Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus66 ksew 232304 
afdsew

Area forecast discussion
National Weather Service Seattle Washington
404 PM PDT Tue may 23 2017

Synopsis...a strong cold front will sweep through western
Washington this evening, bringing windy conditions to areas near
the Strait and Admiralty Inlet. A much cooler air mass will be in
place on Wednesday. Warmer and drier upper ridging will rebound
off the coast on Thursday, then pass western Washington on
Saturday.

&&

Short term...an upper trough with a strong height fall center
will move east-southeast across southern b.C. Through tonight.
This will sweep a strong cold front out of the northwest through
western Washington this evening. Strong onshore flow has now
developed in response, with the observed 3 PM uil-bli pressure
gradient at an impressive +6.6 mb. A strong surge of westerly
winds continues to gather steam through the Strait of Juan de
Fuca, which will take aim at Whidbey Island and the Admiralty
Inlet area early this evening. Later this evening around 8-10 PM,
the west-northwest winds will penetrate all the way into western
Snohomish County, where gusts to 40 mph are expected. For the
Everett area in particular, west-northwest is a less common direction for
stronger winds, which could increase the impacts of 40 mph gusts.
The current configuration of wind highlights looks good, so no
update to the wind advisories and warnings in the near future.
Westerly flow will gradually weaken during the overnight hours.
Otherwise, a few light showers are forecast tonight where upslope
flow into the mountains or low- level convergence downwind of the
olympic mtns can maximize low- level lift.

An enhancement of showers is expected on Wed morning as the main
upper trough axis swings through. The main upper trough will then
exit to the northern rockies later Wednesday. A piece of the
upper trough along with some colder air aloft will linger over the
inland northwest until Thursday night, so afternoon showers and
thunderstorms near the Cascade crest are forecast on Thursday PM.
The Lowlands are forecast to be rain-free on Thursday.

A well-defined upper ridge axis will move to near 130w on Friday,
bringing the biggest day of day-over-day temperature rises in the
week ahead, aided partially by significant weakening of cooling
low-level onshore flow. Haner

Long term...a warm and dry upper ridge axis will pass directly
over western Washington on Saturday. The upper ridge axis will
be over eastern Washington on Sunday. Cooling seabreezes will
strengthen at the immediate coast, but the rest of western
Washington should have little change on Sunday. Some towering
cumulus and an isolated shower could form near the Cascade crest
in the afternoon as a turn to light southerly flow aloft takes
place.

For Monday and next Tuesday, the long-range models are pretty
close with their 500 mb height values. The main difference is that
the GFS is a little more amplified with the pattern than the
European model (ecmwf). So the gfs's forecast of a stronger southerly component to
the upper flow is more supportive of late afternoon and early
evening shower/thunderstorm development along the Cascade crest.
The European model (ecmwf) in contrast does not show this outcome, given its more
westerly component to the upper flow. For The Lowlands, both
solutions are rain-free. Haner

&&

Aviation...a cold deep upper level low over central b.C. At 23z
will move southeast across southern b.C. Arriving over southern Alberta
around 15z Wednesday. Strong northwest flow aloft will prevail over West Washington.
Low level onshore flow will be quite strong through this evening.
Areas around the north interior including the Strait of Juan de
Fuca have localized very windy conditions.

Overall, the air mass will remain generally dry with VFR
conditions. Stratus currently along the coast will move part way
inland through the Strait of Juan de Fuca and between khqm and
kshn, but strong mixing from the onshore flow should prevent
widespread coverage of MVFR/IFR stratus. In addition, low level
convergence may develop between kpae and ksea 00z-08z for
localized ceilings bkn025-060.

Ksea...good VFR conditions through 00z. A convergence zone may be
in the vicinity from 02z through 06z with ceilings possibly down to
bkn025 for short periods of time. With the convergence zone lying
across central Puget Sound winds at kbfi and especially will be
erratic. Confidence is low. Best guess is that SW winds 5-15 knots
will last until around 02z then become variable 02z-04z as the cz
is overhead. Around 05z winds may shift east-NE 5-12 knots as the cz
shifts S and weakens. Kam



&&

Marine...an upper level low is moving southeast across southern b.C.
This afternoon and tonight. Onshore flow has increased
dramatically with a 6.6 mb gradient through the Strait at 3 PM.
Race Rocks was 44g49kt at 3 PM so in coordination with Canada the
central and east Strait will be upgraded to a Storm Warning for a
few hours through 8 PM. The gale through the Strait will spread
over the north inland waters around the San Juans and through
Admiralty Inlet albeit with slightly weaker winds. A gale is also
in effect for the north part of Puget Sound, north of Seattle.
This gale is a little shakier and northwest winds could end up topping
out around 30 knots. Winds will likely peak in the Strait early this
evening then weaken gradually overnight. Winds in Admiralty Inlet
and north Puget Sound will peak a couple hours later and ease a little
later than winds in the Strait. Winds should ease fairly rapidly
after midnight, but Small Craft Advisory conditions in the Strait may persist into
Wednesday morning.

The surface ridge offshore will maintain light to moderate
onshore flow the rest of this week.

A steep fresh swell 10 to 13 feet at 8 to 10 seconds is expected
to reach the coastal waters later this evening and persist into
Thursday morning. Kam

&&

Sew watches/warnings/advisories...
Washington...High Wind Warning until 1 am PDT Wednesday for Admiralty Inlet
area.

Wind Advisory until 1 am PDT Wednesday for eastern Strait of
Juan de Fuca-San Juan County.

Wind Advisory until 1 am PDT Wednesday for Everett and vicinity.

Pz...Small Craft Advisory until 5 am PDT Thursday 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 for rough bar until 9 am PDT Thursday for
Grays Harbor bar.

Storm Warning until 8 PM PDT this evening for central U.S.
Waters Strait of Juan de Fuca-East Entrance U.S. Waters
Strait of Juan de Fuca.

Gale Warning from 8 PM this evening to 5 am PDT Wednesday for
central U.S. Waters Strait of Juan de Fuca-East Entrance
U.S. Waters Strait of Juan de Fuca.

Gale Warning until 2 am PDT Wednesday for Admiralty Inlet-
northern inland waters including the San Juan Islands.

Small Craft Advisory until 5 PM PDT Wednesday for West Entrance
U.S. Waters Strait of Juan de Fuca.

Gale Warning until 1 am PDT Wednesday for Puget Sound and Hood
Canal.

&&

$$

National Weather Service Glossary of Abbreviations