FXUS66 KSEW 230310

Area Forecast Discussion
National Weather Service Seattle WA
810 PM PDT Mon Oct 22 2018

.SYNOPSIS...One last fog episode tonight and into Tuesday morning
before a weakening front will arrive late Tuesday into Tuesday
evening. This may not bring much rain but should turn winds
southerly and clear out most of the fog. A stronger system will
arrive on Thursday with another front reaching the area around


.SHORT TERM...Fog is already starting to creep back into the areas
around the Puget Sound and along the coast this evening after a
fairly brief clear period in the late afternoon. Given there has
been no change in air mass and thus no dynamics along with not much
in the way of wind most places, there does not seem to be anything
stopping fog from becoming widespread again tonight. Lows tonight
will be in the upper 30s and 40s.

At some point Tuesday morning the approaching weak system is
expected to turn winds southerly...allowing the fog to lift into
stratus.  As this system draws closer in the afternoon...other
clouds will be increasing too...bringing a chance of rain to the
coast in the morning, then rain will become likely on the coast in
the afternoon. Models remain consistent in having the front fall
apart as it moves inland and as such POPs reflect this as most
interior locations will only see chance category POPs Tuesday night
except for the mountains and coast. Precip amounts with this system
look pretty unimpressive with only a few hundredths of an inch in
general. Highs Tuesday will be similar to today...around 55 to 60.

Wednesday still looks relatively dry, with just residual chance pops
in the morning mainly coast and mountains, and then just a slight
chance on the coast in the afternoon from the next approaching
system. It will still be pretty cloudy on Wednesday, and highs will
only be in the 50s.

At this point the system on Thursday looks to be the best chance for
widespread precip for W WA. Categorical POPs remain in the forecast
for everywhere at some point. Precip amounts are much more
impressive with a quarter to a half inch in the lowlands, and up to
an inch in the mountains. The snow level doesn`t really fall until
after the front passes Thursday night, so most of the precipitation
in the mountains will be rain. It will be breezy in the north
interior. Highs on Thursday will be in the 50s. SMR/Burke

.LONG TERM...From Previous Discussion...Friday will have scattered
showers ending, and Saturday has a good chance of being dry. The
next system will arrive around Sunday, and it looks about as rainy
as the front on Thursday. The snow level will fall to 5000-6000 feet
in post frontal showers on Monday. Temperatures will be near normal
each day. Burke


.AVIATION...An upper ridge over the interior Pacific Northwest will
shift further east Tuesday. A weak front will reach the coast late
in the day and dissipate. Light south-southeast flow aloft becoming
moderate southwest flow aloft Tuesday. Stable air mass and dry
except near-surface based moisture trapped by the inversion. Mid and
high clouds increase over the interior Tuesday afternoon, then
deeper moisture arrives Tuesday evening with some spotty light rain.

Cigs over W WA already starting to deteriorate as fog creeps into
the area again. Most locations and terminals near the Sound have
already dropped into IFR conditions with further deterioration into
LIFR expected overnight. Similar can be said along the coast.
Locations still seeing VFR or MVFR conditions are fairly limited to
those a little further away from the Sound and northernmost
terminals such as CLM and BLI. Expecting a more gradual erosion of
conditions there as fog coverage increases overnight. An approaching
front will begin to induce light southerly flow by daybreak Tuesday.
This should allow fog to begin slowly lifting with a mix of IFR,
MVFR and scattering low clouds Tuesday morning. Light rain should
reach the coast in the afternoon and spread inland by early evening.
Mostly VFR conditions with a few pockets of MVFR as spotty light
rain develops are expected Tuesday evening and overnight.

KSEA...Sea-Tac is sitting right on the fog boundary and...as is
evidenced in current obs...seeing this back and forth in visibility
and cloud cover. Expecting this to shift to more fog than not late
this evening with low IFR possible after midnight. The flow will
become southerly with a light breeze by daybreak Tuesday causing
cigs/vis to improve to mainly MVFR after 14-15z, then more rapid
clearing of fog/low stratus. Mid and high clouds lower in the
afternoon with VFR stratus and spotty light rain by Tuesday evening.


.MARINE...Light flow over the waters will become southerly later
tonight. A weak front will reach the coastal waters Tuesday with
small craft southerly winds. As such...will issue a Small Craft
Advisory for the coastal waters with evening forecast package. The
front will dissipate as it reaches the coast by late in the day and
thusly not much impact with regards to winds is expected over the
interior waters. Another front is expected to arrive late Wednesday
and Thursday with small craft winds over the coastal waters possibly
spreading to much of the north inland waters north of Puget Sound
Thursday. Lighter flow is expected Friday. DTM/SMR


PZ...Small Craft Advisory from 5 AM to 5 PM PDT Tuesday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.



NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather