Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT Thursday may 28 2015
Synopsis...high pressure aloft will prevail over the region today.
A weak upper level low will move into central British Columbia this
afternoon and evening maintaining westerly flow aloft over
Washington through Saturday. Conditions will remain generally dry
with above normal high temperatures through the weekend. There will
be a chance of afternoon and evening showers over the Cascades
Thursday and Friday. A second upper level low will move into the
Pacific northwest from the southwest Sunday night and Monday
bringing increasing showers and the potential for thunderstorms to
parts of the forecast area. High pressure will return by middle week.
Short term...marine stratus that spread into all but the central
Puget Sound sound this morning has begun to erode in the last hour. Expect
low clouds to retreat to the coast by late morning. Upper level
ridging will prevail over the region today with generally dry
conditions and high temperatures a little warmer than yesterday.
Westerly flow aloft will help support a chance of showers over the
Cascades again late this afternoon and evening. Cannot quite rule
out the threat of thunderstorms over the mountains...but the risk is
decidedly less than yesterday.
A closed low working its way south and east into central British
Columbia this afternoon and evening will help boost westerly flow
aloft as well as low level onshore flow through Friday...leading to
afternoon and evening showers over the Cascades and subtly
cooler...but still above normal...high temperatures across the area.
Saturday will likely see similar daytime temperatures and a
diminished threat of showers over the Cascades.
Long term...heights will build over the region Sunday ahead of a
negative-tilted upper level trough approaching from the
southwest...boosting high temperatures Sunday afternoon. The
approaching upper level trough will help pull moisture up from the
south and an increasing threat of showers late Sunday afternoon and
evening over the south...spreading north through the night. This
pattern is a little more conducive to thunderstorms especially as
the upper low will moves inland Monday. Showers and clouds will
linger over the area through Tuesday with highs in the 60s.Weak high
pressure aloft will return to the region by midweek bringing drier
conditions and afternoon sunshine.
Aviation...the morning clouds are shallower and did not come in as
far this morning and they will pull back to the coast quickly today.
There is a chance that even the ocean beaches will clear out for
awhile later today. The air mass over the Cascades will
destabilize again with a scattered buildups and a chance of showers
this afternoon and evening. Coastal marine stratus and fog will push
back inland overnight tonight with probably about the same coverage
Ksea...mostly clear with an afternoon and evening northerly breeze. For
Friday morning...the marine layer will probably again be too shallow
to make it to sea tac.
Marine...high pressure centered along 140w with lower pressure
inland will give light onshore flow through the weekend. The
strongest breeze will be each evening in the Strait of Juan Delaware
fuca...and the forecast for now is for a westerly 15-20 knots wind with 2 or
3 foot waves late this afternoon and tonight for the central and eastern
Strait. Areas of fog will burn off into the afternoon...except for
over the coastal waters...and spread back inland again by Friday
You can see an illustrated version of this forecast discussion at