Weather

934 
fxus65 kcys 110324 
afdcys 


Area forecast discussion 
National Weather Service Cheyenne Wyoming 
924 PM MDT Friday Oct 10 2008 


Discussion...fog has formed more quckly over the East Part of the 
County Warning Area as of 03z. The southern Laramie range and western Laramie 
County...including kcys have near zero visibilities per the web 
cams. Will go ahead and issue a dense fog advisory for that part of 
the County Warning Area from now through 15z Saturday. 


Light precipitation has fallen at times this evening in the west 
part of the County Warning Area as well as over the northern and central Panhandle. 
The snow was mainly confined to the Snowy Range and the far west 
edge of the County Warning Area. As of 02z the more significant precipitation...in 
the form of snow...was located over western Wyoming. Cloud tops 
have been cooling over that area per satellite imagery. The upper 
low was moving into southern Nevada early this evening and the 300 
mb jet and diffluent flow aloft ahead of the feature was over the 
western half of Wyoming. The latest nam12 and RUC have most of the 
precipitation over the far north and west parts of the County Warning Area for the 
rest of tonight. Will lower probability of precipitation some over the East Part of the County Warning Area 
with the lack of dynamics over that part of the County Warning Area as well as a 
shallower moisture depth. In a recent forecast update have made 
adjustments mainly to the probability of precipitation and fine tuned the other elements. 
Will leave current highlights as is for now...but the Winter Storm 
Warning and Winter Weather Advisory for the west part of the County Warning Area 
overnight may be overdone. Models indicate the heavier 
precipitation will be Saturday afternoon and early Saturday night 
even over that part of the County Warning Area. 


&& 


Previous discussion.../issued at 235 PM MDT Friday Oct 10 2008/ 


Short term...tonight through Sunday afternoon. 


Tonight...widespread snow will develop overnight west of the Laramie 
range with decent warm air advection occurring over the cold dome. 
Both NAM and GFS quantitative precipitation forecast depicts this quite well. Have elected to 
upgrade the Winter Storm Watch to a Winter Storm Warning for 
northern Carbon County and also to include the Snowy Range in the 
Winter Storm Warning. Rain and snow chances will be less further 
east to the east of the Laramie range as lift will be less. With 
temperature and dewpoint spreads narrowing and upslope winds...will 
see areas of fog form especially east of the Laramie range and 
across The Summit of Interstate 80 between Cheyenne and Laramie. 


Saturday...as the upper trough pulls closer and into northern Utah... 
mesoscale and dynamical lift will increase markedly across our 
western counties west of the Laramie range and with impressive warm 
air advection indicated with perpendicular winds versus the Theta-E 
ridge axis. Colder guidance highs looks better based on what 
happened today and with cloud cover inhibiting warming. Quite windy 
across the Laramie valley based on surface pressure gradient. 


Saturday night...impressive closed upper trough moves across western 
Wyoming...which will produce impressive warm air advection aloft in 
a deeply saturated airmass. Both NAM and GFS quite impressive with 
quantitative precipitation forecast amounts...although NAM looks suspiciously high as is typical. 
HPC quantitative precipitation forecast grids look good and have accepted with minor modifications. 
Rain will change over to snow in the evening from the Laramie valley 
eastward across the southern Laramie range and the eastern plains of 
Wyoming. Impressive 500-300 mb and 700-500 mb quasigeostrophic 
forcing over the deeply saturated airmass will produce widespread 
snow with rain or a rain and snow mix across western Nebraska. Have 
issued a Winter Weather Advisory beginning with this period for the 
Laramie valley and southern Laramie range with this being the first 
winter weather event of the season and with expected snowfall 
amounts. 


Sunday...the closed and stacked trough aloft moves northeast into 
eastern Montana. Expect a quick cessation of the snow as strong 
downward forcing couplet seen on 500-300 mb and 700-500 mb layers of 
quasigeostrophic forcing moves overhead. Downslope winds in the 
lower levels will produce a quick end of the snow east of the 
Laramie range. Will see snow continue into the morning hours across 
the orographically favored areas of the Snowy Range and also across 
Converse and Niobrara counties in wraparound precipitation zone. 


Long term...Sunday night through Friday 
cyclonic upper flow will be over the County Warning Area Sun night as the upper low 
moves into eastern Montana. The upper trough axis will slowly move into 
the western portion of the County Warning Area Sun night. Went dry across much of 
the County Warning Area Sun night...but depending on the speed of the upper 
low...may have to linger snow showers across the western and 
northern portions of the County Warning Area and mountains Sun night. The upper 
trough axis will move east of the forecast area Monday...resulting in 
dry conditions with downslope winds at the surface. Dry conditions 
will prevail through the rest of the extended period...as zonal flow 
sets up over the area. The upper flow will become northwest on Thursday as an 
upper ridge begins to build over the western U.S. A weak short wave 
in the flow will graze the northern zones on Thursday...but at this 
time it looks like only some clouds will be noted with this wave. 
The northwest flow will continue through Friday as will the dry 
conditions. High temperatures will be from 10 to 15 degrees below 
normal for Monday...will begin to moderate on Tuesday with highs around 5 
degrees below normal...then around normal for Wednesday through Friday. 


&& 


Aviation...tonight...ceilings and visibilities lowering to LIFR and 
IFR in fog over the entire County Warning Area and light rain or snow over the 
northwest half of the County Warning Area. 


Saturday...IFR and MVFR west of the Laramie range with VFR and MVFR 
east of the Laramie range. Icing will become a concern in the 
afternoon hours. 


Saturday night...widespread LIFR and IFR developing with widespread 
icing and mountain obscurement. 


&& 


Cys watches/warnings/advisories... 
Wyoming...Winter Storm Warning from 6 PM Friday to 6 am Sunday for wyz062- 
wyz063-wyz064. 
Winter Storm Warning from noon Saturday to 6 am Sunday for 
wyz059. 
Winter Weather Advisory from midnight Saturday to 6 am Sunday 
for wyz061. 
Winter Weather Advisory from 6 PM Saturday to 9 am Sunday for 
wyz065-wyz066. 
Dense fog advisory tonight to 9 am Saturday morning southern 
Laramie range and western Laramie County. Zones 66..69. 
NE...none. 


&& 


$$ 


Weiland 

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