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fxus65 kriw 060900 
afdriw 


Area forecast discussion 
National Weather Service Riverton Wyoming 
300 am MDT sun Jul 6 2008 


Short term...today through Tuesday night 
a few showers moved through the eastern third of the County Warning Area during 
the late evening hours of Saturday...with the majority of heavier 
shower activity staying to the east and North. Pacific trough 
currently moving off the Pacific coast...with continued easterly 
movement towards Wyoming expected this morning and afternoon. 


Both GFS and NAM models have come into better agreement this 
morning with the progression of the system this afternoon. Pacific 
trough will move into western Wyoming by the late afternoon 
hours...pushing a cold front through the County Warning Area this afternoon... which will 
bring additional moisture across the County Warning Area. Meanwhile...a jet streak 
will also move through the northeastern third of the County Warning Area area this 
afternoon...moving to the northeast of the County Warning Area by this evening. 
Moderate qg forcing also noted to move through the County Warning Area this 
morning and afternoon. The jet streak and forcing will help to bring 
significant instability across the eastern portion of the County Warning Area. 
Showers and thunderstorms are anticipated through the morning and 
afternoon hours. Precipitable water values of 1 to 1.5 inches of liquid are 
present in the 00z NAM run allowing for the thought that storms 
will contain heavy rains. Li/S across Natrona and Johnson 
County...as well as into the Big Horn basin remain in the -3 to -6 
range...with cape/S of 1200 to 2000 j/kg present in the afternoon 
hours. Main change in the models was moving these stronger 
instabilities further west to include the majority of the Big Horn 
basin excluding Cody. Storms in Natrona...and Johnson 
counties...as well as the Big Horn basin have the potential to 
become severe this afternoon...especially after 21z...when 
damaging winds and large hail may accompany storm activity. Other 
strong thunderstorms with small hail and gusty winds may impact 
the majority of the remaining portions of the County Warning Area through the late 
morning and afternoon hours. With all storms...the potential for 
frequent lightning exists. 


These storms should taper off and exit the area overnight 
tonight...as a ridging pattern returns tot he County Warning Area. Northwesterly 
winds anticipated to move over the County Warning Area by Monday morning...helping 
to usher in a more stable environment. Temperatures will warm 
again Monday and Tuesday under this pattern...with values 
returning to the low 90s across the eastern basins. Daytime 
heating will bring some instability on Monday and 
Tuesday...allowing for isolated showers and thunderstorms both 
days. 


Long term...Wednesday through Sunday 
Flat Ridge over the area Wednesday with limited moisture and 
instability. Will leave our isolated probability of precipitation in for the favored northwest 
flow areas. On Thursday...ridge axis shifts slightly east of the 
area with driver west to west-southwest flow developing. Will limit any 
convection to the far northern mountains models begin to diverge 
significantly Friday into the weekend. The new European model (ecmwf) continues the 
idea of a trough digging into the pacnw with subtropical moisture 
moving northward ahead of it into Wyoming Saturday. The GFS shows the 
initial trough moving across Montana on Friday but just a rather 
dry westerly flow over the weekend. This is the first night in the past 
5 that the GFS has switched so dramatically from some sort of 
trough to the west that would allow subtropical moisture to move 
northward. Will keep in our probability of precipitation over the weekend until we get a better 
feel for which way will work out. The European model (ecmwf) would indicate a very 
active weekend of thunderstorms while the GFS would generally say 
dry. Looking at the two models on the hemispheric scale it's hard 
to tell why the European model (ecmwf) develops the secondary southwestern Canada wave and 
the GFS does not. They are in pretty good agreement through 96 
hours but the ecwmf leaves behind more energy when its main or 
first upper low moves across southern Canada on Friday. The potential 
forecast difference is huge and we will look very closely at this 
with the next forecast. 


&& 


Aviation... 
a front will waffle over central Wyoming today as a Pacific trough 
approaches Wyoming. This trough will bring increased thunderstorm 
coverage to Johnson and Natrona counties late this afternoon and 
evening. Some of these storms could be quite strong/severe...with 
large hail...strong outflow winds as well as very heavy rain and 
frequent lightning. MVFR visibilities and some ceilings possible within the 
thunderstorms in these areas. The remainder of areas east of The 
Divide and the northwest should see isolated to scattered activity 
with a slight chance of them becoming severe. The southwest will 
see limited activity. 




&& 


Fire weather... 
a cold front will move through the forecast area today...aiding in 
the development of showers and thunderstorms. A highly unstable 
environment is anticipated to develop across Natrona and Johnson 
counties...along with the Big Horn basin this afternoon. This 
environment will allow some thunderstorms to potentially become 
severe with large hail...damaging winds...and frequent lightning. 
Showers and thunderstorms are expected to weaken overnight. 
Raising temperatures and generally light winds expected Monday and 
Tuesday afternoon across the area...with a few isolated showers and 
thunderstorms possible on Monday and Tuesday afternoons. 


&& 


Riw watches/warnings/advisories...none. 


&& 


$$ 


Short term...branham 
long term...skrbac 
aviation...twb 
fire weather...branham