337 
fxus66 ksew 200350 
afdsew 


Area forecast discussion 
National Weather Service Seattle Washington 
850 PM PDT Friday Mar 19 2010 


Synopsis...a strong upper level ridge just offshore Friday evening 
will shift over eastern Washington by Saturday afternoon...giving 
one more day of sunny and dry weather. A cold front will reach the 
coast Saturday night and move inland early Sunday...spreading light 
rain over the area. An approaching upper level trough will bring 
showers to the area Sunday and Sunday night before moving east 
Monday morning. Showers will taper off behind the trough through 
Monday afternoon. High pressure will build over the area on Tuesday 
bringing dry weather. The next front will bring unsettled weather 
Wednesday night into Friday. 


&& 


Short term...the strong upper level ridge just approaching the 
coast this evening will shift pretty quickly east Saturday as the 
frontal system centered near 40n/140w heads NE toward Vancouver 
Island. Fairly good offshore flow is already developing between the 
surface high over southeast b.C. And the 40n/140w surface low. Theme for 
Saturday is still sunny and warm. Just cirrus is expected over the 
area...even though the GFS suggests the band of high level moisture 
along 130w will move over the area Saturday. Should see good 
radiational cooling again tonight for another round of lows in the 
upper 20s and 30s. 


Models have 850 mb temperatures peaking over the area around midday 
Saturday...9c for the NAM and European model (ecmwf) and almost 7c for the GFS. Will 
lean toward the warmer NAM/European model (ecmwf) solution which usually handles warm 
spells better. Mixing 850 mb temperatures down the surface would yield 65 
or 66 Saturday which is in line with 66 in the current forecast and 
the uw probcast for sea-tac. Surface winds should be S by then so 
the typical cooling effect of the northerly sound breeze should not 
be in play. 


GFS/NAM/ECMWF all agree on bringing a rather weak looking skinny 
front to the coast Saturday evening with some light rain or 
sprinkles developing maybe a little before or a little after 00z. 
Low level flow shifts onshore Saturday afternoon ahead of the 
front...and what jumps out on the NAM is the rapid increase in low 
level relative humidity caused by the shift to onshore flow. The NAM relative humidity fields 
suggest the weak front will be over the Cascades by 12z and thus a 
non-factor for West Washington on Sunday. 


Sunday/Sunday night looks like the wettest period. Continuing 
onshore flow will supply the moisture and an approaching upper level 
trough and 90 knots 300 mb jet streak just ahead of the trough will 
supply the lift for precipitation. This trough looks like it is the one 
currently near 50n/145w...and comes in behind the Saturday night 
system. The NAM shows good upward motion from 850 mb up through 500 
mb on Sunday...which should rapidly moisten the air mass. Current 
likely probability of precipitation are OK for now...but with the latest MOS guidance in the 
categorical range...they may be boosted a bit in a subsequent 
forecast. 


The upper trough axis will cross the area either Sunday night or 
early Monday morning...with Post trough showers in the unstable air 
mass easing off through Monday night. Could see some convergence 
zone activity over Puget Sound sound Monday. Kam 


Long term...previous discussion from the 330 PM afd...Tuesday and 
most of Wednesday look dry as another transient ridge builds. The 
GFS and Euro differ on how this dry period will end with the GFS 
bringing some rain into the coast from the southwest on Wednesday 
afternoon...and the Euro holding it off until Wednesday night. Made 
some attempt to reflect this uncertainty in the zones and grids. 
Shower chance appears to continue through the end of the week in 
both models. Burke 


&& 


Aviation...northwest flow aloft this evening will become 
southwesterly by early Saturday ahead of an upper level trough over 
the offshore waters. Surface high pressure over southern British 
Columbia will produce low level offshore flow through the period... 
which will strengthen tonight and Saturday as a frontal system 
approaches the region from the west. Clear skies will prevail 
tonight and Saturday except for localized valley stratus and fog 
during the early am hours. A front will push through the region 
early Saturday evening. There will likely be little precipitation 
associated with the front...but Post frontal shower activity 
associated with the upper level trough could reach interior 
terminals by 09z-12z Sunday. 


Ksea...VFR with no ceiling through 20z...then increasing high and 
middle level cloud. Northeast wind 8-12 knots easing to 5-9 knots late this 
evening...then becoming southerly on Saturday afternoon and evening. 
27 


&& 


Marine...offshore flow will increase tonight and Saturday as 
surface high pressure sits over southern British Columbia and a 
Pacific frontal system approaches the region from the west. This 
will produce Small Craft Advisory winds over the coastal waters. The 
easterly gradient and winds at Tatoosh Island are running above 
guidance. Have decided to Post a Gale Warning for the West Entrance 
to Strait for early Saturday. An approaching front and associated 
pressure falls will only serve to tighten the gradient overnight. 
The cold front will move across the forecast area Saturday night and 
Sunday...with Small Craft Advisory winds spreading to the East 
Entrance Strait...Admiralty Inlet...and northern inland waters. 
Onshore flow will develop behind the front Saturday night through 
Monday as high pressure builds offshore. The high will shift inland 
on Tuesday...with the next frontal system approaching the region 
from the west on Wednesday. 27 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Gale Warning West Entrance to the Strait of Juan Delaware fuca. 
Small Craft Advisory coastal 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).