993 
fxus65 kmso 220336 
afdmso 


Area forecast discussion 
National Weather Service Missoula Montana 
936 PM MDT Thursday Aug 21 2008 


Update...adjusted sky...weather and temperatures in the 
northwest and sky and chances of precipitation south. 




&& 


Discussion...upper trough continues to push east of the Canadian 
rockies this hour. However...moisture on the south/back side of 
the trough is sliding into northwest Montana. These clouds are 
expected to linger through the night and keep temperatures 
insulated and deter fog development. Across central and 
south...the jet will linger through the overnight hours and keep 
isolated showers in the forecast...mainly over the higher terrain 
through sunrise. Also snow levels have fallen further than 
previously forecast so passing showers will likely be in the form 
of snow near 7000 feet. Surface ridging will build in overnight 
decreasing winds and allowing temperatures to lower into the 40s 
most locations...with some lower 30s in the higher terrain of 
north central Idaho. 




&& 


Aviation...showers and isolated thunderstorms associated with an 
upper level trough will continue across the clearwaters and 
southwest Montana and should exit this region after midnight. 
Reduced visibilities and heavy rain will accompany these showers. 
Gusty winds from a strong jet streak pushing across Lemhi County 
Idaho will gradually diminish by midnight. Skies will clear 
overnight with the exit of the upper level low...allowing areas of 
fog to develop in the valleys of north central Idaho and western 
Montana by daybreak. 




&& 


Previous discussion... /issued 254 PM MDT Thursday Aug 21 2008/ 


Discussion...the unseasonably cold upper level low responsible 
for heavy precipitation over the past twenty-four hours is 
currently passing across northeast Washington along the Canadian 
border. Thunderstorms...producing severe hail of up to 1 inch in 
diameter...have developed in association with the passage of this 
upper level low and it's cold core aloft in Washington. These storms will 
move into northwest Montana this evening bringing another round of 
heavy rainfall and potentially severe hail. Farther south the cold 
front which was stalled over west central Montana has finally 
shifted to the east. However a steady stream of moisture continues 
to move into central Montana ushered in by a strong jet stream 
aloft. Given the convective atmosphere behind the front...expect 
precipitation this evening to be more scattered in nature with off 
and on showers impacting much of central Idaho and western 
Montana. Precipitation may be heavy at times...but over all totals 
will be much less impressive than those which occurred overnight. 
In addition...windy conditions will continue into the evening 
across Lemhi County in Idaho and southwest Montana as the frontal 
boundary produces a tight surface pressure gradient across the 
area. A Wind Advisory has been issued to bring awareness to these 
gusty winds through about 9 PM this evening. In addition...fire 
weather highlights have been issued over southwest Montana given 
the low relative humidity values and strong winds in advance of 
the slow moving cold front. 


Skies will gradually clear from west to east tonight as the trough 
and upper low move rather quickly into central Montana. Given the 
healthy rainfall totals over the past day and clearing skies 
overnight expect areas of fog to develop soon after midnight. Some 
fog may be quite dense leading to reductions in visibility. In 
addition...low temperatures will be unseasonably cold for late 
August as cold air advects into the region overnight and clear 
skies aid in radiational cooling. Although the atmosphere will 
continue to dry tomorrow...enough residual moisture will hang 
around to produce some cumulus clouds over the higher terrain 
Friday afternoon with temperatures staying about 10-15 degrees 
below normal. 


Friday night through Thursday...a ridge of high pressure will build 
into the region again Friday night and remain over western Montana 
and north central Idaho through the weekend. This will produce 
mostly sunny skies and a warming trend. On Monday another Pacific 
trough will move toward western Montana. This will bring in an 
increased chance of showers along with cooler temperatures. The 
trough will move off to the east on Wednesday with a cool northwest 
flow setting up over the region...which will stay over western 
Montana and north central Idaho through the rest of the extended 
forecast. 






&& 


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


$$ 


Update...Mead 
previous discussion....Dickerson/allegretto/Smith 
aviation...derosa