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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Wednesday
• 
• High: 59 °
• Low: 45 °
• Partly Cloudy

• Thursday
• 
• High: 57 °
• Low: 43 °
• Chance of Rain

• Friday
• 
• High: 57 °
• Low: 43 °
• Chance of Rain

• Saturday
• 
• High: 55 °
• Low: 43 °
• Mostly Cloudy

• Sunday
• 
• High: 57 °
• Low: 46 °
• Chance of Rain

Area forecast discussion 
National Weather Service Seattle Washington 
300 am PDT Wednesday may 16 2012 


Synopsis...onshore flow and an upper level trough will lead to 
cooler weather and clouds at times through Friday. A weak 
disturbance moving through the trough will produce a chance of 
showers Thursday and Friday. Weak high pressure will bring dry and 
somewhat warmer weather Saturday. Wetter weather will develop early 
next week as a series of fronts move through. 


&& 


Short term...moderate onshore flow continues this morning as a weak 
trough moves into western Washington. The absence of any significant 
stratus coverage along the coast coupled with a northwest component 
to the onshore flow means sky cover forecasts have been scaled down. 
I went with a partly sunny forecast to account for areas of high 
level moisture moving onshore today along with areas of 
stratocumulus that are likely to develop as the day wears on. 
Temperatures will cool further today under the influence of the 
trough...with high temperatures in many places down 6 to 10 degrees. 


The trough will settle in over the region tonight and Thursday and 
deepen. With lower heights and onshore gradients edging upward a 
notch...high temperatures will be held to slightly below seasonal 
normals on Thursday...mostly in the 50s to lower 60s. The models 
continue to depict a general lack of moisture with this trough...and 
I retained the generally partly sunny forecast along with a small 
chance of showers. An upper level disturbance rotating around the 
trough from the northwest will probably be the best catalyst for any 
shower activity Thursday evening into the early hours of 
Friday...with the Cascades and northwest interior probably having 
the best chance of any precipitation. Friday is likely to be the 
coolest day of the week. Substantial cloud cover to start the day 
along with northwest flow aloft and low level onshore flow could 
mean that even interior areas may struggle to get much above 60 for 
high temperatures. Weak upper level ridging will begin to build on 
Friday leading to a modest warming and drying trend to begin the 
weekend. 


Long term...weak upper level ridging builds into western Washington 
on Saturday for dry and somewhat warmer conditions. Both the GFS and 
European model (ecmwf) depict the ridge axis being east of the Cascades by Saturday 
evening with a front approaching during the day Sunday. 
Precipitation could be on the coast and olympic peninsula by midday 
Sunday...moving into the interior by late in the day. This front 
will be the first of a series of systems that are likely to maintain 
cool and unsettled conditions across western Washington into the 
middle of next week. No significant changes were made to extended 
forecast overnight...but a continuation of the model trends will 
probably lead to higher precipitation chances early next week in subsequent 
forecasts than what is currently mentioned. 27 


&& 


Aviation...an upper level trough will move over the Pacific northwest today 
through Thursday. Light-moderate northwest flow aloft. Stable conditions and 
mostly dry. Moist in the lowest levels...mainly along the coast...SW 
interior...and the Strait. 


Observation indicate stratus has yet to form just about anywhere including 
the S coast. The stratus/fog satellite product is mostly obscured by 
high clouds but a small area around the north coast would indicate some 
patchy fog has formed there. Will watch the trends over the next 
hour and probably need to scale back or remove much of the status in 
the forecast at khqm/kolm/ksea. 


Ksea...north wind 5-10 knots...becoming northwest after 18z. Patchy fog forming 
around the terminal is increasingly unlikely. Northerly winds 
persist with the southwest flow confined to the far S sound. This 
may also help keep drier air in place. Will likely remove any low 
clouds from the stratus this morning. 


&& 


Marine...northwest winds of 15 to 25 knots will likely continue the next 
couple days over the coastal waters. The outer coastal waters could 
reach 30 knots periodically. The Small Craft Advisory remains in effect 
there today with extensions likely. 


Small craft winds continue in the Strait and models show gradients 
staying above +1 or 2 through tonight. Will keep the small craft 
going and probably will need an extension for tonight as well. The west 
Strait is problematic the winds will mostly stay in the 10-20 knots 
range but probably rising to 20-25 knots at times. Decided to keep an 
advisory there as well. 


Models show a diurnal pattern to the flow through the Strait each 
day this week. Westerly winds will pick up to 15 to 30 knots during the 
late afternoon and nighttime hours. There is a slight possibility of 
low end westerly gale during this period. The pattern could change 
by Sunday night as models show a front approaching the coast then. 
Dtm 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Small Craft Advisory coast..entire Strait of Juan Delaware fuca...and 
Admiralty Inlet. 


$$ 


You can see an illustrated version of the forecast discussion at 
www.Weather.Gov/Seattle/gafd/latest_webafd.Html.