Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 220359

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Fri Jul 21 2017

.SYNOPSIS...An upper level trough and weak warm front will brush
Western Washington tonight and Saturday bringing clouds to all of
the area and a chance of light rain to the coast and north part. An
upper level ridge will gradually build north over the region late
Saturday and Sunday. Onshore flow will return for dry and mild
weather through most of next week.


.SHORT TERM...An upper level disturbance is moving through northwest
Washington this evening. The main effect has been to bring clouds to
most of the area, but there have also been sprinkles over the north
half and on the coast this evening. A couple of sites on the north
coast have received a hundredth of an inch or two in the last six
hours. The models are little changed over the last 24 hours: a
little rain is likely on the north coast at times through Saturday;
there is a chance or slight chance of showers over the rest of the
north; and the interior from about Seattle south will stay dry. This
is happening as 500mb heights rise into the mid and upper 580s.
There will be plenty of 80s in tomorrow`s high temperatures, with
70s north and coast.

The strong upper level longwave ridge currently over the central
U.S. will be shifting back over the Southwest on Saturday, and in
the process gradually building north over the Pacific Northwest. The
departing shortwave trough and the building ridge will end the light
rain over the area Saturday night and start a minor warming and
drying trend for mostly sunny skies by Sunday afternoon. Guidance
puts highs in the 70s to lower 80s, but with 500 MB heights around
5840 meters, this could be cool.

A flat upper level shortwave ridge will move across the area on
Monday.  The weak surface ridge moving across southern British
Columbia ahead of the shortwave ridge will give Western Washington
a short period of northerly offshore flow. This little bit of
additional warming combined with sunny skies should produce the
warmest day for the next week. Highs should be in the mid to upper
70s coast and north interior to the lower to mid 80s around Puget
sound and over the southwest interior. Burke/Kam

.LONG TERM...Previous discussion...The 12Z GFS and 12Z ECMWF
solutions have diverged so confidence in the details in the long
term has diminished. Both models eventually bring an offshore upper
level low to the central B.C. coast then carve out an upper level
trough over W WA. The GFS is faster with SW flow aloft developing
over the area on Tuesday. The ECMWF is about a day slower with the
trough. Both models have a shortwave crossing W WA on Thursday with
a little QPF for some showers, but confidence is low.  Both models
also maintain a large surface ridge offshore, which will keep low
level onshore flow going over W WA. Developments with the upper
level features will impact the daily strength of the onshore flow so
there is less certainty that a moderate to strong marine push will
develop Tuesday evening. Best course for now is to stick with the
current forecast and wait for the models to sort out their
solutions. At the least, the continuing onshore flow will maintain
our typically mild summer weather pattern. Kam


.AVIATION...Moderate westerly flow aloft will continue through
Saturday. A weak front moving over the north side of the upper
ridge over the Pacific Northwest will keep skies BKN-OVC with
bases 7000-12,000 ft tonight, with light rain possible at times
over the northwest part of the forecast area. A moister air mass
will arrive behind the front, so ceilings of 4000-7000 ft will be
common on Saturday.

KSEA...Ceilings 8000-12,000 ft this evening will lower to
4000-7000 ft late tonight or Saturday morning. West to northwest
wind 4-8 kt backing to southerly around midnight. McDonnal


.MARINE...A weak warm front will move across Vancouver Island
tonight, with marginal small craft advisory southerly winds likely
over the northern two-thirds of the coastal waters zones.
Otherwise winds should remain 20 kt or less.

The warm front will dissipate on Saturday, allowing the
resumption of routine moderate onshore gradients from Saturday
afternoon/evening through Monday. This should bring small craft
advisory westerlies to the Central and Eastern Strait of Juan de
Fuca each evening, as well as a turn to northwest winds over the
coastal waters. Onshore gradients will become strong on Tuesday
evening, and that looks like the next possibility for westerly
gales through the Strait. McDonnal


PZ...Small Craft Advisory until 5 AM PDT Saturday 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



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