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fxus66 kotx 211132 
afdotx 


Area forecast discussion 
National Weather Service Spokane Washington 
330 am PST Sat Nov 21 2009 


Synopsis... 
after a short relatively quiet break in the weather today...a 
stronger and colder storm system will bring the potential for 
moderate to heavy snowfall over much of the region tonight and 
Sunday. A series of storm systems will continue to pass through 
the inland northwest every 24 to 36 hours through at least the 
middle of next week. 


&& 


Discussion... 
today...while this morning's satellite loop looks messy...in general 
the forecast area is currently under an inter trough cold pool 
aloft which will exit the region and allow a dirty and weak short 
wave ridge to set up over the region for this afternoon. In the 
wake of last evenings cold front passage breezy winds will 
continue through the morning as a cooler air mass fills the basin. 
There will likely be some residual snow showers mainly 
concentrated over the Idaho Panhandle mountains and near the 
Cascade crest...with plenty of clouds and a small chance of a 
stray valley rain or snow shower possible over the rising terrain 
of the eastern basin today as a moist westerly flow aloft 
continues. High temperatures today will have trouble breaking out 
of the upper thirties at most locations other than the lowest 
valleys. 


Tonight through Sunday...a very tricky forecast for the next 24 to 
36 hours. The next storm system...visible on Pacific satellite as 
a developing baroclinic Leaf structure approaching 140w this 
morning...will move ashore sometime this evening and cross the 
forecast area overnight and into Sunday morning. There are some 
significant model differences with the track of the developing surface 
low and the strength of the parent trough. All the models develop 
a strong surface low just off the coast this afternoon and eject 
it inland tonight. The GFS and Canadian models track the low 
through the central Cascades and swing it north into British 
Columbia...while the European model (ecmwf) and NAM guidance track the low through 
the southern Columbia Basin. This low track is critical for snow 
amount potential. Although all models indicate significant 
accumulating snow is likely overnight Saturday and Sunday 
morning...a more southerly track will bring heavier snow amounts 
to the valleys and basin. Thus confidence is high that most of the 
forecast area north of Interstate 90 will experience at least a 
couple inches of snow tonight and Sunday morning...with moderate 
to heavy accumulations in the mountains. Confidence is low at 
this time which model will verify best...and thus uncertainty 
remains about the potential for highlights with this system. Thus 
a slew of watches have been issued...and hopefully further model 
runs today will settle on a confident solution. At this time we 
have opted for a track that is slightly south of the GFS/Canadian 
models and delayed the timing closer to the ec/NAM given the 
satellite extrapolation and aggressive digging of the parent 
trough. This argues for an onset of light to moderate snow in the 
western zones this evening...intensifying overnight then cutting 
out/tapering off with the cold front passage Sunday morning. Onset 
over the east and northern zones will hold off until late 
overnight Saturday or early Sunday morning...reach maximum 
intensity Sunday morning and taper off Sunday afternoon with the 
usual lingering dense snow showers over the Idaho Panhandle. /Fugazzi 




Sunday night through Friday...the weekend storm system continues 
its exit to the east Sunday night allowing for the higher probability of precipitation 
associated with it to show a similar exit out to the east Sunday 
night as weak ridging moves through behind it. This ridging is not 
well amplified and appears prone to overtopping by two storm 
systems. The first storm system drags a well maintained moisture 
taps through eastern Washington and northern Idaho Monday night 
into Tuesday which deflates the weak ridging further. This allow 
the second storm system to exert some influence as early as late 
in the day Tuesday...however the ridge appears to slowly amplify 
as a few shortwaves start to dig on the west side/norths side of 
the jet of the large baroclinic band/moisture tap associated with 
this second system. The result is a possibly shunting of any 
precipitation ban frontal feature back up to the north and west from 
Tuesday night to as late as Thanksgiving. Eventually a digging 
shortwave gets enough significant cyclogenesis going on the 
baroclinic band offshore to get it wrapped up into a compact low 
that dumbbells around with another low to its north as it moves 
inland sometime between the period between Thanksgiving and 
Friday. /Pelatti 


&& 


Aviation... 
breezy conditions in the wake of a cold front passage last night 
will keep generally VFR visibilities at all taf sites this 
morning...with some occasional MVFR and IFR stratus ceilings at 
kgeg/ksff and kcoe until 16z. A weak ridge aloft will promote VFR 
conditions at all taf sites through 03z tonight...with lingering 
snow showers locally obscuring mountains in the Idaho Panhandle. The 
next Pacific storm system will spread MVFR and local IFR cielings to 
the western reaches of the region after 03z tonight...mainly 
impacting the keat and kmwh taf sites. Conditions at eastern taf 
sites will begin to deteriorate by the end of the taf period at 12z 
Sunday. 




&& 


Preliminary point temps/pops... 
Spokane 39 32 37 27 39 31 / 20 70 90 10 40 10 
Coeur D'Alene 39 31 37 28 40 31 / 20 60 90 20 50 20 
Pullman 40 32 36 28 42 32 / 20 60 90 20 40 20 
Lewiston 47 36 41 30 48 35 / 10 60 90 20 30 20 
Colville 39 33 40 28 38 30 / 30 100 90 10 60 20 
Sandpoint 33 32 37 28 36 30 / 40 90 90 20 60 30 
Kellogg 32 30 34 27 35 30 / 70 70 100 40 60 40 
Moses Lake 45 32 43 26 43 30 / 10 80 50 0 40 10 
Wenatchee 43 32 43 29 41 33 / 30 90 50 10 50 10 
Omak 43 31 42 25 40 27 / 40 100 50 0 60 10 


&& 


Otx watches/warnings/advisories... 
Idaho... Winter Storm Watch from late tonight through Sunday morning for 
Coeur D'Alene area. 


Winter Storm Watch from this evening through Sunday morning for 
northern Panhandle. 


Winter Storm Watch from this evening through Sunday afternoon 
above 2500 feet for central Panhandle mountains. 


Washington... Winter Storm Watch from late tonight through Sunday morning for 
Spokane area. 


Winter Storm Watch from this evening through Sunday morning for 
east slopes northern Cascades. 


Winter Storm Watch from this evening through Sunday morning 
above 2000 feet for northeast mountains...Okanogan 
Highlands. 


Winter Storm Watch from this evening through Sunday morning 
above 2000 feet for Okanogan Valley...upper Columbia 
Basin...Waterville Plateau. 


&& 


$$ 

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