Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 280426
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
926 PM PDT SAT AUG 27 2016

.SYNOPSIS...Weak high pressure aloft will prevail over the region
overnight. The remnants of a warm front will bring a threat of
light rain to parts of the area on Sunday. Expect high pressure
aloft on Monday. A low pressure system will approach the region on
Tuesday.

&&

.SHORT TERM...
Weak high pressure aloft or zonal flow will prevail over the
region overnight. The ridge will amplify some during the day
Sunday. This combined with weak low level onshore flow will result
in temperatures being warmer than what they were today.

The remnants of a warm front associated with a vigorous upper
level low moving over the central British Columbia coast may
bring a threat of light rain to parts of the CWA Sunday afternoon.
Confidence in the model details was not high due to uncertainty in
the models. The latest GFS solution indicated a threat of light
rain over parts of the interior, which is different than the
earlier runs. The NAM continued to advertise some light rain over
the Olympic peninsula and far northwest interior, which is
essentially what the current forecast reflects.

Expect the upper level trough offshore to continue deepening
Sunday night through Monday, with the downstream ridge amplifying
in response. This will place the CWA under southwest flow aloft.
Most areas should be warmer on Monday due to generally less cloud
cover.

The models continued to slow down the system anticipated about the
middle of the upcoming week. It appeared that the occluded front
will become parallel to the flow; thus, it now looks like Tuesday
will be dry for much of the CWA.

.LONG TERM from the previous discussion...
The large upper level trough will remain just W of the WA and
B.C. coast Wednesday and Thursday while about 3 shortwave troughs,
rotating around the low, move cross W WA. On Thursday night the
large trough will move E, only to be replaced by another low on
Friday. The GFS, ECMWF, and Canadian models all agree on the large
scale trough affecting the area through next week, but predictably
differ on the strength and timing of the individual shortwaves
crossing W WA. Given the day-to-day uncertainty the forecast has
been broadbrushed with a chance of showers each day, although the
chance of showers will probably be refined as the timing of
individual waves becomes more predictable. Otherwise mostly cloudy
weather will prevail with temperatures near or slightly below
normal. Kam

&&

.AVIATION...An upper trough will move east of the region tonight
with light westerly flow aloft between weather systems on Sunday.
Stable conditions and moist in the lower levels with strong onshore
flow. Somewhat moist in the mid and upper levels.

Stratus along the coast this evening with ceilings in the 1500-2500
foot range. Over the interior weak convergence zone between KPAE-
KBFI with ceilings near 3500 feet. Outside of the convergence
zone over the interior mostly scattered clouds. Low level onshore
flow continuing into Sunday. Stratus along the coast will move
into the interior early Sunday morning with ceilings in the 1500-2500
foot range. Onshore flow will weakening during the day Sunday with
stratus lifting in the morning then scattering out by early
afternoon.

KSEA...3500 foot deck from convergence zone slowly moving south this
evening and could move over the terminal for a couple hours after
06z. Stratus reaching the terminal 09z-11z with ceilings near 2000
feet. Ceilings lifting Sunday morning and scattering out around
19z.Southerly winds 6 to 10 knots through Sunday morning. Felton

&&

.MARINE...Strong onshore flow will begin to ease after midnight and
ease more during the day on Sunday. Westerly gradients have peaked
in the Strait this evening. The gale warning will remain in effect
through later tonight for the central and eastern portions with
winds easing in the early morning hours on Sunday. Small craft winds
will also ease near the eastern exit to the Strait, including the
south part of the north inland waters and Admiralty Inlet.
Onshore gradients will continue to weaken during the day on Sunday
before picking up slightly on Monday. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning in effect until 2 AM Sunday for the central and
 eastern Strait Of Juan De Fuca.

 Small Craft Advisory in effect until 2 AM Sunday for the
 Admiralty Inlet and Northern Inland Waters.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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