975 
fxus65 kbyz 070920 
afdbyz 


Area forecast discussion 
National Weather Service Billings Montana 
320 am MDT Tuesday Oct 7 2008 


Short term...valid for today and Wednesday... 


Main forecast concern in the immediate short term is gap wind 
potential. Northwest drainage winds still observed at klvm...but 30 miles per hour 
SW gusts occurring at Nye as of 2 am. Surface analysis shows pressures 
falling on the order of a mb/hour now in north central Montana. Given 
this would expect klvm gap winds to kick in soon...probably before 
12z. Pressures over Yellowstone Park are a bit higher than models 
suggest...so onset of gap flow early this morning may hit a bit 
harder than previously thought. Local gap flow approximator gives 
35 miles per hour sustained maximum wind. Will up wind speeds a bit but stay 
below advisory criteria...which would seem to be difficult given 
no snow cover in the park and modification from mixing by 18z... 
the time when models show the pressure gradient becoming tightest. 


Sticking with winds...will see a burst of west-northwest winds with frontal 
passage this evening beginning around 00z. Mixed west-northwest winds will 
continue Wednesday...strongest east. Along with the wind will come very 
dry air for tomorrow and should see dewpoints in the teens to low 
20s. Will highlight gusty winds and low relative humidity in the fire weather 
forecast. 


Shortwave will skirt across northern Montana tonight. Should see 
some sort of band of light showers come across along middle level 
front tonight...beginning possibly a bit before 00z in our northwest 
parts given speed of approaching wave. Baroclinic zone sags to our 
south for tomorrow...and will keep low probability of precipitation only over our southern 
mountains strong/dry downslope flow will prevail over our plains as 
already mentioned. 


Things begin to become more interesting Wednesday night as next Pacific 
energy approaches and induces frontogenesis to our south. Models 
show strongest warm advection to our south late Wednesday night...but 
with q-vector convergence spreading over and diminished downslope 
flow cannot rule out a slight chance of precipitation late Wednesday night across 
our west and south. Dry airmass in place will need to be moistened 
and better chance of precipitation will begin Thursday. See extended discussion 
for that. 


As for temperatures...mild today with low 70s common in surge of warmth 
ahead of approaching front. Should be a good 10 degrees cooler on 
Wednesday. Have scaled low temperatures down Wednesday night in dry airmass and 
perhaps clear skies before next wave affects US...especially 
across our N/NE. 


Jkl 


Long term...valid for Thursday...Fri...Sat...sun...Mon... 


Extended forecast in pretty good shape overall...with models in 
good agreement through late in the extended...most importantly 
through the extent of the next storm system. There is some temperature 
variance...but generally a couple degrees in the same general 
pattern. Did make several changes throughout the extended...mainly 
confined to the period of the passage of the next storm 
system...cooled temperatures and adjusted probability of precipitation slightly. 


Thursday remains the start of the large upper trough digging a large 
trough into northwest U.S. This looks like it will begin bringing 
in some warm air and associated lift across the southern zones...so 
pretty much kept probability of precipitation as they were...with only minor collaboration 
adjustments 


Much colder and snow appears to be the story for Friday through 
Saturday and possibly Sunday. Storm system seems to deepen and 
proceeds across the region...finally exiting on Sunday. Made 
minor adjustments to probability of precipitation...but mainly for storm track and 
collaboration and generally kept in the same value range. Biggest 
change surrounded temperatures...where cooled the entire period a few 
degrees. Models appear much colder then forecast...and were well 
above mex numbers...so generally trended toward mex numbers. Still 
some question regarding how much snow...vs how much rain...vs how 
much mixed precipitation...but looks like is trending to more of a snow event 
than a rain or rain/snow event...as temperatures keep coming in cooler. 


Temperatures appear to pop back up to middle to upper 50s for Monday and 
Tuesday following the passage of the low. Models very a bit on how 
much it will rebound...so generally tried to split the difference. Gilstad 




&& 


Aviation... 


VFR conditions will prevail across the region this morning through 
the afternoon. Gusty west winds from 15 to 25 kts...with gusts around 35 
kts...will impact the klvm area today. A cold front is expected 
to cross the region this evening and overnight and may cause a few 
showers across the area. Gilstad 




&& 


Preliminary point temp/pops... 


Today Wednesday Thursday Friday Sat sun Monday 
------------------------------------------------------- 
bil 072 044/060 034/047 033/040 028/038 027/042 029/053 
0/b 20/u 22/r 33/o 23/o 32/west 21/b 
lvm 071 032/060 029/047 027/039 021/040 021/044 028/056 
1/north 20/u 24/o 44/o 43/o 22/west 21/north 
hdn 074 041/063 033/051 032/041 028/039 026/044 028/056 
0/b 20/u 23/r 33/o 23/o 32/west 22/r 
mls 074 042/061 032/049 032/044 030/042 028/045 030/058 
0/b 20/north 12/r 32/o 23/o 32/west 22/r 
4bq 072 041/059 034/050 029/044 029/040 028/043 029/058 
0/u 20/b 23/r 43/o 34/o 43/west 22/r 
bhk 073 039/057 027/048 026/044 026/041 026/046 028/055 
0/u 20/north 12/o 33/o 33/o 33/west 22/r 
shr 073 039/061 032/047 028/042 027/037 026/041 028/057 
0/u 20/b 34/r 54/o 44/o 42/west 22/r 


&& 


Byz watches/warnings/advisories... 
Montana...none. 
Wyoming...none. 


&& 


$$ 
Weather.Gov/Billings