460 
fxus65 kbyz 212145 
afdbyz 


Area forecast discussion 
National Weather Service Billings Montana 
245 PM MST Sat Nov 21 2009 


Short term...valid for sun and Monday... 


Airmass fairly dry in a Post frontal environment with gusty 
northwest winds persisting over the eastern zones. Upper level 
trough over the area will begin yielding to a shortwave ridge 
tonight induced by a much stronger system pushing into the Pacific 
northwest. Areas of precipitation lingering South West of kbil and think 
that as this activity shifts eastward it will persist since it 
will be shifting away from downslope influences. This activity 
should clear after midnight and will have a chance for 
temperatures to fall sharply with clearing skies and low dewpoints. 
However...by morning pressure falls begin quickly along the 
northern rocky front as the next wave moves eastward across 
Washington. A brief period of gap flow winds will develop late 
tonight and early Sunday with strong surface high pressure over 
southeast Idaho waiting for the leeside trough to form. This 
system is expected to fill as it moves eastward but is still 
pretty strong so expect an increase in mountain precipitation and 
have accumulations mentioned but think that low level airmass 
over the plains will be fairly dry and precipitation will have a hard 
time reaching the ground. Colder heights aloft will keep 
temperatures cooler than today but close to 40 degrees is 
seasonable for this time of year. Gradients for winds weaken a bit 
on Sunday but airmass will be unstable and momentum aloft will 
have an easy time mixing down. Upper low fills as it GOES from 
northwest Montana to southeast Montana Sunday night and while 
airmass is still pretty dry...cannot rule some isolated showers 
being squeezed out of the airmass...especially over southeast 
Montana closer to the coldest air aloft. Heights begin to build 
from west to east on Monday pushing isolated showers into the 
Dakotas and bringing clearing to the western zones. Pressure 
gradients for gap flow winds begin to take shape on Monday night 
and have begin to ramp up speeds. Otherwise looks dry with clear 
skies and low dewpoints again supporting a cool night. Borsum 


Long term...valid for Tuesday...Wed...Thu...Fri...Sat... 


Just some minor changes made to the extended forecast this 
afternoon. Upper level ridge will be over the forecast Tuesday 
morning and have dried out most periods for this period. Upper 
level trough slides into North Dakota Tuesday afternoon and 
Tuesday night with some weak energy moving into the forecast area 
at this time. Models do hint at a little precipitation for this weak 
system so have spread the low probability of precipitation a little farther west. Did add 
probability of precipitation for the central and western zones as well for Tuesday night. 
This system quickly exits by Wednesday morning leaving dry 
conditions and warmer temperatures through much of Friday. Models bring 
in the next system across southern Canada on Friday night and 
Saturday where the European model (ecmwf) brings the system right over the 
forecast areas. Have kept in low probability of precipitation for most areas for now 
until the models come into better agreement. Solum 




&& 


Aviation... 


Gusty winds up to 30 knots will continue across the area through 
early evening before tapering off. MVFR conditions and mountain 
obscurations will continue to be possible through early evening 
before becoming dry overnight as some isolated rain and snow 
showers continue to move through. Solum 


&& 


Preliminary point temp/pops... 


Sun Monday Tuesday Wednesday Thursday Friday Sat 
----------------------------------------------------------- 
bil 021/040 028/042 024/046 029/051 031/055 034/051 033/044 
02/j 21/b 12/west 20/b 00/u 02/west 22/west 
lvm 018/036 023/035 018/043 027/050 030/054 030/046 030/044 
04/j 32/S 12/west 20/b 00/u 03/west 32/west 
hdn 022/043 024/044 019/047 025/052 028/057 029/053 027/049 
02/west 22/o 12/west 20/b 00/u 02/west 22/west 
mls 021/045 024/043 020/047 023/046 026/056 030/051 031/042 
01/b 22/o 22/west 00/b 00/u 02/west 22/west 
4bq 020/042 021/042 019/045 022/046 025/056 028/051 028/044 
21/north 23/o 12/west 20/b 00/u 02/west 22/west 
bhk 020/045 021/040 019/043 023/040 025/054 026/051 028/043 
11/b 22/o 22/west 00/b 00/u 02/west 22/west 
shr 019/043 022/039 014/046 022/050 024/056 027/051 024/046 
22/west 22/o 12/west 20/b 00/u 02/west 32/west 


&& 


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


&& 


$$ 
Weather.Gov/Billings