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fxus66 kotx 071248 
afdotx 


Area forecast discussion 
National Weather Service Spokane Washington 
445 am PST Sat Nov 7 2009 


Synopsis... 
heavy snow is expected over the high elevations of the inland 
northwest today and tonight. The mountains of northeast 
Washington...northern Idaho...and portions of the Cascades can 
expect big accumulations by Sunday morning. Monday will be a 
relatively dry day for the inland northwest...but another storm 
system will bring rain and high elevation snow to the region 
Monday night into Tuesday. 


&& 


Discussion... 


... High elevation snows today and tonight... 


Today and tonight...an upper level low anchored over the Gulf of 
Alaska will remain nearly stationary for the next 24-48 
hours...and the fast zonal flow into the Pacific northwest will 
continue to provide unsettled weather to region. A pair of 
vigorous shortwaves will move through the fast flow today and 
tonight. Water vapor imagery as of 10z shows the first wave on the 
Washington coast...and this wave will bring a round of 
precipitation to the Spokane forecast area this morning through 
early this afternoon. The second wave is about 500 miles due south 
of the Alaska coast...and this wave will bring another round of 
precipitation tonight. Neither of these shortwaves has a 
particularly impressive moisture tap. The AMSU satellite derived 
product depicts two-thirds of an inch precipitable water with each 
shortwave. Even with the limited moisture...significant 
precipitation will likely accompany the passage of both waves. 
Deep layer instability and good orographics will compensate for 
the limited moisture. The models are in good agreement that each 
wave will produce a band of precipitation between .25 and .33 
inches over the mountains of the Panhandle and northeast 
Washington. Given the favorable west-southwest wind into the 
mountains...some of the southwest facing slopes could receive 
upwards of a half inch of liquid equivalent with each wave. With 
the highly convective nature of the precipitation...elevations 
above 4000 feet can expect snow ratios of between 12 to 1 and 15 
to 1. Snow totals of 6 to 12 inches seem plausible by Sunday 
morning for the mountains around Sandpoint...Chewelah...and 
Kellogg. The Winter Storm Watch for these areas has been upgraded 
to a Winter Storm Warning for elevations above 4000 feet in the 
central Panhandle...and above 3500 feet for the mountains near the 
Canadian border. Accumulations of 10-18 inches are likely for the 
Cascade crest...with lesser amounts west of the crest due to the 
considerable westerly down-slope wind. 


Locally breezy winds will occur this afternoon behind the first 
shortwave. Winds will not be as strong as yesterday...but with 
850mb winds in around 30kts and favorable lapse rates for low 
level mixing...gusty conditions will be experienced again over the 
Palouse...West Plains...and Spokane area. /Gkoch 


Sunday through Tuesday...the models begin this period of the 
forecast with a fair spread in quantitative precipitation forecast values across the forecast area 
on Sunday morning...however with a trend toward much improved 
conditions by Sunday afternoon well established. The NAM has 
consistently been slower than the GFS/Canadian with the 
progression of Saturday night's system to the east of the 
area...and tonight's 00z and 06z runs were no exception. As a 
result...the forecast trends a bit closer toward the GFS/Canadian 
consensus and neglects the rather impressive quantitative precipitation forecast generated by the 
NAM over the southeastern portion of the area on Sunday morning. 


By 12z Sunday...the GFS is tracking the main vorticity well into 
northwestern Montana...while the NAM trails it a bit father west. 
The main differences in the models center around strong quantitative precipitation forecast 
generated by the NAM along the cold front as it sags southward 
into the southeastern portion of the area. Harnessing of 
instability and coupling it with very impressive differential positive vorticity advection 
to generate strong omegas over a very narrow region along the 
front yield quantitative precipitation forecast values around 0.60 inches from The Blues across 
the Palouse on Sunday morning. However...given the large sale 
westerly to southwesterly flow...this scenario seems rather 
unlikely to materialize...given the propensity of the large scale 
flow to advect any particular vorticity maximum east rather than allow it 
to sag due south along the front. In addition...with ridging 
popping up immediately to the west of the main vorticity lobe...it 
seems increasingly unlikely that any additional incoming vorticity may 
run down the front...effectively re-activating lift along the 
frontal band. As a result...the forecast features much lighter 
diminishing up-slope showers rather than organized frontal 
rain/snow through Sunday. 


After forcing along the cold front shifts eastward Sunday 
afternoon and evening...the GFS starts to shift the thermal 
gradient a bit farther farther north...with incoming large scale 
southerly flow increasing the northward translation of the front 
overnight Sunday night as a warm frontal push. Some isentropic 
lift is noted along the front as it shifts northward Sunday 
night...however condensation pressure deficits in the 290k area 
generally stay near or above 30 mb over eastern Washington...with 
the only area of reduced condensation pressure deficits near the 
Cascades. As a result...precipitation chances have been relegated to the 
Cascades...with the rest of the area dry along and behind the warm 
front Sunday night into Monday. 


Better model agreement exists in clearing behind the warm front 
for Sunday night and Monday. Additionally...the 00z GFS and 06z 
NAM have come into much closer agreement for tracking the trailing 
cold front into the area on Monday night with a 120 knots jet streak 
left entrance region coming on shore and deep layer positive vorticity advection spreading 
stratiform rains into the area Monday night. Layer temperatures 
after the warm frontal passage surge above freezing well up into 
the atmosphere...and wet bulb zero heights reach nearly 6000 feet 
over many areas. As a result...many locations that will have seen 
snow this weekend are likely to see rain with the next system on 
Monday night and Tuesday. /Fries 




&& 


Aviation... 
a pair of fast moving storm systems will move through Washington 
today and tonight. Look for precipitation to begin in the 
Spokane...Pullman...and Lewiston areas around 15z and continue 
through 18-19z. Temporary MVFR ceilings are possible for a couple of 
hours at kgeg...kcoe...ksff...and kpuw before precipitation quickly 
clears these airports. The second round of precipitation will spread 
across eastern Washington shortly after 00z. Look for possible MVFR 
ceilings again at the same terminals between 04z and 08z. /Gkoch 






&& 


Preliminary point temps/pops... 
Spokane 45 34 43 32 46 35 / 90 80 20 10 0 50 
Coeur D'Alene 45 34 43 32 47 34 / 90 80 20 10 0 40 
Pullman 47 35 48 30 53 36 / 70 70 20 0 0 40 
Lewiston 54 38 53 36 55 41 / 40 40 20 0 0 30 
Colville 45 34 46 32 46 32 / 70 90 20 10 0 60 
Sandpoint 42 35 41 32 43 33 / 100 90 40 10 0 50 
Kellogg 40 33 38 31 43 33 / 90 100 50 10 0 60 
Moses Lake 52 34 50 32 50 33 / 20 70 10 10 10 30 
Wenatchee 50 35 48 35 47 36 / 10 30 0 10 10 50 
Omak 48 33 46 34 46 29 / 20 70 10 10 10 60 


&& 


Otx watches/warnings/advisories... 
Idaho...Winter Storm Warning from 7 am this morning to 4 am PST Sunday 
above 3500 feet for northern Panhandle. 


Winter Storm Warning from 7 am this morning to 4 am PST Sunday 
above 4000 feet for central Panhandle mountains. 


Washington...Winter Storm Warning from 7 am this morning to 4 am PST Sunday 
above 3500 feet for northeast mountains. 


&& 


$$ 

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