557 
fxus66 ksew 211105 cca 
afdsew 


Area forecast discussion...corrected warnings line 
National Weather Service Seattle Washington 
340 am PDT sun Mar 21 2010 


Synopsis...a dissipating cold front will move through the area this 
morning. A cool upper level trough offshore will approach the coast 
this afternoon and move over western Washington tonight. The upper 
level trough will shift east on Monday with an upper level ridge 
building off the coast. The ridge will slowly move over the area 
Tuesday and Wednesday. The next front will bring unsettled weather 
Wednesday night through Friday. 


&& 


Short term...hard to pick up the exact location of the weakening 
cold front moving into the area this morning. Satellite imagery 
before the satellite went into eclipse was not impressive with cloud 
top temperatures not cooling and plenty of holes in the cloud cover. 
Current observations across the area just reporting cloudy skies 
and the WSR-88D does not have much in the way of echoes on it at 
this point. Model guidance suggests some sort of frontal passage 
this morning but with the front looking weak will just word the 
forecast as showers. This afternoon the air mass destabilizes as 
cooler air moves in behind the front. In the lower levels not much 
cooling with 850 and 700 mb temperatures only dropping a couple of 
degrees c. Further aloft 500 mb temperatures down to near -30c by 
00z this afternoon. Lifted indexes this afternoon near 0 and 
convective temperatures near forecasted maximum temperatures. Current infrared 
imagery not showing much in the way of cold cloud tops offshore and 
the flow aloft is southwesterly from right off the ground up to 300 
mb with little speed shear. Will stay with the showers wording and 
not put any convection in the forecast. Upper level trough moving 
onshore tonight keeping showers in the forecast. Trough axis moving 
east of western Washington by 18z Monday. Surface high pressure 
building along the coast with a Puget Sound sound convergence zone 
possible Monday afternoon into the evening hours near the East 
Entrance of the Strait eastward into the Cascades. For the remainder 
of the area showers coming to an end but with the onshore flow there 
will still be plenty of cloud cover. Surface high pressure building 
inland Monday night with a flat upper level ridge building off the 
coast. Warm front riding by to the north Monday night and at this 
point does not look like it will be far enough south to bring any 
light rain to western Washington. Surface high pressure over the 
region on Tuesday with the upper level ridge off the coast resulting 
in a dry day for the area. With the lack of any offshore flow have 
kept daytime highs just a couple of degrees above normal. 


Long term...extended models in pretty good agreement early on in 
the extended this morning with the next front stalling offshore on 
Wednesday as the trough behind the front digs south. This is a 
similar situation to what happened on Saturday. For now will go 
above MOS by about 5 degrees and go for highs mostly in the 60s for 
Wednesday. Negatively tilted front moving inland Wednesday night 
into Thursday morning. The system will be splitting as it moves 
inland so have stayed with the chance probability of precipitation. Some differences 
in the models beginning later Thursday into Friday with the European model (ecmwf) 
indicating a little bit stronger trough moving through the area 
versus the GFS. The GFS then builds an upper level ridge over 
western Washington later Friday into Saturday while the European model (ecmwf) is 
much weaker with this feature and actually has some precipitation on 
Saturday as the weaker ridge moves east. GFS ensembles show the 
operational run matches up with the ensemble mean but the 500 mb 
heights are not that high...in the 550s dm. For now will stay with 
the climatology probability of precipitation for Thursday night through Saturday. Felton 


&& 


Aviation...an upper level low centered just west of the central b.C. 
Coast will maintain moist SW flow aloft across West Washington through 06z. The 
air mass over West Washington is moist...compliments of SW onshore flow and a 
weak front that seems to be falling apart over the area. An upper 
level trough further offshore along 135w will move east...reaching 
the Washington coast around 06z...an causing showers to develop in the moist 
air mass. Ceilings have been lowering a little slower than 
expected...with the lowest ceilings seeming to have settled in the 
bkn020-035 range. Will go with that...and plan on gradual lowering 
of the higher ceilings around the area to that range. 


Ksea...expect ceilings have dropped to around bkn050. The front over the 
area seems to be falling apart...and with lots of moisture around 
will still plan on ceilings dropping a bit more. However...observation to the west 
have not gone down below bkn020 as previously thought...so will keep 
ksea ceilings above bkn020 this morning. Expect S-SW winds to continue 
through today. Kam 


&& 


Marine...a weak cold front draped over West Washington should be making 
progress inland. However the main player is the surface low near 
51n/135w or just west of Queen Charlotte Sound. The surface low has set 
up SW onshore flow across West Washington this morning...which will become more 
westerly this afternoon. Low end gale force winds are hanging on 
tenaciously at Tatoosh Island this morning...so will have to extend 
the gale a few more hours before lowering the winds. Otherwise 
expect west flow through the Strait to increase today...with winds 
occasionally reaching Small Craft Advisory levels. Went with the WRF-GFS solution 
since it handles west flow in the Strait better than the NAM...but even 
the WRF-GFS only had intermittent Small Craft Advisory winds through the Strait. 


Winds along the coast seem to be at the lower edge of the Small Craft Advisory range. 
Will hang on to Small Craft Advisory winds for a few more hours before transitioning 
to a Small Craft Advisory for hazardous seas. Small Craft Advisory for rough bar conditions at Grays 
Harbor is already up...but rough conditions are really only expected 
on the Ebb currents. 


Models remain consistent in weakening the onshore flow Monday...as a 
surface ridge builds over the offshore waters...then moves inland 
Monday evening. The ridge hangs around over West Washington though Tuesday then 
shifts east...allowing offshore flow to increase. The next frontal 
system should be approaching the area Wednesday...then move inland 
Wednesday evening. Kam 


&& 


Sew watches/warnings/advisories... 
Washington...none. 
Pz...Gale Warning West Entrance. 
Small Craft Advisory coast...central and east Strait of\ 
Juan Delaware fuca...Admiralty Inlet and north inland waters. 
.Small Craft Advisory for rough Grays Harbor bar. 


$$ 


Www.Weather.Gov/Seattle 


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

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)