Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 140022
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
422 PM PST Thu Dec 13 2018

.SYNOPSIS...A strong frontal system will move into Western
Washington on Friday. This front is associated with a developing
low that will move into Western Washington Friday or Friday
evening. Damaging winds are expected on the coast, and they are
possible over potions of the interior lowlands of Western
Washington. After a little break between systems on Saturday the
active weather pattern will resume with additional weather systems
reaching the area Sunday through the middle of next week. These
systems may produce heavy rainfall and windy conditions at times.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SUNDAY/...Satellite imagery
shows developing low pressure near 40N 139W this afternoon. Models
develop this low into a 980 mb storm that moves onto northern
Vancouver Island Friday afternoon. The associated frontal system
will drag inland across Western Washington on Friday. The wrinkle
of the day concerns the amount of secondary development that -may
or may not- develop to the south of the main low off the Oregon
coast then move NE into Western Washington. The 12Z mesoscale
models (the WRFARW, University of Washington WRF, and HRRR) all
show most zones across the area getting a couple hours with
pockets of high wind with the frontal system as it passes. The
larger scale models and NAM show a baggy front with little wind,
and the 18Z gradients with the front have backed off a bit. All in
all, there remains a good deal of uncertainty in wind -especially
in the interior- for Friday`s system despite it now in the 24 hour
forecast zone.

The high wind watch for the coastal zones was upgraded to a
warning. The watch will be maintained in the interior zones for
now. The 00Z runs and additional satellite imagery should help us
refine where to go to warnings or advisories as appropriate.

With southerly flow aloft and the progressive nature of the front,
expect snowfall amounts in the passes to be `garden variety` at
just below advisory level.

Saturday will be a day between systems with lower pops and lighter
winds. The next rather strong frontal system will move through the
area on Sunday. This front will bring breezy to windy conditions
to the area and some rain. Albrecht

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...Extended models show
the potential for an atmospheric river aimed into our latitude
with the source region for the air mass from around Hawaii. Snow
levels will be high, rainfall amounts will likely be high and
rather persistent, and many area rivers could flood by the middle
of the week. Forecasts represent a model blend. Albrecht

&&

.AVIATION...Wet and breezy conditions have largely been the rule
today as a quick moving system moves across BC. Cigs have hovered in
the VFR and MVFR range today for most sites. As a warm front lifts
across most of western Washington tonight and early Friday, there is
indication of some lifting of ceilings for most along with some
clearing. This is expected to be short lived as the next storm
system moves in during the day Friday. A strong cold front is
expected, carrying with it the potential for some gusty winds with
low level wind shear being a concern yet again. Widespread rain is
also expected along the front, which will likely lower cigs back to
at least MVFR as the front moves from west to east.

KSEA...Overall TAF trends look to be on track this afternoon. Cigs
are hovering along MVFR/VFR border but do expect cigs to improve
overnight with the potential for some break in the constant overcast
skies. The exception thru this evening will be in any heavier shower
activity. Most of the shower activity will continue to lift north
thru the evening and overnight hours, giving the airfield a brief
break in precip before the next storm system arrives. Winds will be
a concern on Friday, with the potential for gusty winds and low
level wind shear. Widespread rain showers will be possible by the
afternoon with cigs likely lowering back to MVFR.

Kovacik

&&

.MARINE...Winds will continue to pick back up again this evening as
a strong storm system approaches western Washington. Current
headlines remain unchanged with a Gale Warning in place for most
waters, the exception being the central Strait and Puget Sound where
a SCA is in place. Puget Sound should see Gale force winds on Friday
so SCA will transition to Gale Warning. Fridays storm
system has the potential to be rather strong and although some
uncertainty exists in regards to track and strength, this forecast
errs on the side of a stronger system. This places very borderline
high end Gale to Storm Warning criteria winds across coastal waters.
Will let evening shift take one more look before making any changes
so for the afternoon update will leave Gale Warning in place, which
matches the WFO Portland forecast. In addition, although still
uncertain, there is potential to see a strong surge of wind down the
Strait late Friday-a Gale Watch has been issued for the central
Strait. Although the winds, if they occur, would occur across all
parts of the Strait, given Gale Warning already in place for western
and eastern Strait, left that as is for now. Rough Bar for Grays
Harbor will continue into Friday night as well. Finally, High Surf
Advisory for the coast was left as is given very minor changes in
wave height timing and strength with latest data. Tidal issues do
not seem to be a concern at the moment.


Kovacik

&&

.HYDROLOGY...Flooding continues on the Skokomish River in Mason
County this afternoon. The river is forecast to recede to below
flood stage later Friday. The flood watch for the Olympic
Peninsula will continue this afternoon and evening until streams
stop rising, but the threat of flooding appears to be rapidly
decreasing and the watch will likely be allowed to expire.

Extended models show a strong potential for an atmospheric river
aimed into our latitude with the source region for the air mass
from around Hawaii. Snow levels will be high and rainfall amounts
may be high and rather persistent. Many area rivers could flood
by the middle of next week. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Wind Watch from Friday morning through Friday evening for
     Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
     Vicinity-East Puget Sound Lowlands-Everett and Vicinity-
     Hood Canal Area-San Juan County-Seattle and Vicinity-
     Southwest Interior-Tacoma Area-Western Skagit County-
     Western Whatcom County.

     Flood Watch until 10 PM PST this evening for Admiralty Inlet
     Area-Central Coast-Eastern Strait of Juan de Fuca-Hood
     Canal Area-Lower Chehalis Valley Area-North Coast-Olympics-
     Western Strait of Juan De Fuca.

     High Wind Warning from 4 AM to 4 PM PST Friday for Central Coast-
     North Coast.

     High Surf Advisory from 10 AM to 10 PM PST Friday for Central
     Coast-North Coast.

PZ...Small Craft Advisory for rough bar until 10 PM PST Friday for
     Grays Harbor Bar.

     Gale Warning until 10 PM PST Friday for Admiralty Inlet-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-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands-West Entrance U.S. Waters Strait Of Juan
     De Fuca.

     Gale Watch from Friday afternoon through late Friday night for
     Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 1 PM PST Friday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 9 AM PST Friday for Puget Sound and
     Hood Canal.

     Gale Warning from 9 AM to 10 PM PST Friday for Puget Sound and
     Hood Canal.

&&

$$

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