793 
fxus65 kboi 210320 
afdboi 


Area forecast discussion 
National Weather Service Boise Idaho 
820 PM MST Friday Nov 20 2009 


Discussion...cold front has just about cleared the County Warning Area to the 
east...with it having moved through Twin Falls and Jerome within the 
last hour. Precipitation was developing behind the front and latest data 
indicates light snow across SW Idaho mountains with little if any 
precipitation occurring in the Treasure valley. Widespread light snow will 
continue overnight in the Boise/west central mountains for light snow 
accums. Elsewhere...precipitation will be scattered with snow levels around 
4500 feet then falling to near valley floors by Saturday morning... 
but by then the precipitation will have pretty much ended in the valley. 
Think probability of precipitation are a little too high in southeast Oregon where only expecting 
scattered showers...thus will trim them back to scattered overnight. 
Otherwise...forecast is on track. 


&& 


Aviation...surface cold front has already moved through the 
region. Surface winds behind the front have switched to the 
southwest with speeds between 10 and 20 kts. Very little 
precipitation with this front with light snow over the west central 
and Boise Mountains for local MVFR conditions. More showers will 
move over the region after 05z as an upper level trough moves east. 
This will bring mountain obscuration along with areas of MVFR 
conditions in rain and snow showers and local IFR conditions. 


&& 


Previous discussion... 
short term...tonight through Sunday...pre-frontal gusty east to 
south winds have been locally very windy in the Mountain Home area 
and parts of southern Harney County today. Local blowing dust was 
observed as well. Winds are expected to decrease to 15-30 miles per hour 
through the late afternoon then shift to the SW and northwest 15-35 miles per hour 
with the cold frontal passage. The front appeared to have crossed 
into the Lee of the Cascades in west central Oregon as of 2 PM and 
should reach the Oregon-Ida border 5-6 PM MST. Precipitation looks to lag the 
front by 2-3 hours for a quick shot of rain and snow this evening in 
Oregon and precipitation changing to snow above 3000 feet in Idaho tonight. 
Valleys look to dry out Saturday morning as snow showers linger in 
the Baker mountains until noon and Idaho mountains through 
mid-afternoon. Cold and dry but considerable cloudiness around 
Saturday night along with another round of gusty southeast to south 
winds. Near normal temperatures for lows tonight through highs 
Sunday after warming up into the 40s and 50s this afternoon. 


A stronger and colder system into southeast Oregon late Saturday night 
and across Idaho Sunday looks to be a potential winter storm. Warm 
advection and moisture with around 120 percent of normal precipitable waters  
quickly transitions to cold unstable conditions with widespread snow 
showers through the afternoon in the mountains. Up to a foot of 
new snow is possible Sunday in the west central and Boise 
Mountains along with areas of blowing and drifting snow. Last two 
runs of the NAM have shown 8-16 inch snow accumulations Sunday 
but the GFS is closer to 8-10 maximum amounts and the ec 4-7 inches 
based on quantitative precipitation forecast...so there is still some uncertainty in the 
qpf/snowamt. The storm tracks are similar so a weather story has been 
issued to highlight the possibility of a winter storm Sunday 
across the northern mountains. Snow levels will be at valley 
floors in the morning and an inch or so is expected at Boise early 
Sunday morning. 


Long term...Sunday night through Friday...snow showers will linger 
across the central Idaho mountains and western Magic Valley Sunday night 
before drying out on Monday. This will transition toward a period of 
high pressure as an upper level ridge will build over Oregon Tuesday 
through Thursday. The ridge will amplify over the forecast area on 
Wednesday and Thursday for predominantly dry conditions /and minimal 
afternoon mixing/ through early Friday. This could bring an 
inversion to the low elevation valleys over the Holiday travel 
period for air quality and visibility impacts. Have adjusted the 
forecast grids to reflect inverted temperatures based on elevation 
..along with weak southeast winds through Thursday night. Models are 
in reasonable agreement bringing the next Pacific trough into the 
area late Friday or Saturday. 


&& 


Boi watches/warnings/advisories... 
Idaho...none. 
Or...none. 
&& 


$$ 


Weather.Gov/Boise 


Discussion...bw 
aviation.....Ja 
previous short term...vm 
previous long term....cd/LH