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Area forecast discussion 
National Weather Service Seattle Washington
930 PM PST Tuesday Nov 25 2014

Synopsis...a warm front will remain near the Canadian border
tonight and Wednesday with rain at times over the north part and
rain likely elsewhere tonight. Rain will diminish with just a chance
of rain on Wednesday. A front will move across the area Wednesday
night for more rain and windy conditions. Rain will continue on
Thanksgiving day as another cold front approaches from the north
then crosses the area Thursday night. Cool dry and breezy weather
will develop by Friday afternoon and continue through the weekend as
high pressure builds over British Columbia. Weak weather systems may
bring a little rain early next week.

&&

Short term...the surface warm front is just north of Puget Sound sound at
8 PM with southerly winds increasing S of the front and remaining mainly
light from near the East Entrance to the north interior. Alki was reporting
wind gusts to 43 knots this past hour and other observation sites around the
sound are gusty as well. Rain will persist to the north of the front so
another inch or locally 2 inches is possible tonight in the north
Cascades. The warm front should lift north of the WA/b.C. Border by
Wednesday morning with the bulk of moisture to the north of Washington. There
will be some rain at times in the Olympics and areas north of Puget Sound
sound and the Cascades but amounts do not look hydrologically
significant.

Rain will pick up again Wednesday night into Thursday as a strong
cold front pushes into western Washington. The strong SW flow aloft will favor
heavier rain along S/SW facing slopes of the Olympics and north
Cascades. 3 to 5 inches of rain could fall renewing the flood threat
for the skokomish basin and also prone rivers in the north Cascades.
See the Hydro discussion for details. It will also rain across The
Lowlands but the strong SW flow will probably shadow portions of
Puget Sound sound with less rainfall amounts expected.

Models have trended toward a second frontal boundary hanging up over
the area on Friday. This is ahead of a colder air mass over b.C.
Trying to push southward. The westerly flow which starts off strong during the
morning will weaken considerably through the day. This will limit
orographic enhancement along the Cascades. While an inch or two is
possible across the northern part of the forecast area...the impact on
rivers will probably be minimal. Will need to look over all the 00z
runs before making any adjustments to quantitative precipitation forecast.

Colder air will be damming up along the Canadian rockies later
Friday and Friday night. Fraser outflow is expected to develop and
colder air aloft will also start to filter into the area. Some
models show residual moisture in place...low level convergence...and
colder air filtering in. Will need to watch this situation closely
as this set up can sometimes result in localized lowland snow. Will
review this potential for the morning forecast issuance.

Long term...from previous discussion...based on agreement between
the 12z GFS and 12z European model (ecmwf) Saturday and Sunday were cleared out and
the mention of rain dropped...except for the S part late Sunday.
With cool dry north flow aloft over the area and cold dry NE flow from
the Fraser valley...skies will be mostly sunny...temperatures
cool...and north-NE winds breezy. Lows will drop into the 20s Saturday
night with highs Saturday and Sunday in the upper 30s to middle 40s.
However...confidence in the models is still rather low...and the 18z
GFS has just reversed course and brings rain up from Oregon on
Sunday. The European model (ecmwf) has been more consistent so remains the preferred
solution. The European model (ecmwf) brings moisture and rain northward from Oregon
Sunday night. With cold air in place over West Washington there may be a
chance of some light snow...particularly over the northern zones as
the precipitation moves north. So far this is not accounted for in the
forecasts and will have to be re-evaluated once model consistency
and agreement improves. Kam

&&

Hydrology...the area of heaviest rain in the Cascades extended from
the Snoqualmie basin to the north Cascades this evening...and it
continues to shift slowly northward. The heaviest rain for the event
so far has occurred in The Mount Rainier area...where locally up to
7 inches has fallen since Monday afternoon. The Cowlitz... Nisqually
and Carbon rivers all flooded. That area will only receive another
inch or so of rain tonight and Wednesday. The Nisqually and Carbon
rivers have receded below flood stage...and the Cowlitz at Randle
will crest late this evening.

Up to 5 inches of rain has fallen over the basins of the central and
north Cascades...but so far only the Snoqualmie river is flooding.
More heavy rain will fall in this area tonight as the baroclinic
band shifts northward...with another 1 to 3 inches likely as the
snow level remains 7000 to 8000 feet. So other rivers could still
flood...and in fact the upper reaches of the Snoqualmie and
Skykomish have started rising again. In addition the Elwha
river...flowing off the north side of the Olympics...is rising more
quickly than expected and it will need watching for the next several
hours.

The next frontal system will bring another round of heavy rain
Wednesday night through Thursday night...renewing the possibility of
flooding on Cascades rivers and also perhaps driving the Skokomish
River above flood stage. There will probably be 3 to 6 inches of
rain...with the Olympics and north Cascades receiving the heaviest
amounts. The snow level will be 5500 to 7000 feet. Mcdonnal

&&

Aviation...a warm front just north of Puget Sound sound will lift toward
the b.C. Border by Wednesday morning. Strong SW flow aloft will
persist through Wednesday. Stable and moist conditions will persist
but rain will become more spotty in nature. A front will push into
western Washington Wednesday night.

IFR ceilings and visibilities have been stubborn to improve north of
the front...but after the front passes ceilings should lift to low
end MVFR at least...1-2k feet...and visibilities will improve to 4-6sm
br. Light winds at the surface will increase tonight behind the
front.

Ksea...MVFR conditions likely through Sunday with little rainfall
expected due to shadowing off the Olympics. SW winds 15-20 knots west/
gusts 30 knots...easing to 10 knots on Wednesday.

&&

Marine...a front north of Puget Sound sound will move into southern
British Columbia by Wednesday morning. Small Craft Advisory strength
winds will spread north behind the front...except gale force winds
in Puget Sound sound.

Winds will increase Wednesday as a cold front approaches the coast.
At least Small Craft Advisory level winds are likely over most
waters with gales possible on the coast and over exposed inland
areas Wednesday night. A gale watch is in effect for the coast...the
Strait entrances...and the northern inland waters Wednesday night.

Another cold front will move through the waters on Friday. This
front will be followed by strong Fraser outflow in the northern
inland waters and moderate northerly winds over the other waters
later Friday into Saturday. Chb/Mercer

&&

Sew watches/warnings/advisories...
Washington...various flood watches and warnings in effect for portions of
western Washington.
Pz...Gale Warning for Puget Sound sound through 6 am Wednesday morning.
Small Craft Advisory all waters except East Entrance and Puget Sound
sound.
Small Craft Advisory for rough Grays Harbor bar.
Gale watch coast...entrances...and northern inland waters
Wednesday night.

&&

$$

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

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