Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251023
AFDSEW

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

.SYNOPSIS...Exiting high pressure will keep conditions mostly dry
through late 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...Current radar imagery is clear this early
morning...but zooming out to satellite imagery...starting to see
clouds associated with next weather system out over the far coastal
waters signaling more of what most of the Pacific NW has grown far
too accustomed to.

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...running 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

.LONG TERM...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.  SMR

&&

.AVIATION...An upper ridge will move onshore today and then
weaken tonight with westerly flow aloft over Washington. At the
surface...a weakening Pacific frontal system offshore will approach
the coast this afternoon and move inland tonight. Air mass stable
ahead of the front. Areas of lower level moisture this morning then
increasing mid/high level moisture through the day ahead of the
front.

Precipitation with the front is expected to begin along the coast
during the afternoon and in the interior late today or this evening.

KSEA...The advertised lower level MVFR ceilings developed a few
hours early and should linger through much of morning. They should
break out and give way to increasing mid/higher level clouds through
the rest of the day. Rain to begin just after 00z. Southerly winds
of 6-10 kt to prevail through the day and rise to 10-15 kt tonight.
Buehner

&&

.MARINE...A ridge of higher pressure over Western Washington this
morning. The next Pacific frontal system about 450 nm off the coast
early this morning is expected to weaken as it approaches the coast
today. Coastal water winds should pick up to small craft advisory
levels today ahead of this system as well as in the eastern Strait
and northern inland waters this evening.

Post-frontal onshore flow should be quite healthy with a good
potential of gale force winds in the Strait late Wed as higher
pressure builds behind the frontal system. A Gale Watch now in place
for the central and eastern Strait. The strong onshore flow should
ease by Fri as higher surface pressure builds onshore. Buehner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory coastal waters, eastern Strait of Juan de
     Fuca and northern inland waters.

     Gale Watch for the central and eastern Strait of Juan de Fuca
     later Wed afternoon and evening.

&&

$$

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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