Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 241609
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
909 AM PDT SAT SEP 24 2016

.SYNOPSIS...A weak warm front will bring mainly mid and high level
cloudiness to the northern two-thirds of the area through
tonight. A strong ridge of high pressure aloft will build into the
area late tonight through Sunday giving drying and warming
conditions through Monday. Temperatures will rise to above normal
for late September Sunday and Monday. Onshore flow with a marine
push will cool things down on Tuesday. An upper level trough
moving out of the Gulf of Alaska in the middle of the week will
bring a threat of some light rain to the area late in the week.

&&

.SHORT TERM...Morning satellite pictures and observations show a
mixed bag across Western Washington this morning. Low clouds with
some patchy drizzle have been reported across the central Puget
Sound area this morning while clouds and patchy fog are being
reported elsewhere. A warm front extends from a 1008 mb low
centered near 49N 139W east-southeast to the northern Oregon
coastal waters. Visible and infrared satellite images show
extensive mid and high clouds northeast of the warm front
spreading into the northern 2/3 of the area. Short term models
show warm advection clouds continuing to spread into the area
through tonight while light precipitation remains mainly over the
offshore waters northeast into Vancouver Island. While a sprinkle
or two cannot be ruled out over the northern Cascades and far
northwestern Olympic peninsula, the GFS in particular appears to
spread precipitation a bit too far to the southeast as 500 mb
heights rise strongly later today through tonight.

Sunday will see clearing, drying and warming as 500 mb heights
build to near 590 dam and a thermally induced trough of low
pressure at the surface develops northward from the interior of
western Oregon into the southwestern interior of Washington. Under
clearing skies and warming aloft, expect temperatures to rise into
the 70s across much of the area on Sunday.

It still appears that Monday will be the warmest day across the
interior as the upper ridge shifts east and a frontal system
approaches the offshore waters. 850 mb temperatures peak on Monday
around 16C. High temperatures will reach well into the 70s on
Monday and the 78 degree high anticipated at SeaTac airport will
likely tie a record. A rather strong marine push ahead of the
incoming front Monday evening will usher cooler marine air inland
and provide an increase in clouds. The incoming front will be
weakening as it moves inland, so rain chances will be minimal
Monday night and Tuesday.

A short term update to the zones and grids was issued this morning
to add patchy light drizzle to the central Puget Sound area this
morning and to increase mid and high clouds across the northern
2/3 of the forecast area through early Sunday morning. Otherwise,
the forecasts appear to be in good shape. Albrecht

.LONG TERM...From the previous long term discussion: Upper level
ridge well to the east on Tuesday. Marine push Monday night will
bring low clouds back to the interior Tuesday morning. With the
low level onshore flow highs Tuesday will be 10-15 degrees cooler
than on Monday for the interior and a few degrees cooler on the
Coast...mostly in the 60s.

Upper level trough digging well offshore on Wednesday with the
flow aloft becoming southwesterly. Low level onshore flow weakens
resulting in less morning cloud cover on Wednesday. Temperatures
aloft cooling with 500 mb heights near 570 dms by 00z Thursday.
Wednesday looks to be a pretty normal late September day with
highs in the 60s.

Some changes to the forecast on Thursday with all of the models digging
the upper level trough further south keeping Western Washington
dry. Onshore surface gradients increasing keeping highs in the
60s.

Model solutions differ on Friday with the Canadian model bringing
the trough inland...the GFS moving the trough to just offshore and
the ECMWf digging the trough further south. Will stay with the
chance pops for now on Friday. If the model trends continue look
for the precipitation to get taken out of the forecast on Friday.
When the trough finally moves inland it has the potential to be
the coolest air mass to move into the area for awhile with snow
levels possibly as low as 5000 feet. Felton

&&

.AVIATION...High pres aloft will prevail over the region today for
strong WNW flow aloft. The surface flow will be nearly flat.
Meanwhile, areas of MVFR CIGs and patchy LIFR CIGs/VSBYs will
dissipate or lift by 2000 UTC.

KSEA...Anticipate MVFR CIGs to improve to VFR by 1900 UTC. Winds
will generally be light and vrbl.

&&

.MARINE...
A 1028 mb high near the WA coast will keep the flow light and
variable through Sunday morning. Expect a warm front to brush the
outer WA coastal waters zones tonight as it moves across the
offshore waters. Light northerly flow will develop Sunday
afternoon before becoming onshore or westerly on Mon.

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...None.
&&

$$

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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