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Area forecast discussion 
National Weather Service Seattle Washington 
900 am PDT Thursday Aug 21 2008 


Synopsis...showers will decrease this afternoon as an unusually 
deep upper level low over Vancouver Island shifts east and weakens. 
An upper level ridge will develop over the area Friday and Friday 
night...then will shift east Saturday. The ridge will bring dry and 
warmer conditions to the area. Another frontal system will move 
through the area Sunday or Sunday night. This front will bring 
another round of wet conditions to the area. 


&& 


Short term...deep 550 dam upper level low is past Vancouver Island 
and shifting into the interior of British Columbia. The associated 
trough axis is passing over western Washington this 
morning...helping to maintain scattered shower activity. 
Unseasonably strong jet streak of 120 knots is located over the 
Washington/or border and pushing east. Area of enhanced shower activity 
with cooler cloud tops is over the northern Cascades this morning 
probably associated with an upper shortwave rotating around the 
upper low. Washington Pass nwac station shows temperatures of 33f at 
6630' 8am hour...so snow level may be down below 6000 feet in some 
places. 


Flat upper ridge will begin to build in today...with the ridge axis 
passing across the area midday Friday. Although shower activity is 
already on the decrease this morning across The Lowlands...airmass 
will remain cool aloft and soundings support an unstable lower 
atmosphere capable of producing showers through at least the early 
afternoon. One other Point of interest is that the NAM and Canadian 
Gem continue to hint at a Puget Sound sound convergence zone this 
afternoon as surface high pressure builds in from the southwest. Not 
very much quantitative precipitation forecast being shown with this feature between 21-03z 
today...but should help maintain ample cloud cover the Puget Sound sound 
and keep temperatures on the cool side. 


Light surface flow and period of subsidence will take effect later 
tonight and Friday morning ahead of the upper ridge. This will help 
trap residual low level moisture after our real soaker of a 
system...giving areas of stratus throughout and patchy fog in 
favored outlying river valleys. Friday will eventually become mostly 
sunny with temperatures closer to normal for this time of year...but 
high cloud from warm frontal feature aimed at central b.C. Coast may 
dampen some of the sunshine tomorrow afternoon. 


Not much change in thinking for Saturday...temperatures will warm 
further as flow becomes southerly and heights bump to around 580 dam 
ahead of approaching frontal system. Saturday has the potential to 
be the warmest day with lower 80s possible for the central sound 
away from the water and the southwest interior. Damico 


Long term...from previous discussion...progressive short waves will 
affect the area every couple days during the extended period. Global 
models show a couple of rather deep troughs moving across the area 
during the period...one later Sunday or Sunday night and another the 
middle of next week. The one for later this weekend looks 
impressively wet with both the European model (ecmwf) and GFS hinting at 
precipitation amounts similar to the system that just recently 
brought locally heavy rainfall to the area. As a result probability of precipitation will be 
increased...especially Sun night. 


Model timing becomes less consistent with the next wave expected to 
move through the area Tuesday or Wednesday...so forecasts beyond the weekend 
will be left alone for now. Later forecast shifts may need to adjust 
forecast to a cooler and wetter than normal patter next week. 
Albrecht 


&& 


Aviation...upper trough moving east while surface trough shifts inland and 
fills. Westerly flow aloft turning northwest this afternoon. Air mass remains moist but 
will gradually dry in the upper then middle levels through the day. Chance 
of the pscz enhancing showers in the kbfi-kpae area 
midday/afternoon. Mountains will stay mostly obscured through tonight. Skies 
clear Friday after some areas of morning low clouds or fog. 19 


Ksea...the bkn015 will lift or scatter out and that moisture will 
bubble up into scattered-broken cumulus with daytime heating. If the pscz sets up 
it will likely be more of a problem for kpae than as far south as 
sea tac. Still...the wind will become vrb05kt from middle afternoon 
through evening as pscz winds work down into Seattle at times. A shift 
to northerly wind this evening wont last...the 00z 4km WRF shows 
light southerlies back over ksea by midnight tonight. Will add a 
tempo group 12-16z Friday as low level moisture could give some IFR 
St/fog and the lowest level cloud forecast from the 00z 4km WRF does show 
areas of stratus kpwt-ksea-kolm and south along I-5 to Kelso at 15z 
Friday which would be pretty typical under weak high pressure and light 
winds after a rain event. 19 


&& 


Marine...onshore flow will become light tonight. Seas coastal 
waters a notch lower than I forecast ydy but current forecast OK and Small 
Craft Advisory continues. No significant updates this morning. 19 


&& 


Sew watches/warnings/advisories... 
Small Craft Advisory for hazardous seas coastal waters. 
Small Craft Advisory for wind Puget Sound sound/Hood Canal this morning. 
Small Craft Advisory for rough bar conditions Grays Harbor bar. 


$$