Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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864
FXUS66 KSEW 201012
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
312 AM PDT Sat May 20 2017

.SYNOPSIS...A weak upper trough will move through the area early
this morning. An upper level ridge will build offshore later today
into tonight then move over the area on Sunday. The ridge will
remain in place on Monday then weaken on Tuesday. The warmest days
of the year are forecast for Monday and Tuesday. A strong marine
push is likely Tuesday night and Wednesday with much cooler
weather for the second half of next week.

&&

.SHORT TERM...Satellite imagery shows upper level trough over
Western Washington early this morning. Plenty of cloud cover over
Western Washington but the trough is weak with breaks in the
cloud cover already occurring over Seattle ( roof observation ).
Skies in the northerly flow behind the trough over southern
British Columbia have cleared out quickly in the last few hours.
Temperatures with the cloud cover were in the upper 40s and 50s at
3am/10z.

Trough will move through the area early this morning. Enough mid
level moisture around to not rule out isolated light showers but
that will be about it for weather with the trough. Dry northwesterly
flow aloft behind the trough will increase later this morning
into the afternoon hours clearing skies out over most of the area
with just some upslope cu over the Cascades expected this
afternoon. Temperatures aloft even with the trough passage rebound
some this afternoon with 850 mb temperatures near plus 8c and 500
mb heights back into the lower to mid 570 dms. With plenty of
sunshine expect highs to be similar to what they were on Friday
with 60s and lower 70s common.

Upper level ridge building tonight and Sunday with the ridge axis
near the coast by 00z Monday. Temperatures aloft warming with 500
mb heights into the lower 580 dms by 00z Monday and 850 mb
temperatures in the plus 12-14c range. Surface gradients remain
northwesterly onshore which will put a cap on the high
temperatures for Sunday with the warmer locations getting into the
lower 80s and 70s for a majority of the rest of the area. Lows
tonight will be in the mid 40s to mid 50s.

Upper level ridge continues to strengthen Sunday night and Monday
with the ridge axis spilling over into Western Washington. 500 mb
heights continue to increasing peaking out 00z Tuesday in the mid
580 dms. Model 850 mb temperatures continue to warm to around plus
16c. Thermally induced trough makes its way up the coast during
the day on Monday and model 850 mb winds turn easterly. All of
these variables add up to the warmest day of the year so far with
80s common from Seattle southward and mid 70s to lower 80s north
of Seattle and along the coast. Lows Monday morning will mostly be
in the 50s.

.LONG TERM...Confidence in the forecast for Tuesday waning this
morning. Extended models have done a turn around and instead of
Tuesday being the warmest day in this series the models are now
showing a faster transition to onshore flow with an upper level
trough approaching from the northwest. This solution is much
colder for the coast, 10-20 degrees, than the previous run and
cooler for the interior as well, 5-10 degrees. GFS ensembles show
the operational run on the cool side of the ensemble mean so
will not completely go with the cooler solutions for Tuesday.
Will trim the interior temperatures just a few degrees, back down
to near Mondays readings in the mid 70s to mid 80s, and cool the
coast off considerably, into the upper 60s, with most of the
solutions indicating onshore flow at least in the afternoon hours.
Cooler Wednesday with an upper level trough moving into the area
and low level onshore flow with highs returning to near
normal...in the 60s. Cool northerly flow aloft and low level
onshore flow continuing Thursday and Friday keeping high
temperatures near normal. Felton

&&

.AVIATION...The weak upper trough brushing western Washington
this morning producing limited, light showers mainly over the North
Cascades. VFR ceilings will dip to MVFR levels around daybreak
before scattering out by late morning. Moderate west-northwest flow
aloft will become northerly today as a strong upper ridge begins to
build over Vancouver Island. The moist and generally stable air mass
will dry by afternoon as the ridge begins to build into the region.

KSEA...VFR conditions with north winds 4-6 kt. Expect ceilings to
lower to MVFR levels around daybreak before burning off by late
morning.

&&

.MARINE...High pressure rebuilding over the offshore waters will
drive onshore flow through tonight. Strongest westerlies will occur
in the central and eastern Strait again this evening. Onshore
gradients will weaken somewhat on Sunday, with a stronger northerly
component to the flow developing. Low level flow will become
offshore Monday and Tuesday mornings, returning to onshore s
expected Tuesday afternoon.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

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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