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fxus66 ksew 120027 cca 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
400 PM PST Thursday Mar 11 2010 


Synopsis...a frontal system that sagged south over the area today 
has stalled over southern portions of the area this afternoon. A new 
low is beginning to develop well off the Oregon coast. This system 
will move northeast across the area Friday morning followed by rain 
turning to showers midday and produce gusty winds. The upper trough 
axis will pass through Friday night with decreasing shower activity 
on Saturday. Another front will approach the coast Sunday while a 
ridge of high pressure builds east of the Cascades. 


&& 


Short term...a strong frontal system associated with a deep low 
heading into Southeast Alaska has sagged and stalled over the southern part 
of the state. The heaviest precipitation is near the Washington 
border and south while shower activity can be seen further north. 


Generally up to 2 inches of precipitation fell in the coast and Olympics 
with up to 1 inch in the Cascades. Snow levels have been low enough 
and quantitative precipitation forecast light enough that there has been only modest rises on the 
major rivers. Roughly 6 to 12 inches of snow has been reported at 
the different nwac stations so far. Will keep the Winter Storm 
Warning going through Friday morning but precipitation rates tonight 
will not be as heavy as earlier today. 


The next wave to affect the area can be seen as a baroclinic Leaf on 
satellite near 35n/135w. The triple point feature of this new wave 
will be positioned off the north Oregon coast by 12z Friday and pass up 
and through western Washington around 18z. This will be accompanied by a 
quick ramp up in precipitation over the area with a jump in snow 
levels. Newest model runs shunt the warm up east of the Olympics 
Friday morning...bumping the snow levels in the Cascades up to 4000 
feet in the north and 4500 feet in the south. The front pushes 
inland Friday afternoon. Snow levels will quickly fall early Friday 
afternoon over the Olympics and later in the afternoon for the 
Cascades. Pressure rises along the coast and shifting into the interior 
will combine with an unstable Post frontal air mass to mix down 
strong gusts along the coast and interior. The high wind watch has 
been dropped along the Washington and north Oregon coast. 
Still...gusts over the interior on Friday afternoon will be strong 
with up to 45+ knots over the coast and 30+ range over the interior. 
Would not be surprised if a few gusts qualify for Wind Advisory over 
the interior with a few sun breaks in the afternoon. 


The trough axis is prognosticated to move through late Friday night. 
Showers will decrease over The Lowlands on Saturday as a shortwave 
ridge builds offshore. Although there will be a fair amount of 
sunshine in the afternoon ... will remain a few degrees 
below normal. 


Another system offshore will lifts warm frontal precipitation near 
the coast on Sunday. GFS brings precipitation inland Sunday 
night...while European model (ecmwf) keeps precipitation offshore. Keep a dry forecast for 
the interior with chances probability of precipitation for rain on the coast. Damico 


Long term...in a similar vein as Sunday...GFS and Canadian keep 
upper ridge axis slightly further east and allow frontal systems to 
in southwest flow to cover most of western Washington Sunday night 
through at least part of Tuesday. European model (ecmwf) continues to stand the front 
up offshore...keeping most of the area dry. Have continued the 
forecast trend of keeping the sound and central Cascades dry with a 
chance of rain elsewhere. New wave that develops in the southwest 
flow finally lifts a low to our north and east bringing rain Tuesday 
night followed by showers on Wednesday. Westerly flow keep showers 
in the forecast Wednesday night and Thursday. Damico 


&& 


Hydrology...generally up to 2 inches of precipitation has fallen 
over the coast and Olympics with the nwac Cascade gages reporting up 
to 1 inch of precipitation thus far. This amount of quantitative precipitation forecast combined with low 
snow levels has kept the rivers in check. A brief bump in snow 
levels....especially in the central Cascades...and another 1 to 2 
inches of precipitation with the secondary wave Friday will cause a 
secondary rise in the rivers tomorrow afternoon. Still...no main 
Stem flooding is expected at this time. A front passing through 
Friday afternoon will quickly lower freezing levels overnight. 


Showery weather will persist through Saturday. A large trough in the 
NE Pacific will turn the flow aloft to SW and direct most of the 
precipitation into central/southern British Columbia. Just light 
bouts of rain are likely over the area...especially the 
Olympics...Sunday through early Tuesday. A quick moving system will 
cross the area Tuesday followed by cooler weather and showers 
midweek. 


The flooding risk is low for the Green River for the next 10 days. 
Damico 


&& 


Aviation...a stationary front over SW Washington will support locally 
reduced visibilities in rain and IFR/MVFR ceilings through today and tonight. 
Rain will become more widespread again overnight as the front starts 
to move back north. A low pressure center moving north just off the 
coast on Friday will push a quick-moving cold front through western Washington. The 
front will be accompanied by a brief burst of strong cold 
advection...which is usually favorable for particularly gusty 
conditions and convective turbulence. 


Ksea...temporary breaks in the rain are expected through 09z...but 
rain will continue at times with ceilings generally remaining MVFR. A 
quick-moving compact cold front will pass sea-tac around 20z 
Friday...bringing a few hours of low-level wind shear in advance of the 
front and then followed by particularly gusty winds at the surface 
from 20z-24z Friday. The front will also be followed by a slow 
decreasing trend in cloud coverage but a few showers Friday afternoon. Haner 


&& 


Marine...small craft winds tonight will be hit or miss...probably 
more miss than hit. Still...a subtle wave moving onshore right now 
could enhance winds a bit this evening...so kept small craft going 
through tonight. 


A low pressure center will lift north between the coast and 130w on 
Friday. Increasing offshore flow and easterlies are expected at 
first...but then the system will push a quick-moving cold front 
across western Washington from the SW during the day. A brief burst 
of strong cold advection with this feature could bring particularly 
gusty winds to gale force along the coast with high-end small craft 
conditions over the interior as a surge of wind moves in from the 
S-SW. Haner 


&& 


Avalanche...increasing amounts of wind driven snow should be 
deposited over a variety of existing weak layers over an old crust 
Thursday as the first of several frontal systems moves over the 
region. Along with strong Ridgetop winds and slow warming this 
weather should produce a significant increase in the avalanche 
danger. Although a slight decrease in precipitation rates are 
expected later Thursday as the first storm weakens and stalls...a 
secondary front should spread further moderate to heavy rain or snow 
northward over the region overnight and Friday morning...along with 
another surge of strong winds and further warming. This should allow 
developing high avalanche danger to spread to progressively lower 
elevations. As a result of these increasingly dangerous avalanche 
conditions likely over the next 24 to 36 hours an avalanche warning 
will be issued shortly. Back country travel in avalanche terrain is 
not recommended. Consult nwac.US for further details. Moore 


&& 




Sew watches/warnings/advisories... 
Washington...Winter Storm Warning through 6 am Friday morning for the 
Olympics and west slopes north and central Cascades and passes. 
.High surf advisory 10 am this morning through Friday morning 
for the north and central Washington coast. 
.Avalanche warning from 10 PM tonight through noon on Friday 
morning for the Olympics and Cascades near and west of the 
crest. 


Pz...Gale Warning Friday coastal waters and West Entrance. 
Small Craft Advisory all other waters except central Strait. 
Small Craft Advisory for rough bar Grays Harbor bar. 


$$ 


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

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