Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181600
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT SAT JUN 18 2016

.SYNOPSIS...An upper level low is bringing showers and isolated
thunderstorms to the region. The air mass will dry on Sunday with
warm sunny weather in store for Monday. A weak weather system will
move by to the north of the area on Tuesday. Another upper level low
will drop down toward the region later next week.

&&

.SHORT TERM...The upper level low centered just off the coast is
still giving the region showers. The satellite image this morning
shows a little break in the clouds along the coast at 8am, but also
more showers just offshore in the center of the upper low. The
showers and isolated thunderstorms over the coastal waters under the
coolest air aloft will spread inland into the afternoon as the low
comes ashore.  Shower activity will decrease as the low moves inland
tonight, with only some residual showers left in the mountains due
to stronger low level onshore flow and stronger westerly flow aloft.
On Sunday, after lowland morning clouds and perhaps a shower or two
in the mountains, the air will dry into Sunday afternoon and night
as the low level flow turns northerly. Skies will clear and Monday
will be sunny and warm as the low level flow turns more offshore.

.LONG TERM...Some changes to the extended forecast this morning.
Both the ecmwf and the gfs bring the remnants of the upper level
low through western Washington on Tuesday. This will turn the low
level flow back onshore resulting in at least a marine push with
cooler daytime highs. 500 mb heights near 570 dms and the 00z
model runs have the low tracking over the northern portion of the
area. Have also added a chance of showers to the northern portion
of the area on Tuesday. A little break on Wednesday as Western
Washington sits between an upper level ridge to the east and a
digging upper level trough offshore. Current timing brings the
trough through Thursday night into Friday for another round of
mostly cloudy skies and a chance of showers. Felton

&&

.AVIATION...
An upper low over the Washington coastal waters will shift inland
across northwestern Washington this evening. Southerly flow aloft
will become variable this evening. Light surface flow will become
west to southwest this afternoon and evening. The air mass will
be moist and unstable today, then become more stable tonight with
drying at the mid and upper levels.

Multiple cloud layers are seen across the area this morning with a
scattered to broken layer around 015 and layers at 035 and 070.
Pockets of ceilings 005-010 are seen over the central Puget Sound
region and around KOLM. The layers below 035 will erode around 21Z
this afternoon leaving mainly VFR conditions across the area.
Shower can be expected through 03Z and the air mass is unstable
enough that there will be a chance of TSTMS across the area as the
upper low moves inland. The TSTM chance will not be high enough to
include in the terminals.

Conditions at the mid and upper levels will dry tonight as the
upper low moves off the the east, but expect the lower levels to
remain moist enough in the interior to allow ceilings to fall to
BKN-OVC020 10Z-17Z Sunday. Albrecht

KSEA...Variable wind less than 6 kt will shift to south 5-8 knots
at 18z then increase to SW 10-15 kt with gusts to 22 kt after 20Z.
Winds will diminish to S 5-7 kt after 06Z. Expect cigs to increase
from 015 to 030-050 after 20Z, but showers will continue through
about 03Z. There is about a 25-30 percent chance of TSTMS coming
within 20 nm of the terminal through about 03Z this evening.
Albrecht

&&

.MARINE...
Flow will become increasingly west to southwest during the
afternoon hours as an upper level disturbance over the coastal
waters shifts inland through northwestern Washington later today.
Small craft advisories are in effect for the coastal waters except
for the inner coastal waters from Cape Shoalwater to Point
Grenville. A small craft advisory is also in effect for this
afternoon and evening for the central and eastern portions of the
Strait of Juan De Fuca and Puget Sound and Hood Canal.

The flow will turn northerly on Sunday and then briefly offshore
on Monday, as a thermally induced trough extends northward into
the Pacific Northwest. Onshore flow will return Monday night or
Tuesday and continue through Wednesday. Albrecht

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory from noon today to midnight PDT tonight for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-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.

     Small Craft Advisory from 3 PM this afternoon to midnight PDT
     tonight for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Puget Sound and
     Hood Canal.

&&

$$

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You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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