Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 120425
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
925 PM PDT Wed Oct 11 2017

.SYNOPSIS...A large cool upper level trough over Western Washington
will bring showers and cool temperatures to the area through Friday.
A weak upper level ridge will bring drying and a little warming this
weekend although a dissipating warm front could bring a little light
rain to the coast on Saturday. Strong westerly flow aloft offshore
early next week should bring several weather systems to the region.

&&

.SHORT TERM...A large upper level trough resides over the Pacific
Northwest this evening, and should be a resident through Friday. The
mean trough axis is over the Cascades or E WA, so W WA will be under
the influence of NNW flow aloft paralleling the B.C. coast.

At 03Z/8 PM this evening, RADAR shows less activity than this
afternoon with spotty showers mainly over the Cascades. This should
be a brief lull since both the models and water vapor imagery show
an approaching upper level shortwave trough over the central B.C.
coast this evening. Models bring this feature down over W WA on
Thursday for numerous showers. GFS lifted indices get down to about
-1 and NAM bufr soundings for KOLM/KHQM show fairly deep but weak
instability. There should be lots of showers around and with the
freezing level around 4500 feet there will probably be some ice
pellets or small hail in the showers. The main question is whether
there will be thunderstorms. Can`t rule out a few isolated lightning
strikes but the instability seems borderline enough to hold off
including them in the forecast for now. I`ll pass it along to the
midshift who will see the 00Z models later tonight.

It looks like there will be enough precipitation on Thursday over
the Cascades of Pierce and Lewis counties, i.e. the Mt. Rainier
area, so that a Winter Weather Advisory for 5-9 inches of snow may
be needed. Snow levels should be below the levels of Paradise and
White Pass.

On Friday the upper level trough axis will be shifting E, putting W
WA under drier but still cool northerly flow aloft. The air mass
will be more stable and showers should be diminishing in the
afternoon over the coast and north part, and becoming more scattered
over the interior south of Seattle. High temperatures both Thursday
and Friday will be in the lower 50s, which would be 6 to 8 degrees
below normal.

As the upper level trough continues E on Saturday, a low amplitude
upper level ridge will slide inland over southern B.C.  The GFS and
ECMWF agree that a weak warm front will approach the coast on
Saturday but will probably stall and dissipate just offshore. There
could be some spotty light rain along the coast. At least the warm
advection should nudge the afternoon highs up closer to normal. Kam

.LONG TERM...Previous discussion from the 404 PM AFD...The ECMWF was
the preferred solution for this period since it has been the most
consistent of the models. Thus, expect the upper ridge to break down
early in the period, allowing strong zonal or westerly flow aloft to
develop over the Pacific Northwest. This will allow a series of
Pacific storms to move across the region during this period for a
wetter period. Temperatures are anticipated to be near normal.

&&

.AVIATION...An upper level trough will remain over the Pacific
Northwest through Thursday night. An upper level disturbance
arriving from the northwest will cause an increase in showers later
tonight and Thursday. Light southwest flow aloft tonight will become
west-northwesterly Thursday. Air mass moist and weakly unstable.

A weak residual convergence zone may produce localized showers
around KBFI/KSEA through around midnight before dissipating.
Residual low level moisture will result in local MVFR cigs tonight,
otherwise VFR sct-bkn cigs 3-4K ft through much of tonight.

Showers or areas of light rain will develop across the area Thursday
morning. MVFR cigs will become widespread as precipitation quickly
spreads into the area around daybreak. A few spots like KOLM/PWT
could see a few hours IFR during the morning. Thunderstorm activity
with this next system will likely remain confined to extreme
southern WA and Oregon on Thursday. However, an isolated strike
further north near the Washington coast or around Puget Sound cannot
be completely ruled out. Will not include in TAFs due to the low
probability.

KSEA....A few showers with a residual convergence zone ending by
around midnight. Mostly MVFR but some brief improvement to VFR
possible around midnight or early this morning. MVFR should develop
again as rain or showers increase Thursday morning. S winds at the
surface 5 to 10 kt. dtm

&&

.MARINE...Northwest winds will continue to ease through midnight.
Swell near 10 feet may continue for several hours with a small craft
for hazardous seas in effect for the coast. Seas should subside
below 10 feet later tonight.

A weak 1015 mb surface low will move E-SE across the waters on
Thursday. Northwest winds will increase behind the low Thursday
night, likely reaching small craft strength over the coastal waters.
Winds will begin to ease Friday and especially Saturday as surface
high pressure builds over the waters. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory for hazardous seas coastal waters until
      3 AM PDT Thursday morning.
     Small Craft Advisory for rough Grays Harbor bar until 1 AM
      PDT Thursday morning.

&&

$$

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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