Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
340 PM PDT Thursday may 28 2015
Synopsis...high pressure aloft will will weaken somewhat as a weak
upper level low moves into central British Columbia this afternoon
and evening. Westerly flow aloft and generally dry conditions with
above normal high temperatures will continue through the weekend.
There will be a chance of afternoon and evening showers and
thunderstorms over the Cascades this evening and again 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...low clouds have retreated to the coast leaving sunny
skies over most of the area this afternoon. Upper level ridging will
weaken somewhat tonight but remain generally dry as a closed low
works its way into central British Columbia this evening. This 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. Have been seeing convection bubble
along the crest with a few lightning strikes along and just east of
the Ridgeline. Have updated the forecast for tonight to include a
chance of thunderstorms. Saturday will likely see similar high
temperatures but a much diminished threat of showers over the
Long term...medium range models continue to highlight building
heights over the region on Sunday Sunday ahead of a negative-tilted
upper level trough approaching from the southwest. This will likely
boost high temperatures Sunday afternoon as well as help pull
moisture up from the south and bring an increasing threat of showers
late Sunday afternoon and evening over the south...spreading north
through the night. The flow 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 Tuesday and Tuesday
night with highs in the 60s. And lows in the middle to lower 50s. Weak
high pressure aloft will return to the region by midweek bringing
drier conditions and a typical pattern of morning clouds and
Aviation...marine low clouds and fog are over the coastal waters
and were still squirting east into the Strait of Juan Delaware fuca at
times...otherwise it has been a sunny afternoon most places. Over
the Cascade crest there are scattered cumulonimbus and thunderstorms where the air is
the most unstable this afternoon. Low marine stratus and fog will
push back inland overnight tonight with probably about the same
coverage Friday morning as there was this morning.
Ksea...mostly clear with an afternoon and evening northerly breeze. For
Friday morning...the marine layer will once again likely be too
shallow to make it to sea tac.
Marine...high pressure centered offshore 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 the rest of the afternoon and evening in the central
and eastern Strait is for a westerly 15-25 knots wind with 2 to 4 foot wind
waves. Areas of fog and low clouds will continue to move from the
coastal waters east into the Strait...and then partly into the
inland waters by daybreak with another typical burnoff Friday.
Pz...Small Craft Advisory central and eastern Strait of Juan Delaware fuca.
You can see an illustrated version of this forecast discussion at