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Area forecast discussion 
National Weather Service Seattle Washington 
300 PM PDT Thursday Aug 21 2008 


Synopsis...scattered showers will continue to decrease this evening 
as the deep upper level trough over the Pacific northwest progresses 
eastward. A ridge will build in on Friday trending towards dry and 
milder conditions. Mostly sunny skies will prevail on 
Saturday...before another cool and wet frontal system moves in late 
Sunday. 


&& 


Short term...due to the copious amount of rainfall over the 
mountains in the last 48 hours particularly this morning's rainfall 
for the north Cascades...a Flood Watch for Whatcom County was issued 
late this morning for the Nooksack river. If any flooding does occur 
it will be minor...likely cresting this evening and falling quickly 
below flood stage before midnight. 


Showers have tapered off across the County Warning Area...mostly 
confined to the Cascades at this hour. A weak Puget Sound sound 
convergence zone has formed over near Kingston/Edmonds but is having 
trouble producing widespread precipitation due to the general 
low-level instability disrupting it's formation. A somewhat flat 
upper level ridge will build overnight...and with plenty of low 
level moisture to be trapped...except stratus and some fog for 
tomorrow morning. Light gradients and a middle-High Deck will slow the 
break out process tomorrow...but expect partly sunny skies in the 
afternoon with daytime highs just a little below daily normals. 


Saturday still looks to be on track for the warmest day this 
week...with near 580 heights at 500mb and southwest flow aloft. 
Locations in the east Puget Sound sound lowlands...southwest 
interior...and Cowlitz valley could push into the lower to middle 80s 
if some of the warmer numerical solutions are correct. Another 
strong frontal system reaches the coast Sunday morning...spreading 
rain inland later that afternoon. Impressive deep upper trough over 
Gulf of Alaska pushes inland overnight Sunday. Model guidance again 
suggesting that this trough will drop snow levels into the 6500-7500 
range Sunday night. Damico 


Long term...Monday looks to be not too much unlike today...Post 
frontal with northwest flow aloft and widespread showers at least to 
start the day. As the upper ridge builds near 140w we dry out 
according to the GFS but maintain the cooler than normal northwest 
flow. Euro and GFS disagrees on timing on disturbance sliding down 
the ridge Tuesday - Thursday...but have introduced chance of showers into the 
Cascades Wednesday night - Thursday Onward to account for onshore flow and 
the possibility that a weak disturbance will swing through. Damico 


&& 


Aviation...weak high pressure building into the area will allow skies 
to clear later this evening. Isolated showers will end by sunset. 
Areas of fog and low clouds are a not a bad bet Friday morning after 
these recent rains. Clearing skies and calm wind overnight. The 12z 
4km WRF low level cloud valid at 15z Friday shows stratus over Puget Sound 
sound south of about sea tac...plus stratus over the coast and 
Strait...and over the Chehalis gap. Fog in some of the river valleys 
seems likely as well by daybreak. 19 


Ksea...skies clear later this evening...cross section suggests around 
06z for Seattle. Winds should go calm after midnight. Areas of fog 
and low clouds Friday morning...mainly south of sea tac after daybreak 
per 4km WRF. 19 


&& 


Marine...onshore flow becomes light overnight under weak high pressure. 
Winds Friday and Sat will be light and variable...maybe 10kt northwest wind 
afternoon and evening hours. Fronts go by to the north Friday through Sat 
night...giving increasing southerly wind over the coastal waters this 
weekend. Models have trended a bit faster and stronger with the 
front now arriving coastal waters Sunday and the 18z nam12 shows a 
wave deepening and tracking along the front offshore Saturday night 
which bears watching...that forecast suggests a chance for gales northern 
coastal waters late Sat night and 12z uw WRF shows 30kt southerly off the 
north coast by daybreak sun. 19 


&& 


Sew watches/warnings/advisories... 
Flood Watch for Whatcom County through midnight tonight. 
Small Craft Advisory for rough bar conditions Grays Harbor bar. 


$$