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Area forecast discussion 
National Weather Service Seattle Washington 
930 PM PDT Wednesday Mar 17 2010 


Synopsis...surface high pressure just offshore will build east 
across British Columbia and Alberta through Thursday. An upper level 
ridge will build over the region. Dry weather with cool night time 
low temperatures can be expected...while daytime highs warm through 
Saturday. A cold front will reach the area Sunday with a cool upper 
trough Monday. 


&& 


Short term...the evening slope analysis shows a 1038mb high centered 
just off Vancouver Island. Satimg is mostly clear. The time height 
cross section from the NAM model this evening nicely shows the northwest 
flow aloft and cool air mass gradually warming through Sat morning as the 
upper ridge offshore shifts over the area. 500mb heights rise into 
the 560s...and the ridge axis reaches the coast Friday night with a 
570dm contour to kast. The NAM has some low level moisture for a few 
hours Thursday and Friday morning...patchy fog or low clouds possible. After 
the upper ridge shifts inland an upper trough and cold front approach 
the area sun. Until then 850mb temperatures warm from -4c this evening to 
maybe as warm as +9c Saturday...and with the surface high pressure building 
through SW Canada and then sagging southeast inland offshore flow will 
develop...probably strongest Saturday morning as thermally induced 
low pressure over northern calif on Friday spreads out and merges with what 
looks like a bit of a warm front just offshore Sat. The forecast looks 
fine...we may see some cirrus Sat...but otherwise fair skies and 
warming daytime highs after a couple of frosty mornings in all the 
normal cool spots. 19 


Long term...a compact upper low will rotate near the Queen 
Charlottes on Sunday. A warm front may brush the coast and northern 
interior with some light rain beginning Saturday night and lifting 
by on Sunday. The cold front comes through Sunday evening. Models 
seem to be back to the idea of a cold upper low over the area Sunday 
night and Monday...with low snow levels and showers. Shortwave 
ridging will try to build in briefly Tuesday and Tuesday night 
before another system approaches from the west on Wednesday. Damico 


&& 


Hydrology...the flood risk across western Washington and the Green 
River is low through the next 10 days. A strong upper level ridge 
building offshore tonight will provide dry weather through Saturday. 
A system approaching from the west will produce light amounts of 
precipitation Saturday night and Sunday. Heavier precipitation 
Sunday evening/night will be accompanied by low snow levels. A cold 
showery pattern will persist through Tuesday. A series of cool and 
quick moving systems will affect the area later in the week. A 
warmer...wetter storm is possible going into the following 
weekend...although uncertainty is high this far out. Damico 


&& 


Aviation...high pressure aloft will build offshore tonight through 
Thursday with dry northwesterly flow aloft over the area. Surface 
gradients light with fog...ceilings below 500 feet visibilities below 1 
sm...a possibility in the usual locations like kolm 10z-17z Thursday 
morning. Clear skies for the remainder of the area through Thursday 
evening. 


Ksea...clear skies with north northeast wind 6-10 knots through 
Thursday evening. Felton 
&& 


Marine...surface high pressure over the coastal waters will build into 
southern British Columbia tonight into Thursday morning. The high 
will remain in place through Friday with increasing offshore flow 
Thursday night into Friday as surface pressure lowers along the 
coast. Offshore flow will weaken on Saturday with the flow switching 
back to onshore on Sunday as the high pressure east of the Cascades 
dissipates. 


Swell along the coast in the 10 to 13 foot range tonight will 
subside below 10 feet Thursday morning and remain below 10 feet into 
the weekend. Felton 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory for hazardous seas is in effect for 
the coastal waters and West Entrance to the Strait of Juan 
Delaware fuca. 


A Small Craft Advisory for rough bar Grays Harbor bar 
conditions remains in effect until 9 am Thursday. 


$$ 


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