutions separate beyond Thursday though. The 00z
Monday GFS shows the upper trough moving east of the Cascades
Friday...with dry northwest flow over western Washington Saturday
and Sunday. The Canadian and European models are similar...keeping
an upper low over the region through Saturday and holding off dry
weather until at least Sunday. Our forecast...for now anyway...
leans heavily toward the GFS solution. Mcdonnal
Aviation...a weak front will hang up over the northern portion of
the area today. Light rain will slowly spread south this morning
finally reaching kolm around 18z. Ahead of the rain ceilings 4500-
6000 feet. Ceilings lowering to 3000-4000 feet with the light
rain...2000-3000 feet along the coast and north interior. Light rain
will continue into the afternoon hours then dissipate in the
evening with little change in the ceilings.
Ksea...ceilings near 5000 feet lowering to near 3000 feet with the
onset of the light rain around 15z. Ceilings remaining near 3000 feet
into the evening hours. Southerly winds 10 to 15 knots. Felton
Marine...frontal system hung up over the northern portion of the
waters will keep southerly gradients intact today. Expect Small
Craft Advisory southerly winds over the inland waters...Admiralty
Inlet and the East Entrance to the Strait. Winds will ease tonight.
A pair of weak systems will move through the waters Tuesday and
Wednesday. Weak high pressure will build over the waters on Thursday
and remain through Friday. Felton
Pz...small craft advisories East Entrance to the Strait...
Admiralty Inlet...and the inland waters.
You can see an illustrated version of this discussion at