Weather

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Area forecast discussion 
National Weather Service Seattle Washington 
300 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...heavy snow continues to fall in the mountains though 
it should taper by morning. Snoqualmie Pass has changed to rain or a 
rain/snow mix but could easily change back to snow overnight. 
Another foot or so of snow is likely tonight in the Cascades. 


Next problem is heavy rain. Starting about 18z Tuesday a warm front 
will drape itself over western Washington and not leave the area 
until Thursday morning. The result will be rain...heavy at times in 
the mountains...along the coast...and over the southwest interior 
and lower Chehalis valley. Amounts are definitely copious looking. 
Mountains will be mostly 5-10 inches with some southwest facing 
slopes 15-20 inches. Perhaps 6 inches will fall on the coast and SW 
interior. Seattle area could see 4 inches over the two days and 
northern lowland areas will get an inch or two. 


This sums to a real flood potential...both for river flooding and 
urban and small stream flooding. Unlike some of the false alarms of 
the previous two weeks...this flooding will be driven by heavy rain. 
Even if model amounts turn out to be too high by 50 per central or 
so...there will probably be enough to cause some flooding. The 
modeled amounts...and the amounts in the National Weather Service grids...are enough to 
cause major flooding on many rivers. Will go with a Flood Watch for 
practically everywhere. 


Snow levels will rise to 7000 feet in the Olympics and 5500 feet in 
the Cascades...meaning most of the precipitation will be rain. High 
temperatures in The Lowlands will approach 50 degrees each of the 
next three days. It might feel warm if not for all the rain. Burke 


Long term...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...onshore flow continues this afternoon and is limiting 
ceilings over most areas to around 1500 to 2500 feet and a few 
showers. Little change in the air flow is expected overnight...so 
would expect this trend to continue. Some areas of IFR should form 
as temperatures cool and rain begins again later 
tonight...continuing into Tuesday as the next weather system moves 
inland. Another frontal system is expected on Wednesday. 


Ksea...low level onshore flow continues with MVFR ceilings 1500 to 
2500 this evening. Expect ceilings 500 to 1000 feet with visibility 
3-5sm in rain after 10-12z. Little improvement Tuesday as main 
portion of system crosses West Washington. South to southwest winds 10 to 15 
knots with gusts to 18 to 25 knots this evening...easing to 6 to 10 
knots after 09z. Prange 


&& 


Marine...onshore gradients ease overnight with winds diminishing 
most areas. Small Craft Advisory for winds remain over the coastal 
waters and until 06z for the entire Strait. Winds increasing again 
late in the day...Tuesday as another strong frontal system 
approaches. Gales are possible over the coastal waters Tuesday night 
into Wednesday with this system. Prange 


&& 


Sew watches/warnings/advisories... 


Washington...Winter Storm Warning mountains through 6 am Tuesday. 
Avalanche warning mountains through Tuesday. 
Flood Watch most of western Washington Tuesday night through Friday. 


Pz...Small Craft Advisory coast and entire Strait. 
Small Craft Advisory for rough Grays Harbor bar. 
Avalanche warning 
$$ 


Weather.Gov/Seattle