Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211056
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
356 AM PDT Sun May 21 2017

.SYNOPSIS...An upper level ridge will remain over the region
through Monday, then weaken Tuesday as an upper trough approaches
from the northwest. The trough will move over the area Tuesday
night and remain over the area into at least Friday but weaken
towards the end of the week.

&&

.SHORT TERM...Satellite imagery shows cloudy skies along the coast
with low stratus down the Strait of Juan de Fuca into Whidbey
Island and Admiralty Inlet. To the south, the stratus has surged
inland as far east as just west of Shelton at 10z/3am.
Temperatures were in the mid 40s to mid 50s.

Looking at the new GOES-16 satellite imagery as the stratus moved
through the Strait into Whidbey Island the stratus deflected
around Whidbey. Pretty good indication that the layer is shallow.
In addition, the onshore gradients have dissipated with the KUIL-
KBLI gradient near 0 at 10z so there will be no reinforcing of
the stratus this morning by any onshore flow. The shallow layer
will burn off quickly leaving sunny skies for the entire area by
mid to late morning. Temperatures aloft warming with the 500 mb
heights rising into the lower 580 dms today and 850 mb
temperatures rising to around plus 14c. The combination of the two
will result in 5 to 10 degrees of warming today. This will put
highs in the 70s and lower 80s with only locations near the water
on the Strait staying in the upper 60s.

Upper level ridge continuing to build over Western Washington
tonight and Monday with the 500 mb heights in the mid 580 dms by
00z Tuesday and the 850 mb temperatures around plus 16c. Surface
gradients never really turn offshore with the thermally induced
thermal surface trough remaining south of the area. Without the
offshore flow temperatures will only warm up to 5 degrees or so
versus todays highs with 80s common from Seattle southward and 70s
to the north and along the coast. Lows tonight will mostly be in
the 50s.

The trend of Monday being the warmest day remains intact in the
00z model runs with the ridge weakening faster and the trough and
increasing onshore flow arriving sooner on Tuesday. With this
development have dropped the high temperatures a few more degrees
with mid 70s being the warmest max temps on Tuesday. Locations on
the Coast will have a hard time getting out of the 50s.

.LONG TERM...Extended models in good agreement this morning with
the upper level trough moving over the area Tuesday night and
remaining over the area into Friday. The trough will slowly weaken
with time with weak shortwaves dropping down over the area in the
north northwesterly flow aloft. Onshore flow at the surface. The
strongest onshore flow looks to be Tuesday night and Wednesday
which will keep highs on Wednesday a little below normal, in the
upper 50s to mid 60s. As the onshore flow decreases as the week
wears on high temperatures will creep back up to normal, in the
60s. There is a chance of showers each day. The 00z runs of both
the ECMWF and GFS show an upper level ridge building offshore on
Saturday. A little shift in the ridge eastward would give the area
a dry Memorial day weekend. Felton


&&

.AVIATION...The upper level ridge over the offshore waters will
slowly build eastward and onshore through Monday. High pressure at
low levels over the offshore waters will maintain onshore flow
through Monday with light northerlies through the interior south of
Admiralty Inlet and southwesterlies north of the inlet. The air mass
will remain generally dry and stable, with marine stratus and fog
affecting the Washington coast, Chehalis Gap, and Strait of Juan de
Fuca this morning. IFR and LIFR conditions have been observed along
the coast and strait. Expect stratus and fog to clear relatively
quickly this morning. Clear skies and VFR conditions will prevail
through the remainder of the interior.

KSEA...VFR conditions North to northwest wind 8 to 12 knots.

&&

.MARINE...High pressure over the offshore waters will drive
northwesterly onshore flow through Monday night. The strongest flow
is expected during the late afternoon and evening hours over the
southern two-thirds of the coastal waters. Have issued a small craft
advisory for these areas this evening to reflect this.

A strong upper level disturbance is expected to drop southeastward
from the Gulf of Alaska into Eastern Washington Tuesday. Mesoscale
models continue to highlight solid gale conditions across portions
of the Strait of Juan de Fuca starting Tuesday, with Small Craft
Advisory conditions over the remaining waters. Onshore flow will
continue Wednesday onward, but will relax somewhat as temperatures
in the interior cool.

&&

.CLIMATE...The average date for the first 80 degree plus day of
the year in Seattle is today ( May 21st ). Through May 20th
in Seattle 102 out of the first 140 days ( 73 percent ) of the
year have had high temperatures at or below normal. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to midnight PDT
     tonight for Coastal Waters From James Island To Point
     Grenville 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville Out 10 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm.

&&

$$

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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