Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 141213

Area Forecast Discussion
National Weather Service Seattle WA
413 AM PST Thu Dec 14 2017

.SYNOPSIS...A strong upper ridge over the Pacific Northwest will
quickly flatten and break down through tonight, allowing a cold
front to move onshore with some rain on Friday morning. A warm
front will then try to spread onshore on Saturday and Saturday
night. A cold front will sag southward through Western Washington
Sunday night and Monday. A developing low center could move near
the area around Tuesday, followed by cool showery weather next


.SHORT TERM...This long streak of dry December weather is only
about 24 hours from reaching its end. The strong and amplified upper
ridge axis which has clung to the Pacific Northwest for the past
week will flatten quickly today and tonight, and westerly flow
aloft will become strong tonight. This will drive a cold front
onshore late tonight and Friday morning, accompanied by light to
moderate rain and mountain snow. Rain should first reach the North
Coast after midnight tonight, then arrive in the Puget Sound
region around 4-6 am. The front will be followed by weak, shallow
shower activity. Onshore flow will focus shower activity on
windward terrain and near a short-lived Puget Sound Convergence

Early Saturday morning, deep warm advection will develop, bringing
warm frontal lift and the development of a large stratiform precip
shield that will persist for most of Saturday and Saturday

.LONG TERM...Models are in good agreement with the longwave
pattern over the eastern Pacific and western North America through
next Wednesday.

An east-west oriented cold front will sharpen over Vancouver
Island early Sunday and then sag southward with precip later
Sunday and Sunday night. The front should turn up stationary over
Southwest Washington on Monday.

Meanwhile, an upper trough trailing southwest from the Gulf of
Alaska will advance eastward on Monday. Large-scale lift in the
right entrance region of an upper jet preceding the trough will
interact with Monday`s leftover front to support cyclogenesis off
the coast. The 00z ECMWF and the 06z GFS ensemble mean rapidly
deepen the low offshore and bring it on an E-NE track across
central Vancouver Island. The deterministic 06z GFS`s track
through Western Washington appears to be a rightward outlier at
this time. Still, it will support moderately windy conditions on
Tuesday, first in southeast-wind prone areas in advance of a
strong trailing cold front, then in the Puget Sound region
following the front. This will be something to watch in the days
ahead. In fact, the biggest change with this forecast package is
to significantly increase precip chances on Tuesday.

Once the deepening low passes east of our longitude, a fairly cold
ocean-modified air mass will spread across the Pacific Northwest
next Wednesday, along with typical post-frontal showers. For being
a Day 7 forecast, there is strangely good agreement in models
depicting Fraser Outflow initiating late next Wednesday or on
Thursday, bringing even colder but drier continental air from the
interior of British Columbia. Only two of the 06z GFS`s 20
ensemble members shows over 1 inch of snow at SEA and BLI next
Wed. In other words, the odds of significant lowland snow appear
very low right now through the middle of next week. Haner


.AVIATION...Surface high pressure is inland and the axis of the
upper ridge is just off the coast. There is offshore flow with dry
stable air--except for some high clouds and also patchy fog. Clouds
will increase tonight and Friday morning, with areas of rain friday
morning in a cold front.

KSEA...Some cirrus at times and a chance of some fog this morning,
winds will be light northeast and with the SEA-YKM gradient at -8mb
we might see an east breeze develop today. A little rain and a
southerly breeze should develop for Friday morning with a cold front.


.MARINE...Light offshore flow continues. Some areas of fog should
develop by daybreak. East winds at the West Entrance to the Strait
of Juan de Fuca and west of Grays Harbor will rise to around 20
knots at times today. A cold front will move through the waters
after midnight tonight through Friday morning with onshore flow.
Another frontal system will reach the area Sunday.


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


WA...Air Stagnation Advisory until 2 PM PST this afternoon for
     Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
     Vicinity-East Puget Sound Lowlands-Eastern Strait of Juan
     de Fuca-Everett and Vicinity-Hood Canal Area-Seattle and
     Vicinity-Southwest Interior-Tacoma Area-Western Skagit
     County-Western Whatcom County.



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