Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 231613
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
914 AM PDT Sun Jul 23 2017

.SYNOPSIS...An upper level trough over British Columbia will
produce mostly cloudy skies and cooler temperatures over Western
Washington today, although the clouds will begin dissipating during
the afternoon. An upper level ridge will bring mostly sunny and
warmer weather Monday and Tuesday. Another upper level trough over
British Columbia and onshore flow will allow more clouds and cooler
weather to return Wednesday and Thursday. Another ridge will build
over the area next weekend.

&&

.SHORT TERM...Visible satellite imagery from 1530Z/830 AM showed W
WA pretty much covered in low clouds this morning. An earlier PIREP
from around KBFI put the cloud tops around 5500 feet indicating a
deep marine layer. The clouds will begin gradually dissipating this
afternoon as drier air at low levels filters in behind the departing
B.C. trough. However, there should still be considerable cloud cover
over the area through mid afternoon.

An upper level shortwave ridge will move E over the area Monday and
Tuesday for a minor warming trend. The GFS shows low level flow
going weakly offshore for short periods of time. This shows up best
in the 925 MB wind field. This little bit of offshore flow should
help push max temps up to the lower to mid 80s over much of the
interior lowlands.

The GFS and ECMWF agree that the upper level shortwave ridge will
shift E over the ID panhandle Tuesday evening while an upper level
trough and a very weak front approach the coast. The shift in these
synoptic scale features should trigger a marine push Tuesday
evening. Kam

.LONG TERM...A marine push Tuesday evening should cool down the
lower levels and produce morning stratus through Wednesday morning.
At this point the GFS indicates the push should be shallow enough
for a late morning burn-off. Another round of onshore flow Wednesday
evening should produce a deeper marine layer and a later burn-off on
Thursday. The GFS and ECMWF both still show a weak upper level
shortwave trough moving NE across W WA on Thursday. The GFS keeps it
dry while the ECMWF shows some light precip. The models have the
large upper level ridge centered over the SW U.S. building northward
over the area Friday through Sunday. The 06Z GFS shows 500 MB height
around 5900 meters Saturday afternoon while the ECMWF has 5880 meter
heights. At least weak westerly onshore flow should hold back the
warming a little bit. Kam

&&

.AVIATION...Westerly flow aloft today and an upper trough is
moving through British Columbia. The flow will become northwest
tonight as an upper ridge builds offshore. Low level onshore flow
will continue with high pressure offshore and lower pressure inland.
Morning clouds will scatter out for the afternoon.

KSEA...Morning clouds, afternoon sun, and a northerly breeze.

&&

.MARINE...Varying degrees of onshore flow will prevail today
through Wednesday, with high pressure over the offshore waters and
lower pressure east of the Cascades. There are small craft
advisories up for the coastal waters and Strait of Juan de Fuca with
the stronger onshore/nw flow that is probably being beefed up due to
the upper trough moving through British Columbia.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory coastal waters and Strait of Juan de Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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