Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 230959

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PDT Sun Jul 23 2017

.SYNOPSIS...A weak upper trough over British Columbia will allow for
clouds and cooler weather today. High pressure aloft will build over
the area Monday and Tuesday for a warming trend. Stronger onshore
flow will cool highs back to near normal Wednesday through Friday.


.SHORT TERM...Cloudy conditions are in place this early morning for
much of the area as the combination of onshore flow and an upper
level trough well to the north look to result in a gray start to the
day...though there does seem to be a bit of a hole over the south
Sound area at the time of this writing.

The upper trough will continue eastward today and as such will allow
for skies to gradually clear through the day...fully clearing out
tonight. This will do nothing for any chances for precip...which
remain slim to none for W WA. Daytime highs will be on average 5-7
degrees cooler in the interior lowlands today while little change in
temps are expected over the coast.

Upper level ridging is expected to build over the Pacific Monday and
Tuesday and as such...increasing 500mb heights will result in
warming temperatures. Highs Monday are expected to return to the 80s
in the interior lowlands...then climb further into the mid to upper
80 for Tuesday. Further facilitating this warming trend...expecting
weakening onshore flow Monday and the development of a thermal
pressure trough for Tuesday...which may allow for some very weak
offshore flow Tuesday afternoon. This feature does not hang on for
very long onshore flow looks to resume Tuesday night. SMR

.LONG TERM...Dry conditions will remain in place for Wed as the
upper level ridge continues eastward. A quick moving upper level
trough may push over the area Wednesday evening but long range
models agree on it being a very progressive system and that any
moisture associated with it will likely remain in Canada...thus
allowing the current dry streak to continue. A shortwave ridge
Thursday will allow for more dry conditions. Closing out the
week...models show another upper level low out over the Pacific but
again...thanks to the fairly stationary Four Corners high...said low
will likely be forced once again north of the area. As
such...chances for precipitation look pretty minimal for the
entirety of the long term forecast.  SMR


.AVIATION...Westerly flow aloft over Western Washington early this
morning will continue today as an upper trough moves through
British Columbia. The flow will veer to northwest tonight as the
trough moves east and an upper ridge builds offshore. At the
surface, onshore flow will continue with high pressure offshore
and lower pressure inland. The lower layer of the air mass will be
moist today and become drier tonight.

A layer of clouds with bases 050-060 is over the northern part of
the forecast area. In addition, there is patchy low stratus with
bases 003-015 especially in areas along the coast. The low stratus
will probably increase during the next few hours, giving areas of
BKN-OVC low end MVFR and IFR ceilings to much of the area
including the central Puget Sound region. Gradual improvement is
expected beginning around midday, with widespread VFR conditions
late in the afternoon.

KSEA...Low clouds will probably move over the terminal during the
next few hours, with ceilings 006-012 likely at times roughly 12Z-
Northerly wind 5-10 kt will become northwest this afternoon.


.MARINE...Varying degrees of onshore flow will prevail today
through Wednesday, with high pressure over the offshore waters and
lower pressure east of the Cascades. The Central and Eastern
Strait of Juan De Fuca should have small craft advisory west winds
most evenings, and gale force winds are possible there Tuesday and
Wednesday evenings. The Coastal Waters will also have small craft
advisory northwest winds at times through Tuesday night, especially
the outer waters. McDonnal


PZ...Small Craft Advisory until 6 PM PDT Monday 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 from noon today to 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to midnight PDT
     tonight for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.



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