Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190444
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
944 PM PDT Tue Jul 18 2017

.SYNOPSIS...A low pressure system offshore will keep the flow
aloft from the southwest through Wednesday. This system will move
across the region on Thursday for a threat of showers. Expect
temperatures to be near or slightly above normal Wednesday but
below normal on Thursday. An upper level ridge will build over the
region this weekend with temperatures trending above normal.

&&

.SHORT TERM...Main weather feature of interest is the upper low
that is spinning out near 135W. This feature has been nearly
stationary for the past 24 hours, but it is about to start moving
eastward. It will then accelerate eastward across Vancouver
Island on Thu AM. 500 mb heights will fall nicely into the 565-572
decameter range as the trough axis swings through Western
Washington early Thu. Lower upper heights and large-scale lift
will cause the marine layer to become quite deep early Thu, with
model soundings showing moisture extending up to near 700 mb from
Seattle on north. Given moisture of that depth in combination
with some larger scale lift and smaller-scale orographic and PSCZ
lift, some light rain and showers will be around on Thursday. Most
likely places to get measurable rain will be the coast, the Puget
Sound Convergence Zone north of Seattle, windward sides of the
Olympics and Cascades, and near the Canadian border where moisture
depth and larger-scale lift will align most favorably.

In advance of the upper trough, cooling low-level onshore
has already increased this evening, with the UIL-BLI gradient
currently at +3.1 mb. As a result, temperatures on Wed will trend
downward a few degrees. For Thu, clouds and a few showers will
hold temps back even further, with a forecast high of only 69F at
Sea-Tac.

On Friday, upper heights will rebound, and westerly zonal flow
aloft will replace Thursday`s cyclonic flow. In fact, 850 mb
temps and winds indicate a weak warm advection pattern on Friday,
a bit reminiscent of a warm front. Temperatures will
correspondingly rise on Friday afternoon, but the presence of
some weak warm advection lift could very well squeeze out an ever
so small amount of rain near the North Coast on Friday. Haner

.LONG TERM...From Previous Discussion:  Heights will rise this
weekend as a ridge builds over the region. However, the models
disagree on how strong the ridge will be. The GFS and Canadian
models are more amplified compared to the ECMWF, for example. For
now, a blend was used to raise temperatures a few degrees warmer
than normal - highs in the lower to mid 80s in the interior. The
models continue to disagree on the ridge timing and strength early
next week. But it looks like dry weather will continue. 33

&&

.AVIATION...Southwest flow aloft will continue tonight and
Wednesday as an offshore upper trough slowly approaches the coast.
At the surface, onshore flow will continue with high pressure over
the Coastal Waters and lower pressure inland. The air mass is dry
and stable except unstable over the Cascades through this evening
before stabilizing tonight. Low clouds are expected to fill in at
the coast tonight and spread partially inland Wednesday morning.

KSEA...Clear skies this evening. Low clouds at the coast will
approach from the southwest Wednesday morning. At this point they
are not expected to reach the terminal but it may be close.
West to northwest wind 4-8 knots will become southerly tonight.
Schneider/SMR

&&

.MARINE...Varying degrees of onshore flow will prevail over the
next several days with high pressure offshore and lower pressure
inland. The flow is expected to turn a bit more northerly late in
the week.

Small craft advisory west winds are likely each evening in the
Strait of Juan De Fuca. Inherited headlines are holding up well...so
no need for updates for evening forecast package. Schneider/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Wednesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 AM PDT Wednesday for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 3 PM PDT Wednesday for Central 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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