Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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949
FXUS66 KSEW 271559
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Mon Sep 26 2016

.SYNOPSIS...Skies will clear from the north today. Southwest flow
aloft will maintain a generally dry air mass Wednesday and Thursday
except for some morning low clouds and patchy fog. A large upper
level trough will bring cool and showery weather Friday through the
weekend.

&&

.SHORT TERM...A very weak front is over southern Washington and is
moving southeast. There is already good clearing behind the front
evident on satellite imagery and in surface observations. The
general idea of clearing from the north today is well supported by
increasingly northerly gradients and north wind down to Tacoma.
Highs today will be mainly in the 60s with a few low 70s in far
southern areas.

With no weather features of note expected Wednesday and Thursday,
generally dry southwest flow aloft will prevail over the region.
Onshore flow will remain weak, but still strong enough to produce
some patchy low clouds and fog around the interior in the morning.
Temperatures will remain near normal with highs in the 60s to lower
70s and lows in the 40s to lower 50s. Burke

.LONG TERM...Previous discussion...The main weather feature Friday
through next Monday will be a large upper level trough. The trough
is well offshore this morning just S of the Gulf of AK, but will
continue moving E through Wednesday, then begin digging SE and
deepening into a large longwave trough  as it nears the PacNW. The
large trough will eventually end up centered just offshore near
45N/130W on Friday. Both the GFS and ECMWF bring a small shortwave
trough around the base of the large trough and up over W WA Friday
afternoon for an initial chance of showers.

The models agree that the large trough will continue to bring mostly
cloudy and cooler weather with at least a chance of showers to W WA
this weekend and into Monday. However after the initial shortwave
trough Friday afternoon the model consensus with details falls
apart. The GFS is wetter, particularly Sunday and Sunday night so
POPs have been left in the likely range for now, but confidence
remains low. The models bring the snow level down to around 5000
feet or so this weekend, with the GFS slightly lower. Kam

&&

.AVIATION...A weak trough brushing by Western Washington will
maintain light to moderate westerly flow aloft through tonight. The
associated weak front will move east of the Cascades by midday.
Onshore flow is keeping the low levels moist, and only somewhat
moist aloft this morning. As northerly flow near the surface picks
up today, expect drying at all levels and residual clouds scattering
out. Stable conditions. Patchy IFR or MVFR stratus and fog will
reform in place later tonight, mainly in prone sheltered valleys.

KSEA...north wind 5-9 kt...becoming northeast 4-6 kt this evening
and overnight. Cigs should continue improving this morning and
scatter out by midday. There is a 30 percent chance for IFR stratus
or fog to form in place at the KSEA terminal, mainly between 12-16z
Wednesday morning. dtm

&&

.MARINE...Increasing northerly winds over the interior waters may
reach up to 20 kt this afternoon. Northwesterly small craft winds to
25 kt are expected over the coastal waters where gradients remain
strongest. Low pressure east of the Cascades will maintain light
onshore flow through the week. Another weather system should reach
the area Friday and Saturday. dtm

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small craft advisory coastal waters.


&&

$$

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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