999 
fxus65 kboi 231726 
afdboi 


Area forecast discussion 
National Weather Service Boise Idaho 
1026 am MST Monday Nov 23 2009 


Discussion...no updates so far. Very cold start will challenge high 
temperatures for today. Sounding gives 42 which looks on target. May 
need an update around noon depending on how fast/slow surface 
inversion erodes. Look for the subsidence inversion /due to 
sinking and warming aloft/ to strengthen Wednesday/Thursday after a weak 
system crosses north Tuesday. This combined with snow melt looks 
to lead to a moist inversion through Thanksgiving. There are two 
main areas of concern so far. Expect stratus and fog developing 
in the upper snake plain to drift westward into the northwest Magic Valley. 
Second is areas of stratus and fog which are expected to form in 
the lowest elevation of the lower Treasure valley. Wednesday looks 
to be the most inverted /greatest stratus coverage/ day and 
stratus could persist in the Vale Oregon to Black Canyon Idaho 
region past noon or even all day. By Thursday afternoon southeast winds 
should pick up ahead of a cold front for earlier breakup. 


Of course...any stratus/fog has not formed yet and that brings 
much more than normal uncertainty to the shorter term forecast 
periods for the Treasure and Magic Valley forecasts through 
Thursday night at least. 


Models still agree that a front is due across Boise around 
sunrise Friday. High pressure surface/aloft is likely to rebuild behind 
the front for the rest of the upcoming Thanksgiving weekend for 
inverted conditions. Even more uncertainty on moist vs. Dry 
inversion next weekend due to questions about low level moisture 
sources such as any precipitation with front and low/middle elevation snow 
melt. Will it be moist with stratus and fog or will it be dry with 
increasing haze and decreasing air quality? Tough forecast 
dilemma for temperatures but much below normal probability of precipitation are called for 
either way Sat/sun/Mon as in current package. 


&& 


Aviation...VFR for SW Idaho and southeast Oregon today. Increasing 
middle-level ceilings tonight over the west central and Baker County 
mountains with widely scattered snow showers and IFR conditions on 
Tuesday. Expect lower conditions to remain north of kbke-kmyl line. 


Other story will be developing inversion. Expect local IFR 
conditions to develop in the lower Treasure valley and possibly the 
western Magic Valley Tuesday morning. Coverage of IFR conditions 
will increase Wednesday morning to possibly include the Boise 
terminal. 


&& 


Previous discussion... 
short term...today through Tuesday...high pressure building over 
the forecast area for benign weather the next two days. However...a 
weak disturbance is forecast to slide southward late tonight from 
the Pacific northwest...eventually washing out over central Idaho 
within the building upper ridge. Models handling this feature 
slightly differently with the NAM more aggressive showing a more 
southerly position and limited probability of precipitation...while the GFS keeps the 
feature north of our forecast area in central Idaho across the 
Salmon river and points north. Have chosen a compromise between the 
two and allowed a smattering of slight chance probability of precipitation across Baker 
County Oregon and Valley County Idaho late tonight and Tuesday. 
Otherwise...with the above mentioned ridge taking hold...weather 
remains quiet with a good amount of sunshine today and partly to 
mostly cloudy skies Tuesday. Temperatures cool today with surface 
based inversion strengthening...but warming becoming more 
significant Tuesday as high center drifts over eastern Idaho 
bringing easterly component flow back to our region aiding low level 
warming. 


Long term...Tuesday night through Monday...the pattern through the 
period is progressive. Model agreement and run to run consistency 
are quite good through Friday. A ridge over our area Wednesday moves 
east of The Rockies Thursday followed by a shortwave trough crossing 
the region Thursday night through Friday night. Over the weekend the 
Canadian diverges from the gfs40/GFS ensemble mean/European model (ecmwf) by bringing 
US a much colder pattern with a ridge building offshore rather than 
over the West Coast states. We are staying with the milder 
guidance...with temperatures 5 to 10 degrees above normal on 
Thanksgiving day...cooling to near normal for Saturday through 
Monday. Best chances of precipitation will be with the trough 
passage Thursday night through Friday night. For the rest of the 
period probability of precipitation will be below normal for our southern zones and near 
normal for Baker County and the west central Idaho mountains. 


&& 


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


$$ 


Weather.Gov/Boise 


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