Scientific Forecaster Discussion
fxus66 ksew 202131
Area forecast discussion
National Weather Service Seattle Washington
231 PM PDT Thu Jul 20 2017
Synopsis...a weather system lifting across Vancouver Island and
into southern British Columbia will bring a chance of showers into
this evening. After drier conditions later tonight and Friday,
another disturbance will move by to the north of the area Friday
night and Saturday giving a chance of light rain to the northern
portions of the area. The pattern will return to a dry, sunny and
warmer than normal one Sunday through Monday. There will be an
increase in night and morning clouds and temperatures will cool to
near normal starting about Tuesday.
Short term...radars show scattered light showers continuing
along the coast, along the west slopes of the mountains, and in an
area of convergence from around Port Townsend to east of Mount
Vernon. These showers will gradually dissipate this evening as the
disturbance that is producing them pushes well off to the northeast
of the area.
500 mb heights will rise later this evening through Friday as a
weak shortwave upper ridge moves across the area. Conditions aloft
will dry and warm in response to this weak ridge. Then another
short wave trough, this one now centered along 138w longitude,
will move east across Vancouver Island late Friday through
Saturday. Despite the high 500 mb heights in the mid 580 dam
range across western Washington Friday night and Saturday, the
12z models all bring some precipitation into the northern third or
so of western Washington during this period. Time-height diagrams
based on forecast upper air soundings show more mid and high
level cloudiness and higher humidities in the lower levels than
with the system that moved across our area this morning, and this
next system will be accompanied by a stronger jet streak moving by
to the north of the area.
Despite continued onshore flow on Sunday, high 500-mb heights and
warm conditions aloft will give temperatures in the mid 70s to
lower 80s in the interior as well ad dry conditions. Albrecht
Long term...Monday will be much like Sunday with 500 mb heights
remaining in the lower 580 dam range and low level flow remaining
onshore. Then Tuesday Onward we will see lowering heights aloft
as a mid 560 dam upper low passes through southern British
Columbia and onshore flow increasing somewhat. The forecast from
Tuesday Onward will feature more extensive night and morning
clouds and temperatures around or slightly below normal for late
July. A few model solutions hint at some isolated showers mid week
as the upper trough passes by to the north, but we are entering
our driest time of year and most solutions are dry. Albrecht
Aviation...a weak upper ridge will move over the area tonight.
Upper heights will continue to build over the area on Friday with
westerly flow aloft. At the surface, onshore flow will prevail
tonight and Friday with a ridge along the coast and lower
pressure inland. Slightly unstable air will become stable this
Ceilings are generally 5000-6000 feet although some locations
have scattered and a few patches of MVFR low clouds continue to
linger near the coast and Strait. In general, clouds will continue
to slowly decrease this evening. Areas of MVFR low clouds are
expected to reform again later tonight and Friday morning.
Ksea...ceilings should be generally 5000-6000 feet through early
Friday morning although clouds might scatter for a few hours this
evening. MVFR low clouds are expected to redevelop Friday morning
and then break up later in the day. South wind 5-10 knots through
early evening. Winds are expected to switch to north later this
evening but they will be pretty light by then and become variable
after midnight. Schneider
Marine...a surface ridge will be along the coast with lower
pressure inland tonight and Friday with Small Craft Advisory
strength west winds in the central and eastern Strait tonight.
A front will move into the offshore waters Friday and dissipate
offshore on Saturday. This will be followed by onshore flow Sunday
through Tuesday with high pressure offshore and lower pressure
Pz...Small Craft Advisory from 6 PM this evening to 3 am PDT Friday
for East Entrance U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory until 3 am PDT Friday for central U.S.
Waters Strait of Juan de Fuca.
Small Craft Advisory until 2 PM PDT this afternoon for Puget
Sound and Hood Canal.