Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171554

Area Forecast Discussion
National Weather Service Seattle WA
855 AM PDT Sat Apr 17 2021

.SYNOPSIS...Warm and dry conditions will continue through the
weekend as high pressure remains in place, with afternoon highs
expected to be the warmest in the forecast period. Onshore flow
return Sunday and persist allowing for cooler highs and increasing
cloud cover but continued dry conditions. A few showers may
materialize over the high terrain on Tuesday. There is the potential
for a pattern change late in the week, potentially bringing rain and
cooler temperatures back to the Pacific Northwest.


.SHORT TERM /TODAY THROUGH MONDAY/...Clear skies again over much of
W WA this morning as many locations already find themselves in the
lower to mid 50s at the time of this writing, setting a good pace
for high temperatures to get to the upper 70s along the coast, right
around 80 in the Seattle Metro Area, lower 80s in the SW interior
/Olympia, Chehalis/ and mid to upper 70s for northernmost portions
of the CWA. Helping contribute to this is a thermal pressure trough
out over the nearshore coastal waters, allowing for light offshore
flow. Current warm spell does highlight some potential dangers to be
kept in mind. While no heat criteria are expected to be met, those
taking advantage of the warmer weather are encouraged to stay
hydrated. In addition, those seeking refuge from the heat in area
waters should remember that water temperatures are still quite cold,
in the 40s, and thus can lead to the onset of hypothermia rather
quickly. Lastly, as W WA has been fairly dry lately, finer fuels are
dry, allowing for the prospect of roadside or brush fires. Those out
and about this weekend may want to check car undercarriage to make
sure nothing is dragging so as to limit any potential sparks.

Onshore flow returns Sunday, kicking off a gradual cooling trend for
the remainder of the short term. The difference in temps will only
be a degree or two Sunday, but will be far more noticeable Monday as
temps area-wide fall into the 60s to lower 70s. However, as upper
level high pressure remains in place, generally clear skies /aside
from some AM clouds, particularly along the coast/ and benign
weather are expected.

Inherited forecast is on track. No need for any morning updates.  18

.LONG TERM /TUESDAY THROUGH FRIDAY/...From Previous Discussion...The
beginning of the long term forecast period will feature the local
area in a weak pattern aloft, essentially between high pressure just
offshore over the Pacific and troughing east of the Rockies. Onshore
flow will continue in the lower levels as high pressure resides over
the Pacific. Provided this setup, there does exist some uncertainty
in regards to the evolution of the upper level pattern throughout
the course of the day on Tuesday. Some model guidance (GEFS, CMC
ensemble) is suggestive of better shortwave influence, stemming from
the weakening trough mentioned at the end of short term, while other
guidance (ECMWF ensemble) prefers more of a high pressure influence.
Based on the WPC cluster analysis mean that takes into account
individual ensemble member solutions, prefer more of a high pressure
influence for now, but will not discount the potential for a few
terrain-influenced showers during the afternoon and evening.
Overall, most locations will remain dry.
High temperatures will continue a downward trend, reaching only
into the upper 60s for many (upper 50s-near 60 along the Coast).

Somewhat of an upper level ridge recovery will take place on
Wednesday for a dry day, with temperatures a degree or two cooler
than Tuesday given stronger marine push/better lower level cloud
cover. Should still see clearing in the afternoon.

By Thursday, noticeable deepening of a large scale trough is noted
over Northern Canada in generally all model guidance. There is
some indication of some stronger shortwave activity potentially
approaching or clipping portions of the region on Thursday. Rain
chances may increase over northern counties if this general trend
persists in the coming days. There is also some indication of more
significant troughing developing over the NE Pacific, which may
also begin to influence the region towards the end of this
extended forecast period. Confidence in this pattern evolution
starts to increase some towards Friday and then into next weekend.
This signals a return to wetter and cooler conditions. Current
high temperatures for Friday are back into the upper 50s/near 60.



.AVIATION...An upper ridge will remain over the area today and
tonight with northerly flow aloft and low level offshore flow. The
air mass will be stable and dry.

KSEA...Clear skies. Northerly wind 4-8 knots. Schneider


.MARINE...Offshore flow today. Offshore flow will continue into
Sunday for the interior but a weak flow reversal will occur along
the coast. Weak offshore flow will continue on Monday then onshore
flow will gradually increase Tuesday and Wednesday. Schneider


.HYDROLOGY...No river flooding is expected over the next 7 days.



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