Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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543
FXUS66 KSEW 310417
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
917 PM PDT MON MAY 30 2016

.SYNOPSIS...
Upper level ridging will move slowly east across western
Washington tonight and Tuesday bringing mainly clear skies and a
short warming trend. A marine push will begin Tuesday night and
strengthen on Wednesday as a weak weather system moves across the
area. The system will bring increasing clouds Wednesday and a
chance of showers Wednesday night through Thursday evening. A
stronger upper level ridge will build into the area next weekend
giving sunny skies and much above normal temperatures.

&&

.SHORT TERM...
An upper level ridge axis will pass across western Washington on
Tuesday morning, bringing 500 mb heights of 573-576 decameters.
For a time on Tuesday morning, both the UIL-BLI and SEA-EAT
gradients will be weakly offshore. Under mainly sunny skies, this
will support further warming on Tuesday. Afternoon seabreezes
along the coast will not penetrate far inland, and northerlies
will prevail through the Puget Sound lowlands.

Upper heights will fall quickly on Tuesday night and Wednesday
morning, and westerly flow aloft will strengthen. This will
initiate a cooling marine push into western Washington on
Wednesday morning, with marine air deepening and surging inland
during the afternoon. By Thursday morning, a deep marine layer
will extend upward to about 8000 feet. Strong onshore flow and
such a deep marine layer typically lead to orographic showers and
areas of drizzle over the lowlands, especially in the Puget Sound
convergence zone. Models show this very outcome. A few cloud
breaks on Thursday afternoon, but still more clouds than sun. Most
max temps struggling to hit 70F on Thu afternoon.Haner

.LONG TERM...
From Previous Discussion: An active weather pattern over the east
central Pacific with a storm track outside of 130-140W will
promote ridging over the western United States. Global models and
their ensemble members all agree that a hot and strong ridge over
the Desert SW on Thursday will build into the area from the
southeast over the weekend, with its amplitude and subsidence
aided by an upper level low near San Francisco on Sunday that will
tend to move northward with time. 500 mb heights on Saturday rise
to around 588 dam while 850 mb temperatures rise to 18-20C. With
the ridge building in from the southeast, thermal troughing will
tend to develop east of the Cascades leaving high heights and warm
air aloft to combine with light onshore flow at low levels. This
will promote highs in the mid to possibly upper 80s in the
interior of western Washington with dewpoint temperatures in the
mid and upper 50s. Low temperatures over the weekend will have a
hard time dropping much below 60. With the trend in the long term
model solutions, high and low temperatures were increased Friday
through Sunday.

The ridge moves off to the east a bit later Sunday through early
next week while the upper low to our south opens up and lifts
northward through our area. This will promote an increase to low
level onshore flow for a cooling push and a chance of showers or
thunderstorms -mainly over the Cascades. Albrecht

&&

.AVIATION...An upper ridge will be over the pacific northwest
tonight and Tuesday with westerly flow aloft. At the surface, a
thermal trough will build north along the west coast tonight,
then shift into eastern Washington Tuesday afternoon and evening.
The air mass is dry and stable with high clouds at times.

KSEA...High clouds at times. Northerly wind 5-10 knots. Schneider

&&

.MARINE...A thermal trough along the coast will bring small craft
advisory northwest winds to the inner coastal waters this evening.
The thermal trough will build north along the coast tonight,
shift over western Washington Tuesday, then into eastern
Washington Tuesday evening. Small craft advisory west winds are
likely for the central and eastern Strait of Juan De Fuca Tuesday
evening as onshore flow increases.

Small craft advisory west winds are likely each evening for the
remainder of the week for the Strait as onshore flow continues
although the onshore flow may ease a bit late in the week.
Schneider

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 11 PM PDT this evening for Coastal
     Waters From James Island To Point Grenville Out 10 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.weather.gov/seattle/gafd/latest_webafd.html



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