Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 260401 AAA
AFDSEW

Area Forecast Discussion...UPDATED
National Weather Service Seattle WA
901 PM PDT Tue Jul 25 2017

Updated warnings section to account for gale warning
Central and East Strait of Juan de Fuca marine zones for this
evening.

.SYNOPSIS...Onshore flow will gradually increase tonight through
Thursday as an upper level ridge over Western Washington moves off
to the east. The increase in onshore flow will result in a day-
to-day increase in late night and morning clouds and cooler
high temperatures. A warming trend is possible later this weekend
as another upper level ridge moves into the area from the
southwest. Dry weather is expected to persist for the next week or
longer.

&&

.SHORT TERM...The short wave upper level ridge responsible for
this afternoon`s warm temperatures is pushing east of the Cascades
this evening. In response to the troughs eastward movement,
low level westerly flow has increased with westerly gradients
through the Strait of Juan de Fuca over 2.5 mb and westerly winds
in the strait now 20-30 knots. With the increase in onshore flow,
stratus has moved onto the coastal beaches this evening and
should progress east through the strait to about Whidbey Island
and possibly Everett by morning and through the Chehalis Gap to
near Seatac airport. The marine push is not very strong, so expect
any stratus to break and recede back toward the coastline by
midday. Lows tonight will be in the 50s to around 60, while highs
on Wednesday will be in the mid and upper 60s on the coast and in
the 70s to around 80 inland.

Onshore flow will be stronger Wednesday evening. This will result
in a more complete inland surge of marine air inland Wednesday
night and further cooling on Thursday.

The marine air will moderate later Thursday through Friday
resulting in temperatures rising back to a couple degrees above
normal and less marine stratus, especially in the interior.

We will continue to add onto our consecutive days without
measurable rain at SeaTac airport, and no end is in sight at this
time.

The current short term forecast is in good shape, and no updates
are needed. Albrecht

.LONG TERM...The pattern will change little in the longer term
with a 500 mb long wave trough centered around 145W and a ridge
centered from the Canadian Prairies southward into the Four
Corners region of the Desert Southwest. While the operational GFS
shows higher heights and warmer temperatures aloft than the
operational ECMWF, both sets of ensemble means show similar
solutions (somewhere in the middle) for the weekend through early
next week with 500 mb heights around 588 DAM over our area. Low
level flow will be onshore.

This pattern in late July and early August favors temperatures
above normal, highs in the mid and upper 80s in Seattle, and dry
weather. The current forecast favors the cooler operational ECMWF
solution that showed temperatures 4-8 degrees cooler than the
ensembles would suggest and 6-10 degrees cooler than the
operational GFS. Will wait for the complete suite of 00z solutions
to come in before making changes, but the current forecast may be
a few degrees too cool from Sunday onward. Albrecht

&&

.AVIATION...A broad upper level ridge will shift east tonight
while a weak trough forms offshore. The flow aloft over Western WA
is W/SW. The air mass is dry. Low level stratus clouds will push
farther inland tonight with IFR to MVFR cigs possible in the
interior Wednesday morning. Low clouds will burn back to the coast
by 18-20z with VFR conditions expected. 33

KSEA...Clear skies and N winds to 08 kt this evening. Winds
flipping to S 09-12z early Wed. Low level stratus clouds in the
vicinity with MVFR cigs possible 14-18z. 33

&&

.MARINE...High pressure over the NE Pacific will maintain onshore
flow across Western WA through Friday. Westerly gales are
possible through the Strait of Juan de Fuca late Wednesday
afternoon and evening with a stronger onshore push - a Gale Watch
is in effect. A weak front will pass through the Coastal Waters
Saturday night. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until midnight PDT tonight for Coastal
     Waters From James Island To Point Grenville Out 10 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out
     10 Nm.

     Small Craft Advisory until 6 AM PDT Wednesday 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 Watch from Wednesday afternoon through late Wednesday night
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PDT Wednesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

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

     Small Craft Advisory from midnight tonight to 3 PM PDT Wednesday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until midnight PDT tonight 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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