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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 151206

Area Forecast Discussion
National Weather Service Seattle WA
406 AM PST Fri Dec 15 2017

.SYNOPSIS...A weak cold front will spread light rain across
Western Washington this morning as it moves southeast. An upper 
level trough trailing the front will bring a few showers this 
afternoon and evening. A warm front will spread more rain and 
mountain snow across Western Washington Saturday, then a slow 
moving cold front will keep Sunday wet at times. After a minor 
lull on Monday, a deepening low center should pass across 
southwest B.C. on Tuesday, possibly bringing windy conditions and 
heavy mountain snow. Drier high pressure will build across the 
Pacific Northwest next Wednesday and Thursday.


.SHORT TERM...A weak cold front will gradually move southeast through
Western Washington this morning. This front will be most notable 
for producing the first rain since early last week, albeit light 
rain in places. Radar already shows light bands of rain spreading
southeast through Northwest Washington early this morning. The 
front will exit by early afternoon. Some onshore flow behind the 
front will keep a few showers going into this evening, mainly in 
the Cascades and in a diffuse Puget Sound Convergence Zone. In 
terms of cooling, the mountains will see the most cooling after 
such a long stretch of warm air aloft. 

Some flat upper ridging will rebound over the coastal Northwest on
Saturday. This will push a warm front onshore. Rain and mountain 
snow will spread onto the coast on Saturday morning then spread 
inland later in the day. 

As the ridge flattens and retreats to the south on Sunday, an 
east-west oriented cold front will slowly sag in from the north. 
The tail end of a shortwave in strong west flow aloft will enhance
lift along the front. With the slow movement of the front and 
strong west flow aloft, heavier precip amounts are likely with 
moderate snow accumulation in the mountains. The front will 
eventually reach Southwest Washington on Monday evening before 
turning up stationary. Haner

.LONG TERM...The weather on Tuesday bears a watchful eye. A strong
shortwave will travel east along the leftover front just to our
south. Cyclogenesis is expected along the front on Monday between
130W and 140W. Under the influence of strong upper divergence, 
the low will rapidly deepen while it tracks east on Tuesday to 
central or southern Vancouver Island. The 00z ECMWF deepens the
central pressure by about 20 mb in 24 hours, and the 06z GFS is
not far behind. GEFS ensemble means show similar trend, so
confidence is good at this point. The 00z ECMWF shows the low 
continuing to rapidly deepen below 995 mb as it crosses the B.C. 
Coast Range and passes our longitude. The continued strengthening
is key, because such lows bring stronger wind and more wind 
impacts than lows that are weakening. Again, this setup bears 
watching for wind impacts. In addition to wind, heavy mountain 
snow appears likely thanks to a sharply defined trailing cold 
front that will move across the area concurrent with the large- 
scale forcing and lift of a deepening parent low.

Very cold air aloft will follow Tuesday PM's front, with 500 mb
temps falling to -34C on Wed morning. So plenty of post-frontal
convective showers and snow levels down to 2000 feet or less will

Later Wednesday, an upper ridge axis around 135W will strengthen,
amplify, and spread toward the Pac NW. The air mass should quickly
stabilize. By next Thursday, northwest flow aloft with 500 mb
heights nearing 570 decameters along the coast should lead to dry
weather. All 20 members of the GEFS ensemble system show no precip
at SEA next Thursday, so dried out the forecast for all but the
Mount Rainier area. Haner


.AVIATION...A weak low pressure system and cold front will move over 
Western Washington Friday morning. The air will become moist with 
areas of light rain or showers developing. Southwest flow aloft will 
become northwest later today. Areas of light rain or showers 
tapering off by evening except for showers in the PSCZ.

KSEA...Clouds will thicken up with lowering cigs as light rain or 
showers develop this morning. A southerly breeze could shift to 
northerly around 22z with a weak fropa but variable is a good bet too 
through evening as convergence sets up with some shower activity.


.MARINE...A 1020mb low and cold front will move over Western 
Washington this morning. Small craft advisory northwest winds 
will develop over the coastal waters behind the front and then 
spread into the Strait of Juan de Fuca by evening. A warm front will 
move through the waters late Saturday night into Sunday. Some small 
craft advisory conditions are likely across the waters, especially 
Sunday, as flow aloft interacts with the area terrain. 

A deepening low will move onto Central Vancouver Island on Tuesday
then move east more or less along the Canadian border. Widespread
small craft advisory conditions are expected with this system and
gale force winds are possible over some waters.


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


PZ...Small Craft Advisory from 6 AM this morning to 3 AM PST Saturday 
     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.

     Small Craft Advisory from 4 PM this afternoon to 3 AM PST 
     Saturday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-West 
     Entrance U.S. Waters Strait Of Juan De Fuca.