185 
fxus63 klbf 022126 aaa 
afdlbf 


Area forecast discussion 
National Weather Service North Platte NE 
325 PM CST Tuesday Dec 02 2008 


..get ready for snow and blowing snow tonight nctrl...Followed by 
cold temperatures Wednesday and Thursday... 


Synopsis... 
deeply cyclonically curved upper level jet located around eastern Continental U.S. 
Trough...with axis extending through the appalacians. Surface low 
located in western Ontario with cold front extend through the Black 
Hills of South Dakota. Associated upper trough in northern plains will be system to 
bring snow and wind tonight followed by drastic cooldown of 30 
degrees from todays highs to Wednesday highs. 


&& 


Discussion... 


Tonight...NAM/GFS remain similar with resulting qpfs tonight in northern 
forecast area in area of 850 mb-700 mb frontogenesis and associated lift in dendritic 
layer. Favorable lift in the -10c to -15c layer will cause efficient 
Snowflake growth and thus higher snowfall rates. Although most of 
snowfall accumulation may only last 3 hours...hrly amounts of an inch 
per hour are possible. Will bump up pop into categorical across 
nctrl nebr mainly north of hwys 2 and 91. Winds will be an 
additional concern with winds 02z through 12z northwest at 20 miles per hour with 
gusts to 35 miles per hour. This will cause areas of blowing snow and visibility 
reduced to less than a half mile at time. Due to the combination of 
snowfall amounts of 2 to 4 inches and winds...will be issuing a 
Winter Weather Advisory tonight generally north of a line from 
Ellsworth through Ainsworth and Lynch. Areas near and north of Highway 
20 will be most impacted. Due to the fast and progressive nature of 
the system...-sn accumulations of an inch or two are possible in 
central counties. However...near and south of i80...a dusting is all 
that is expected. 


Wednesday...left front quadrant of upstream upper jet and low level cold air advection 
Wednesday justifies the need for light snow until 18z and afternoon 
flurries with colder highs 25 to 30 nctrl to the Lower/Middle 30s 
south. NAM/GFS now coming into agreement but will favor the colder 
GFS mav guidance. 


Wednesday night and Thursday...right entrance region of h30 jet will 
provide for a chance of snow for southwestern forecast area. 850 mb to 700 mb frontogenesis 
focuses across southwestern nebr into Thursday...could yield up to an inch of 
snow with a prolonged light snow. Weak cold air advection will continue through 
Thursday with highs 2 to 4 degrees colder ranging from 23 to 27 
degrees nctrl and 27 to 32 degrees southwest. Lows Wednesday night from 8 
to 15. 


Friday...after a cold start to the morning of 8 to 15 degrees...western 
nebr will be on the western side of polar jet resulting in a warmup into 
the 40s southwestern half of forecast area. Winds will turn southwest with downslope 
warming. Northestern side will remain cool in the upper 30s. 


Extended...(saturday through tuesday). 
Strong upper level jet stream will stream east across downstream 
side of upper ridge across the western Continental U.S. As broad upper trough fills 
as it lifts northeast into eastern Canada. The result will be a warmup. 
Although jet stream will be positioned further east...backdoor cold 
front will bring middle 30s back the the northeast with temperatures again in 
the middle/upper 40s southwest. Next upper trough digging into the northern 
rockies of Montana and Idaho will bring another warmup Sunday with highs 
back into the middle 40s to lower 50s most areas. For Monday...adjusted 
temperatures back down to near climatology in the lower 40s as next cold 
surge timing remains uncertain. Although moisture will remain 
limited...retained slight chance of snow Sunday night through Monday as 
northwest flow aloft turns more zonal with trough passage and frontal passage. 


&& 


Aviation... 


At 20z the cold front was pushing south across northern Nebraska 
with VFR conditions. Cloud ceiling and visibilities are expected to 
lower to MVFR conditions across the north to include the kvtn 
terminal tonight beyond 03z. Models in good agreement with an area 
of snow to develop across the north and shift southeast through 
daybreak Wednesday. Main impacts will be seen at the kvtn terminal 
overnight as the moisture deepens and lift increases. The potential 
exists for a rain and snow mix that will transition to all snow 
around 10 PM. The combination of snow and wind could reduce 
visibilities to 2 miles from 06z to 11z. To the south of Highway 2 
to include klbf terminal clouds are expected to lower but remain VFR 
as light snow or flurries move southeast from 06z to 09z. Beyond 12z 
Wednesday ceiling and visibilities are expected to improve as the 
main forcing shifts east. 


&& 


Lbf watches/warnings/advisories...Winter Weather Advisory in effect 
from 02z until 12z Wednesday for nez004-094. Winter Weather Advisory 
in effect from 03z until 15z Wednesday for nez005>008. 


&& 


$$ 


Public...roberg 
aviation...Keck