Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181630
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 AM PDT WED MAY 18 2016

.SYNOPSIS...
An upper level low pressure system near the coast of southeastern
Alaska will gradually drop southeastward into the Pacific
Northwest through the weekend. The upper low will then linger over
the western states through next week. The upper low will result in mostly
cloudy skies, a chance of showers, and temperatures a little
below normal.

&&

.SHORT TERM...
A sprawling vertically stacked upper low sits just off the coast
of southeastern Alaska this morning. On the large scale, models
are consistent in showing a pattern change over the Pacific
Northwest as this upper low drops southeastward through Friday.

Today will be a transition day as a weak frontal system ahead of
the upper low drops southeast across the area. A mid level cloud
deck has already moved into the interior of Western Washington.
Satellite imagery and observations show thickening clouds along
the coast that will move in midday. The only precipitation seen on
radar at this time sits from southwestern British Columbia
southwestward to around Victoria. Shower activity will gradually
increase this afternoon and evening as lift with the incoming
front and upper low moistens the lower and mid levels of the
atmosphere. With the clouds, expect cooler high temperatures today
with around 60 on the coast to lower to mid 60s inland.

Pressure gradients, already onshore, are expected to increase this
afternoon and evening with strong inflow developing through the
Strait of Juan De Fuca and Chehalis Gap. The increased flow will
produce a good increase in showers along the west slopes of the
mountains and will produce a convergence zone initially around
Skagit and north Snohomish counties. The convergence zone will
gradually slide southward into southern Snohomish and King
counties late tonight into Thursday. Mountain snow levels are
expected to drop to about 4500 feet tonight and 4000 feet on
Thursday as the cool upper low moves into the area. Snow showers
are likely on the higher Cascade passes and some locations above
4000 feet by Thursday could see a couple inches. Accumulations are
not expected to be a problem on the roadways as temperatures will
be just above freezing and a good deal of insolation will make it
down through the clouds.

The GFS and ECMWF both drop the upper low into southern Oregon and
Northern California on Friday, then wrap a deformation zone around
theupper low and into mainly eastern Washington Friday night and
Saturday. The Canadian GEM is more robust in bringing the
deformation zone westward into the area for a wetter period. The
weather for Friday through Saturday will be dependent on the exact
timing and placement of difficult to forecast vorticity features
and deformation zones moving around the upper low so a broad brush
forecast for partly to mostly cloudy conditions and a chance of
showers at times appears appropriate. Temperatures may reach the
mid 60s Friday and Saturday if we are able to get some sunshine.

Albrecht

.LONG TERM...
From the previous long term discussion...A long wave trough over
the West will prolong the cool and unstable weather pattern
through early next week. There is a chance of showers each day
through the period. Temperatures will be near to slightly below
normal. There is little day-to-day variance and the forecast leans
toward persistence. 33


&&

.AVIATION...
An upper level trough offshore will move over the region
late today or early this evening. Strong sw flow aloft /at 500 mb/
will develop over the area this afternoon. Also, strong low level
onshore flow will develop this afternoon in the wake of a cold
front.

Areas of MVFR cigs this morning will become more widespread this
afternoon. Expect localized LIFR cigs and mvfr vsbys over the
coast. The mountains will become partially obscd this afternoon.

KSEA...
VFR conditions are anticipated to prevail over the terminal
today. Winds will increase from the sw at 10-15 knots this
afternoon and there may be ocnl gusts near 20 kt after 2100 UTC.

&&

.MARINE...
High pressure offshore with lower pressure inland will result in
onshore flow today. Strong onshore flow will develop late this
afternoon into this evening in the wake of a cold front. Onshore
flow will weaken Thursday due to falling pressure over the coastal
waters. A weak surface low will move across the offshore waters
on Friday. Felton/05

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 2 PM PDT Thursday
     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 6 AM PDT Thursday 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 3 AM PDT
     Thursday for Northern Inland Waters Including The San Juan
     Islands-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 3 PM this afternoon to 6 AM PDT
     Thursday for Admiralty Inlet.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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