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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 170549

Area Forecast Discussion
National Weather Service Seattle WA
949 PM PST Sat Feb 16 2019

.UPDATE 940 PM PST...The latest IR/WV image shows the upper level
circulation just off the south Washington coast this evening with
a weak 1012 mb low off the north coast. Given the system is not
completely vertically stacked, precipitation will probably become
more disorganized over the next several hours. The ECMWF shows no
QPF after 06z which is unlikely give the low level convergence and
upper level deformation producing an area of mostly light rain but
with reports of mixed rain/snow on hilltops north of Seattle. The
air mass is expected to cool further overnight and light
precipitation is lingering a bit longer than models had indicated.
There is also some upslope along the north side of the Olympics
along the Strait. Port Angeles to the Kitsap could see a minor
accumulation but drier air should gradually clear clouds and
precipitation later tonight. The BLI-YWL gradient is near -8 mb
and forecast to peak near -15 mb during the morning. Fraser
outflow should begin during the early morning hours and become
strong for a time through the afternoon. Probably not quit
advisory level but will keep a watch. Gales seem probable over the
waters. Other areas through early Sunday morning could see some
mixed precipitation down to a few hundred feet or so with minor 
spotty accumulation less than 1 inch, except potentially a bit 
more along the Strait which will be watched. All precipitation 
should end by morning and drier, windy and cooler air will filter 
into Western WA. Mercer 


.SYNOPSIS...Low pressure off the Washington coast will weaken and
move south tonight and Sunday. Northerly flow aloft Sunday 
through Monday will give generally dry but cold conditions to the 
area, with Fraser outflow winds in the north interior. The ridge 
will flatten Monday night with a system arriving from the 
northwest on Tuesday. Unsettled weather is expected later Tuesday 
through the end of the week, with perhaps with a break between 
systems on Thursday. 


.SHORT TERM /TONIGHT THROUGH TUESDAY/...Satellite imagery shows
the upper low giving scattered showers to the area now over
Western Washington. This upper low is expected to slide southward
and out of the area late tonight into Sunday. Showers over the
coastal waters are moving into central and southern portions of
Washington while a weak deformation zone immediately north of the
center of the upper low is giving showers to the central Puget 
Sound area. All of these showers will drop slowly to the south 
tonight and out of the area, except maybe along the Cascade Crest 
and far south portion of the area where they will linger longer.
While cooler air will be filtering into the area overnight, lows
will be held in the upper 20s to mid 30s due to lingering cloud

Arctic high pressure centered over the Canadian Northwest 
Territories and Yukon is forecast to move southward into the 
central interior of British Columbia later tonight into Sunday. As
the upper low to the south moves away, expect north to northeast 
pressure gradients to increase allowing cold Fraser outflow to 
resume starting on Sunday. Both the NAM12 and the GFS give -15mb 
Bellingham to Williams Lake BC pressure gradients Sunday morning 
for breezy windy northeasterlies in Western Whatcom County and 
the San Juan Islands. There continue to be indications that the 
winds will stay below advisory strength in those areas, however. 

The north to northeasterly flow Sunday into Sunday night will
result in drying and clearing across the area. Highs will
generally be in the 30s to lower 40s and lows will fall mainly
into the 20s Sunday night and Monday night. 

Clouds will increase on Tuesday as a disturbance approaches the 
area from the northwest. There is a threat of some light 
precipitation on Tuesday that will increase Tuesday night. At 
this time it appears that conditions will be cold enough that many
locations in the interior will see some light snow. However, 
impacts are tough to predict at this time. Conditions are not 
expected to be as cold as what we experienced with the snow events
of the previous two weeks, and heavy precipitation -if it occurs-
may be more limited in area than with recent events. Small 
changes in temperatures and locations of stronger forcing will 
result in large differences in area impacts, so please watch 
latest forecasts for updates. Albrecht

chance of accumulating snow in the interior lowlands of Western
Washington is Tuesday night through early Wednesday as a
disturbance moves through the area. Models generally show
accumulations between a trace to about 3 inches with the
operational GFS and experimental FV3/GFS models the heaviest. 
Behind the disturbance, the air mass cools and dries somewhat so 
that there may be a break between systems Thursday and Thursday 
night. Temperatures remain well below normal.

Friday and Saturday's system is now handled inconsistently between
the models. The recent GFS solutions are a bit warmer and wetter
with Friday's system resulting in snow being limited to the far
north interior and snow levels in the central and southern
portions of the area remaining at 800-1500 feet or so. The 
operational ECMWF has taken a twist in that the approaching 
disturbance for Friday now gets wrapped southwestward under the 
offshore block, reestablishing the REX block along about 140W. 
This would leave Western Washington dry and cool.

At this point the extended forecasts are a blend of model
solutions. Expect changes to the forecasts, their timing, and
potential impacts we move forward in time. Albrecht 


.AVIATION...Light flow will become northerly to 10 kt overnight.
Low MVFR is forming around the area and pockets of IFR will be
possible before precipitation ends later tonight, maybe lingering
into the early morning from KSEA south. Mixed rain/snow is
possible as the air cools a bit, but little or no accumulation is
expected except for KCLM and KPAE which could see slushy
accumulations up to 1 inch. Less or none elsewhere.

KSEA...Low MVFR cigs will likely prevail through early Sunday
morning. Periods of IFR will be possible after 09z before
precipitation tapers off from the north. Light wind will become N
to 10 kt, at KSEA and lighter N wind at KBLI. Cigs will improve to
MVFR by 18z and then likely VFR by midday and gradual scattering
of clouds.


.MARINE...A somewhat disorganized system off the coast will drift
southward tonight. The weak 1012 mb low off the north coast and
upper closed low off the south coast will drift south. The only
concern would be if the system becomes vertically stacked and 
could slow the exit of the system, but this is not likely at this 
point. High pressure developing over southern British Columbia 
late tonight and Sunday will help northeasterly outflow winds to 
develop over the northern interior waters and offshore flow 
through the strait. This will support low-end gales over the 
northern inland waters and small craft conditions over most other 
waters. Westerly swell 10 to 11 feet at 11 seconds tonight and 
Sunday will subside to 8 or 9 feet Sunday afternoon and evening. 


.HYDROLOGY...Precipitation that falls through tonight will mainly
be in the form of rain, or a rain snow mix. The precipitation, 
combined with some snow on the ground that continues to melt and 
recent snowfall that has saturated the soil, has increased the
threat for landslides more than rainfall-based indices would
suggest. So, the Special Weather Statement for the landslide 
threat will be continued for one more day. 

As far as river flooding is concerned, none is expected for the 
next 7 days. Albrecht


PZ...Small Craft Advisory until 4 PM PST Sunday for Central U.S. 
     Waters Strait Of Juan De Fuca-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-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 4 PM PST Sunday for 
     Grays Harbor Bar.

     Small Craft Advisory until 4 AM PST Monday for East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until midnight PST Sunday night for Northern Inland 
     Waters Including The San Juan Islands.

     Small Craft Advisory from 4 PM Sunday to 4 AM PST Monday for 
     Admiralty Inlet-Puget Sound and Hood Canal.