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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 192253

Area Forecast Discussion
National Weather Service Seattle WA
253 PM PST Tue Feb 19 2019

.SYNOPSIS...A frontal system will bring rain and locally heavy
mountain snow to the area tonight. A cool upper trough will
result in showers continuing through Wednesday. Drier northerly
flow will develop starting Wednesday night and continuing through
Thursday night. A second frontal system will reach the area on 
Friday followed by an upper low later on Saturday. Cool and
unsettled weather at times will continue through next week.


.SHORT TERM /TONIGHT THROUGH FRIDAY/...Another very challenging
forecast shift today; when snow levels are below 1000 feet, every
100 foot error results in huge impact changes across the region.
Add convergence zones, water, terrain, and microclimates to the
mix and you have a real challenge.

Snow levels have generally risen to above 1000 feet this afternoon
as warm advection and precipitation has moved in ahead of the
incoming frontal system. The front will quickly move in overnight
with precipitation becoming showery and snow levels lowering back
to around 500 feet by morning. Snow will be heavy in the mountains
now through early Wednesday morning. A winter weather advisory is
in effect for the Cascades of Whatcom and Skagit counties while a
winter storm warning is in effect for up to 18 inches of snow for
the Cascades of Snohomish County southward to the Cascades of
Lewis County.  

Challenge number 1: Many models form a convergence zone somewhere
around the central Puget Sound area 4AM to Noon on Wednesday. 
High resolution mesoscale models generally show the zone 
developing from about the Hood Canal Bridge ESE along the King- 
Snohomish County to Stevens Pass. Many of these show trace to up 
to 3 inches of snow accumulation by 10 AM at higher locations from
Everett (Paine Field) eastsoutheastward. Temperatures are 
marginal though. The GFS shows convergence farther south moving 
into the Cascades faster and little to now snow in the lowlands 
below about 800 ft. Much will remain unknown until the convergence
zone actually forms. Also, snow accumulations in this type of 
marginal situation depends on precipitation rates, persistence of
the band, etc.

Showers decrease during the day on Wednesday and snow levels
increase a bit with late February insolation. Then drier
conditions Wednesday night through Thursday night will result in
quite cool conditions with lows in the mid 20s to lower 30s.
Daytime highs under late Feb sunshine should result in highs in
the lower to mid 40s despite the cool air mass in place.

Now for challenge number 2: Another frontal system will move
quickly southeast into the area on Friday. The track of the system
will be similar to today's system with warm advection
precipitation followed by cooling aloft and possible convergence 
zone activity. The GFS is faster as well as many members of the 
blended models. Precipitation moving in during the morning hours 
would result in precipitation beginning as snow at rather low 
elevations before turning to rain midday. If the ECMWF verifies, 
precipitation comes in a few hours later -in the afternoon and 
evening- and results in all rain. Then there is the question about
post frontal convergence zones, where they form, whether they
persist, etc. For now the forecast will call for a chance of rain
or snow most places with little or no snow accumulation in the
lowlands. A lot will need to refined between now and then. 

.LONG TERM /SATURDAY THROUGH TUESDAY/...The forecast challenges of
the short term continue into the long term today. 

The past 3 runs of the GFS model show upper level troughing 
amplifying over the area late Saturday into Sunday. As an upper 
low drops just south of Seattle, The GFS forms a deformation band 
into the central Puget Sound area Saturday night into Sunday night
and indicates possibly several inches of snow down to near sea
level (the 18Z solution is farther north with the band, more into
Skagit County). The ECMWF is cold but drier. At this point, it is
anyones guess, but it does appear that cold and unsettled 
conditions will keep things interesting through the weekend.

A southern stream system moving into cold air in place over the 
region later Monday or Tuesday may make for more interesting 
times, though models are having a hard time as the pattern shows 
some signs of breaking down. 

At this time, the forecast was initialized with the National Blend
of Models and will be refined as we get closer to the events. But
it does appear that temperatures will remain below normal and
weather will generally be active for the next week or so. Albrecht


.AVIATION...Flow aloft becoming westerly today as an upper level trough
shifts southward along the British Columbia coast. The air mass 
is moist and stable. Widespread low MVFR and areas of IFR are 
expected into Wednesday morning with potential improvement later 
in the day Wednesday. Showers have become more numerous this 
afternoon and activity will continue into Wednesday morning before
gradually tapering off later Wednesday. A Puget Sound Convergence
Zone is also expected to form some over the central Puget Sound 
overnight into Wednesday morning which may bring heavier showers 
and additional degradation in flight conditions.

KSEA...Low MVFR and periods of IFR expected through the forecast 
period. Showers will continue into Wednesday morning. Possible 
convergence zone activity after 12Z may briefly lower snow levels 
near the surface but confidence is low in exact location or snow 
accumulations. Will leave out of TAF for now. Surface winds S-SW 5
to 15 knots. CEO


.MARINE...A frontal system will continue to bring small craft
advisory strength winds to all waters into Wednesday morning.
Rough bar expected this evening into Wednesday for Grays Harbor
Bar. The flow will turn more northerly on Wednesday and then
northeasterly Wednesday night and Thursday as high pressure moves
over southern British Columbia. Another frontal system will drop
down from the northwest on Friday with a trailing weak low moving
over the area on Saturday. CEO


.HYDROLOGY...River flooding is not expected for the next 7 days.


WA...Winter Storm Warning until 6 AM PST Wednesday for Cascades of 
     Pierce and Lewis Counties-Cascades of Snohomish and King 

     Winter Weather Advisory until 6 AM PST Wednesday for Cascades of 
     Whatcom and Skagit Counties.

PZ...Small Craft Advisory until 10 PM PST Wednesday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal  
     Waters From James Island To Point Grenville 10 To 60 Nm-Coastal 
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.

     Small Craft Advisory for rough bar from 10 PM this evening to 4 
     PM PST Wednesday for Grays Harbor Bar.

     Small Craft Advisory until 10 AM PST Wednesday for Admiralty 
     Inlet-Coastal Waters From Cape Flattery To James Island Out 
     10 Nm-Coastal Waters From James Island To Point Grenville 
     Out 10 Nm-Coastal Waters From Point Grenville To Cape 
     Shoalwater Out 10 Nm-Northern Inland Waters Including The 
     San Juan Islands.

     Small Craft Advisory until 7 AM PST Wednesday for Central U.S. 
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters 
     Strait Of Juan De Fuca-Puget Sound and Hood Canal.

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