848 
fxus65 kriw 211020 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
420 am MDT Thursday Aug 21 2008 


Short term...today through Saturday night 
upper low over Vancouver with its associated convection and lightning streaming 
east to western Montana will be tracking east along the Canadian Montana 
border through tonight. Diffluent flow and weak convection with 
the most favorable total totals will be producing scattered thunderstorms 
across the northern tier zones this afternoon and evening. South 
of this area...warm...dry and windy conditions will prevail with 
right front quadrant of the 110 knot jet maximum forcing the stronger 
winds toward the surface today. 700mb winds prognosticated to be west 
southwesterly at 35 knots today. The Fire Weather Watch that was 
in affect today for much of southern and central Wyoming due to 
increased winds and low relative humidity in the low to middle teens has been 
upgraded to a red flag warning in effect from 12 noon today 
through 9 PM this evening. Southwest winds in these areas of 15 to 25 miles per hour 
with gusts to 35 miles per hour will be common. An associated cold front will 
push south across the County Warning Area tonight. Strong west southwest winds 
will blow ahead of the approaching front. Winds will shift to the 
north across much of The Divide tonight. 3 hourly surface pressure 
rises of 7mb will cause Post frontal winds to blow strongest 
across the northern Big Horn basin and northern Johnson County at 
20 to 30 miles per hour. The north winds should extend as far west as the 
eastern portion of the gun barrel fire in Park County. With the 
threat of increased winds before and after the frontal passage...lightning 
starts and a wind shift...have upgraded the Fire Weather Watch to 
a warning across northwest Wyoming as well as The East Slope of the 
Wind River mountains...The Owl creek mountains and the southern big horns 
from 12 noon today through midnight tonight. By Friday 
morning...700mb temperatures will drop to less than 1c across the far 
north. With the right rear quadrant of the jet maximum and strong positive vorticity advection 
producing lingering precipitation in northern Wyoming tonight...some light 
snow should fall in the northwest mountains late tonight and early 
Friday morning with less than one inch expected in some areas. 
Skies will clear afterward later Friday morning with subsidence 
moving in under confluent flow. Some high level monsoon moisture 
will likely stream north on Saturday as a high center establishes 
itself over the 4 corners region. Should see high based isolated T 
storm activity sneak north across the western mountains late Saturday. 


Long term...Sunday through Thursday 
major model differences in regards to potential trough towards the 
end of the medium range period. GFS did a total flip-flop with a 
strong closed low forming over the Great Basin Wednesday night and 
Thursday. The European model (ecmwf) shows a trough swinging across Montana. The 00z 
UKMET is a compromise showing a stronger trough moving into the 
pacnw Tuesday afternoon which definitely supports the potential for a 
stronger trough in the Great Basin around midweek. Hard to see where 
this storm system originates looking at the 500 mb hemispheric charts. 
It appears to come out of the Bering Sea area but the development 
doesn't become trackable until about Sunday. Current forecast is 
following the European model (ecmwf) until we see more consistency with what would be 
a significant change. Models still depicting a minimal amount of 
moisture rotating around monsoonal high leading to potential for 
isolated high based storms late Sunday across the S and central 
portions. Hot...dry SW flow develops Monday with both the GFS and 
European model (ecmwf) showing +19c at 700 mb Monday afternoon around the central basins. 
Have continued trend of bumping up temperatures with widespread 90s east of 
The Divide and 80s out west. European model (ecmwf) shows a series of weak 
disturbances in the flow just brushing the far north early next 
week. Precipitation chances seem minimal with most of the energy in Montana. 
However...any southward shift of this trough or further digging as 
indicated on the UKMET and GFS would change the forecast 
dramatically. Will keep some isolated probability of precipitation across the far north at 
times based on European model (ecmwf) timing. Monday should be the hottest day with 
some moderation toward midweek with several weak frontal boundaries 
moving down from the north at times along with cooler middle level 
temperatures. 


&& 


Aviation... 
a strong Pacific trough will affect Wyoming through this evening. 
Expect windy conditions to develop late this morning into the 
afternoon over much of the area. Southwest to west winds between 15 
and 30 kts will be common over much of the area. Some isolated to 
widely scattered convection could occur this afternoon and 
evening...mainly north/northwest of a line from kjac to kcod to 
kgey. During the evening...a cold front will move south across the 
area. Some low clouds could occur across the Cody 
foothills/Johnson County around midnight or later. 


&& 


Fire weather... 
a warm and dry southwest wind of 15 to 25 miles per hour with gusts to 35 miles per hour will 
blow across much of southern and central Wyoming today along with 
a minimum relative in the low to middle teens. This has prompted a 
red flag warning for those areas today from 12 noon through 9 PM 
this evening. Strong west southwest winds ahead of an approaching 
cold front...the threat of lightning starts and an expected wind 
shift with a cold front has prompted a red flag warning to be in 
effect today for northwest Wyoming as well as The East Slope of the 
Wind River...The Owl creek and the southern Big Horn Mountains today 
from 12 noon through midnight tonight. Some light snow may fall 
across the northwest mountains late tonight. Temperatures will be as 
much as 15 degrees cooler east of The Divide behind the front 
Friday. During the weekend some high level monsoon moisture will 
stream north which could produce some isolated high based 
thunderstorms across the western mountains 


&& 


Riw watches/warnings/advisories... 
red flag warning from noon today to 9 PM MDT this evening for 
wyz277-279>281-283-289. 


Red flag warning from noon today to midnight MDT tonight for 
wyz140-276-285>288. 


&& 


$$ 


Short term...lipson 
long term...skrbac 
aviation...wm 
fire weather...lipson 

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