Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 290359
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT Wed Jun 28 2017

.SYNOPSIS...A surface ridge offshore will maintain onshore flow
into early next week. This will produce typical early summer weather
with mild temperatures, morning low clouds, and afternoon sunshine.
An upper level ridge will weaken the onshore flow and produce a
minor warming trend Thursday and Friday. Stronger onshore flow
Friday night will bring more cloud cover on Saturday. Clouds from a
weak front may cross the area Sunday and Sunday night.

&&

.SHORT TERM...Low level onshore flow kept pumping moisture inland
this afternoon, so clouds over the lowlands from Puget Sound
southward were more tenacious, never really clearing east of Puget
Sound. There will be partial clearing in some areas this evening,
but tonight`s round of weak onshore flow should be enough to allow
stratus to fill back in overnight.

The axis of the offshore upper level ridge slowly approaching the
region has reached 130W this evening and models agree that the axis
will move over the WA coast Thursday afternoon, then slide over E WA
by Friday afternoon. Subsidence from the approaching ridge will help
erode the marine layer as well as warm the air mass. So the GFS and
NAM along with MOS guidance all look good in clearing out the marine
layer clouds by Thursday afternoon. Afternoon highs should warm from
the upper 60s today to the 70s on Thursday.

Onshore flow will be weak Thursday night, not enough to add
significant moisture to the marine layer. So not much stratus is
expected Friday morning. What does form will probably be restricted
to the coast. More sunshine and warming from the ridge will push
afternoon highs into the upper 70s and lower 80s over the interior
lowlands with highs near 70 along the coast.

With the upper level ridge axis moving over the ID panhandle Friday
evening and an upper level trough approaching the coast, a marine
push is expected to develop. The GFS shows low level onshore flow
becoming at least moderate with the UIL-BLI pressure gradient
reaching 4 MB Friday evening and the SEA-HQM gradient up to 3.8 MB.
This should bring another round of stratus to the interior Saturday
morning but probably shallow enough to burn off in the afternoon.
Highs should drop back into the 70s. Kam

.LONG TERM...The long range models have been consistent in bringing
the remnants of a weak front across the NE Pacific and to the WA
coast Saturday night and Sunday. Previous model runs had been
showing spotty precip over the area but the latest solutions have
shown even less precip. What is still consistent is a lot of low
level moisture below 800 MB spreading inland. The latest GFS timing
is a little slower, bringing the moisture to the coast on Sunday
then over the interior later Sunday afternoon and Sunday evening.
Increasing onshore flow Sunday afternoon will help moisten the lower
air mass and possibly cause a weak Puget Sound convergence zone to
develop.

The GFS suggests that a drier air mass behind the front will bring
clearing to the coast and strait by Monday afternoon, but that low
level onshore flow and convergence will keep the interior
mostly cloudy.

For Independence Day it looks like onshore flow will be continuing
along with moderate W or SW flow aloft. The GFS shows a flat ridge
moving over the coast Tuesday afternoon while the ECMWF has an upper
level trough over E WA departing with the upper level ridge farther
offshore. Best guess is more of the same, morning clouds, afternoon
sunshine and mild temperatures. Kam

&&

.AVIATION...High pressure is just offshore with low pressure
inland. Marine layer clouds are deeper and have persisted in some
areas this evening--the coast is cloudy, and a good portion of the
west slopes of the Cascades is still cloudy. Skies will cloud up
quicker overnight and most of Western Washington should be cloudy
Thursday morning--but a sunny afternoon is still a pretty good bet,
except of course on the coast.

KSEA...At 03z Sea-Tac is right on the edge of a clear area over
Puget Sound that looks like it is slowly expanding in most
directions. Still, the slower than expected burn off today does
probably mean that clouds will redevelop faster tonight, so the 03z
TAF was adjusted.

&&

.MARINE...High pressure offshore with low pressure east of the
Cascades will maintain onshore flow of varying strength through the
week. The evening westerlies in the Strait are pretty weak with only
15kt at Race Rocks, so the small craft advisory will be cancelled.
The westerly push in the Strait might be on the weak side again
Thursday evening--but after a warmer day on Friday there should be a
stronger push in the Strait Friday evening.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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