Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
307 PM PDT Tuesday Aug 4 2015
Synopsis...a low pressure system aloft will prevail over the
region through Wednesday. This system will bring further cooling
and scattered showers to the area on Wednesday. High pressure
aloft will return on Thursday and continue on Friday.
an upper level trough centered over British Columbia will remain
over the region through Wednesday. This combined with moderately strong
onshore flow and increased cloud cover will lead to additional
cooling on Wednesday...especially across the interior.
Scattered showers will likely develop on Wednesday...particularly during
the afternoon. Modest upward forcing due to the proximity of the
upper low should enhance shower development over the north
Cascades. For this reason...bumped up probability of precipitation over this area. It
looks like the risk of thunderstorms will stay over Canada and east of
the north Cascade crest.
The upper trough will move into the northern Great Basin Wednesday
night...and will be followed by a weak upper level ridge. The
shower threat will gradually diminish Wednesday night and...by Thursday
morning...the threat will be confined to the Cascades and within
a dissipating Puget Sound sound convergence zone. Light onshore flow
and increased sunshine will lead to warmer conditions Thursday
afternoon. Temperatures should rebound to near normal after falling
below normal on Wednesday.
Look for the upper ridge to move east of the area on Friday. Most
areas should experience some additional warming on Friday before the
onshore pressure gradient tightens in response to an approaching
upper level trough.
an upper trough will dominate the weather pattern over the Pacific
northwest during this period. The main differences in the medium
range solutions concerned the details. However...the overall
message was the same. Anticipate cool and unsettled conditions
Saturday through Tuesday. Temperature guidance appeared too warm...so
undercut the models and kept maximum temperatures below climatology.
Aviation...an upper level low is spinning over b.C. With westerly
flow aloft over western Washington. Onshore flow will continue at the
surface and stratus will reform overnight for MVFR to IFR
conditions. The upper low will shift south into western Washington on Wednesday
for a chance of showers...mainly in the interior and Cascades. 33
Ksea...VFR conditions this evening then stratus forming overnight.
MVFR conditions are likely by 12z. Surface winds will remain
southerly to 10 knots. Expect scattered showers in the vicinity Wednesday
Marine...moderate to strong onshore flow will continue across
western Washington this week with high pressure over the NE Pacific and lower pressure
in eastern Washington. Models continue to show westerly gales through the
central/east Strait tonight. Winds will ease through the Strait Wednesday
morning but will likely rise back to Small Craft Advisory speeds
late Wednesday afternoon and evening. 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 until 5 am Wednesday for the central and
eastern Strait of Juan Delaware fuca.
Small Craft Advisory in effect until 3 am Wednesday for the
An illustrated version of this forecast discussion can be seen at