Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251631
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
931 AM PDT Tue Apr 25 2017

.SYNOPSIS...High pressure aloft will keep conditions mostly dry
this morning. Rain will return by this afternoon and continue into
Wednesday morning as a couple of weather systems move onshore.
Showers will linger later Wednesday then decrease Thursday and
Friday. High pressure will build Saturday followed by a weak front
Sunday.

&&

.SHORT TERM...
There were a few light showers over the Cascades this morning
occurring well ahead of a frontal system presently offshore. Rain
associated with the frontal system was starting to move onto the
coast this morning.

Models hold together well for the incoming front and its lingering
effects. Said front definitely takes its time...as it looks only to
reach the coast by around noon today and will not reach the interior
until mid to late afternoon...depending on location. Given the
orientation of the front...oriented NW to SE...southern portions of
the CWA more likely to see rain move in during the early afternoon.
Seattle itself will be a bit interesting...as precip there may hold
off till closer to 00Z. Once it is here though...rain will linger
through the night tonight and continuing into the day Wed as the
front stalls before finally breaking up into more showery precip Wed
night and into Thu. Previous model runs had suggested the
possibility of the need for some winter weather headlines for the
Cascades for Wed...as snowfall there could potentially reach low end
advisory amounts. Models have backed away from this...avoiding
putting a cherry on the very depressing sundae that has been this
combined winter and spring period. Model agreement starts to fall
apart on Friday. Both models show very similar building of a ridge
to the west of the area...but GFS suggests drying conditions while
the ECMWF provides a wetter solution. Have opted to side more with
the GFS purely out of sheer rain fatigue.

High temperatures will continue to run cool...with most locations
not escaping the mid 50s for the near term period.  SMR/05

.LONG TERM FROM THE PREV DISCUSSION...Model solutions fall back
into alignment Saturday with ridging overhead and dry conditions
with...wait for it...afternoon highs in the lower 60s. Cautiously
optimistic for that...but on the other hand...models have been very
inconsistent with the long term over past several
runs...particularly when it comes to placement of ridges and
duration of dry breaks so much of this section may end up falling
into the realm of so many grains of salt. Sunday sees rain again
over W WA...but placement of parent Low is wildly different between
the two models...as is the coverage of precip after Sunday. GFS is
generally more showery except for late Monday afternoon which sees
more organized activity along a front that quickly moves through.
The ECMWF shows two distinct waves...one Sunday morning and the
other late Monday afternoon...but is very quick in building a ridge
in for Monday night and actually offers a drier solution.

The short version of all this is that while models seem okay when
taken on their own...the fact that this current run looks very
little like runs from 24 hours ago and those looked very little like
runs 24 hours previous to that and it becomes clear that any degree
of confidence in the long term forecasts is not warranted at this
time.

&&

.AVIATION...There are a few showers and areas of clouds with cigs
from 025-060 over Western Washington this morning. A front just
offshore will reach the coast this afternoon and move inland
tonight. Rain will develop on the coast this afternoon and spread
inland this evening.

KSEA...current TAF seems to be in the ballpark, there will be a
southerly breeze today and tonight that should not shift til
Wednesday afternoon when a strong PSCZ pattern will develop.

&&

.MARINE...Southerlies will pick up today, especially over the
coastal water. Decent south winds will spread inland with the front
this evening. Fropa reaches the coastal beaches this evening and
westerlies move down the Strait of Juan de Fuca overnight--and the
westerlies in the Strait should reach gale force Wednesday
afternoon. There will be a strong PSCZ wind pattern in place
Wednesday afternoon and night--for that matter, those winds
gradually ease but the pattern stays the same through Thursday, with
westerlies coast and Strait and a PSCZ over the Puget Sound area.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 1 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.

     Small Craft Advisory until 3 AM PDT Wednesday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     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.

&&

$$

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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