Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 291556

Area Forecast Discussion
National Weather Service Seattle WA
856 AM PDT Mon May 29 2023

.UPDATE...Onshore flow this morning with extensive stratus across
Western Washington, clearer skies at higher elevations of the
Cascades and Olympics. Clouds will likely continue through the
morning with some clearing this afternoon, however, some areas may
stay generally cloudy through much of today. Temperatures will
likely be a bit cooler for this reason with highs in the upper 50s
to low 60s on the coast, and 60s for the interior.


.SYNOPSIS...Onshore flow will continue through mid week for
morning clouds and some afternoon sunshine. Troughing will
develop along the coast Tuesday through Thursday for cooler
temperatures and potential for drizzle. Warmer temperatures likely
next weekend into the following week.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Zonal pattern remains in
the upper levels to start the short term forecast period. An
upper-level low stays down in California and a trough to the east
will progress giving way to a long-wave trough coming from the
Pacific by Wednesday. At 300 mb the jet core will be over WA by
Tuesday night/Wednesday which will support rising motion in the
right entrance region. There`s good consistency with this in
between the ECMWF GFS and NAM with the aggressiveness of this

Dynamically there is some weak vorticity advection at 500 mb and
700 mb Tuesday night/Wednesday to support some lift. However
moisture aloft remains relatively dry in the model guidance with
no support at the aforementioned levels. Down near the surface a
weak surface trough sets up along the Cascade Ridge with onshore
push continuing through the short term period. There is sufficient
moisture near the surface with RH values approaching 80-90 percent
with the marine stratiform pushes coming on the coast.

Given the dynamical environment and the onshore pushes will keep
temps cooler with the expected morning marine stratus to push
inland during the next few mornings. As temperatures climb do
expect most areas to clear out for the afternoon and evening for
all mornings. The weak synoptic environment will only allow for
drizzle in the morning especially near the coast with the
moisture near the surface and the drier air higher up. Winds
remain light out of the west of 5 to 10 mph with the exception of
the Strait of Juan De Fuca with winds 20 to 30 kts.


.LONG TERM /THURSDAY THROUGH SUNDAY/...The upper level pattern at
the start of the long term forecast period will feature broad
troughing extending from the Aleutian Islands into the PNW. A
shortwave trough embedded within this flow will skirt across W WA
through the day on Thursday. Weak dynamics and meager moisture
will prevent much in the way of sensible weather impacts across
the area, however an isolated shower or thunderstorm cannot be
completely ruled out over the North Cascades. Continued onshore
flow in the low levels will make for similar temperatures as what
was experienced for much of the short term period.

By Friday, the large scale upper level pattern will amplify, with
strengthening high pressure over much of Canada and the northern
tier of the US and a deepening trough over the far NE Pacific. As
the ridge strengthens over Canada, its influence will build
westward towards the PNW. Current ensemble solutions favor more of
a high pressure influence over W WA vs Pacific troughing,
however, continued onshore flow will help keep temperatures in
check, allowing them warm back only into the low to mid 70s.

Through the weekend, ensemble solutions favor increasing high
pressure influence aloft, making for a warming trend. Some
uncertainty remains in the degree of warming, but confidence is
currently high in a return to temperatures in the upper 70s to
near 80. There still remains some concern for a period of above-
normal temperatures at the very end of the forecast period and
just beyond, along with the potential for the development of weak
offshore flow.



.AVIATION...An upper level trough in the Gulf of Alaska will bring
zonal flow aloft over W WA for much of the day before gradually
turning more southwesterly as this feature starts to sag southward.
Surface flow remains onshore with generally stable conditions present.

Satellite shows marine stratus has made it inland and all the way
to the Cascades this morning, with the majority of area terminals
under stratus mainly seeing IFR to LIFR conditions. Expect to see
slow, but gradual improvement back to VFR conditions for area
terminals this afternoon - likely around 21Z. Winds are generally
light across the area, persisting at speeds less than 5 kts over
through the morning hours. Winds will then pick up slightly after
18Z, ranging generally between 4-8 kts.

KSEA...LIFR conditions in stratus. Expect stratus to linger into
the early afternoon, before approximately lifting and scattering
near 21Z. VFR conditions will resume thereafter. Light S/SW winds
persisting this morning at the terminal. A slight uptick in winds
expected this afternoon, with winds expected to generally persist
between 4-8 kts. 18/14


.MARINE...High pressure will remain situated over the waters the
next several days, helping to maintain breezy northwesterly winds
over the outer coastal waters at times and diurnal westerly pushes
down the Strait of Juan de Fuca. Guidance remains consistent with
a strong westerly push expected along the Strait this afternoon
and evening and as such have maintained the Gale Warning for the
Central and East Strait. Additionally, conditions look right for
SCA conditions to develop in the adjacent Northern Inland Waters
and Admiralty Inlet waters. In the meantime, northwesterlies also
look to meet SCA criteria over the coastal waters and Western
Strait, meriting additional headlines there.

Current seas ranging 3-5 ft will build during the day, getting to 5-
7 ft this afternoon and 7-9 ft tonight before peaking at 9-11 ft
Tuesday morning in the outer coastal waters. With seas dominated by
short period fresh swell, expect quite choppy conditions to continue
over the next few days. 18/14


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed.


PZ...Small Craft Advisory from 2 PM this afternoon to 11 PM PDT this
     evening for West Entrance U.S. Waters Strait Of Juan De

     Small Craft Advisory until 5 PM PDT this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 5 PM this afternoon to 8 AM PDT Tuesday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Tuesday for Admiralty Inlet-Coastal Waters From Cape
     Flattery To James Island Out 10 Nm-Coastal Waters From
     James Island To Point Grenville Out 10 Nm-Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 8 AM PDT Tuesday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.


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