Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 250401
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Mon Jul 24 2017

.SYNOPSIS...Upper level ridging will keep sunny and warmer
weather over the region Tuesday, except for some isolated morning
low clouds near the coast. Increasing onshore Tuesday evening will
result in more clouds and cooler weather Wednesday and Thursday. A
transition to southwest flow aloft and weaker onshore flow will
result in mild and dry weather this weekend into early next week.

&&

.SHORT TERM...Clear skies will continue tonight with stratus
redeveloping late tonight along Central Washington coast. Weak upper
level ridging will shift onshore Tuesday with northerly low level
offshore flow. The ridge and weak low level offshore will help boost
high temperatures into the lower to mid 80s through much of the
interior and low 70s along the coast and strait.

Increasing onshore flow Tuesday evening ahead of an approaching
upper level trough will result in a moderate marine push resulting
in a shallow marine layer working into the Central Puget Sound
Wednesday morning before dissipating by midday. The morning clouds
will help keep areas with stratus around 5 degrees cooler than
Tuesday and only slight cooling elsewhere. Stronger onshore flow
Wednesday evening will result in a more solid marine push and a
deeper marine layer Thursday morning with afternoon clearing. The
additional cloud cover and deeper marine layer will keep high
temperatures Thursday in the mid to high 70s over the interior and
upper 60s along the coast and strait. Weak onshore flow Thursday
night will reinforce the marine layer and the typical pattern of
morning clouds and afternoon clearing again Friday with slightly
warmer temperatures.

.LONG TERM...from previous discussion...Thursday nights onshore flow
should maintain the fairly deep marine layer Friday morning. It
still looks like there will be breakout Friday afternoon.
Temperatures should be about the same or slightly warmer. By this
time a large upper level trough will have formed over the NE
Pacific, centered near 50N/145W. W WA will be under SW flow aloft
between the trough and the large SW U.S. upper level ridge through
the weekend. The GFS and ECMWF now agree that an upper level low
swinging around the large trough will move NE toward the central
B.C. coast Saturday night. The associated weak front extending S
from the low could bring a few showers to the coast late Saturday
night or Sunday morning. For now, the models agree on the showers,
but this could easily change over the next several model runs.

The models have now postponed building the big SW U.S. ridge
northward to Monday. We`ll see how that evolves over the next bunch
of model runs, as well. Kam

&&

.AVIATION...A broad upper level ridge will bring dry and stable
conditions to Western Washington tonight and Tuesday. The flow aloft
is NW to W. Low level stratus clouds will likely form along the
coast tonight for IFR to MVFR conditions. VFR conditions will
prevail in the interior. 33

KSEA...Clear skies and N/NE winds to 08 kt. 33

&&

.MARINE...High pressure over the NE Pacific will maintain onshore
flow across Western WA through Friday. Westerly gales are possible
through the Strait of Juan de Fuca Wednesday night with a stronger
onshore push. A weak front will pass through the Coastal Waters on
Saturday. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 6 AM PDT Wednesday 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.

     Small Craft Advisory until midnight PDT tonight for 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.

     Small Craft Advisory until 3 AM PDT Tuesday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

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



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