Weather

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Area forecast discussion 
National Weather Service Riverton Wyoming 
102 PM MDT Friday Oct 10 2008 


Update... 


Persistent band of light to moderate snow has developed across the 
lower Wind River basin and the Lander foothills this morning...and 
model guidance indicates that isentropic lift and jet dynamics 
will cause light to moderate snow to continue through tonight in 
these areas. Upgraded Winter Storm Watch for the lower Wind River 
basin and the Lander foothills to a Winter Storm Warning starting 
at 18z today. 12z runs of the GFS and NAM point to 700 mb temperatures 
falling below minus 7 c in Natrona and Johnson counties Saturday 
afternoon. The intrusion of colder air across eastern Wyoming 
Saturday afternoon combined with significant moisture moving 
northward ahead of the upper low will produce moderate to 
occasionally heavy snow across Natrona and Johnson County Saturday 
afternoon through Sunday morning. Winter storm watches have been 
issued for Natrona and Johnson counties for Saturday afternoon 
through Sunday. 


&& 


Previous discussion... /issued 517 am MDT Friday Oct 10 2008/ 


Short term...today through Sunday night 
a low pressure center is deepening over the Great Basin along with 
an evident developing circulation on satellite. Upper diffluent flow along 
with jet stream dynamics is producing a streak of snowfall with 
rain at the lowest elevations extending from Johnson County 
southwest to Rock Springs and northeast Utah. This snowfall will 
continue through today as isentropic lift in the southeast flow 
ahead of the low pressure center enhances the lift. Temperatures 
should be warm enough at the lower elevations of the eastern zones 
of Natrona and Johnson County for the snow will mix with rain 
through the afternoon today so much of the snowfall will melt as 
it falls in these areas due to the warm trowal on the east side of 
the storm. Even the Wind River and Big Horn basins will be warm 
enough so that much of the snow that falls today will melt as it 
falls...especially this afternoon... given the warm ground and 
highs expected above freezing this afternoon even though most of 
the precipitation will be snow. It is still 33 degrees here at riw. Have thus 
cancelled the hilites for the lower elevations of Johnson and 
Natrona counties and delayed the Winter Storm Watch in the central 
basins until Saturday. The next stronger jet maximum will round the 
base of the trough Saturday. The combination of lfq jet 
dynamics...diffluent flow...strengthening isentropic lift and q 
vector forcing of -65 k/m^2/1e16s will enhance the snowfall 
considerably Saturday through 06z Saturday night...not to mention 
the seeder feeder action of warm air advection over running moist 
low level northeasterly upslope flow. Delayed the Winter Storm 
Watch for the central basins for Saturday and Saturday evening as 
well as initiated one for the Upper Green river basin and the 
adjacent salt and Wyoming ranges for Saturday. The southeasterly 
isentropic lift nosing right into the Upper Green river basin will 
enhance the lift there. The GFS has been persistent in indicating 
a 20mb surface pressure gradient between the northern and 
southern borders of Wyoming for Saturday. The boundary level winds 
are prognosticated to be east northeast at 45 knots across the southern 
wind corridor for Saturday and Saturday evening so went with a 
blizzard watch for 12z Saturday through 06z Sunday for Sweetwater 
County and southern Lincoln County. South Pass should see white 
out conditions as well during this time frame but this was 
covered in the Winter Storm Warning for that zone. The low 
pressure center will be northeast of our County Warning Area by 12z Sunday taking 
the widespread precipitation northeast with it. However...even with the 
less favorable northwest flow associated with the backwash behind 
the low...there will be wrap around isentropic lift as well as 
diffluent flow on Sunday along with a troughpa Sunday night as a 
trough pinwheels around the main low...which will be in the 
northern plains by then. Probability of precipitation for additional snowfall look good 
for Sunday and Sunday night with higher chances east of The 
Divide. However...increased snowfall slightly in The Tetons where 
northwest flow favors that area and it matches up better with pih. 


Long term...Monday through Friday 
high amplitude pattern in the short-term transitions to more of a 
zonal flow during this period resulting in a dry pattern with a 
gradual warming trend. 


On Monday...main upper low lifts across northern plains into central 
Canada...with baggy trough lingering back across the southern 
rockies. Cool northerly flow remains over Wyoming with high 
temperatures 15 to 20 degrees below normal. 


Tuesday...kicker shortwave races across western Canada into the 
northern plains. Precipitation with this system is expected to stay 
well north of Wyoming. Temperatures will recover but highs will 
still be around 10 degrees below normal. 


Wednesday...GFS is slightly faster bringing a shortwave across the 
northern rockies than the European model (ecmwf). The main impact of this shortwave 
will be to kick up the winds but temperatures will remain similar or 
slightly milder than Tuesday with better mixing. 


Thursday and Friday...both GFS and European model (ecmwf) show a ridge of high 
pressure building over the western U.S. With the European model (ecmwf) placing this 
ridge further west than the GFS...the European model (ecmwf) keeping Wyoming under a 
northwest flow. Forecast dry for this period with temperatures returning 
to near normal. 


Aviation... 
a variety of weather is expected over the region today and tonight. 
Areas of rain and snow will occur with snow in the mountains. 
Ceilings will be IFR to LIFR over and near the mountains. VFR to 
MVFR conditions will prevail over the lower elevations of the south 
today. MVFR to IFR conditions will occur over the lower elevations 
of the central and north. 


Fire weather... 
much colder temperatures along with significant snowfall is expected 
across much of western and central Wyoming through Saturday night as 
a strong winter storm over the Great Basin tracks toward Wyoming 
this weekend. Very strong east winds are expected from South Pass 
through Sweetwater County and southern Lincoln County Saturday and 
Saturday evening. 


&& 


Riw watches/warnings/advisories... 
blizzard watch from Saturday morning through Saturday evening 
for wyz027>030. 


Winter Storm Warning until 6 am MDT Sunday for wyz014-015- 
017>019-022. 


Winter Storm Watch from Saturday afternoon through Sunday 
morning for wyz010-011. 


Winter Storm Watch from Saturday morning through late Saturday 
night for wyz016. 


Winter Storm Watch from Saturday morning through Saturday 
afternoon for wyz024>026. 


Winter Storm Warning until 12 PM MDT Sunday for wyz001>003- 
007>009. 


Winter Storm Watch from Saturday afternoon through late Saturday 
night for wyz020. 


Winter Storm Watch from Saturday morning through Sunday morning 
for wyz004>006. 


&& 


$$ 


Update...Smith 
short term...lipson 
long term...aem 
aviation...Arkansas 
fire weather...lipson 

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