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fxus65 kriw 082036 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
236 PM MDT Friday Aug 8 2008 


Short term...tonight through Monday 


Plume of monsoon moisture will be over Wyoming this evening with 
precipitable water values exceeding one inch over the entire County Warning Area in 
the 12z forecasts of the GFS and NAM. Both models indicate that a 
vortmax will move across western Wyoming in the evening and into 
northeast Wyoming after 06z. Will allow the flash flood advisory for 
southern Wyoming to continue through 03z. Will keep scattered convection 
across the northeastern County Warning Area after 06z and isolated convection across 
the rest of the County Warning Area...as slightly more stable air moves in from the 
west behind the vortmax. Partly to mostly cloudy skies tonight will 
cause low temperatures to run 5-10 f above normal. Plume of 
monsoonal moisture will shift slightly eastward Saturday...as the 
upper ridge axis crests over the central and northern rockies. 
Although there will be little dynamic forcing beneath the ridge 
axis...NAM and GFS are both projecting weak instability with lifted 
indices in the minus 1 to minus 2 c range. Expect isolated showers 
and thunderstorms to develop Saturday afternoon and evening in the 
lower elevations and scattered activity in the higher elevations. 
Light steering winds aloft...500 mb winds between 10 and 15 knots...will 
result in slow moving showers and thunderstorms with the potential 
for very heavy rainfall and flash flooding Saturday afternoon and 
evening. High temperatures Saturday will vary with cloud cover...but 
with 700 mb temperatures in the 12 to 16 c range should still be near to 
slightly above normal. 


Strong shortwave will move eastward across northern Washington into southern 
Alberta. This system will cause 500 mb heights to fall 3-4 dm across Wyoming 
and will produce a difluent flow aloft. Precipitable water will 
still be around 0.75 of an inch across the County Warning Area Sunday. The 
combination of above normal moisture...dynamics associated with the 
shortwave and weak instability will produce scattered showers and 
thunderstorms across the region Sunday afternoon and evening. A few 
thunderstorms will have the potential to produce damaging winds 
Sunday...as stronger winds aloft at 500 mb could be transported to the 
surface. A cold front will move across Wyoming behind the shortwave...and 
high temperatures will be near to slightly below normal. 
Drier...more stable air will advect across the region from the 
west...yet lingering moisture in the northeast County Warning Area will produce a 
slight chance of showers in the Bighorn Mountains...Johnson County 
and northern Natrona County. 


Long term...Monday night through Friday 


Medium range models are in decent agreement at the beginning of the 
extended...with an upper low across south central Alberta...with a 
dry westerly flow pushing into the area. However...there is 
disagreement on what to do with this system after Monday...with the 
European model (ecmwf) model moving it into central Manitoba slowly by 12z 
Thursday...while the GFS has it across the Dakotas. Both models 
would keep it mainly dry across the area Tuesday and Wednesday...but 
the GFS would be a tad cooler...especially east of The Divide. By 
Thursday and Friday some monsoonal moisture might begin to work into 
the area...with better shots of convection. However confidence is 
low due to the wide spread of model solutions. Have gone a few 
degrees above mex MOS guidance...especially Wednesday through 
Friday...anticipating that the GFS is too far south...and too cool 
with this closed low. 


&& 


Aviation... 


Monsoonal moisture will bring slow moving storms with heavy rain 
through this evening...especially impacting the southwestern half of 
Wyoming. The storms with heavy rain may produce periods of MVFR 
conditions across krks...kbpi...and kjac this evening. Showers will 
weaken overnight... but will return across the area on Saturday 
afternoon. Another round of showers and thunderstorms will be 
possible across the state...with the chance for periods of MVFR 
conditions due to ceilings and light to moderate rainfall. Gusty 
winds and lightning will also remain a possibility with all 
thunderstorm activity through Saturday. 


&& 


Fire weather... 


Main surge of monsoonal moisture will move around the large upper 
high centered in northern nm across Wyoming today through Saturday. 
Scattered to numerous showers and thunderstorms are expected to 
continue into this evening across the region with the highest 
chances for wetting rain across southern Wyoming and in the higher 
elevations. As the plume of moisture shifts slowly to the east 
Saturday...daytime mixing of the atmosphere will bring slightly 
drier air aloft to the surface...and minimum relative humidities 
will be slightly lower. There will still be enough moisture present 
to produce isolated to scattered afternoon and evening thunderstorms 
Saturday. Wetting rain will again be common in thunderstorms 
Saturday. Winds this afternoon through Saturday will be fairly light 
and terrain driven with high pressure aloft and a weak surface 
pressure gradient. An upper level system will cross the northern 
Pacific northwest Sunday...and will produce isolated to scattered 
showers and thunderstorms with strong outflow winds possible. 


&& 


Riw watches/warnings/advisories... 
Flash Flood Watch until 9 PM MDT this evening for wyz019-024- 
026>030. 


&& 


$$ 


Short term...Smith 
long term...wm 
aviation...branham 
fire weather...Smith