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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 202234

Area Forecast Discussion
National Weather Service Seattle WA
234 PM PST Wed Feb 20 2019

.SYNOPSIS...Dry northerly flow aloft developing tonight and 
continuing into Thursday. The next frontal system will arrive from
the northwest on Friday. Another cool upper level trough will 
follow the front for the weekend into the early part of next week.
Low snow levels will continue through the period. 


.SHORT TERM /TONIGHT THROUGH SATURDAY/...Showers will continue to
decrease this evening as an upper level trough exits. Dry 
northerly flow will allow some clearing with a cold night ahead - 
lows into the mid 20s to lower 30s. May see freezing fog in the 
south sound. Thursday will be dry with highs in the lower to mid 

Moisture will spread into the area on Friday as our next front
arrives. Initially, we may see reports of rain/snow mix as snow
levels will remain low through the morning. We should see mainly
rain in the lowlands during the afternoon as onshore flow
increases. The best shot at seeing snow will be areas near the 
Cascades, over 1,000 feet. In fact, we may need winter weather 
headlines for the Cascades as the passes will likely see 4-8". 

Saturday will be a cool and showery day as an upper level low drops
down from B.C. Snow levels are still low, around 1,000 feet, with
a rain/snow mix expected on the higher hilltops. The afternoon 
forecast high temperature is only 40 degrees which is 10 degrees 
below normal. 33

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...A few showers will linger
into Sunday as the cool upper level low slowly exits. Shower
activity is mainly isolated to scattered, but as mentioned above,
we may see a rain/snow mix at higher elevations. The air mass
starts to dry out again as we move toward Monday, while offshore
flow strengthens. But this sets us up for the next round of
lowlands snow, possibly Tuesday or Tuesday night. By then we have
moisture moving up from the south with a warm front while the low
level air mass will remain cool enough for snow. The models are
not exactly aligned so overall confidence remains low. The ECMWF
is drier compared to the GFS at this point. We'll see how this 
pans out over future solutions. Looking further ahead, a warmer 
system may bring higher snow levels and warmer conditions toward 
the middle of the week - back to rain in the lowlands. 33


.AVIATION...There are still some clouds and scattered light showers 
over Western Washington this afternoon. Northerly flow and drier air 
will arrive tonight for clearing skies and it should be sunny on 
Thursday. There will probably be a few patches of low clouds or fog 
Thursday morning, but they will not persist too long. 

KSEA...Skies will clear this evening. A north breeze will persist 
into Thursday.


.MARINE...Small craft advisories are up for the coastal waters for 
the swell. Breezy northeast winds are expected to develop tonight in 
the northern waters as high pressure builds over British Columbia. A 
cold front will reach Western Washington on Friday.


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


PZ...Small Craft Advisory for hazardous seas until 6 AM PST Thursday 
     for 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 8 AM PST Thursday for 
     Grays Harbor Bar.

     Small Craft Advisory from 11 PM this evening to noon PST 
     Thursday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern 
     Inland Waters Including The San Juan Islands.