Weather

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Area forecast discussion 
National Weather Service Cheyenne Wyoming 
915 PM MDT sun Sep 7 2008 


Update... 
moist upslope flow has significantly reduced visibilities across 
the southern Laramie range this evening. Highway patrol reported 
visibilities of less than 100 yards in fog near mile marker 325 on 
Interstate 80. Persistent moist upslope flow overnight will 
maintain this dense fog through just after sunrise. So have opted 
to issue a dense fog advisory for this area overnight. Toyed with 
adding foothill locations as well...but think winds plus better 
chance of rainfall should limit areal extent of fog. 
However...these trends will need to continue to be monitored 
closely. Hahn 


&& 


Previous discussion... /issued 140 PM MDT sun Sep 7 2008/ 


Short term...tonight through Tuesday afternoon... 


Tonight...quite a Stark contrast in temperatures across our counties 
early this afternoon with temperatures ranging from the lower to middle 
50s at Douglas...Torrington...Scottsbluff...Chadron and Alliance in 
the cold air dome...to the lower 70s at Rawlins...still in the warm 
sector. 


Thank you to our neighboring offices at Riverton...Rapid 
City...North Platte...Denver and Grand Junction for forecast 
coordination. 


Widespread showers...producing light to moderate rain...will occur 
this evening along and east of the Laramie range as the atmospheric 
column becomes deeply saturated while decent lift is provided by low 
level upslope...mesoscale and synoptic lift from incoming shortwave 
trough and decent warm air advection lift as evidenced by the 
impressive middle level Theta-E advection...warm air advection...above 
the cool dome at low levels. With some elevated instability...expect 
isolated thunderstorms this evening. 


Widespread showers will move southeast into our far eastern Wyoming 
counties and western Nebraska counties after midnight. Will try 
again tonight for areas of fog...although fog will be areally 
minimized by falling rain. Have also included areas of frost in 
those locales where temperatures will be 36 fahrenheit or below. 


Warmer GFS minimum temperatures look best over colder NAM 
temperatures based on dense low cloud cover retarding cooling and 
also on upstream dewpoints. 


Monday...rain will quickly end from west to east as mesoscale and 
dynamic lift moves east of our forecast counties. Prefer cooler NAM 
guidance highs over warmer GFS highs as NAM usually does best in 
this situation with recovery days after a shortwave trough exiting 
stage right and with low level upslope continuing keeping the 
shallow cool air entrenched. 


Monday night...quiescent overnight as flow aloft backs to west and 
low level return south flow commences. Expect patchy fog to develop 
along and east of the Laramie range and the Interstate 25 corridor 
as temperature and dewpoint spreads narrow. 


Tuesday...decent warmup on tap as 1000-500 mb thicknesses rise to 
5760 meters producing maxima in the 70s and lower 80s. South wind 
at Cheyenne will limit warming as usual. Later shift teams may need 
to add isolated afternoon thunderstorms along the Laramie range in a 
low level convergence axis that develops and with a low and middle 
level Theta-E ridge axis developing. Rubin 


Long term...Tuesday night through sun...some moisture in west-southwest flow aloft 
spreads over the region by Tuesday night and continues into Wednesday. Models 
also depicting a weak ftr ejecting on Wednesday across Wyoming ahead of the 
main developing upper trough in ID/Mt...which should bring some late 
day thunderstorms to the region. Models comtinue to have some serious 
differences with the evolution of this main upper trough...with the 
European model (ecmwf) runs of 7/00z and 7/12z both showing a cut off low developing 
over northestern Nevada by 12z Thursday. Other models showing more of an open 
positive tilt trough moving across the region...although with perhaps a 
strong vorticity in the base of the trough over WY/co/UT. Will lean towards 
more of compromise for now as tough to give the nod to any one model 
at this time. It does look like the system will be slower to move 
out of the area...although the European model (ecmwf) appears to be too slow with its 
cutoff solution. So will continue with some probability of precipitation into next Friday... 
then go mainly dry into the weekend. Temperatures with this system likely 
not as cool as current one affecting US...so only a little below 
normal readings expected. 


Aviation...widespread IFR and LIFR developing tonight east of a 
Rawlins to Laramie line...with VFR and MVFR elsewhere. 


Ceilings and visibilities improving to VFR by Monday afternoon. 


VFR Monday night...except for localized MVFR in patchy fog along and 
east of the Laramie range after midnight. Rubin 


&& 


Cys watches/warnings/advisories... 
Wyoming...dense fog advisory until 8 am Monday for wyz066. 
NE...none. 


&& 


$$ 


Short term/aviation...Rubin