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afdsew

Area forecast discussion
National Weather Service Seattle Washington
246 PM PDT Mon Apr 22 2019

Synopsis...a frontal system will weaken as it moves across
western Washington this afternoon. A trough of low pressure behind
the trough will move across the region tonight into Tuesday giving
a chance of showers. Weak high pressure will build over the
region for Wednesday and remain over the area into Thursday night.
A cool upper level trough is likely to shift over the area Friday
through next weekend giving a chance of showers, mainly to the
mountains.

&&

Short term /tonight through Thursday/...satellite and radar data,
including observations and ascat wind data, show a weakening
frontal system from central portions of Vancouver Island out into
the coastal waters moving slowly southeastward. Behind the front,
satellite and ascat data show a Post frontal trough moving through
the offshore waters. The front will continue to weaken as it moves
inland later this afternoon and tonight, and the Post frontal
trough will move through the area on Tuesday. Light rain ahead of
the front has moved into the interior; expect this to continue
into the evening hours before precipitation turns more showery.
Shower chances will continue into Tuesday, especially over the
west slopes of the Cascades and in the central portions of the
Puget Sound area with a convergence zone. Showers will taper off
Tuesday evening.

A passing low amplitude ridge aloft will give dry conditions to
the area Wednesday into Thursday. Model blends are showing
significant cloud cover by Thursday, but operational runs down't
indicate an increase of southwesterly onshore flow until Thursday
evening. Forecasts were trended toward the drier operational European model (ecmwf)
and GFS solutions. Expect temperatures Wednesday and Thursday to
be near the seasonal norms. Albrecht

Long term /Friday through Monday/...extended models appear to be
trending toward the European model (ecmwf) which carves a cold but low amplitude
trough into the region later Friday into Saturday. Snow levels
fall to just below 3000 feet, but models show at most low chance
probability of precipitation in the mountains -so snowfall will likely not be an impact in
the passes. Troughing will likely linger across the region into
early next week. Temperatures at lower elevations are expected to
average near or perhaps slightly below normal with partly sunny
days and at most isolated showers. Today's forecast will be a
blend of the various model solutions. Albrecht

&&

Aviation...a frontal system continues to gradually push eastward
through the area. Flow aloft remains westerly and will stay that way
into Tuesday.

Cigs mostly VFR out there however some locations...particularly
along the coast...have fallen into MVFR conditions. With rain and
showers expected for most locations as the front passes...would not
be surprised if additional sites dipped down into MVFR due to
precip. Cigs expected to lower tonight in the wake of the front with
plenty of low level moisture present. This will likely be enough to
put most locations in MVFR although locations prone to lower
cigs...looking at hqm in particular...will probably dip down into
IFR conditions. Conditions look to be slow to recover with these
MVFR cigs continuing into late Tuesday morning.

Ksea...VFR ceilings will continue most of the day before dropping to
MVFR overnight and into Tuesday morning. Rain from passing front
already started at the terminal given the 21z ob expected to
continue through the evening and into tonight before tapering off by
early Tuesday morning. South wind increasing to 8-12 kts. Smr

&&

Marine...winds beginning to relax over some of the waters...with
speeds over the coastal waters as well as the eastern
Strait...Admiralty Inlet and northern inland waters falling below
Small Craft Advisory criteria. Given limited number of obs...may allow Small Craft Advisory for
coastal waters to continue through 5 PM PDT as per inherited
headline while the other aforementioned waters look good to expire
at 2 PM PDT. Still seeing gusts near the Puget Sound/Hood Canal
waters above Small Craft Advisory criteria...so will leave that headline unaltered.

Once this frontal system passes thru the local area, winds will turn
more to the west/northwest in its wake. No headlines needed in immediate short
term but could start to see a return of Small Craft Advisory winds as early as
Tuesday night...mainly for the coastal waters...Strait and adjacent
waters. Speeds do not seem to be quite as strong as previous model
runs suggest...putting them in low-end Small Craft Advisory...so will leave for next
shift to evaluate. Smr

&&

Hydrology...no river flooding is expected over the next 7 days.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory until 5 PM PDT this afternoon for coastal
waters from Cape Flattery to James Island 10 to 60 nm-
coastal waters from Cape Flattery to James Island out 10 nm-
coastal waters from James Island to Point Grenville 10 to
60 nm-coastal waters from James Island to Point Grenville
out 10 nm-coastal waters from Point Grenville to Cape
Shoalwater 10 to 60 nm-coastal waters from Point Grenville
to Cape Shoalwater out 10 nm.

Small Craft Advisory until 2 am PDT Tuesday for Puget Sound and
Hood Canal.

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