960 
fxus65 kbyz 210946 
afdbyz 


Area forecast discussion 
National Weather Service Billings Montana 
246 am MST Sat Nov 21 2009 


Short term...valid for today and sun... 


Progressive weather pattern to be over the forecast area this 
weekend and into the day Monday. Large trough currently moving 
onto and across the Pacific northwest and into the northern 
rockies region. Satellite showing a deep southwest flow aloft 
across the state with plenty of middle and high level clouds moving 
across our area. Radars across Idaho showing convective type 
precipitation now moving up into the southwest portions of the state and 
approaching our Southwest Mountains. A few lighting strikes have 
also been observed across central Idaho indicating fairly strong 
dynamics and instability for this time of year. Will continue 
with likely probability of precipitation for the Southwest Mountains with chance probability of precipitation for 
the central and western zones this morning with best chances 
west. Snowfall could be heavy for a short while over the 
beartooth/Absaroka Mountains resulting in several inches of snow 
this morning. 


A cold front is expected to move across the central and western 
portions of the forecast area between 12-15z and the eastern zones 
by early afternoon. Dynamics and moisture decrease behind the 
front resulting in only chance probability of precipitation over the mountains...with 
slight chances spreading east during the afternoon. Expect windy 
conditions to develop behind the front across the plains. This 
downslope flow will help to inhibit precipitation across the plains. 
Dynamics also look to weaken as the trough moves across the 
forecast area so for now will continue with only a slight chance 
for showers across the plains today. 


Ridging quickly moves across the central portions of the state 
tonight bringing an end to any shower activity. However...Lee- 
side troughing develops across the forecast area late tonight in 
advance of the next fast moving trough across the Pacific 
northwest. Pressure gradients tighten once again across the 
upper Yellowstone and Stillwater valleys late tonight through 
Sunday morning. As a result...have put sub-advisory winds across 
these areas for this time. Another cold front moves through late 
Sunday morning which will shut the stronger winds off in the 
Livingston area. The GFS continues to shift the bulk of precipitation 
south across western Wyoming so will continue with just a slight 
chance to chance probability of precipitation for our plains with likely probability of precipitation returning 
for the mountains. Temperatures today will be cooler but still 
remain above normal before more normal temperatures arrive for 
Sunday with readings in the upper 30s to lower 40s. Hooley 




Long term...valid for Monday...Tue...Wed...Thu...Fri... 


A progressive pattern develops over the forecast area for the 
extended. 3 systems will impact the area next week...but 
ridging on Thanksgiving brings warm...dry and breezy conditions to 
the area. The first system is over the area Monday...shifting 
southeast Monday night. Increased probability of precipitation over eastern areas Monday 
as well as keeping some probability of precipitation for Western Mountain and foothills 
locations. Temperatures don't look terribly cold with this system 
in the lower 40s...and could be a tad cooler in some locations 
where clouds and precipitation occur. An Alberta clipper system 
quickly moves through eastern areas on Tuesday afternoon...though 
not before temperatures climb into the low to middle 40s. If timing 
of the clipper is faster will need to cool eastern areas 5 to 10 
degrees. Ridging builds into the area Wednesday and Thursday with 
downslope wind gradient in place pushing temperatures back into 
the 50s. The final system arrives Friday or Saturday depending on 
model preference. The European model (ecmwf) digs the late week system further to 
the south and brings cooler air into the area Friday 
afternoon...while the GFS is less deep with the system keeping the 
energy over Montana and Wyoming and brings colder air in for 
Saturday. 00z ensemble data supportive of the operational GFS 
keeping trough further north...and trended forecast toward the GFS 
for the later periods...including increasing probability of precipitation and lowering 
temperatures just a tad. Chambers 


&& 


Aviation... 


A cold front will move through the area this morning. MVFR with 
occasional IFR conditions will develop early this morning over 
western areas...including klvm...lasting into early afternoon. As 
the front moves eastward conditions should remain primarily VFR 
with local MVFR conditions associated with isolated shower 
activity. Mountain obscuration will last into late afternoon for 
western mountains...and into this evening for the Big Horn 
Mountains...as conditions improve from west to east. Gusty 
northwest winds are expected across the area behind the front this 
afternoon. Chambers 


&& 


Preliminary point temp/pops... 


Today sun Monday Tuesday Wednesday Thursday Friday 
------------------------------------------------------- 
bil 048 023/040 028/042 024/046 030/051 036/055 035/051 
2/west 02/west 21/b 11/b 00/b 00/u 02/west 
lvm 041 020/036 020/035 018/043 025/050 029/054 031/046 
4/j 04/j 32/j 12/west 10/b 00/b 03/west 
hdn 051 024/043 024/044 019/047 025/052 031/057 030/053 
2/west 02/west 22/west 12/west 00/b 00/u 02/west 
mls 053 022/045 024/043 020/047 023/046 030/056 031/051 
1/north 02/west 22/west 22/west 10/b 00/u 01/u 
4bq 051 023/042 021/042 019/045 022/046 028/056 029/051 
1/north 22/west 22/j 12/west 00/b 00/u 01/u 
bhk 053 021/045 021/040 019/043 020/040 024/054 027/051 
1/north 11/b 22/j 22/west 10/b 00/u 01/u 
shr 047 020/043 022/039 014/046 024/050 028/056 028/051 
3/west 22/west 22/j 11/b 00/b 00/u 01/b 


&& 


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


&& 


$$ 
Weather.Gov/Billings