487 
fxus65 kmso 200227 aaa 
afdmso 


Area forecast discussion...updated 
National Weather Service Missoula Montana 
827 PM MDT Tuesday Aug 19 2008 


Discussion...showers and isolated thunderstorms are persisting over 
southwest Montana where surface convergence still exists this evening and 
over north central Idaho...where another upper level impulse is approaching 
in the southwest flow aloft. Local heavy rain has been reported 
with storms. The red flag warnings will be canceled as 
temperatures cool and rainfall versus gusty winds is the main 
impact. The GFS model is outperforming the NAM in overall 
features this evening. By following the GFS...look for the 
convective activity over southwest Montana to move east of The Divide 
as the evening progresses and the showers and the isolated showers 
over north central Idaho to move over the Clearwater and Bitterroot 
Mountains by midnight. After midnight...convective activity should 
die off as the airmass stabilizes but a chance of showers will 
develop on the western fringe of north central Idaho and northwest Montana 
as the upper level trough along the West Coast moves inland. 
Forecast will be updated to adjust to current and projected 
weather conditions. 


&& 


Aviation...thunderstorms should persist along higher terrain this 
evening over the Clearwater Mountains and across the Bitterroot 
Sapphire range. Heavy rain and small hail have been the main threat 
from storms that have developed this evening. Expect thunderstorm 
activity to diminish after midnight. Additional showers will track 
into the northern rockies by daybreak. Ceilings will lower by 
afternoon leading to obscured terrain at times and winds will 
become gusty as well. Smoke will continue to linger in the 
Missoula and Kalispell valleys in the morning and may reduce 
visibilities. 




&& 


Previous discussion... /issued 219 PM MDT Tuesday Aug 19 2008/ 


Discussion...a deep upper level trough continues to move across 
the Pacific towards British Columbia and the Pacific northwest. 
As it approaches...a moist and unstable southwesterly flow has 
developed over the northern rockies. Two rotating shortwave 
disturbances are moving through this flow causing showers and some 
strong thunderstorms to develop. Given increased middle level wind 
speeds...enhanced shearing in the atmospheric profile and the 
advection of additional moisture the Storm Prediction Center has 
opted to issue a Severe Thunderstorm Watch until 8pm MDT for 
portions of southwest Montana. This does include the city of 
Butte. The largest threats with these storms will be 1 inch 
hail...gusty winds to 60 miles per hour and brief heavy rainfall. 
Thunderstorms are expected to continue across west central and 
southwest Montana into the evening hours. Convective activity 
will decrease gradually overnight with only isolated showers and 
thunderstorms expected across central Montana by morning. 


Tomorrow the combination of a tightening surface pressure gradient 
and a strong jet beginning to nose into northwest Montana will 
create windy conditions...with gusts up to 35 miles per hour in some of the 
higher elevations. A cooling trend in temperatures will be 
noticeable Wednesday as highs are expected to reach into the middle 
to upper 70s behind the front. Showers and thunderstorms will 
develop once again in advance and along the front. These storms 
will most likely produce more precipitation than in past days due 
to the increasingly moist atmosphere over the area. 


Wednesday night through Thursday night...a broad upper trough of 
low pressure will be moving over the northern rockies region 
providing much cooler...breezier conditions. A strong 300kt jet 
streak with provide added synoptic lift for some enhanced 
precipitation...mainly across northern Idaho and western Montana 
Wednesday night and Thursday. Valley temperatures will cool into 
the 60s and lower 70s...and 50s in the mountains Thursday. 
Westerly winds will be increasing as the surface pressure gradient 
and west winds aloft work in concert with one another...this will 
be especially true late Thursday. 


Friday through Sunday...in the wake of an exiting upper trough... 
a low amplitude ridge of high pressure will be developing early 
Friday and continuing through Saturday night. The air mass will be 
warming and drying during this period...with generally light 
diurnal winds on tap. 


Sunday night through Tuesday...another broad upper level trough 
will be approaching the West Coast on Sunday night...moving into 
the inter-mountain region Monday and Tuesday. Temperatures will be 
cooling down once again with an increased chance of precipitation. 
Expect westerly winds to increase. 




&& 


Mso watches/warnings/advisories... 
Montana...none 
Idaho...none 


&& 


$$ 


Short term...felsch 
long term....msj 
aviation...derosa