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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 182212
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
212 PM PST Mon Feb 18 2019

.SYNOPSIS...A weak low over Vancouver Island will continue to
spread low level moisture into the area through tonight. Snow
levels will generally remain low, between 500 and 1500 feet. A
system will move southeastward across the area Tuesday through
Wednesday bringing rain, or a mix of rain and wet snow. A
convergence zone may form Wednesday morning bringing local
snowfall accumulations where it occurs. After a break on Thursday,
another system will track southeast across the area Friday through
Saturday. This system will be similar to its predecessor with
mostly rain or a rain snow mix - but low snow levels.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...Observations show a weak
1028 mb low centered near Comox BC this afternoon. Light onshore
flow is rotating marine air inland, and some light showers are 
seen from Forks southward along the coast and over the coastal 
waters. Some light showers have also been seen across the interior
today, but they have been spottier. Snow levels remain low. 
Earlier, a weather spotter at 500 feet elevation in North Bend 
reported a snow shower. Precipitation amounts through tonight will
be quite light with most places tracing out -if they even get 
any. Clouds and moisture will hold temperatures in the lower to 
mid 30s in most places.

The next system is still well upstream in the northern Gulf of
Alaska per latest satellite imagery. This system is forecast to
quickly drop southeastward. Heaviest precipitation with this
system will likely occur from noon on Tuesday through Wednesday
morning. Onshore flow will keep snow levels up higher than earlier
thought through much of Tuesday night, starting around 700-1700
feet midday Tuesday, rising to 1000-2000 feet Tuesday night, then
falling back to 500-1000 feet Wednesday morning. Some higher hill
locations in the lowlands will start off as possibly a wet snow,
or rain snow mix, but will transition quickly to rain in the 
afternoon through much of Tuesday night. The tricky part of the 
forecast will be during the morning hours on Wednesday when snow 
levels fall back to 500-1000 feet and heavier precipitation rates 
occur with a potential convergence zone. Some models center the 
convergence zone over the higher terrain of southern Snohomish 
County while other models center the convergence from about 
Bremerton and Seatac airport southeastward. We really won't know 
for sure where it develops until a few hours or less before it 
forms. 

At this time, we will be issuing a Winter Storm Watch for 
the Cascades and Passes from the Canadian Border southward to 
Lewis County for the good potential of 1 to 2 feet of snow during 
the period noon Tuesday through noon Wednesday. Most of the 
precipitation will likely fall during the late afternoon Tuesday 
through Tuesday night and hazardous travel conditions are quite
possible over the passes.

Showers will wind down from the north Wednesday afternoon, then a
break is expected Wednesday night into Thursday. With some
clearing Wednesday night, expect temperatures to fall into the mid
to upper 20s most places. Temperatures will rebound into the lower
to mid 40s Thursday afternoon with strengthening late February 
insolation. Albrecht

.LONG TERM /FRIDAY THROUGH MONDAY/...All of the extended models
keep the cold and periodically unsettled conditions going through
the first of March! We may get close to breaking the record for 
coldest February at Seatac Airport (35.6 degrees set in 1956) and
should get into the top 5!

A deepening upper trough dropping southeast into the long wave
trough position on the west coast promises to give precipitation
to the area Friday through Saturday along with low snow levels.
Models generally support enough onshore flow in the lowlands to
keep locations below the highest hills in the form of rain, though
a mix of rain and snow or a wet snow can't be ruled out at this
time. 

Dry and cold weather will likely return from the tail end of next
weekend into early next week with lows in the 20s to lower 30s and
highs in the upper 30s to mid 40s. Albrecht

&&

.AVIATION...Northerly winds aloft will continue through today 
before becoming more west/northwest on Tuesday. Light flow continues 
at the surface. Ceilings are bouncing around between VFR and MVFR 
early this afternoon with cloud cover and highly-scattered showers. 
Brief improvement possible this afternoon/evening (confidence 
remains low) before MVFR ceilings return tonight into Tuesday 
morning as next system approaches. Showers associated with the 
system may hold off until the afternoon hours in most locations. 

KSEA...MVFR cigs continue this afternoon. May still see improvement 
to VFR later this afternoon into evening but confidence is low in 
timing/occurrence. Another round of MVFR cigs expected tonight into 
Tuesday ahead of next system. Showers likely after 21z Tuesday. 
Light southerly surface winds under 10 knots will continue. CEO

&&

.MARINE...Light winds today will become southerly and 
increase on Tuesday as a low pressure system moves into the waters. 
Small craft advisories issued for most of the waters for winds and 
for rough bar in Grays Harbor Bar. Surface low will drift south on 
Wednesday with lighter northerly or offshore winds into Thursday. A 
stronger system will move inland on Friday with increasing southerly 
winds and the possibility of additional small craft advisories. CEO 

&&

.HYDROLOGY...No river flooding is expected for the next seven
days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Tuesday afternoon through Wednesday 
     morning for Cascades of Pierce and Lewis Counties-Cascades 
     of Snohomish and King Counties-Cascades of Whatcom and 
     Skagit Counties.

PZ...Small Craft Advisory from 1 PM Tuesday to 10 AM PST Wednesday 
     for Admiralty Inlet-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-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.

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

&&

$$

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