Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 161548 CCA

Area Forecast Discussion
National Weather Service Seattle WA
847 AM PDT Sun May 16 2021

.SYNOPSIS...An upper level ridge remains in place today, maintaining
another mild and dry day across Western Washington. This ridge
breaks down by early Monday as an upper low slides down the coast of
British Columbia. This will usher in a much cooler pattern that will
remain across the region for much of the coming week as well as
bring a return of cloudier and wet conditions.


.SHORT TERM /TODAY THROUGH TUESDAY/...One last warm day for the
interior today as upper ridging weakens. Shallow marine moisture
will evaporate across the interior by late morning, but is likely to
linger along the immediate coast and portions of the Strait of Juan
de Fuca through the day. Onshore gradients will ramp up
significantly by late afternoon and send the marine stratus surging
inland tonight. The marine layer is expected to deepen quickly
overnight. It wouldn`t be too surprising to see some areas of
drizzle around Puget Sound on Monday morning. The deep marine layer
will be slow to clear...if at all...for the lowlands tomorrow. And,
accordingly, high temperatures will take a tumble. Eyeing the
guidance...I suspect that places like Seattle will be looking at
high temperatures 12 to 14 degrees cooler tomorrow. A full scale
pattern change will dominate much of the week ahead with unsettled,
showery conditions and cooler temps. No forecast updates planned
this morning.  27

Stronger onshore flow down the Strait of Juan de Fuca will bring
some breezy winds along the Strait and across Whidbey Island and
through Admiralty Inlet this evening. Latest convection allowing
models hint at some potential for a weak convergence zone to
develop late Sunday evening. An upper low advancing south along
the B.C. coastline will be close enough early Monday to bring some
rain chances across the northern tier of the area - roughly along
and north of a line from La Push to Mount Vernon. As the upper
trough moves closer, rain chances increase from north to south
through the day on Monday. While chances are relatively high for
much of the area to get some rain, QPF does remain relatively
light away from the coast and Whatcom/Skagit/San Juan counties.
Afternoon temperatures will be much cooler, only in the upper 50s
to mid 60s for the most part, with abundant cloud cover through
the day.

Ensembles remain in good agreement for Tuesday, with the upper
trough axis likely extending from the central B.C. coast to the
southwest across northern Vancouver Island and out over the
Pacific. A shortwave will likely round the trough and provide
another focus for showers across the area on Tuesday. Temperatures
will also trend a few degrees cooler, struggling to get out of
the 50s. With the colder air mass in place, snow levels will
likely fall to around 4,000 ft by Tuesday afternoon. Given the
longer days and showery nature of the precipitation, significant
impacts and even much in the way of accumulation are not
expected, but it wouldn`t be surprising to see some light snow
falling over the higher stretches of SR 20 and US 2, for example,
around midweek. But again, accumulations are not expected and
perhaps only an inch or two in the high Cascade elevations.

in fairly strong agreement that the upper trough will remain the
primary focus for weather into the middle and late stages of the
week, though spread in how fast it moves eastward becomes larger
late in the week. As it slides south and east on Wednesday,
instability may increase with steepening lapse rates, and cannot
entirely rule out a few lightning strikes out of the stronger
showers in the afternoon. As a result, have introduced a slight
chance mention of thunderstorms for portions of the area. Even as
the upper low eventually pushes southeast toward the Great Basin
late in the week, there will remain some potential for wrap-around
moisture to bring showers Friday and Saturday, though temperatures
likely trend back warmer toward seasonal normals or into the mid
60s with an increasing mix of sun/clouds.      Cullen


.AVIATION...High pressure will flatten over the region today but
the air mass will remain stable. The flow aloft is westerly. The low
level air mass remains moist with onshore flow, leading to stratus
clouds over portions of the interior. Low clouds and/or fog will
burn back to the coast late this morning with VFR conditions
expected. Low level stratus clouds will return overnight, and extend
in coverage, with likely MVFR to IFR ceilings. 33

KSEA...VFR today. Variable wind becoming more westerly by 18-21z.
Low clouds and MVFR to IFR cigs by 12z Mon. 33


.MARINE...High pressure over the NE Pacific will lead to onshore
flow across western WA today. A stronger onshore push will develop
late this afternoon with gales likely through the Strait of Juan de
Fuca. The flow will remain onshore through early next week as a
series of weak fronts move inland. 33


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


PZ...Small Craft Advisory until 2 PM PDT this afternoon 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.

     Gale Warning from 3 PM this afternoon to 7 AM PDT Monday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to 7 AM PDT Monday
     for Admiralty Inlet-Northern Inland Waters Including The
     San Juan Islands.


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