Weather

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fxus66 ksew 110446 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
945 PM PDT Friday Oct 10 2008 


Synopsis...strong high press aloft offshore is expected to slowly 
drift east into the Pacific northwest through the weekend. This 
weather pattern will provide generally clear skies..crisp mornings.. 
and areas of night and morning fog and low clouds. The high pressure 
aloft is forecast to move east of the Cascades Monday as a weak warm 
front approaches for a threat of rain. Westerly flow aloft will 
prevail next week bringing a few weak Pacific fronts through the 
region. 


&& 


Short term...satellite imagery shows upper level ridge offshore 
extending into central British Columbia. Just a few high clouds over 
western Washington at 04z. Temperatures vary widely at 04z with wind 
sheltered areas already in the 30s while places exposed to the north 
winds like Eastsound are still near 50. Kolm-kbli gradient hanging 
near -1 mb for the last couple of hours and little change is 
expected overnight into Saturday morning. Fog study by retired wseo 
bob Jackson puts the prime kolm-kbli fog gradient in the 0 to -2 mb 
range. Dew point temperatures are mostly in the 30s where most of 
the lows will be Saturday morning. At this point there are a couple 
variables that look like they will limit the amount of fog coverage 
to areas Saturday morning. One is the air mass is drying out with 
24 hour changes showing dew points 4 to 6 degrees cooler. In 
addition multiple acars soundings and the Sandpoint profiler show 
northerly winds in the 500 to 2000 foot layer 15 to 20 knots. Model 
cross sections have these conditions continuing through Saturday 
morning. Because of this will stay with just areas of fog for 
Saturday morning. With some places already in the 30s frost advisory 
for The Lowlands for Saturday morning also looks on track. Little 
change in the 00z model solutions with the upper level ridge slowly 
sagging over western Washington during the weekend. The differences 
in the model solutions continue to occur on Monday with the NAM much 
weaker with the approaching warm front versus the GFS and the old 
Euro run. GFS continues to have high 500 mb heights...near 580 
dm...and high surface pressure as well...near 1030mb at the time of 
precipitation Monday with the warm front. With the old Euro model 
still showing a stronger solution will stay with the chance of rain 
forecast for Monday. If the new Euro comes in weaker look for the 
precipitation to be taken out of the forecast for Monday. No updates to the 
forecast this evening. 


Long term...not much consistency in the extended models...not 
surprising given the disagreement on mondays solution. The general 
consensus is zonal flow returning to the Pacific northwest with 
weather systems embedded in the flow moving through the area. Timing 
these systems at this point is difficult. There is some agreement 
for a dry window sometime in the middle of next week. Current 
forecast is dry for Tuesday night. 00z GFS indicating dry conditions 
Wednesday...continuing the solution of the previous GFS run. If the 
Euro comes in dry as well will take chance of rain out of the 
forecast for Wednesday. With the uncertainty current broadbrush 
chance probability of precipitation for most of the period looks good. Felton 


&& 


Aviation...upper level trough will continue to dig over Nevada this 
evening...with upper level ridge building offshore. West Washington remain in 
the middle with northerly flow aloft. Skies clear across the area this 
evening and main concern will be fog formation by dawn. Airmass is 
dry and cold so favored areas for fog will be near the water. Low 
clouds will dissipate by middle morning with clear skies expected by 
the afternoon. 33 


Ksea...northerly flow will continue this evening with clear skies and dry 
weather. Low clouds possible in the vicinity early Sat as dew points 
stay in the middle 30s and temperatures drop...along with light northerly winds. Not 
expecting freezing temperatures/icy conditions though. Clear skies expected 
by the late morning and continuing through the afternoon. 33 


&& 


Marine...strong area of high pressure centered along 130w is leading to 
northerly flow across West Washington this evening. Winds across the Puget Sound sound/Hood 
Canal area have weakened thus will reduce wind and waves and cancel 
the Small Craft Advisory. Light winds/offshore flow expected through 
the weekend as area of high pressure offshore shifts farther east and 
into West Washington. Onshore flow will increase early next week as a cold 
front passes through the region. 33 


&& 


Sew watches/warnings/advisories... 
frost advisory for The Lowlands. 


$$ 


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