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Area forecast discussion 
National Weather Service Seattle Washington 
320 am PST Sat Nov 7 2009 


Synopsis...a broad upper level trough with cool moist unstable air 
will continue to direct showers and breezy weather into western 
Washington through Saturday night. A warm front will move through 
the area Sunday afternoon and evening...followed by a cold front on 
Monday. A weaker cold front should cross the area Tuesday followed 
by showers from an upper level trough on Wednesday. A weak upper 
level ridge on Thursday should lower the chance of rain...but 
another frontal system will probably reach the area Friday or 
Saturday. 


&& 


Short term...the large scale hemispheric 5-wave pattern looks 
pretty stable over the next week and a half...so western Washington 
will remain in a wet pattern with a broad strong long wave trough 
offshore most of the time. 


GFS and European model (ecmwf) are in good agreement through middle next week 
maintaining a longwave trough offshore...with a parade shortwave 
troughs propagating through the longwave trough and crossing western 
Washington. The leading edge of the next significant shortwave feature to 
arrive was just approaching 130w at 08z and should cross western Washington 
late this morning through afternoon. Expect the air mass to remain 
unstable with more showers and thunderstorms. Cape could be 
better...but what stands out with this wave is the low level speed 
shear up through 5000 feet...from about south-southwest 15 knots just above the 
surface to SW 30 knots at 2500 feet and SW 45 knots at 5000 feet. Helicity 
gets above 200 east and southeast of Puget Sound sound which suggests that 
individual thunderstorms may be more likely to develop rotation. 


Otherwise...850 mb flow will remain around SW 40 knots with lots 
moisture available...so expect snow showers to continue piling up 
the snow over the Cascades. The upslope flow finally decreases after 
12z Sunday...so will keep the Winter Storm Warning for the Cascades 
going until then. 850 mb temperatures hover around -1c...so expect the snow 
level to remain around 3500 feet...but briefly lower in heavier 
showers. 


Both GFS/European model (ecmwf) bring a stronger somewhat larger shortwave trough southeast 
across the Bering Sea Saturday...that eventually matures into a 
frontal system off the Pacific northwest coast on Sunday. A small warm 
front moves across West Washington late Sunday afternoon followed by a stronger 
cold front on Monday. This system appears to pack pretty Standard 
stuff...breezy southeast winds with the warm front...then windy the cold 
front. Could end up with another Wind Advisory for the north interior 
with the cold front. Will address precipitation amounts and snow levels in 
the Hydro section below. Kam 


Long term...GFS and European model (ecmwf) remain in good agreement regarding the 
overall pattern through the end of next week...but after Wednesday 
begin diverging on timing...with the GFS faster. The overall pattern 
remains progressive as the parade continues. Models have good 
agreement with another cold front Tuesday...followed by an upper 
level trough Wednesday. Minor break Thursday with a ridge briefly 
overhead...but the GFS keeps some light precipitation in the area. Another 
frontal system arrives either Friday or Friday night. Kam 


&& 


Hydrology...it is unlikely that river flooding will be a problem 
over then next week. The pattern remains too progressive with 
systems only affecting West Washington for a brief time each. Snow levels down 
to 3500 feet will limit runoff problems during the heavier precipitation 
period through Sunday morning. 850 mb winds south-southwest 40 knots will produce 
good upslope flow with sundays warm front...but the short precipitation 
period will probably only produce 1-2 inches over the mountains even 
with snow levels around 5000 feet. After a short break between 
systems mondays cold front packs another short period of heavy 
rain...850 mb winds south-southwest 40-50kt...snow levels still around 5000 feet 
and another 1-2 inches of rain. Tuesdays cold front is much weaker. 
The frontal system around Friday or Saturday also appears to be the 
Standard progressive type. In the 8-10 day period beyond the reaches 
of any real forecast skill...our current progressive pattern appears 
to continue. Kam 


&& 


Aviation...unsettled weather will continue across western 
Washington today. A strong upper level low continues to spin off 
the Alaska Panhandle with cyclonic flow aloft across the Pacific northwest. Infrared 
satellite shows an extensive cumulus field over the ocean drifting 
inland. Through the morning...showers will be off-and-on...then an 
uptick in activity anticipated this afternoon as a surface trough 
moves inland. Models show strong vertical shear profile with a few 
hundred cape over the region...isold-sct thunderstorms will be 
possible along with small hail. Showers will decrease in coverage 
tonight. 33 


Ksea...off-and-on showers expected through the morning...then an 
increase in activity this afternoon as the next wave move through. 
Air mass remains unstable with isolated-scattered thunderstorms developing 
once again. Brief heavy downpours will reduce visible and/or ceilings into 
the MVFR range. Will continue to see gusty southerly winds this 
afternoon...then winds easing this evening. Showers will decrease in 
coverage tonight. 33 


&& 


Marine...strong southwest to west flow will continue across western 
Washington today...with possible gales over the coastal waters late 
morning/afternoon as surface trough passes through. Winds across all 
waters expected to ease tonight. The next cold front will arrive on 
Monday and may see southerly gales across the coast/entrances jdf/north inland 
waters just ahead of this system Sun night into early Monday. Another 
front is slated for Tuesday. 


Meanwhile...large westerly swells over 25 feet continue across the 
coastal waters today...will continue the high surf warning for the 
beaches. Swells will gradually decay to 20 feet on sun. 33 


&& 


Sew watches/warnings/advisories... 
Washington...Winter Storm Warning for the Cascades through Saturday 
night. 
Weather advisory for the Olympic Mountains through Saturday 
evening for the Olympic Mountains. 
High surf warning for the coast through 4 PM Saturday. 
Pz...Small Craft Advisory all waters. 
Small Craft Advisory for rough Grays Harbor bar. 


$$ 


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www.Weather.Gov/Seattle/gafd/latest_webafd.Html. 

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