Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191617
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PDT Wed Apr 19 2017

.SYNOPSIS...A slow moving front offshore will move through the
area later this afternoon into the evening hours followed by an
upper level trough on Thursday. An upper level ridge will build
over western Washington on Friday. The break in the weather will
be brief with weak weather systems arriving Saturday and Sunday. A
stronger system will arrive around Monday.

&&

.SHORT TERM...Skies are overcast over Western Washington and there
is light rain over western sections. A front will move inland this
afternoon and rain will develop over all areas. Highs will be in the
50s to near 60. Rain will change to showers this evening. Some
breaks in the cloud cover early Thursday morning will allow lows to
drop back into the 40s.

An upper level trough offshore tonight will move inland Thursday.
Convective variables are similar to Tuesday`s numbers with lifted
indexes near 0, 500 mb temperatures near -30C and convective
temperatures in the lower 50s in the afternoon. This will keep a
slight chance of thunderstorms in the forecast for Thursday
afternoon in addition to the shower activity. Highs will be in the
50s.

An upper level ridge building offshore will shove the upper level
trough east of the area Thursday night. Shower activity will dry up
quickly in the evening with the possible exception of over Snohomish
county with a convergence zone. The upper level ridge will move over
Western Washington on Friday with 500 mb heights rising to around
570 dm. Temperatures aloft warm but are not very impressive with 850
mb temperatures by 00z Saturday around plus 5C. Model soundings do
show easterly winds right off the ground all the way up to 700 mb.
This offshore flow will help warm temperatures up into the 60s
Friday afternoon with the warmer locations near 70 degrees. Burke

.LONG TERM...Previous discussion...As has been the case since
October the dry spell will be short lived as the upper level ridge
shifts east Friday night with yet another negatively tilted front
moving into the area on Saturday. Upper level trough moving through
Western Washington Sunday with a much more consolidated front
reaching the area Monday. Broad upper level trough over the area on
Tuesday with Western Washington on the cold side of the jet
keeping a chance of showers in the forecast. Multiple runs in a
row of the extended model solutions have had this scenario for the
weekend into the first part of next week. Felton

&&

.AVIATION...An upper level trough of low pres offshore will keep the
flow aloft from the sw. A surface trof off the WA coast will move
across the area today. Expect areas of MVFR CIGs/VSBYs today. The
mtns will be mostly obscured.

KSEA...VFR conditions anticipated during the day today. CIGs should
lower into the MVFR category tonight. Winds will increase from the
south this afternoon.

&&

.MARINE from the previous discussion...A surface trough will bring
Small Craft Advisory wind speeds to the area today. Onshore flow
will prevail behind the front Wednesday night and Thursday as high
pressure builds in from the southwest. The high pressure will shift
inland Thursday night and Friday for a period of offshore flow.
Another front will arrive on Saturday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisories are in effect for all waters.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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