Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 250412
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
912 PM PDT Wed May 24 2017

.SYNOPSIS...Temperatures will slowly rebound on Thursday, with
enough moisture and instability for a few afternoon showers and
thunderstorms over the Cascades. A warm and dry upper ridge axis off
the coast on Friday will pass directly over Western Washington on
Saturday. Its warm and dry influence will continue from Sunday
through Tuesday, including Memorial Day.

&&

.SHORT TERM...Northerly flow aloft will prevail until late
Friday. A weak upper level disturbance now over central portions
of British Columbia will move southward into central portions of
Western Washington on Thursday. This weak disturbance will contain
enough lift and instability to allow showers and thunderstorms to
form over the Cascades midday Thursday. The thunderstorm threat
will come to an end during the evening hours as the lower levels
stabilize with the loss of insolation and the weak disturbance
pulls away to the southeast.

The strong upper level ridge now just outside of 140W will shift
eastward into the offshore waters on Friday. The surface ridge
well offshore combined with lower pressure east of the Cascades
will result in onshore flow, although it will be weaker. High
temperatures will reach well into the 70s over the northern half
of the forecast area on Friday, while areas of the interior south
of Seattle reach into the lower 80s.

The upper ridge will finally move over the forecast area on
Saturday, and low level flow will become light. With strong late
May insolation, temperatures will rise another 3-5 degrees on
Saturday with many areas in the interior and away from the water
will rise into the lower to mid 80s. A beautiful start to the
Memorial Day weekend.

A minor forecast update was issued for tonight to remove the
shower threat across the area and for Thursday morning to reduce
the extent of the shower chances to mainly the northern portions
of the Cascades. Otherwise, forecasts are in great shape this
evening and no updates are required. Albrecht

.LONG TERM...From the previous long term discussion: The upper
ridge axis will move to Eastern Washington on Sunday and then stay
in place there until Tuesday. The turn to a mid-upper level
southerly flow component will start to support a couple of
afternoon showers and thunderstorms near the Cascade Crest each
afternoon and evening from Sunday through at least Tuesday.
Seabreezes along the immediate coast will hold afternoon highs in
the 60s there, but inland temperatures will be well above normal
each day.

By next Wednesday, it appears the upper ridge axis to our east
will flatten, with a more zonal flow pattern trying to develop.
Cooling onshore flow will spread further inland, but afternoon
temperatures will still remain above normal. The operational GFS
was cooler than most of its ensembles next Wed.  Haner

&&

.AVIATION...Wrn WA will remain between a ridge of high pres offshore
and an upper level trof over the interior W overnight. Contd nly
flow aloft. The low level flow will be weak onshore. Anticipate
areas of MVFR CIGs to dvlp overnight over the coast, possibly
spreading as far W and N as KPWT and KTCM between 0900 and 1200 UTC.

KSEA...VFR. Winds will generally be light and variable overnight,
becmg NW near 10 knots Thursday afternoon.

&&

.MARINE...
High pressure centered over the northeast Pacific along with lower
pressure east of the Cascades will result in light onshore or
westerly flow through Friday. The offshore high will weaken and
redevelop farther west on Saturday, causing the flow to become light
northwest.

&&

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

&&

$$

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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