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Area forecast discussion 
National Weather Service Seattle Washington
900 PM PST Wednesday Dec 17 2014

Synopsis...a weak upper trough will move east of the Cascades late
tonight. The next frontal system will bring rainy and locally windy
weather to western Washington later Thursday and Thursday night...
with showers decreasing Friday. Windy rainy weather will continue
this weekend as a vigorous frontal system moves through the Pacific
northwest. Somewhat drier weather is likely early next week.

&&

Short term...a weak upper trough is moving across western
Washington this evening. Areas of light rain continue across the
area...especially in the Puget Sound sound region. The upper trough will
move east of the Cascades late tonight...and a small upper ridge
will move into the offshore waters. Then the ridge will progress
inland quickly on Thursday...moving into eastern Washington by late
afternoon.

A Pacific frontal system will start spreading rain into western
Washington Thursday...probably just behind the upper ridge
axis...with precipitation continuing Thursday night as the system
moves inland. The upper trough following the cold front will move
through western Washington on Friday. This is not an especially
strong front for December...but it will bring steady rain and
locally windy conditions to the coast and north interior.

Storm total precipitation for the 36 hour period ending Friday
afternoon should be 0.75 to 1.5 inches over most of the mountains...
with up to 1 inch at the coast...and 0.25 to 0.75 over most of the
interior lowlands. Then snow level will rise to 4000 to 5000 feet
Thursday night...then fall to around 3500 feet Friday. Storm total
snowfall at the higher passes and ski resorts should amount to 3 to
8 inches or so.

A vigorous frontal system will move quickly into western Washington
late Friday night and Saturday. It will bring another round of
locally windy conditions...steady rain to the western Washington
lowlands...and rain and snow to the mountains with somewhat higher
snow levels. Mcdonnal

Long term...here is the long term section from the afternoon
forecast discussion -- models now show a break or at least decrease
in rainfall rates Sunday night as weak high pressure temporarily
builds offshore. The ridge will lift the flow northward so moisture
directed into southern Washington/Oregon will lift back over western Washington on Monday.
The ridging is quite a bit stronger on most of today's model runs
which would shut off precipitation by late Monday or Monday night.
The good news is that if heavy rain does develop it appears there
will be some shifting between the Cascades of western Washington/western Oregon.
This should limit potential quantitative precipitation forecast in any one particular area. Models
now show 24 to 36 hour quantitative precipitation forecast amounts of 4 to 6 inches more evenly
distributed along the Washington/Oregon Cascades. This is still enough for
some minor flooding on the more prone rivers so forecasts should be
monitored closely.

Another surface low will track into western Washington or northern Oregon around
Tuesday. Models no longer tap this low for a continuous moisture
stream which instead arrives after a break in rainfall. Models are
also trending much weaker with this low so additional rain amounts
look to be light and not hydrologically significant.

Long range models are indicating a possible pattern change by the
middle of next week. However...models differ on the positioning of a
high over Kona low pattern over the Pacific which will determine the
strength of northerly flow over western Washington. The 12/18z GFS is further east with
the low and dumps most the cold air into The Rockies. Colder air
would arrive across western Washington if the European model (ecmwf) solution verified with the
low further west. Either way...a transition to cooler temperatures is
likely enough to trend high/low temperatures downward a bit for Wednesday.
Mercer

&&

Hydrology...river flooding is not expected through Saturday. A
moist westerly flow pattern will develop Saturday night and continue
through Sunday night with heavy rain at times in the mountains. The
latest 12z GFS/European model (ecmwf) are faster with the trailing frontal band and
pushes it southward into Oregon by Monday. This could help mitigate any
river flooding that develops but does not end the threat as models
have not all settled on a particular solution. 24 to 36 hour rain
amounts are still averaging about 4 to 5 inches over the Washington Cascades
which could cause flooding on prone rivers. At this time...primary
concern for any river rising above flood stage is the skokomish but
also need to keep an eye on other Cascade flood prone streams. Mercer

&&

Aviation...a weak upper level trough is over western Washington
this evening with light westerly flow aloft. The air mass is moist
and stable with weak surface gradients. A cold front about 650 nm
west of Seattle at 04z will push into the region Thursday evening
with strong southwest flow aloft.

Areas of light rain will persist across western Washington overnight
with widespread MVFR ceilings and localized IFR. Some temporary
improvement to VFR is possible midday Thursday as a transitory upper
ridge moves over the area and surface gradients become increasingly
offshore. Widespread rain will spread back into the area by early
evening ahead of the next front with ceilings falling back to MVFR.

Ksea...MVFR through the night with light rain at times. VFR ceilings
at or above 5k feet possible 18z-00z but will drop again with more
rain Thursday evening. Light south-southeast wind
overnight...becoming southeast 7 to 12 knots Thursday afternoon.

&&

Marine...a weak trough lies over the waters this evening for light
winds. A strong front associated with a 970-975 mb low west of
haida gwaii will move through the offshore waters on Thursday...then
inland Thursday night. A period of gale force winds can be expected
over the coastal waters Thursday afternoon through Thursday night
with Small Craft Advisory conditions for the Strait
entrances...northern inland waters...and Admiralty Inlet. Long
period swell near 22 feet are still expected on the coast late
Friday afternoon and Friday night. Tidal anomalies along the coast
are just over 1 foot above normal and high tide occurs Saturday
morning. If swell remains high into Saturday morning
as currently expected...some coastal flooding is possible. Another
strong front will arrive on Saturday with gale force winds once
again likely over the coastal waters...the northern inland
waters...and the Strait entrances. 27

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Gale Warning Thursday and Thursday night coastal waters.

&&

$$

You can see an illustrated version of this forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

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