Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 PM PDT Sat Mar 28 2015
Synopsis...a stalled frontal boundary will bring light rain to the
north interior and coast at times through Monday. The cold front
will reach western Washington Monday night...bringing rain back into
the rest of the interior. Upper level low pressure over the region
next week will maintain a cool and showery weather pattern.
Short term...skies are partly cloudy across interior western Washington with
just a few weak diurnally driven showers on radar this afternoon.
Satellite imagery shows a thick mass clouds moving in from the
Pacific...so an increase in middle/high clouds are expected tonight.
The klgx coastal radar shows a band of steadier rainfall moving onto
the north coast. Models indicate a warm frontal feature will lift across
western Washington tonight and stall mainly over British Columbia Sunday into
Monday. Most of this rainfall will skirt the north
coast...Olympics...Strait...and far north interior. Puget Sound sound to the
SW interior will probably stay mostly dry although some sprinkles
are not out of the question...especially tonight. The gfs40 looks to
aggressive with quantitative precipitation forecast with the drier NAM-12 favored which keeps the
Puget Sound sound region southward generally dry.
High pressure amplifies over The Rockies and a trough deepens
offshore. The cold front lurking offshore will finally push through
western Washington Monday night...a little quicker than some previous model runs
showed. Likely probability of precipitation are in the forecast Monday night and Tuesday for
the front and trailing upper trough. Temperatures the next couple
days should make the low or middle 60s around Puget Sound sound but highs
will come down a good 10 degrees on Tuesday.
Long term...models have not been at all consistent in the extended
range. The overall pattern looks progressive and split with a series
of weak short waves passing near western Washington. Timing these systems is
difficult and it doesn't seem prudent to try pick out short blocks
of dry weather. Will keep the climatology broad brush chance of showers in
the forecast. Mercer
Hydrology...flooding is unlikely over the next 7 days.
Aviation...the flat upper level ridge over West Washington at 21z will slide
over East Washington by 12z as a warm front moves over Vancouver Island.
Moderate west flow aloft will continue over the area...and will become
increasingly moist from the approaching warm front.
The lower air mass over West Washington was somewhat moist and weakly unstable
at 21z this afternoon with areas of VFR stratocumulus. Middle and high
level moisture just offshore associated with the approaching warm
front was beginning to reach northwest Washington and will cover mainly the north two
thirds of Washington with ceilings broken-ovc050 and higher from around 00z to
beyond 12z. Pressure falls over southern b.C. Will allow gusty S
winds to develop over the interior late tonight.
Ksea...VFR ceilings will prevail over the terminal through 12z. Moisture
from a warm front headed toward b.C. Will spread over the terminal
as ceilings broken-ovc050 and higher. Surface winds be S 12g20kt through
about 00z then weaken to S 5-10kt overnight. Kam
Marine...the small weak surface ridge building over West Washington this
afternoon will remain in place through Monday morning. The dominant
feature will be the warm front that will move over Vancouver Island
tonight. Pressure falls along the b.C. Coast ahead of the warm front
will allow moderate S flow to develop over Washington waters tonight and
Sunday. Small Craft Advisory are up for the coast and for all inland waters except
for the central Strait.
The trailing cold front will cross West Washington Monday evening...then strong
onshore flow will develop behind the front Monday night. Pressure
gradient depicted on the GFS and European model (ecmwf) seem strong enough for
gales...similar to the cold front last night...but the WRF-GFS holds
winds in the Small Craft Advisory range. The GFS and European model (ecmwf) also seem to delay the
strengthening onshore flow just a bit while the Canadian is faster.
I will stick with the WRF-GFS timing for now but strong west flow
through the Strait could begin earlier Monday evening...and
could reach gale force. Kam
Pz...Small Craft Advisory all waters except the central Strait.
Small Craft Advisory Grays Harbor for rough bar.
You can see an illustrated version of this forecast discussion at