Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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787
FXUS66 KSEW 131035
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
335 AM PDT Fri Oct 13 2017

.SYNOPSIS...An upper level trough will move east of the area
today. A weak warm front will brush the north coast on Saturday
but otherwise the high pressure will keep weather systems deflected
to the north through Monday. A weather system arriving on Tuesday
will be the first of a pair of systems keeping the area wet into
next Friday.

&&

.SHORT TERM...Satellite imagery shows a cool upper level trough
still over the area this morning. Convergence zone in one of its
favorite locations, along the King/Snohomish county line, with
more convergence near the east entrance of the Strait of Juan de
Fuca being picked up well by the doppler radar. Temperatures have
not changed much from the highs yesterday afternoon with 3 am/10z
readings in the 40s.

Upper level trough moving east today with the flow aloft becoming
northerly. This will start the process of drying the air mass out
beginning this afternoon. In the meantime, convergence in the
lower levels will continue over Snohomish and northern and central
King county. Snow level near 3000 feet today. Will issue a winter
weather advisory this morning for the Cascades of Snohomish and
King county for up to 6 inches of snow with the convergence. The
higher snow amounts are expected around Stevens pass with
Snoqualmie pass getting lesser amounts. The convergence in the
lower levels will dissipate this afternoon with the shower
activity decreasing significantly. Outside of the convergence,
showers activity decreasing as well this afternoon. High
temperatures will remain well below normal, in the lower to mid
50s.

Northerly flow aloft continuing tonight with the upper level
trough well to the east. Still a chance of showers in the
foothills and the Cascades until midnight. Skies clearing
overnight and with the air mass aloft still cool it will be a
chilly morning on Saturday. The colder locations will drop to near
freezing with mid 30s to lower 40s common over the remainder of
the area. The clearing skies combined with light surface gradients
will result in some areas of fog forming by daybreak Saturday
morning.

Flat upper level ridge building into western Washington on
Saturday with a weak warm front trying to move over the top of the
ridge. Right now the ridge looks to build enough to push the warm
front mostly north of the area. Will leave just chance pops for
the Olympics westward and the northern portion of the Northwest
Interior. Mid and high level clouds out ahead of the front over
the area during the day. Will word the forecast partly sunny.
Highs will be a little warmer but with the cool start and the not
very strong October sunshine highs will only be in the mid 50s.

Ridge continuing to build Saturday night into Sunday keeping the
front north of the area. Surface gradients still light leading to
another round of areas of fog Sunday morning. More sunshine in the
afternoon and warmer temperatures aloft will push highs back to
near normal Sunday, in the upper 50s to mid 60s. It will be
another cool morning on Sunday with lows in the mid 30s to mid
40s.

.LONG TERM...Extended models continuing the trend that started a
few runs ago with a little bit stronger ridge on Monday keeping
weather systems out of the area. The ridge breaks down Monday
night with the first of two systems moving through the area on
Tuesday. A much stronger and wetter system arriving on Wednesday
into Thursday. An upper level trough will remain over the area on
Friday keeping the wet weather pattern intact. Felton

&&

.HYDROLOGY...Some differences in the extended models in terms of
rainfall amounts with the system on Wednesday/early Thursday. The
GFS is about twice as wet as the ECMWF with as much as 4 inches of
rain along the south slopes of the Olympics in an 18 hour period
ending early Thursday morning. That amount of rain brings up the
possibility of some problems along the Skokomish river. Felton

&&

.AVIATION...An upper level trough over the region will slowly shift
inland today. Light northwesterly flow aloft today will become west-
southwesterly tonight and Saturday. Air mass moist and becoming
stable during the morning with some drying expected at all levels
during the afternoon. Residual showers will end this morning with
widespread MVFR cigs and some local IFR conditions. Clouds will lift
and scatter with VFR conditions by afternoon.


KSEA...Mostly MVFR stratus this morning with some periods of IFR
stratus possible at times. Residual showers will dissipate early.
South wind 5-9 kt will switch to northerly flow by around midday.

&&

.MARINE...Northwesterly small craft winds over the coastal waters
will ease by this afternoon. Seas should remain just below 10 feet
today over the coastal waters. A weak front will brush the coastal
waters Saturday. High pressure will temporarily build over the area
Sunday and Monday with lighter flow over the area. A series of
stronger fronts will begin to affect the waters by Tuesday.

&&

.CLIMATE...Did it seem a little chilly out there on Thursday? The
high in Seattle of 50 degrees was the coldest day in the first two
weeks of October in 33 years. The last time there was a high
temperature in Seattle of 50 degrees or less during the first two
weeks of October was a 49 degree day on October 14, 1984.
Including the Federal Building records in Seattle which go back to
the 1890`s there has only been 10 other days in the first two
weeks of October that were cooler than yesterday. The coldest day
on record during the first two weeks of October in Seattle is 46
degrees on October 12th, 1966. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 5 AM PDT early this morning
     for Cascades of Pierce and Lewis Counties.

     Winter Weather Advisory until noon PDT today for Cascades of
     Snohomish and King Counties.

PZ...Small Craft Advisory until 11 AM PDT this morning 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.

&&

$$

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