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Area forecast discussion 
National Weather Service Seattle Washington 
400 PM PST Sat Nov 21 2009 


Synopsis...a vigorous frontal system will move through western 
Washington this afternoon and evening bringing strong winds...rain 
and heavy mountain snow to the area into Sunday. A weaker front will 
move through the area Monday and Monday night...and another weak 
front will later in the week. 


&& 


Short term...a warm front will lift across western Washington early 
this evening followed by an occluded front and surface low late 
tonight into Thursday morning. Mesoscale models advertising a more 
southerly track for the the surface low...moving onshore near khqm 
just after midnight tonight...then northeast into the south sound 
and then east of the Cascades by late Sunday morning. This 
will produce windy conditions across the area. Strongest winds will 
likely come as a one two punch with the southerlies ahead of the low 
and occluded front tonight...and again with the westerlies behind 
the low and front as they track inland early Sunday morning. Some 
areas like the coast...Admiralty Inlet...north interior will 
experience both periods of strong winds...most other areas will 
experience the strongest winds with and behind the front late 
tonight into early Sunday morning. Winds should begin to come down 
by late Sunday morning and into the afternoon hours all areas. 


The snow level will continue to rise to around 3000 feet tonight as 
the warm front lifts over the area...falling again to around 2000 feet 
tomorrow morning as the occluded front pushes through. Both fronts 
will bring significant precipitation to the coast...Olympics and 
Cascades tonight through Sunday morning. Heavy snow for the 
mountains remains on track with impressive precipitation amounts 
through Sunday morning for the Olympics...3 to 4 inches...and the 
Cascades...1 to 3 inches. Even with the rise of snow levels to 
around 3000 feet...30 to 40 inches of snow are expected for the 
Olympics and 10 to 30 inches for the Cascades. Even the relatively 
drier Hurricane Ridge area should receive Winter Storm Warning 
amounts...12 inches in 12 hours or 18 inches in 24 hours. 


Flow is strong enough through the day tomorrow that convergence zone 
is unlikely...with most of the Post-frontal showers pushed up onto 
the mountains. Showers will taper off Sunday afternoon evening as 
the upper ridge moves overhead. The ridge kicks east quickly Monday 
ahead of another frontal system. This next system could still bring 
decent precipitation amounts to the coast and olympic peninsula 
Monday afternoon...but weakens as it moves inland Monday night. 
Compared to last several systems this will be an uneventful 
system...marking the beginning of a period of quieter weather. Cook 


Long term...a weak upper ridge will settle in over the region 
Tuesday and Wednesday with a chance of rain in the forecast for 
Tuesday and Wednesday. A weak front will slowly move across western 
Washington Wednesday night and Thursday. This system looks to bring 
more precipitation to the area than mondays weak system...mostly due 
to its slow transition across western Washington...which might bear 
some watching as we move into early next week. Medium range models 
in a bit more agreement on the frontal wave that slides into the 
area Thursday night and Friday. 


&& 


Hydrology...todays system will bring impressive amounts of 
precipitation to the Cascades and Olympics...but relatively low snow 
levels will mitigate the impact on western Washington rivers except 
the Skokomish River which is already at or near flood stage going 
into this event. The snow level will only rising to around 3000 feet 
tonight...and then fall Sunday to around 2000 feet...making this 
mostly a mountain snow event and not a rain event. 


The Skokomish River fell to just below flood stage early Saturday 
afternoon. New forecast guidance shows the river rising again to 
just below 17 feet by early Sunday morning. In addition...there has 
been planned release of 3000 cfs from Cushman dam #2 into the North 
Fork Skokomish River this afternoon. Have accordingly extended the 
warning for the skokomish into Monday. 


Guidance and observations for the Lower Reach of the Chehalis river 
in Grays Harbor County continue to support the idea that it will 
remain well below flood stage so have accordingly ended the Flood 
Watch. 


Mondays weaker system looks to be progressive enough to be of little 
concern with respect to flooding. The air mass over the northwest 
for the remainder of the week will be warmer with higher snow 
levels...however the systems that are expected to affect the area 
will be fairly weak and are not expected to cause any additional 
flooding concerns. 


No flooding is expected at any time on the Green River. Cook 


&& 
aviation...a warm/occluded front just offshore this afternoon will 
lift across western Washington early this evening. Southerly flow aloft will become 
strong. The atmosphere will be stable and moist with rain increasing 
across the area. Surface gradients become more Ely overnight with 
strong southerly flow just off the surface creating the potential for 
low level wind shear. Tafs were updated earlier to include this for most 
terminals...generally in the 02z-12z time frame. 


Surface low pressure will deepen to around 985-990 mb tonight and 
track into southern Washington near khqm around 12z. Strong pressure rises are 
expected behind the low from the central coast eastward into the SW 
interior which will result in a burst of strong winds sometime 
between 12-16z Sun morning. The atmosphere will be somewhat unstable 
near the low center...allowing winds aloft to mix down to the 
surface in gusts. A burst of wind down the Strait will also develop 
Sun morning. 


Ksea...low level wind shear is possible from around 02z-12z as surface gradients 
become Ely and southerly flow of 30-40 knots develops around 2k feet. 
Otherwise...rain will increase this evening with MVFR conditions 
expected. Southerly winds around 10 knots through tonight...but a dramatic 
jump in SW winds is anticipated after 12z as a surface low tracks 
inland near or south of ksea. SW winds of 20-30 knots with higher gusts 
are possible from 12-16z. Dtm 


&& 


Marine...winds are slowly picking up across western Washington in advance of a 
warm/occluded front just offshore. The 12z 4km MM5 came in much 
stronger and widespread with 40 knots winds across the north interior. The 
high end Gale Warning for that area looks good through this evening. 
Gales will also develop on the coast which will extend into Sun 
morning with a second burst of winds with the surface low. The MM5 
is showing widespread 40 knots there as well which is short of storm 
force...so I will maintain the high end gale. 


A potentially strong 985-990 mb surface low will enter the coastal 
waters after 09z tonight and track into the central/S Washington coast 
around 12z. Winds behind the front will rise drastically with high 
end gales expected through Sun morning across the coastal 
waters...especially over the south/central portions. A 3-4 hour 
burst strong westerly winds down the Strait into the Whidbey Island and 
Admiralty Inlet area is also expected after 12z. Dtm 


&& 


Sew watches/warnings/advisories... 
Washington...flood warnings remain in effect for portions of western 
Washington... 
Winter Storm Warning in effect for the Olympic Mountains to 4 
am PST Sunday and for the Cascades from to 1 PM PST 
Sunday... 
High Wind Warning for the central and South Washington 
coast...lower Chehalis valley...southwest interior...and 
Admiralty Inlet area through 10 am Sunday... 
High Wind Warning for the north interior of western Washington 
through midnight tonight... 
Wind Advisory for the central Puget Sound sound area and Strait of 
Juan Delaware fuca zones midnight tonight through Sunday morning... 
pz...a Gale Warning is in effect for the coastal waters...Admiralty 
Inlet and the entrances to the Strait of Juan Delaware fuca through 
Sunday morning. 
.A Gale Warning is in effect for the northern inland waters 
through midnight tonight. 
.A Gale Warning is in effect for the central Strait from late 
tonight through Sunday morning. 
.A Small Craft Advisory is in effect for Puget Sound sound through 
Sunday morning. 


$$ 


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For an illustrated version of the forecast discussion...please see 
www.Weather.Gov/Seattle/gafd/latest_webafd.Html. 

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