853 
fxus65 kboi 211638 
afdboi 


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


Discussion...updated forecasts. Cold frontal cooling will limit 
highs today to the 30s to middle 40s...and morning sounding indicates 
44 for a high at Boise. Valley showers last evening scrubbed 
dust/dirt out with reports from Marsing to Middleton to Emmett 
consistent with yesterdday/S blowing dust reports and multi sensor 
precipitation estimates those locations as well. Cold front 
bringing in cleaner air in as well. About 7c/km lapse rates below 
550 mbs aiding fast moving snow showers through the Boise 
Mountains as of 9 am. Dry across southeast Oregon and valleys of SW Idaho 
today with diminishing snow showers in the central mountains. Next 
system is prognosticated 3hrs later in the sref...6 NAM/GFS..and 9 hours 
later in 00z ec. So have adjusted timing by 3 hours for now. This 
allows mins to be a couple degrees colder and supports up to an 
inch of wet snow accumulation on grassy/ground areas on the north 
half of the Treasure valley zones. Some roadways may get a 
dusting. Snow ends in the valleys towards noon and temperatures 
warm in to the upper 30s by noon Sunday...while moderate to heavy 
snow is expected to linger in the west central and Boise Mountains 
through early to mid-afternoon. Will update weather story as well for 
this new timing and still expected amounts from light in the southern 
valleys and Harney/Malheur counties to moderate...bke/Idaho 
City/Fairfield to moderate or heavy...zone 11/13 mountains. 


&& 


Aviation...brief IFR conditions will accompany widely scattered 
snow showers in the Boise and west central mountains today. Lower 
valleys of SW Idaho and all of southeast Oregon will be VFR. Another 
storm brings IFR conditions to southeast Oregon after 06z Sunday morning 
and after 10z in SW Idaho. Expect a 4-6 hour period of IFR 
conditions at kboi with brief periods of moderate snow possible. 
IFR conditions continue in the SW Idaho mountains and western Magic 
Valley through Sunday afternoon. Strong westerly winds aloft on 
Sunday of 40-60 kts. 


&& 


Previous discussion... 
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. 


&& 


Boi watches/warnings/advisories... 
Idaho...Winter Storm Watch from late tonight through Sunday afternoon 
for idz011-idz013. 
Or...Winter Storm Watch from this evening through Sunday afternoon 
for orz062. 
&& 


$$ 


Weather.Gov/Boise 


Discussion...vm 
aviation.....Dg 
previous short term...gs 
previous long term....Mt/jt