Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211004
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
304 AM PDT SAT MAY 21 2016

.SYNOPSIS...Showers over the Cascades early this morning will
spread west across most of western Washington today. The coast and
parts of the north interior, however, should have dry weather. A
large upper trough will remain over the Pacific Northwest Sunday
through next week for typical May weather with occasional
showers, some sunny periods, and highs in the 60s.

&&

.SHORT TERM...A large upper level low remains over the Pacific
Northwest, with its 5480 meter center over southeast Oregon early
this morning. A shortwave feature rotating around the north side
of the upper low is moving westward across the Washington
Cascades. Showers have mainly been over the Cascades during the
night, with partly cloudy to cloudy skies over the lowlands.
Temperatures at 3 am were in the 40s to lower 50s.

The upper low will move slowly northward across eastern Oregon
today. The shortwave feature on the north side of the low will
spread showers into western Washington from east to west. Heaviest
precipitation amounts today will be over the Cascades, with
most areas receiving 0.25 to 0.75 inch and a snow level around
6000 feet. Less than 0.25 inch will fall over most of the
lowlands, and the coast and most of the north interior should have
dry weather. Highs today will be around 60, which is a few degrees
below normal.

The upper low will continue north along the Washington/Idaho
border tonight and into southern Alberta on Sunday. Showers will
generally decrease tonight and Sunday, and again the heaviest
precipitation will be over the Cascades with less precipitation
and a better chance of dry weather at the coast and in the north
interior. Highs Sunday will again be near 60.

The upper low center will drift slowly eastward into Saskatchewan
Sunday night and Monday, but a weak shortwave will move slowly
south across Washington. That will keep a chance of showers in the
forecast. Precipitation that does occur should be fairly light,
but weak onshore flow and plenty of moisture below about 7000 ft
should mean another mainly cloudy day. McDonnal

.LONG TERM...The longer range models agree that the upper trough
that is currently over the area will begin to exit further east
on Tuesday, only to be replaced by another upper trough moving
over the area from the northwest. The low level flow should remain
onshore through the period. The forecast for Tuesday through
Friday will be necessarily short on specifics, but we should have
pretty typical late May weather will a chance of showers, some
sunny periods, and highs in the 60s. There will be more shower
activity over the Cascades and Olympics, and the snow level will
be around 6000 to 7000 ft. McDonnal

&&

.AVIATION...An upper level low over southeastern OR will keep upper
level flow east to northeast today and allowing the chance for
showers to spread from east to west throughout the day today. Air
mass will destabilize some during the afternoon today...but is
expected to be generally stable.

Cigs have already started to lower...although widespread VFR
conditions remain in place over the area. Most locations will see
cigs drop to MVFR by mid-morning and remain there before returning
to VFR this evening.

While the upper level low will weaken Saturday night and Sunday as
the center drifts up over the northern Rockies...western WA will
remain in a generally trough-y pattern Sunday and Monday.  SMR

KSEA...VFR cigs expected to continue through around 16z...as
approaching showers will push cigs down into MVFR range. Winds light
and variable this early morning...becoming SW 4-7 kts by 16z...with
speeds increasing to 8-12 kts this afternoon and continuing into
tonight.  SMR

&&

.MARINE...Westerly onshore flow weakens a bit today before picking
back up again...this time from a west to southwesterly direction
this afternoon and evening. With winds expected to diminish through
the morning...and not having much luck hitting SCA threshold during
the overnight hours...will likely take down SCA in effect through 5
AM for Central and Eastern Strait. Models agree that winds for these
two zones expected to pick up once more into SCA category by this
afternoon...so have opted to issue an SCA for this afternoon and
tonight. In fact...winds expected to reach higher end of SCA
criteria in this area. Speaking of SCA winds...models try to hint at
the prospect of SCA winds in Puget Sound this evening as well...but
seems isolated to near Seattle area. Given the isolated nature of
this...will opt to not issue anything there for now...but will
advise day shift to keep an eye open. Higher pressure offshore will
continue through early next week allowing varying degrees of onshore
flow to continue. Kam/SMR

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory from 1 PM this afternoon to 3 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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