Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

000
FXUS66 KSEW 150447
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Wed Feb 14 2018

.SYNOPSIS...Lingering showers will end after midnight. The next
front will arrive Thursday night and Friday. A much stronger frontal
system will move through the region late Friday and Saturday. Fraser
outflow develops Sunday with colder temperatures and possible
lowland snow showers. Dry offshore flow with cold temperatures early
next week.

&&

.SHORT TERM...A trough is shifting southeast of the region tonight
with showers diminishing after midnight. Much of the area will
remain dry most of Thursday between systems. However, the leading
edge of light rain with the next system may move onto the coast late
in the day.

Warm frontal rain will spread into the area Thursday night. The
upper trough and associated surface front will push southeast across
Western Washington by around midday Friday. Breezy winds will
probably develop, especially around greater Puget Sound as a meso
low northeast of the Olympics increases southerly flow. Colder air
aloft will arrive with this system and strong northwesterly onshore
flow will be favorable for heavy snow in the mountains, especially
the Cascades. Advisory amounts of snow, over 6 inches, appear likely
by Friday night. This includes the passes.

A brief lull in showers and mountain snow may develop Friday night.
The next much more potent system will arrive Saturday. Models remain
in good agreement that strong south-southwesterly winds develop
Saturday with an even heavier round of mountain snow. Travel through
passes will be very difficult with blowing and driving of snow
causing reduced visibility.

.LONG TERM...Previous discussion...A more vigorous frontal system
will move through the area Saturday. Saturday looks wet and blustery
and the mountains ought to get some heavy snow. Saturday night and
Sunday will see much colder air shifting south through the region.
Sunday morning is probably the best chance for lowland snow with the
modified arctic front. Monday and Tuesday should be clear and cold.
Sunday night will see the cold snap begin, but with breezy weather
it will be more wind chill that night. After the gradients relax
Monday, the coldest low temps of the winter should occur Monday
night with a range of 15 to 25 degrees across Western Washington in
the forecast. For reference, the coldest it has been at Sea-Tac this
winter so far is 28 degrees and the forecast low Monday night is
currently 24 degrees and if there is snow on the ground in the
outlying areas it can easily be ten degrees cooler there than Sea-
Tac.

&&

.AVIATION...Moderate to strong north-northwest flow aloft will
remain in place tonight and Thursday. The air mass will be moist and
somewhat unstable below 6,000 feet msl, with dry and stable air
above. Surface flow will turn to light low-level onshore flow
tonight allowing current widespread VFR conditions to lower into
more prevalent MVFR conditions late tonight and Thursday morning.
Frontal passage late Thursday afternoon/early evening will introduce
precip as well as some instability. The instability will allow for
cigs to rebound somewhat...becoming more of a mix of borderline MVFR
to VFR conditions. However precip may prove to be an offsetting
factor...limiting the degree of cig improvement.

KSEA...Generally dry conditions are expected through the first half
of the TAF period with cigs gradually diminishing in light of winds
turning from dry northerly flow to a more S to SE direction. Clouds
and cigs should gradually improve Thursday afternoon before precip
associated with frontal passage may bring cigs down again Thursday
night.  SMR

&&

.MARINE...Moderate northerly flow is expected gradually transition
to weaker onshore flow by Thursday morning. Inherited headlines are
supported by obs...although somewhat borderline...so will keep them
in place with the evening forecast package. A weak frontal system
will move across the area on Friday, followed by a much stronger
system this weekend. The weekend system may bring gale force winds
to parts of the area. Haner/SMR


&&

.HYDROLOGY...River flooding is not expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.

PZ...Small Craft Advisory until 9 AM PST Thursday 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 for rough bar until 9 AM PST Thursday for
     Grays Harbor Bar.

     Small Craft Advisory until 6 AM PST Thursday 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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.