Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 092235
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
335 PM PDT Mon Oct 9 2017

.SYNOPSIS...A cold front will sweep across the area late tonight
through early Tuesday morning. Expect mostly light precipitation
behind the front. The upper level low pressure system will move
over the area Tuesday night and then linger into Thursday for cool
and unsettled conditions. The air mass will become unstable
enough for a slight risk of thunderstorms on Wednesday.

&&

.SHORT TERM...
An upper level trof off the northwest Canada coast will continue
moving southeast through tonight. The associated cold front will
sweep across the CWA during the late tonight through early Tuesday
morning time frame. The precipitation is expected to mainly occur
behind the front. This front will be weak, so not expecting
amounts to be significant.

A colder air mass will invade the region Tuesday night and the
atmosphere will become weakly unstable for a threat of tstms over
the coastal waters. There is a possibility of an isolated
lightning strike within the Puget Sound Convergence Zone Tuesday
night but, most likely, there will probably just be small hail
accompanying the heavier showers.

The upper level trof will move over the area Tuesday night and
then move slowly east during the day Wednesday. The air mass will
destabilize further Wednesday for a risk of isolated tstms over
much of the CWA. Expect the tstms to contain small hail.

By early Wednesday, snow levels will be near 3500 feet. The
heavier showers will likely pull the snow level down to near 2500
feet for a brief time, so don`t be surprised if you see some
snowflakes while driving through Snoqualmie Pass on Wednesday. We
are not expecting widespread significant snowfall amounts due to
the mostly hit and miss nature of the showers.

A chilly upper level trof will remain over the region through
Thursday for cool and unsettled conditions. Anticipate a threat,
albeit low, of thunderstorms over the far southwest part of the
CWA during the day Thursday.

Temperatures will be below normal during this period.

.LONG TERM...
There were differences in the medium range solutions during the
extended period. The GFS was faster with moving the trof out of
the area than the ECMWF solution. The Canadian model was a
compromise between the two. Therefore, kept a threat of showers in
the forecast for Friday for mainly the interior.

Anticipate strong northerly flow aloft to prevail over the region
Thursday through Saturday. Weak disturbances embedded in the flow
could bring a threat of precipitation to the CWA at times. The
flow aloft will become more zonal early next week. There may be
dry periods Sunday and Monday.

Overall confidence in the forecast for the extended period was not
high.

&&

.AVIATION...Dry weather will continue tonight with mainly high
clouds over the region. Patchy fog may develop early Tuesday
morning, especially at KOLM. An upper level trough will move
inland on Tuesday for a chance of showers and thunderstorms. The
flow aloft is westerly with the air mass becoming moist and
unstable. Models show a Puget Sound Convergence zone forming
along the King/Snohomish border Tuesday evening as onshore flow
increases. 33

KSEA...Increasing clouds tonight but expecting VFR conditions.
Light north winds at the surface flipping to southerly by 12z.
Showers in the vicinity by 21z with a cold front. PSCZ forming
north of the terminal Tuesday evening. 33

&&

.MARINE...Northerly flow will continue tonight with low pressure
over Oregon. North to northwest flow will increase late tonight
into Tuesday over the Coastal Waters as a cold front moves in
from the west. Onshore flow will increase behind the front Tuesday
afternoon and evening. Weak low pressure will slide southeast
across the waters on Wednesday and again on Thursday. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 4 AM Tuesday to 5 AM PDT Wednesday for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory from 6 AM to 11 PM PDT Tuesday for Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-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

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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