067 
fxus66 kotx 202345 
afdotx 


Area forecast discussion 
National Weather Service Spokane Washington 
345 PM PST Friday Nov 20 2009 


Synopsis... 
a cold front will move through the region this evening and bring 
light valley rain and mountain snow to the region. After a short 
break in the weather on Saturday...a stronger and colder storm 
system will bring the potential for moderate mountain and valley 
snow accumulations Saturday night and into Sunday. A series of 
storm systems will continue to pass through the inland northwest 
every 24 to 36 hours well into next week. 








&& 


Discussion... 
tonight and Saturday...the satellite loop is showing a trough digging 
south along the Washington coast. The 12z run of the models were 
very similar showing the cold front on the Cascades around 18z 
and weakening as it moved through the forecast area by 06z this 
evening. At 21z precipitation had been falling along the Cascade 
crest for several hours while keln just reported rain...in 
addition surface pressure was still falling along the east slopes 
of the Cascades. This puts the timing of the front 3-4 hours 
slower than previous thinking due mainly to the 150kt jet digging 
the low and slowing eastward progression. For late this afternoon 
and early evening...precipitation will move across the forecast area 
in the warm sector ahead of the cold front and probability of precipitation were adjusted 
accordingly. Up-Glide along the 285-295 isentropic surface is 
impressive but the stretching front has cut-off the moisture tap 
from out in the Pacific. In addition local soundings are indicating 
a very dry sub- cloud layer and across the eastern zones southerly 
winds have dried out the lower atmosphere further. While just 
about all locations will pick up measurable precipitation with the 
front it will take a while to saturate the lower atmosphere and 
precipitation will be lowered. The southerly flow ahead of the 
front has kept snow levels at or above 4000 feet through early 
evening. 


For the rest of tonight expect drying from the west behind the 
front. Southwest winds will increase resulting in the typical rain 
shadow along the east slopes of the Cascades and promoting up- 
sloping flow across the eastern zones. Snow levels are expected 
to drop behind the front to near the surface. But by that time rain and 
snow will turn showery and the best chances for measurable 
precipitation will along the Cascade crest and the Panhandle 
mountains. Southwest winds will increase to breezy with gusts 
overnight 20-25 miles per hour across a large portion of the forecast area. 
This should keep fog and stratus formation to a minimum. 


Saturday weak and fast moving high pressure will build in behind 
the cold front. Westerly flow into the eastern mountains will 
keep orographic showers in the forecast for those zones. 
By Saturday afternoon the next storm system will be knocking on 
the door. Southwest flow will increase warm air advection and 
moisture into the north west zones. The 12z and now the 18z models 
are all pretty similar bringing in a weak short wave disturbance 
through the northwest zones between 18-00z Saturday afternoon. The 
combination of the isentropic lift and the wave will be enough to 
support likely or better probability of precipitation across the northwest zones by late 
in the afternoon. Snow levels will be down to the valley floor and 
1 to 2 inches of new snow will be possible by late Saturday 
afternoon. Temperatures will be 5-7 degrees cooler Saturday and 
very near seasonal normals. /Tobin 


..moist Pacific storm to bring chance for moderate snows above 
2000 feet Saturday night through midday Sunday... 


Saturday nt through Sunday nt...the next in a line of 
moist Pacific storms will impact the region bringing rain and snow 
as well as moderate winds. Model guidance is generally fair to 
poor with this system and this is making for quite the forecast 
challenge trying to sort out the details. Looking at the big 
picture...a moist 1.4" precipitable water plume...cold core shortwave 
energy...and strong upper-level jet dynamics all come together 
tomorrow afternoon off the Washington coast resulting in strong 
cyclogenesis which will transition across the Pacific northwest Saturday nt 
through midday Sunday. Models are having an extremely difficult 
time with the details regarding the strength of the low- level 
cyclogenesis as well as the track. The trend off the NAM/Gem/ECMWF 
has been toward a southern track which brings the surface cyclone 
along the Washington/or border and heaviest precipitation across the 
southern Columbia Basin while the last few runs of the GFS 
continue with much deeper low (nearly 10mb deeper than other 
guidance) and track directly across the County Warning Area which would result in 
heavy precipitation across the central and northern tier zones. With 
the ec/NAM/Gem in much better agreement and showing some 
consistency within the last two runs...will lean in that direction 
and put less weight with the outlying GFS. 


Strong isentropic ascent along the 280-295k surface developing 
Saturday night will just about guarantee precipitation for all 
locations...starting in the Cascades and northern mountains first then 
spreading across the basin after a shallow dry layer left in the 
wake of saturday's system becomes moistened. But it will be the 
precipitation shield that develops along the northern periphery of 
the low-level cyclone...or trowal airstream which becomes the 
focus for the heaviest amounts as the cyclone tracks across the 
region. Again...the forecast has trended toward the NAM/ec/Gem and 
probability of precipitation/quantitative precipitation forecast were reduced along the northern tier and increased for 
near the Washington/or border. The second major challenge will be snow 
levels. With such a warm moist tap of moisture (nearly 0.50" 
tongue nosing into the basin)...expect snow levels to rise within 
the 2000-3500k feet range along the southern basin to 1500-2000k feet 
layer across the north. This may lead to another event where snow 
falls across the West Plains including the Spokane Airport with a 
few inches but rain/snow mix downtown with little to no 
accumulations. With confidence generally poor with the storm 
track...and where the the heaviest precipitation bands will set 
up...will leave out exact snow amts in the afd but Post 
preliminary snow amts on weather story 2 located 
(weather.Gov/spokane). By late Sunday morning...the system begins 
to weaken and tracks into central Idaho with strong westerlies 
rapidly increasing as strong cold advection slides down the 
Cascades. This will quickly shut off the precipitation from west 
to east at the expense of strong winds down the east 
slopes...Waterville Plateau...and western basin. With 850mb winds 
increasing to 40-55kts...expect winds in locations like Wenatchee 
to gusts in excess of 35-40kts. /Sb 




Monday through Friday...the region will be affected by a weak 
frontal system on Monday...followed by a drying ridge midweek. A 
stronger system will affect the forecast area on Thanksgiving day. 


Warm frontal precipitation will spread across the inland northwest 
on Monday. There are some differences in the track of the shortwave 
trough but generally the best forcing remains to our north. This 
will keep the best chance of precipitation across our northern zones. 
Precipitation is expected to be either rain or a mix of rain and 
snow south of I-90 with mainly snow to the north. The northern 
valleys and the Waterville Plateau could see light snowfall 
accumulations but any significant snowfall will be confined to the 
elevations above 3500 feet. As the cold front pushes through Monday 
night the focus for precipitation will shift to the eastern 
mountains and become more showery in nature. On Tuesday a middle level 
ridge will start to build over the region. There is some moisture in 
the westerly flow aloft along with some isentropic ascent across the 
northern tier but any precipitation that occurs will be light. 
Daytime temperatures will be on a gradual warming trend through 
midweek. By Wednesday night...clearing skies and light winds will 
allow fog and low stratus formation in the basin. Expect this will 
spread north to the northeast valleys not affected by clouds 
lingering along the British Columbia border. 


..Thanksgiving day looks to be wet but not white across the inland 
northwest... 


A stronger Pacific weather system will affect the region by 
Thanksgiving day. This system has a nice moisture tap with precipitable waters  
around 150 percent of normal. We could see a good shot of 
precipitation but low level southerly flow will have pushed snow 
levels above most valley floors. By Thursday night the cold front 
swings through and snow levels will drop overnight. By this time 
most of the precipitation will have moved east to the Panhandle 
mountains. Temperatures will return to near seasonal normals. /Kelch 




&& 


Aviation... 
a cold front will move across the forecast this evening and be 
east of the area later tonight. Conditions will be mainly VFR 
with localized and brief MVFR ceilings during the heaviest 
precipitation. Conditions will dry out from the west behind the 
cold front and expect enough wind to keep the lower atmosphere 
mixed enough for only isolated stratus development Saturday 
morning. After 16z Saturday...southwest winds will increase with 
gusts 20 to 25 kts. Precipitation will affect the western taf 
sites after 18z. /Ek 












&& 


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


&& 


Otx watches/warnings/advisories... 
Idaho...none. 
Washington...none. 
&& 


$$ 

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