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fxus66 ksew 202201 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
300 PM PDT Sat Mar 20 2010 


Synopsis...a cold front will approach the coast tonight and move 
inland Sunday. Light rain tonight will change to scattered showers 
Sunday afternoon...which will then taper Monday as an upper trough 
moves east. High pressure will build on Tuesday and Wednesday 
bringing dry weather. The next front will bring unsettled weather 
Wednesday night through Friday. 


&& 


Short term...clouds have moved onto the coast as a front moves in 
and will spread over the interior tonight. Rain should develop on 
the coast tonight and there will be a chance of light rain inland. 
Winds have picked up on the coast so this system seems to have a 
little punch. Kind of hard to pick out a time of frontal passage but 
it will probably be sometime during the day on Sunday. As the upper 
trough approaches...precipitation will become more showery and will 
continue into Sunday night. High temperatures Sunday will be at 
least 10 degrees lower than today...in the 50s rather than the 60s. 


An upper ridge builds offshore on Monday...putting Washington under 
cool northwesterly flow. Showers are still possible on Monday but 
will taper as surface high pressure builds in the afternoon. Monday 
could be a good day for a Puget Sound sound convergence zone. High 
temperatures will be in the low to middle 50s. 


Things dry out Monday night and Tuesday will have strong upper and 
surface ridges nosing into western Washington. High temperatures 
should jump to near 60 in many places and skies should clear. Burke 


Long term...Wednesday also looks dry. Both the GFS and Euro have 
slowed down the approach of the next system and now hold rain off 
the coast until after 00z Thursday...so have limited afternoon rain 
probability of precipitation on Wednesday to the coastal waters only. Temperatures will rise 
into the 55-65 range. 


The next system does sweep in Wednesday night with showers Thursday. 
For now have kept climatological average probability of precipitation...chance...for those 
periods as there is still time for the models to slow the system 
down again. The system moves through fairly fast and by Friday a 
surface ridge is building which could dry things out again. For 
Friday and Friday night have gone below climatological average probability of precipitation 
but still kept them in the chance category. 


Day seven...Saturday...seems to have another front approaching but 
the GFS and Euro are different in timing. So loaded another batch of 
climatological average chance probability of precipitation in the grids. Burke 


&& 


Aviation...southwest flow aloft will continue tonight and Sunday... 
with a Pacific frontal system moving through western Washington 
during the period. Low level southerly flow will strengthen tonight 
as the front moves inland...then shift to onshore in the wake of the 
front on Sunday. The air mass will remain stable and become moist 
all levels tonight. Light precipitation will reach the coast around 
6 PM...develop over the Olympics and Cascades later this evening... 
and spread to the interior lowlands late tonight. Ceilings will 
likely deteriorate to something like scattered-bkn025 broken-ovc050 around 3 
am Sunday...then improve somewhat Sunday afternoon. 


Ksea...southerly wind 6-10 knots rising shortly to 8-14 knots. Occasional 
MVFR ceilings will probably develop around 3 am Sunday. Mcdonnal 


&& 


Marine...a frontal system will move through the coastal waters this 
evening and inland across western Washington tonight. Winds have 
risen to low end gale force at the West Entrance Strait of Juan Delaware 
fuca and over parts of the coastal waters. I will Post a Gale 
Warning for those zones through late tonight. Small Craft Advisory 
winds should develop later this evening at the East Entrance Strait 
and Admiralty Inlet and northern inland waters. 


Onshore flow will develop behind the front late tonight through 
Monday as high pressure builds offshore. The high will shift inland 
on Tuesday...with offshore flow developing on Wednesday. The next 
frontal system will move through the forecast area Wednesday night 
and Thursday. Mcdonnal 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Gale Warning coastal waters...West Entrance Strait of Juan Delaware 
fuca. 
.Small Craft Advisory East Entrance Strait...Admiralty Inlet... 
northern inland waters. 


$$ 


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