Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS66 KSEW 231040
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
340 AM PDT Tue May 23 2017

.SYNOPSIS...Strong onshore flow will develop today as an upper level
trough rapidly moves toward Western Washington. This will bring
clouds and cooler temperatures for Wednesday. This upper trough will
weaken and shift east Thursday and an upper ridge arrives next
weekend.

&&

.SHORT TERM...Conditions continue to cool this early morning with
temps in most locations dipping into the 50s...with Sea-Tac being
the only exception at the time of this writing sitting at 61. While
given current conditions a peek at the radar seems
superfluous...must admit there have been a series of strange
echoes...particularly just off the coast. Trying to match radar with
IR satellite imagery shows that nothing should be there. Will pass
this discrepancy to next shift.

As stated in the synopsis, and approaching upper level ridge will
result in strong onshore flow today bringing breezy to windy
conditions over portions of W WA. This particular set up with pretty
impressive gradients looks to bring wind advisory conditions this
afternoon and evening to the San Juans, the Eastern Strait and
Admiralty Inlet areas...and as such will be issuing that with the
morning forecast package. Models also suggest some precip moving in
with this front but at this time it looks to be confined to mainly
the Cascades and not really impacting the majority of lowland areas.
It is worth a mention that both NAM and GFS are suggesting a
possible Puget Sound Convergence Zone tonight just short of
midnight...however it looks to be rather short-lived. Did not
include this possibility in the forecast proper but will pass on to
day shift for potential inclusion in afternoon package. Although W
WA ends up on the back side of the trough by late Wed
morning...precip signatures look to linger over the higher
elevations and even bleeding some slight chance pops into some
lowland areas. The big headline for Wed though will be the cool off
in temps down into 60s for much of the area. This cooldown will not
last long...as upper level ridging builds over the Pacific and as
such...temps rebound back into the upper 60s/lower 70s for Thursday.
This will also serve to further limit any precip to near the Cascade
crest...thanks to a bit of instability from minor shortwave
disturbances...and thus resuming a mostly dry forecast for the
lowlands.  SMR

.LONG TERM...By Friday, upper level ridging is in full control of
the area, wrapping up any precip threat and ramping up temps once
again making for a pretty warm weekend...inland lowland highs for
Friday in the mid to upper 70s and for Saturday in the lower 80s.
Some slow, gradual cooling will start on Sunday as the ridge axis
shifts eastward over the Cascade crest. Models once again want to
bring in some precip Sunday and Monday...mostly driven by a mix of
instability and terrain and thusly limited to higher elevations.
Likely will not see a change in the weather pattern until the middle
of next week...although models are in disagreement on when/how this
will occur. GFS is faster at bringing a trough in for next Tue
evening while the ECMWF holds off on the trough til at least next
Wed. Neither solution looks particularly...coherent...leaving a
particularly high lack of confidence.  SMR

&&

.AVIATION...The upper level ridge axis has shifted east of the
Cascades overnight ahead of the approaching upper level low moving
down the British Columbia Coast. Light westerly flow aloft with
moderate onshore flow this morning. The air mass will remain
generally dry today with VFR conditions. The exception is the coast
where onshore flow and stratus will result in IFR CIGs. Strengthening
onshore flow today and the resulting strong push this afternoon and
evening will spread stratus through the Strait and into the interior
Tuesday Tuesday night. In addition, surface winds will become gusty
winds particularly during the afternoon and evening hours.

KSEA...VFR conditions today. Light northerly winds this morning will
become southwesterly 8-10 KT around midday and gusty late in the
afternoon, then northerly 10-12 KT during the evening.

&&

.MARINE...Moderate onshore flow this morning will strengthen this
afternoon as an upper level trough approaching from the NW will
result in a very strong marine push. Gale force westerlies are
expected to develop through the Strait of Juan De Fuca around
midday with peak winds of 35-45 KT possible late in the afternoon or
early evening. Gale force winds are also expected to spread into the
northern interior waters and Admiralty Inlet late in the afternoon
and evening. This system will also support SCA winds and seas
through all other area waters. In addition a steep, fresh swell will
affect the coastal waters through Wednesday night. Onshore flow will
ease late tonight and linger through Friday with strongest winds
and seas occuring during the late afternoon and evening hours.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...Wind Advisory from 11 AM this morning to 11 PM PDT this evening
     for Admiralty Inlet Area-Eastern Strait of Juan de Fuca-San
     Juan County.

PZ...Small Craft Advisory until 5 PM PDT Wednesday 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 9 AM PDT this morning for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 9 AM this morning 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 from 2 PM this afternoon to 2 AM PDT Wednesday for
     Admiralty Inlet-Northern Inland Waters Including The San
     Juan Islands.

     Small Craft Advisory from noon today to 5 PM PDT Wednesday for
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this
     evening for Puget Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.