Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 151724

Area Forecast Discussion
National Weather Service Seattle WA
924 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...The strong, persistent, upper level ridge that`s
been over the region is finally breaking down today as a weak cold
front moves inland. Precipitation amounts have been light but it`s
the first shot at measurable rain for most areas in 11 days. Low
level onshore flow will increase behind the front this afternoon
and models are showing showers due to a Puget Sound Convergence
Zone over King county. Light showers will mainly impact the
Cascades overnight. Temperatures are trending close to normal.

The next system, on Saturday, is a warm front that will mainly
clip the coast then dissipate over southern B.C. Showers will
increase on Sunday as the cold front shifts SE into Western
Washington. By then the snow levels will also drop to around
4,000-5,000 feet and the volcanoes may see a period of heavy snow.
Another warm front will reach the area on Monday for more wet
weather. 33

.LONG TERM...Previous discussion...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 frontal system will move across Western
Washington today. Moderate southwest flow aloft will become
northwest by early evening. The air will become moist at all levels
by midday with areas of light rain or showers, ending during early
this evening.

A puget Sound convergence zone could develop later today into the
mid evening hours and affect the KPAE to north Seattle corridor.
Current thinking is that any convergence zone is unlikely to reach
the KBFI/KSEA terminals, but radar will need to be watched.

KSEA...Cigs may be slower to lower but MVFR conditions are still
expected with another batch of rain likely this afternoon. Expect
some timing adjustments (cigs, rain) with the 18z TAF issuance. S-
SW wind 6-9 kt.


.MARINE...Small craft conditions will prevail today through most
of tonight as a frontal system passes the area. Onshore flow through
the Strait will also likely produce small craft winds in central and
eastern portions during the late afternoon and evening hours. 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 until 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 1 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.


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