Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190311

Area Forecast Discussion
National Weather Service Seattle WA
811 PM PDT Fri May 18 2018

.UPDATE...No changes to the public forecast this evening.


.SYNOPSIS...A weak upper low will bring mostly cloudy skies and a
chance of showers to Western Washington through Saturday. An
offshore upper ridge will strengthen and move inland Sunday
through Tuesday for dry and warmer weather. An offshore upper low
will result in warmer weather Wednesday through Friday, with a
few showers and possible mountain thunderstorms.


.SHORT TERM...A weak cutoff upper low is drifting slowly westward
across south-central Washington this afternoon. Here in the
northwest quadrant of this feature, a few small areas of rain from
leftover convection continues to move westward across the Cascades
and into the Puget Sound lowlands. This has mostly brought trace
amounts of rain over the lowlands. Additional bands of light rain
or sprinkles will continue to move across the Cascades toward the
I-5 corridor through Saturday morning. The 18z NAM brings showers
all the way west to Puget Sound in the hours after sunrise

Later Saturday, the upper low will open into a weak east-west
oriented trough and start lifting northward. Proximity of an upper
trough will lead to afternoon and evening shower development. This
is most likely over the mountains, but given the weak trough
axis, a couple showers could even develop over the lowlands. By
Sat evening, flow aloft will become south-southwesterly, so
showers over the Cascades will become less inclined to drift out
across the Puget Sound lowlands.

On Sunday, an upper low will pinch off over the southwest corner
of Oregon, keeping the focus of shower and thunderstorm activity
to our south. Over Western Washington, the mid-level air mass
will dry out some, so will be back to a dry forecast over the
lowlands. However, low-level onshore flow will strengthen Sunday,
so more low marine clouds and cooler temperatures are forecast

A warming trend is forecast on Monday as an upper ridge pokes in
from the southwest and upper heights rise. Western Washington will
be in the dry zone between an active mid-latitude storm track
pointed at southeast Alaska and an upper low over central or
southern California. Haner

.LONG TERM...The de facto upper ridge will remain over Western
Washington on Tuesday, between the upper low over the southwest
and mid-latitude storm track over northern B.C.. So dry and
warming weather will continue Tuesday.

The upper low over the Southwest will open up into a weakening
trough on Wed and our upper ridge will move on toward the Canadian
Rockies. This will bring weak southeast flow aloft to Western
Washington. This will bring up moister and more unstable air from
the Great Basin and help PM convection to develop over the
mountains, with instability indices at this point conducive for
thunder over the Cascades. With southeast steering flow, this
could push some convection off the Cascades into the Puget Sound
lowlands late Wed.

An upper trough will develop well offshore on Thu, with southerly
flow aloft developing. This is favorable for a few PM showers,
mainly over the Cascades.

There are significant differences in the models by next Friday,
but the ensemble mean shows rising 500 mb heights across B.C. and
the Pacific Northwest, which would favor warm weather. Only about
3 of the GFS`s 20 ensemble members show any precip over SEA next
Friday, so went with a dry forecast next Friday. Haner


.AVIATION...Fairly deep moisture over Western Washington, slight
instability, and light southeast flow aloft continue this evening,
as a weak upper low is centered over southwest Washington. The
upper flow will become more southerly late tonight and Saturday,
as the upper low weakens and drifts slowly westward. At the
surface, moist onshore flow will continue. The air mass is
slightly unstable, mainly over the Cascades.

Ceilings remain in the high-end MVFR range over much of the area
this evening. Conditions should deteriorate overnight as marine
layer clouds form and lower again, with widespread low-end MVFR
and local IFR ceilings late tonight and Saturday morning.

KSEA...High-end MVFR ceiling is likely to persist this evening,
then lower overnight to the 011-015 range, and a drop into IFR is
possible late tonight and Saturday morning. Southwest wind 3-6 kt
overnight and into Saturday. McDonnal


.MARINE...High pressure offshore and lower pressure east of the
Cascades will result in onshore flow the next several days. Small
craft advisory westerlies are likely in the central and eastern
Strait of Juan de Fuca during the diurnal peak late afternoon
through early morning hours each day.

Small craft advisory northwest winds are possible over portions of
the coastal waters starting late Sunday and continuing into early
next week as the forecast has speeds just on the low end of
borderline. SMR/McDonnal


PZ...Small Craft Advisory until 11 PM PDT Saturday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.


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