Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 am PDT Wednesday Aug 5 2015
Synopsis...an upper trough will bring some showers today.
Drier and warmer weather will prevail Thursday and Friday with
high pressure aloft in control. Another upper trough will bring
some showers and cooler temperatures over the weekend.
Short term...the early morning infrared satellite loops shows
clouds will be on the increase over the next few hours as an upper
level trough over southern British Columbia digs southward. A few
showers should develop over the north interior and north coast by
midday or so. For the Puget Sound sound area...the main shot at any
precipitation comes with a convergence zone that will form sometime
after 21z before eventually dissipating over the central Cascades
late tonight. Both the WRF-GFS and NAM models are insistent that we
will get at least some precipitation out of the pscz...between one
and two tenths of an inch in the Seattle metropolitan area. The Cascades
from King County northward could get as much as a half inch. Precipitation
chances were edged up slightly from the previous forecast. Today
will be the coolest day in over a week. It will be interesting to
see if sea-tac manages 70 degrees. If it doesnt get there...our 70
degree or higher streak would end at 53 days which is now 2nd
longest in the Seattle areas 121 year climate record.
The upper level trough axis shifts into northwest Montana by early
Thursday morning with a low amplitude upper level ridge building
into western Washington. Except for a few morning showers in the
mountains...it will be a dry day on Thursday. Inland areas should be
mostly sunny by afternoon while low level moisture could hang on a
little longer near the coast with persistent onshore flow.
Temperatures will rebound closer to climatology on Thursday. The upper
level ridge axis will shift east of the region by Friday afternoon.
850 millibar temperatures peak near 12 or 13c on Friday...which
translates to the warmest spots Seattle southward warming to near 80.
Long term...models are in good agreement with another upper level
trough affecting the region this weekend. None of the models are
quick with ejecting the trough eastward...so we are probably looking
at several days of mostly cloudy skies...near or below normal
temperatures...and chance of showers mainly for the coastal and mountain
areas. The models start to go off the rails by the middle of next
week with some pushing the trough eastward and others forming a
cutoff low to our south. Little change was made to the extended
forecast overnight. 27
Aviation...an upper level low over the central b.C. Coast at 09z
will move southeast down the b.C. Coast and over SW b.C. By 00z this
afternoon. Strong onshore flow developed last night which has
brought more marine air over the West Washington interior. With no visible
stratus on the coast last evening it will take until close to
sunrise this morning for stratus to begin forming over the interior.
The marine layer should be moderately deep with MVFR ceilings expected.
Ceilings will lift this afternoon...but the approaching upper level low
will provide enough lift to maintain VFR broken ceilings after 18z.
Scattered showers should develop in the afternoon as the low moves
down over cyvr.
Ksea...good VFR conditions should continue until around 12z when
MVFR stratus should begin forming. The stratus will lift in the late
morning and afternoon for layers scattered-bkn030 bkn050. A few light
showers may develop this afternoon. Surface winds will remain S-SW
up to 6 knots this morning...then 5-10 knots this afternoon. Kam
Marine...moderate to strong onshore flow will continue across West Washington
this week...between a large surface ridge over the NE Pacific and
lower pressure over East Washington.
Strong onshore flow last night brought gale force winds to the
central and east Strait of Juan Delaware fuca. The gale is still hanging on
but winds should ease back to 15 to 25 knots early this morning. Both
the arw and the Canadian lam maintain 15 to 25 knots Small Craft Advisory level winds in
the Strait today before increasing the winds again to high end Small Craft Advisory
this afternoon and tonight. Both models also push the Small Craft Advisory winds over
the parts of the north inland waters and Admiralty Inlet adjacent to the
Strait...so Small Craft Advisory have been issued for both areas.
A weak ridge will form just offshore Thursday night for slightly
weaker onshore flow.
Moderate to strong onshore flow will redevelop Friday night...just
ahead of a weak cold front approaching the area. The front will move
inland late Friday night or Saturday morning. Behind the front...a
weak surface ridge will build along the coast this weekend. At this
point with the surface ridge over West Washington...the 00z WRF-GFS is showing
sub-Small Craft Advisory winds in the Strait for Saturday and Sunday. Seems weird and
a little hard to believe after the persistent onshore flow we have
had for most of this Summer. Kam
Pz...Gale Warning central Strait and East Entrance tonight.
Small Craft Advisory Admiralty Inlet tonight.
You can see an illustrated version of this discussion at