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Area forecast discussion 
National Weather Service Seattle Washington 
900 PM PST Wednesday Nov 4 2009 


Synopsis...a strong frontal system will move through the region 
Thursday. Rain will be heavy at times in the mountains and the coast 
and north portion of western Washington will be windy Thursday. The 
snow levels will fall to pass level by Friday and remain low through 
the weekend. Strong onshore flow will keep the region wet and 
blustery...and heavy surf will pound the beaches by Friday. Another 
strong frontal system will arrive Monday. 


&& 


Short term...a 945mb storm in the NE Pacific was centered near 52n 142w 
this evening. A broad area of strong southerly flow aloft extends from the 
Washington coast to the cold front and will work through the region Thursday. Cold 
frontal passage will be to the coast late Thursday afternoon and through 
Puget Sound sound and the inland portion of western Washington around 7pm. With strong southerly 
flow aloft the heaviest rain will be on the south side of the 
Olympics...and in the north Cascades. 24hr precipitation 12z Thursday to 12z Friday 
ranges from around a half inch in The Lowlands to 2-4" in the 
mountains and the past three mm5nam solutions are nearly identical. 
Typical rain shadow centered on Dungeness Spit with less than a 
tenth of an inch for that area east through Port Townsend. Southerly gales 
will blow on most of the marine areas...in fact pt Wilson and 
Destruction Island have already started up. There is a marine 
weather statement for the high seas which reach the coast Friday/Sat 
and an Special Weather Statement which talks about that as well as the windy conditions on 
tap and the return to wintry weather in the mountains. 




Long term...shortwave ridging takes place on Sunday...along with a 
brief rise in freezing levels. This will likely be the driest 
day...with lingering showers in the mountains. Another strong 
Pacific frontal system takes aim and plows into the area on Monday 
evening and Tuesday. Quantitative precipitation forecast totals look impressive for the Olympics and 
north Cascades for this system...but snow levels will stay below 
4000 feet for most of the event. Damico 


&& 


Hydrology...strong Pacific frontal system will drop 3-5 inches of 
rain on the Olympics and 2-4 inches in the north Cascades. 
Precipitation will begin in earnest late Thursday morning for the 
Olympics and later Thursday afternoon for the Cascades. Lesser 
amounts are forecast for the central Cascades. Snow levels will 
start off high...around 6500 feet...and drop to 3500 feet between 
06-12z Friday morning. 


Flood Watch will be reissued with little change...the rivers to 
watch at this point are still the skokomish and Nooksack with rivers 
responding to the precipitation on Friday. 


Snow levels will remain low through the weekend...so the two 
shortwaves rotating through Friday night and Saturday evening will 
have little Hydro impact. 


A stronger Pacific frontal system returns Monday. GFS shows 2-3 
inches of precipitation over the Olympics and about 2 inches over 
the north Cascades...but again snow levels will stay below 4000 feet 
for most of the event. Showers will follow on Wednesday...with cool 
northwest flow to follow through the end of the week. No Hydro 
concerns at this time. Damico 


&& 


Aviation...an upper level trough is digging offshore this evening 
with southwesterly flow aloft across western Washington. Rain will spread inland 
early Thursday morning with diffluent flow aloft. Heaviest precipitation will be 
along the coast. The surface cold front...currently located back of 130 
west on the infrared...with pass through the region late Thursday 
afternoon/evening. Winds will be strongest right ahead of the cold 
front with gusty southerly winds expected. Threat for showers will 
continue Thursday night as the broad upper level trough axis passes 
through...with moderate onshore flow at the surface. 


Ksea...clouds will increase overnight as a frontal system offshore 
approaches Washington...with rain spreading into the Puget Sound sound area late 
morning and afternoon. Gusty southerly winds are expected ahead of 
the cold front. Also...winds aloft at 5-10k above ground level will increase to 
S/SW 50 knots. Showers will continue Thursday night as the upper trough axis 
moves through the area. 33 


&& 


Marine...southeasterly flow will increase tonight ahead of a strong 
frontal system offshore. The cold front will pass through the 
coastal waters late Thursday afternoon/evening. Strongest winds 
will be ahead of the front with gale force winds possible across 
most areas. Moderate Post frontal west/southeast flow will persist Thursday night 
into the weekend. 


Meanwhile...this system centered over the NE Pacific will send a west/northwest 
swell train to the Washington coast with seas possibly building over 20 feet 
on Friday. Seas will remain high through the weekend. See special 
marine statement for further details. 33 


&& 


Sew watches/warnings/advisories... 
Washington...Flood Watch Mason and Whatcom counties Thursday afternoon through 
late Friday night. 
Pz...Gale Warning coast...entrances to the Strait of Juan Delaware 
fuca...Admiralty Inlet and north inland waters. 
Small Craft Advisory central Strait and Puget Sound sound. 
Small Craft Advisory for rough bar conditions 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.