Weather

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afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
900 PM PST Monday Jan 5 2009 


Synopsis...rain will develop tonight as warmer air flows into 
western Washington. Freezing levels will rise significantly and 
rain...heavy at times...will last into Thursday. The mountains will 
receive heavy rain and flooding is possible over much of western 
Washington Wednesday and Thursday. There could be a drying trend at 
the end of the week. 


&& 


Short term...snow continues in the mountains this evening but 
lighter than earlier today. Mountain temperatures still support snow 
through most passes with increasing snow later this evening as warm 
frontal precipitation lifts into the western Washington. This warm front will 
eventually boost snow levels Tuesday morning to well above pass 
levels. The Winter Storm Warning will remain in effect for tonight 
for another foot or more. Thereafter...the concern becomes flooding. 


Very strong upstream confluence and resulting stronger jet stream 
will direct a deep moisture Plum into western Washington for a 48 hour period 
starting later tonight. Models indicate freezing levels to shoot up 
over 6k feet on Tuesday...highest central Cascades and Olympics. Also 
very strong west-SW 850 mb flow behind the warm front should produce 
strong orographics all the way through Wednesday night. MM5 and 
global model runs from 00z show widespread 5-10 inch rain amounts 
across the Olympics and Cascades. Not sure how the heavy snow over 
the past couple weeks will affect river rises...but this amount of 
rain should easily produce flooding. Hydro models do take snowpack 
into account and many rivers are shown to easily exceed major flood 
stage or possibly approach record stage. The Flood Watch will remain 
in effect for most of western Washington. River rises will begin Tuesday 
with the first rivers to hit flood stage sometime Tuesday night. 
Continue to monitor flood statements closely. 


Mercer 


Long term...previous discussion follows...GFS and Euro both dry 
things out on Thursday night and keep it dry Friday. Have some 
chance probability of precipitation in there now but the GFS in particular has offshore 
surface flow and high heights...Friday could be a warmish and dry 
day...with conditions continuing into Saturday. On Saturday night 
both the Euro and GFS swing a short wave through for a chance of 
showers Sunday and Monday. Burke 


&& 


Avalanche...further significant warming and heavy snow changing to 
rain or heavy wet snow Tuesday will Load the 2 to 5 feet of snow 
received since new year's...causing the avalanche danger to become 
extreme in many areas especially above 4-5000 feet...hence 
backcountry travel should be avoided Tuesday. Kramer 


&& 


Aviation...a developing warm front extending east from a low near 
48n/162w will lift slowly NE across the area Tuesday. The front will 
reach the SW Washington coast around 18z then stall as it moves into the 
central Puget Sound sound lowlands late Tuesday afternoon. Ahead of the front 
expect rain with decreasing ceilings and visibilities to develop 
from the southwest reaching ksea 08-10z. Low ceilings and 
visibilities will persist through late Tuesday. Once the warm front 
passes...expect winds aloft to increase significantly with winds at 
5000 feet rising to west-southwest 50-70kt. This will give very heavy rain to the 
mountains and windy conditions with S winds increasing to 20 to 30 
knots in the Puget Sound sound lowlands at the surface as the flow aloft 
interacts with the terrain. Little change is expected Tuesday night or 
Wednesday. 


Ksea...ceilings near 1000 feet above ground level through about 16z then falling to 
broken-ovc007-009 16z through 24z. Visibility dropping to 2-4 sm -ra br 
16z-24z. Conditions may improve slightly behind the warm front late 
Tuesday afternoon and Tuesday night with visibility improving 5-6sm -ra br. 
S flow 10-13 knots will likely increase to 19022g30kt around 00z and 
persist through Tuesday night. Albrecht 


&& 


Marine...onshore flow will decrease somewhat overnight...but will 
increase significantly Tuesday as a warm front lifts north across the 
area. As the front pulls up stationary across the central to north 
portions of the waters Tuesday night through Wednesday night...a series of 
strong energetic waves will move east along the front with strong 
flow aloft interacting with the terrain. This will result in nearly 
steady state gales along the coast and high end Small Craft Advisory 
conditions elsewhere. There is a possibility that gale force winds 
will develop over Puget Sound sound...Admiralty Inlet...or the northern 
inland waters as a mesoscale low develops to the NE of the Olympic 
Mountains. 


High pressure will build into the waters Thursday through Friday night. A weak 
front will move by to the north of the waters Sat giving an increase 
in southerly flow. Albrecht 


&& 


Sew watches/warnings/advisories... 


Washington...Winter Storm Warning mountains through 6 am Tuesday. 
Extreme avalanche danger mountains through Tuesday. 
Flood Watch most of western Washington Tuesday night through 
Friday. 


Pz...Gale Warning coastal waters. 
.Small Craft Advisory Strait of Juan Delaware fuca and Puget Sound 
sound and Hood Canal. 
Small Craft Advisory Grays Harbor for rough bar conditions. 




$$ 


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