976 
fxus65 kbyz 231009 
afdbyz 


Area forecast discussion 
National Weather Service Billings Montana 
309 am MST Monday Nov 23 2009 


Short term...valid for today and Tuesday... 


The main weather impact for the short term is windy conditions 
over the western half of the forecast area...particularly late 
tonight through Tuesday. 


Upper low currently over the Lewistown area will continue to shift 
slowly southeast through the day today. Downslope westerly winds 
over western areas should contain precipitation to eastern areas 
as well as mountain locations today. Dynamics are weak with this 
system and most of the moisture associated with the system is 
already moving out of eastern Montana. Still should be enough 
instability with the core of the low to generate isolated to 
scattered snow showers. Did not make much of an adjustment to 
temperatures for today...with 35 to 45 seeming reasonable for 
highs across the area. West winds will increase to 20 to 30 miles per hour 
over western areas this afternoon...with 15 to 25 miles per hour winds as far 
east as Billings. 


Surface pressure gradient tightens up tonight as surface high 
pressure builds into southwest Montana and Idaho...and eventually 
into the park by morning. As warm advection develops at 700mb 
behind the exiting upper low over the Dakotas...conditions become 
favorable for strong gap flow winds from Livingston to Nye. 
Pressure gradient orientation is not optimal and 700mb wind 
direction is right on the cusp of being favorable for highlite 
criteria winds. If everything becomes perfect could see winds of 
45 to 50 miles per hour with gusts near 70 miles per hour at times. However...with 
things not quite optimal on latest run have kept winds just up to 
advisory levels for these areas beginning late tonight. Day shift 
can take another glance to see if the ingredients become more 
favorable leading to an advisory. Before the winds pick up skies 
will be pretty clear over western areas and this should lead to 
some pretty cool temperatures early tonight. Once winds increase 
some mechanical mixing of warmer air aloft should moderate 
temperatures over the west going into Tuesday morning. 


Tuesday will see some flat riding and warm advection aloft through 
much of the day. Gusty downslope winds will spread from the 
foothills east across central portions of the forecast area 
keeping things dry. Some guidance suggestive of near warning type 
winds late Tuesday morning into the afternoon from Big Timber to 
Harlowton. With an west/east oriented pressure gradient this seems 
possible...though would like to see near boundary layer winds a 
bit higher than the 40kts the NAM is showing over these areas. As 
with The Gap flow areas have winds up to highlite criteria for 
these areas and will let the next shift take another look before 
pulling the trigger. 


By late in the day an Alberta clipper system drops into northern 
Montana. The GFS has been more consistent with the track of this 
system over the past 36 hours...pulling a good deal of energy 
across the eastern half of the forecast area. The NAM is a bit 
slower and further to the east with this system. Have leaned 
toward the GFS for this system going into Tuesday night. Good qg 
forcing and frontogenesis is prognosticated to move into the area Tuesday 
evening...but moisture looks somewhat limited. Went slight chance 
for eastern zones for now...with some higher probability of precipitation in and near the 
Big Horn Mountains. If moisture profiles improve may need to beef 
up these probability of precipitation for Tuesday night. Chambers 


Long term...valid for Wednesday...Thu...Fri...Sat...sun... 


The extended period begins Wednesday morning with an upper-level 
ridge building over the region. This feature builds through 
Friday with mild temperatures and dry conditions prevailing. The 
next upper-level trough approaches the County Warning Area late Friday into 
Saturday. Model differences between the GFS and European model (ecmwf) are 
significant at this time. The GFS develops a much broader trough 
with energy reinforcing this feature through Monday. The European model (ecmwf) is 
much more progressive with the bulk of its influence occurring on 
Saturday. Continued to trend forecasts toward the European model (ecmwf) but 
lingered low probability of precipitation into Sunday for portions of the County Warning Area. Both 
models hint at building another upper-level ridge over the region 
for early next week. However...the European model (ecmwf) is quick to break down 
this feature with an upper trough on Tuesday. Therefore...dried 
precipitation forecasts for Monday and introduced slight chance 
probability of precipitation for western zones late Monday into early Tuesday. Schuknecht 


&& 


Aviation... 


An upper-level trough will continue to push eastward across the 
region today. Scattered snow showers will be possible through 
middle-morning across western and central zones. Showers will linger 
over eastern zones through this afternoon. Expect brief periods of 
MVFR conditions within areas of precipitation. Gusty west/northwesterly 
surface winds are expected to develop mainly over western and 
central zones by this afternoon. Expect winds around 15 kts with 
gusts up to 25 kts during this time. Schuknecht 




&& 


Preliminary point temp/pops... 


Today Tuesday Wednesday Thursday Friday Sat sun 
------------------------------------------------------- 
bil 042 026/048 030/051 031/055 033/056 032/045 025/038 
1/u 00/north 10/b 00/u 01/b 22/west 11/b 
lvm 036 021/044 024/050 024/053 029/048 021/038 019/038 
2/j 00/north 10/b 00/u 02/west 32/j 11/b 
hdn 046 023/051 028/050 029/056 029/057 030/048 023/041 
2/j 00/u 20/b 00/u 00/b 22/west 11/b 
mls 045 024/049 024/045 026/055 028/056 031/048 021/039 
2/j 00/u 20/b 00/u 00/b 22/west 11/b 
4bq 043 023/049 026/044 025/055 027/056 031/047 021/037 
2/west 00/u 20/b 00/u 00/b 22/west 11/b 
bhk 043 022/047 022/040 021/053 026/054 029/047 019/036 
2/west 00/u 20/b 00/u 00/b 22/west 11/b 
shr 041 020/048 028/048 025/056 026/057 026/045 020/040 
2/j 00/u 31/b 00/u 01/b 22/west 22/j 


&& 


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


&& 


$$ 
Weather.Gov/Billings