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Area forecast discussion 
National Weather Service Seattle Washington 
330 PM PST Friday Nov 20 2009 


Synopsis...a trough will move through western Washington this 
evening. A frontal system will move through western Washington 
Saturday afternoon and night. A warm front will reach the area 
Monday. A ridge will build over the region Tuesday. A frontal system 
will move slowly through the area later Wednesday and Thursday. 
&& 


Short term...surface and upper trough will move through western Washington this 
evening...blustery on the coast as the surface trough moved through this 
afternoon. Breezy this evening in the interior as well as surface pressure 
gradients increase as the trough moves inland and fills...at 3pm 
olm-bli gradient had jumped to +3.3mb and that gradient should peak at 
+4.5 late this afternoon...windy for a few hours until the pscz sets 
up and winds will decrease...especially north of Seattle in the 
pscz. Westerly winds developing and increasing in the Strait west/ a westerly gale in 
the forecast to aid that pscz. Cooler air aloft will destabilize the air 
mass and showers will move through western Washington...becmg increasingly 
orographic overnight with a good chance of a pscz setting up. Winter 
Weather Advisory in the Cascades...if the pscz sets up might see a 
foot of new snow from Pilchuck to index and up toward Stevens Pass 
overnight. Snow level drops from 2500 to 1500ft...except might be around 
1000ft by midnight in the pscz over the Cascades as 700mb temperature cools 
to -15c. A bit of a break Saturday morning although the NAM keeps it 
pretty drippy so have higher probability of precipitation than I would normally show ahead 
of the next front. That next frontal system and surface low pressure centers in 
the 18z NAM are a much better match to the global models. A first 
low center with the occluded front reaches northern Vancouver Island Saturday 
evening...the front extends south along the Washington and Oregon coast...and 
then the NAM spins up a secondary low to 988mb which reaches the northern 
Oregon coast after midnight...with a track through northwest Oregon by daybreak 
sun. This system will be cooler than previous systems but low level 
thickness values do not support taking the snow level forecast much below 
1500ft. The 18z GFS is a notch further north with the surface low pressure 
track with a 995mb center reaching Long Beach or Grays Harbor by 
10pm Sat...it will be Worth taking a close look at the 00z mesoscale 
models...the GFS track could be windy if it is similar to the 18z 
run...and the mesoscale models are often a notch deeper. In any event 
with breezy solutions there will be enough onshore flow to ensure the 
snow level doesn't drop below much 1500ft late Sat night. 19 




Long term...a warm front moves through western Washington Monday...at 700mb the 
temperature warms from -14c late sun to around -5c Monday afternoon. The surface 
pressure pattern is rather lackluster with this front on Monday...the 
NAM shows a fairly decent southeast gradient Monday morning but then by 
Monday evening the front is dissipating and pressure is rising with 
light gradients as 500mb heights rise into the lower 560s. There is 
a good chance of some dry weather Tuesday as that ridge builds in...then 
a frontal band moves through Wednesday or Thursday...or both. The latest GFS suggests 
the remnants of a weakening frontal band for Wednesday and then a stronger 
system Thursday. There is pretty good agreement that the first half of the 
week will see a fairly high freezing level but not much precipitation...and 
then a lowering snow level late in the week with the best guess for 
a decent cold frontal passage Thursday night...so Friday could be postfrontal and 
cool showery but I haven't gone for that much detail for a day seven 
forecast though I did show the snow level back below the passes by Friday 
for better skiing likely toward the end of next week. 19 


&& 


Hydrology...in the Olympics all that remains is for the heavy rain 
from last night to drain to the ocean. All rivers are falling and 
only two are above flood stage...the Satsop and skokomish. Allowed 
the areal Flood Warning for Jefferson and Clallam counties to expire 
at 230 PM. 


The Skokomish River is forecast to fall below flood stage about noon 
Saturday. There will be a planned release of 3000 cfs from Cushman 
dam #2 into the North Fork Skokomish River on Saturday. This release 
will keep the Skokomish River running high. The forecast calls for 
the river to remain just below flood stage Saturday night and Sunday. 


The lowest reach of the Chehalis will bounce around at a foot or 
less below flood stage through Saturday. More rain Saturday night 
could keep the the lower Chehalis near flood stage...so have 
extended the watch through Sunday. Even local inflow from the weak 
system on Saturday night could cause the lower Chehalis to approach 
flood stage...but any flooding will be minor. 


The Cascades received a burst of rain as the front passed this 
morning. The Nooksack drainage received the most rain and the 
Nooksack river rose slightly...but it appears to have crested at all 
gauges and is heading back down...well below flood stage. Other 
rivers coming out of the north and central Cascades showed small 
rises and are now receding. One exception is the upper Snoqualmie 
and tolt rivers which are still rising. However rainfall amounts 
were not enough to cause any problems and the rivers should head 
back down soon. 


Another system will arrive Saturday with significant precipitation 
late Saturday into Saturday night. However...this system is colder 
with the majority of the precipitation being snow over the 
mountains. New flooding is not expected. 


The next system on Monday should move through quickly and be too 
weak to cause any flooding...even with rising snow levels. For the 
remainder of the week...the air mass over the northwest will be 
warmer with higher snow levels...however the systems that are 
expected to affect the area will be fairly weak and are not expected 
to cause any additional flooding concerns. 


No flooding is expected at any time on the Green River. Burke 


&& 


Aviation...an upper trough axis will move across western Washington early this 
evening. The atmosphere will be slightly unstable tonight with 
showers...and slight chance of isolated thunderstorms until around 5 
PM. Low level mixing overnight should limit lower stratus formation 
with fog not expected. Ceilings will generally run VFR through Sat 
morning. Moderate to strong westerly flow tonight will become strong southerly 
flow aloft on Sat ahead of an approaching surface low and front. 


A Puget Sound sound convergence zone is expected to form early this 
evening north of Everett...then sag southward to near the King/Snohomish 
line by late this evening. The convergence zone should dissipate 
early Sat morning as the flow dampens ahead of the next system. 


Ksea...SW wind 10-14 knots with gusts up to 25 knots overnight. The 
convergence zone may reach as far south as north Seattle but is not 
likely to reach the terminal. Dtm 


Marine...strong onshore flow with SW gradients warrant keeping 
small craft advisories for the waters through tonight. Will keep 
strong gales in the central and east Strait with a strong Post frontal 
westerly surge expected down the Strait this evening. Winds should ease a 
bit later tonight...possibly back to small craft by early Sat 
morning. 


The next system to affect the area arrives midday Sat in the form of 
warm front to lift across western Washington. Models differ considerably on the 
trailing surface low. On model shows two strong sub 990 mb lows 
tracking into Vancouver Island and the other into north Oregon. Another 
model shows one sub 990 mb low going into khqm on the S Washington coast. 
The exact track has a high impact on the wind speeds Sat afternoon 
through Sat night. There does appear to be a fairly strong front to 
still drag across...so I will go with a gale watch for now and let 
the next shift try to figure out if the surface low is heading north or 
S of Washington or right across. 


Seas will remain high through tonight...around 15 feet. On Sat...seas 
will subside slightly to around 12 feet but hazardous seas will 
continue with rough to severe bar conditions. Dtm 


&& 


Sew watches/warnings/advisories... 
Washington...WA...flood warnings and/or flood watches are in effect for a 
portion of the County Warning Area. 
.A Winter Weather Advisory is in effect for the Cascades tonight. 


Pz...a Gale Warning is in effect for the central and east Strait until 
midnight tonight. 
.A Small Craft Advisory is in effect for all remaining waters 
through Sat morning. 
.A Small Craft Advisory for severe bar conditions is in effect 
for the Grays Harbor bar. 
.A gale watch is in effect for the coastal waters...East 
Strait...and north interior from midday Sat through Sat 
evening. 


$$ 


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www.Weather.Gov/Seattle/gafd/latest_webafd.Html. 

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