Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 230315
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
815 PM PDT WED JUN 22 2016

.SYNOPSIS...A weak front will bring increasing showers to Western
Washington tonight. The associated upper level trough will approach
Thursday, moving inland Thursday night and Friday producing a
generally cool and wet period. An upper ridge will build over the
weekend and early next week giving drier and warmer conditions.


&&

.SHORT TERM...IR satellite imagery shows a weak low off the
south-central Washington coast this evening with a cold front
southwest through the Oregon offshore waters. It appears that
previous model solutions showed the low a bit to the north and
west of the current position and models appear weaker that what is
seen on the satellite imagery. The low will curl northeast moving
onshore around Cape Flattery during the early morning hours before
moving into Vancouver Island and weakening.

Rain has already moved onto the coast this evening and will push
into the interior around midnight. The previous forecast was
updated to increase pops on the Olympic Peninsula where it is
already raining, and to change the nature of the precipitation in
the forecast from convective to stratiform overnight. Rainfall
amounts will generally average about 0.5 inches on the coast and a
tenth to quarter of an inch inland, though the latest HRRR
solution is now showing higher amounts possible in the rain
forest areas of the western Olympic Peninsula.

Behind the front the precipitation will be become showery and it
will be a cool and showery day with good onshore flow. A 555 DAM
500-mb low that is currently near 50N/132W will move southeast and
the air mass over the interior of western portions of Washington
will destabilize as the upper levels cool and temperatures at the
surface rise into the lower to mid 60s. Diffluent southerly flow
combined with instability should be sufficient between 21Z and
06Z to trigger isolated thunderstorms from about Hoquiam to
Everett south and eastward. Mesoscale models also hint at the
formation of a Puget Sound convergence zone during the late
afternoon and evening hours.

The upper low will be east of the Cascades Friday but wrap around
moisture and cool onshore flow will continue. A lingering
convergence zone could still produce some local shower enhancement
in the favored north Seattle-Everett corridor, and especially over
the mountains and north. Highs again will top out in the mid 60s.

Upper level low pressure shifts east of the region and high pressure
builds aloft Friday night. Any residual shower activity along the
mountains will end by midnight. Residual clouds my linger into
Saturday morning, then partial clearing in the afternoon. A system
aloft tracking into B.C. Saturday afternoon will miss the area but
will keep heights suppressed near 5750m. Low level northwesterly
onshore flow will also hinder high temperatures near the water. But
with partial sunshine, highs should still warm several degrees into
the low to mid 70s, except 60s coast and strait. Albrecht/Mercer

.LONG TERM...From the previous long term discussion: Global
models generally agree on a pattern change toward ridging aloft
giving dry weather and warming temperatures. Systems will brush by
to the north of Washington next week but no rain is expected. The
main affect will be periods lower heights and a bit more onshore
flow. It is difficult to pin down exactly which days will see
changes, but highs should range from the 70s to the lower 80s.
Skies should be mostly clear Sunday through Thursday, though
marine stratus is possible along the coast at times which may seep
into the Strait or Southwest Interior during the mornings. Most
models hint at Tuesday or Wednesday as being the warmest days with
low 70s possible at the coast and low 80s inland around Puget
Sound. The average high this time of year is in the low 70s for
Sea-Tac. Mercer

&&

.AVIATION...A cold front will move onshore later tonight and
Thursday morning. An upper low offshore will move to just off the
coast Thursday with southwest flow aloft easing. The air mass
will become increasingly moist from the west as the front move
onshore tonight and Thursday morning. Stable air over the area
tonight will become unstable on Thursday.

KSEA...Ceilings will lower overnight as a front nears. Low clouds
will be prevalent Thursday. There is a small chance of thunder
Thursday afternoon but this is too low to mention in the TAF. West
or northwest wind 4-8 knots this evening will switch to south
later tonight and increase to 6-12 knots Thursday. Schneider

&&

.MARINE...A front will push onshore later tonight and Thursday
morning with low end small craft advisory winds for some waters.
A typical summer-time onshore flow pattern will prevail after
that into early next week with high pressure offshore and lower
pressure inland. Expect small craft advisory west winds for the
Central and Eastern Strait each evening in this pattern. Schneider

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory from 1 AM to 8 AM PDT Thursday 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-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until 3 AM PDT Thursday 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.weather.gov/seattle/gafd/latest_webafd.html


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