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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 200444
AFDSEW

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

.SYNOPSIS...A frontal system will move inland tonight with an
upper trough over the area on Wednesday for some lingering
showers. Dry northerly flow aloft will prevail Wednesday night 
and Thursday. A second frontal system will reach the area on 
Friday followed by a cool upper trough on Saturday. An upper 
trough will linger over the area into early next week.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...A front is currently moving
inland and rain has chance to showers. The mountains will pick up
several more inches of snow tonight before showers taper. Showers
will linger into Wednesday with a broad upper trough over the 
area but the trend will be for decreasing activity during the day.
Showers should end Wednesday night with a dry day on Thursday 
under cool northerly flow aloft. A second frontal system will drop
down from the northwest on Friday for another round of rain and 
mountain snow. 

Snow levels will remain low for the next few days with the
lowlands flirting with snow at times but widespread accumulations
are not expected. For tonight and Wednesday morning there could 
be spotty light accumulations on higher hills and locations away 
from the water closer to the Cascades. A Puget Sound Convergence 
Zone could also bring local accumulations. The Friday system could
also bring a little low elevation snow, especially in the morning
before temperatures warm during the day but low level southerly 
winds should be a limiting factor. Schneider

.LONG TERM /SATURDAY THROUGH TUESDAY/...Previous discussion...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...A frontal system will move inland tonight with a broad
upper trough over the area on Wednesday and northerly flow aloft.
At the surface, onshore flow behind the front tonight will turn 
more northerly on Wednesday as high pressure builds over southern 
British Columbia. The air mass is moist and slightly unstable. 

Low clouds will be prevalent tonight and Wednesday morning.
Ceilings should improve later Wednesday as the low levels dry 
with some areas scattering by Wednesday evening.

A Puget Sound Convergence Zone will likely form over Central
Puget Sound tonight, shift south Wednesday morning and then fall
apart. Schneider

KSEA...Showers and low clouds tonight. A rain/snow mix is possible
later tonight but no accumulations are expected. Showers will end
Wednesday morning and ceilings will lift during the day. 
Southerly wind 5-10 knots will switch to northerly Wednesday. 
Schneider

&&

.MARINE...A frontal system will move inland tonight. Most waters
will have small craft advisory strength westerly winds behind the
front into Wednesday morning. The Central and East Entrance Strait
of Juan de Fuca could have marginal gales this evening. There will
also be some 12-15 foot west swell moving into the Coastal Waters
later tonight and Wednesday. 

Winds will turn more northerly on Wednesday as high pressure 
builds into southern British Columbia and then more easterly
Wednesday night and Thursday as the high pressure shifts
southeast. 

A second frontal system will drop down from the northwest on
Friday. Northeasterly offshore flow will develop next weekend as 
high pressure builds over the interior of British Columbia again.
Schneider

&&

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

&&

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

     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-West Entrance U.S. Waters Strait Of Juan 
     De Fuca.

     Small Craft Advisory from 1 AM to 7 AM PST Wednesday for Central 
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. 
     Waters Strait Of Juan De Fuca.

     Gale Warning until 1 AM PST Wednesday for Central U.S. Waters 
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory until 7 AM PST Wednesday for Puget Sound 
     and Hood Canal.

&&

$$

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