Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211510
AFDSEW

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

.SYNOPSIS...An upper level ridge will remain over the region
through Monday. An upper level trough will rapidly push into
the area later on Tuesday. Strong onshore flow with cooler
air will begin to arrive during the day on Tuesday. The trough
will weaken but linger over the region through the end of the week.

&&

.SHORT TERM...Marine layer low clouds and fog are on the coast,
in the Strait of Juan de Fuca, and made it into some areas across
the inland waters--mainly all around Whidbey Island. The low
clouds will burn back to the coastal waters today, and marine
layer clouds should be confined to just off the coast tonight.
It should be sunny everywhere Monday morning except perhaps for
a few hours of fog on the coast and maybe in the Strait of Juan
de Fuca. We will have two warm days, and then a strong marine
push will develop through the day on Tuesday as an upper trough
nears the area. There is not a great deal of shower activity
in the models, but there will be a chance of showers starting
Tuesday night. Maybe a pscz will form Wednesday morning, and
the freezing level will probably drop faster and lower than
the current forecast--it ought to be rather chilly in the
mountains Tuesday night and Wednesday morning with that strong
push of cooler air. The forecast will probably need lower
freezing levels by Wednesday morning.

.LONG TERM...A trough will linger into Friday but 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...An upper level ridge centered 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
Strait of Juan de Fuca. The air mass will remain generally dry
and stable with patchy morning low level moisture today.

Satellite imagery and weather cameras show widespread LIFR stratus
or fog over bodies of water, along the coast, and in the Chehalis
River Valley this morning. Tops of the marine clouds are low at
around 500 feet, so daytime heating will rapidly dissipate the low
clouds by about 18Z; the exception will be over the coastal
waters where the stratus will likely persist into the afternoon.
With the exception of the coastal waters, fog and stratus is not
expected tonight and Monday morning as the lower levels warm and
dry with the approaching ridge. The remainder of the area will
see good VFR conditions that will persist through Monday. Albrecht

KSEA...Fog over the Puget Sound will dissipate by 18Z leaving good
VFR conditions this afternoon through Monday. Light north winds
will rise to northwest 8 to 12 kt during the afternoon and early
evening hours, then turn northeast and ease to 6 to 9 kt later
this evening through monday morning. Winds will return to the NW
at 8 to 12 kt midday Monday. Albrecht


&&

.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. Westerly flow through
the Strait of Juan de Fuca will be lighter today than it was 24
hours ago. A Small Craft Advisory is in effect for the southern
two thirds of the coastal waters for this evening. Conditions
Monday and Monday night will be similar to today but with small
craft advisory conditions limited to the southern nearshore
coastal waters.

A strong upper level disturbance is expected to drop southeastward
from the Gulf of Alaska into Eastern Washington on 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. Albrecht/Cook

&&

.CLIMATE...The average date for the first day that reaches 80
degrees or warmer is today, May 21st. It has been cool this year
so far--through May 20th in Seattle 102 out 140 days, or 73 percent
of the year so far, has been below normal.

&&

.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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