456 
fxus65 kboi 211037 
afdboi 


Area forecast discussion 
National Weather Service Boise Idaho 
335 am MST Sat Nov 21 2009 


Short term...today through Sunday...weak system exiting the 
forecast area at this time with lingering scattered rain and snow 
showers. These current convective showers associated with passage of 
the upper level trough. A decent weather day today with a drier push 
of west to northwest flow and seasonal temperatures ahead of a more potent 
Pacific system for tonight and Sunday. This next system is fast 
moving...embedded within a powerful jetstream flow. Yet...quantitative precipitation forecast looks 
rather substantial in northern areas and with west flow...orographic 
enhancement will aid storm total snow accumulations. Timing into 
Baker County Oregon tonight about 03z which is about 3 hours earlier 
then start time over the Idaho zones. Overall duration to event 
should not last past middle afternoon Sunday with the exception of a 
few lingering snow showers. Main impacts with tonights storm 
expected to be moderate to locally heavy snowfall and poor 
visibility on north south rural roads. Temperatures expected to turn 
sharply colder north of the jet stream which will produce a less 
dense snow than is expected in the southern zones. This in turn 
should drop visibility to less than one half mile at times. Have 
opted to issue a Winter Storm Watch for the northern mountain zones 
and let day crew make final decision of specific winter highlights 
just before the event unfolds. 


Long term..Sunday night through Saturday...snow showers will 
persist across SW Idaho terrain...before tapering off by Monday 
evening. Temperatures will quickly rebound to above normal values on 
Tuesday as an upper level ridge builds over northern California/Nevada. 
By Wednesday...a strengthening surface inversion will keep afternoon 
mixing to a minimum - for cooler valley highs. A similar issue 
possible on Thursday...but current forecast relies on low-level warm 
advection /ahead of an approaching trough/ to increase mixing and 
keep temperatures above normal. Upper trough prognosticated to move into 
the region early Friday...but models vary substantially in location. 
Therefore have remained conservative with near climatology probability of precipitation /Friday 
and Saturday/ due to uncertainty. 


&& 


Aviation...SW Idaho - IFR snow showers across higher terrain 
/mainly west central and Boise Mountains/ will persist through 21z. Mountains 
will be obscured. VFR rain showers across the western snake plain /E of 
Boise/ through 15z. Light to moderate surface winds - with gusts to 
25 kts east and south of kmuo. Southeast Oregon - VFR under broken-overcast skies 
overnight. Light winds. 


IFR conditions will return after 06z tonight...as another storm 
system moves into the area. Expect widespread IFR snow - all 
elevations. 


&& 


Boi watches/warnings/advisories... 
Idaho...Winter Storm Watch from 06z Sunday through 21z for idz011 and 
idz013. 
Or...Winter Storm Watch from 03z Sunday through 18z for orz062. 
&& 


$$ 


Weather.Gov/Boise 


Short term...gs 
long term....Mt/jt 
aviation.....Mt