Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
930 am PDT Tuesday Aug 4 2015
Synopsis...a low pressure system aloft will remain over the
region through Wednesday. Look for weak high pressure to prevail
over the area Thursday and Friday.
an upper level trough with a low center over British Columbia will
continue to dig south during the day...resulting in lowering 500 mb
heights over the County Warning Area. The height falls coupled with moderately
strong onshore flow will result in even additional cooling today...
especially across the interior.
Models show more southwesterly onshore gradients later tonight/Wednesday
morning which would be better for brining more solid low clouds into
the interior. Will stick with the mostly cloudy start to the
day...with partly sunny skies again in the afternoon. The trough
axis will be over the area with 500 mb heights down near 5700m so
Wednesday may end up the coolest day of the week. There could also
be some patchy morning drizzle on the coast. Models show a secondary
trough axis digging southward across western Washington during the afternoon so will
maintain a threat of showers...though quantitative precipitation forecast amounts should remain
light. Showers may linger into the evening as the trough pulls east and
the flow becomes more northwesterly. There could even be a brief convergence
zone over central/north Puget Sound sound.
500 mb heights rebound on Thursday as the low pulls east and a ridge
offshore moves toward the area. Low level northwesterly onshore flow will
prevail but weaken during the day. The trend should be toward
slightly warmer temperatures...reaching the middle 70s to lower 80s
around Puget Sound sound and the SW interior. Mercer/05
Long term from the previous discussion...
virtually all the models show the transitory ridge shifting eastward
Thursday night and Friday. An upper low will approach with lower
heights by later in the day. The main onshore influence will not
be felt until the weekend so another partly to mostly sunny day
appears likely. Highs should reach slightly above average...into
the low 80s around Puget Sound sound and the SW interior...cooler near
the water and toward the north interior/coast/Strait.
Long range models disagree on the timing...strength...and track of
the upper low for the weekend. The general idea is that upper
heights will trend lower and onshore flow will increase once again.
This should introduce more clouds and a threat of showers to western Washington.
Made little change to the extended which has a chance of showers
indicated at times. This can be fine tuned once the
GFS/ECMWF/Canadian come into closer agreement. Temperatures will
probably cool back to near average or even slightly below. Mercer
Aviation...an upper level low is spinning over b.C. With westerly
flow aloft over western Washington. Marine stratus pushed inland this
morning with scattered-broken ceilings near 1000 feet. The stratus in the interior
will lift and scatter this afternoon. The upper low will shift south
into western Washington tonight for a chance of showers...mainly near the
Canadian border. Stratus will return for MVFR to IFR conditions Wednesday
Ksea...bkn-ovc011 this morning then lifting and scattering this
afternoon for VFR conditions. Surface winds will remain southerly. 33
Marine...high pressure over the the NE Pacific will maintain moderate to
strong onshore flow across western Washington this week. Westerly gales are
forecast through the central/east Strait tonight. A weak cold front
will cross through the region Friday night into Saturday. A 1011 mb
low will spin offshore this weekend. 33
Pz...Gale Warning in effect from 5 PM this afternoon until 5 am
Wednesday for the central and eastern Strait of Juan Delaware fuca.
Small craft advisories are in effect for the coastal waters
from Cape Flattery to Cape Shoalwater 10 to 60 nm and the
An illustrated version of this forecast discussion can be seen at