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fxus65 kcys 082059 
afdcys 


Area forecast discussion 
National Weather Service Cheyenne Wyoming 
200 PM MST Thursday Jan 8 2009 


Short term...tonight through Saturday afternoon... 


Tonight...balmy temperatures prevail at early afternoon with temperatures 
from the middle 40s west of Interstate 25...to the 50s and lower 60s 
east of Interstate 25. Will leave the current High Wind Warning 
intact as wind gusts of 50 to 60 miles per hour still showing up and could 
still see gusts to 60 miles per hour with decent 700 mb jetlet of 50 plus knots moving 
overhead. High Wind Warning will likely be able to be cancelled 
prior to midnight. 


Progressive shortwave trough aloft will move across Wyoming 
overnight with a strong Pacific origin cold frontal passage. Expect 
frontal passage around 1 am at Cheyenne. Both NAM and GFS indicated scattered 
to numerous snow showers northwest of a Scottsbluff to Cheyenne line 
which looks plausible with the strength of the shortwave trough 
aloft and baroclinic band passage. Snowfall coverage will be less to 
the East...Lee...of the Laramie range due to downslope wind 
component. 


Friday...primary challenge will be winds. Have issued a high wind 
watch for the western Nebraska Panhandle...where GFS indicates 850 
mb winds of 30 to 40 knots with 700 mb winds near 50 knots...as well 
as strong surface through 700 mb gradients and deep unidirectional 
northwest winds. Strong cold air advection will aid in the downward 
transport of stronger winds aloft to the surface. Currently thinking 
either Wind Advisory criteria winds across western Nebraska or a low 
end High Wind Warning. 


Isolated to scattered snow showers will occur west of Interstate 25 
with numerous orographic snow showers across the Snowy Range. 


Friday night...low level winds will increase again as decent surface 
Lee troughing develops with northwest flow aloft. Snow showers will 
come to an end across the Snowy Range as low and middle level moisture 
wanes. 


Saturday...another typically windy day with brisk northwest flow 
aloft...decent surface through 700 mb isotachs and gradients. 
Surface through 700 mb gradients look minimal enough to keep winds 
below high wind criteria. High temperatures from the middle 20s to 
lower 40s with 1000-500 mb thicknesses near 5400 meters. Rubin 


Long term...Saturday night through Thursday... 
the medium range models continue to show an upper level ridge 
amplifying along the Pacific coast over the weekend. A fast 
northwest flow will be present across the area. This will continue 
to be a windy period along the Front Range of The Rockies...as high 
pressure over The Four Corners region will continue to combine with 
Lee troughs east of the mountains to produce strong pressure 
gradients. A series of disturbances east of the ridge axis will 
bring a cold front across the area early Monday...and a stronger 
cold front Tuesday afternoon and evening. 


Differences between the European and GFS models forecast for the 
upper level pattern become visible by around Tuesday next week. The 
main difference is the position of the amplified upper ridge along 
the Pacific coast. The GFS model allows the upper ridge axis to 
shift on-shore with the location of the axis about 5 degrees 
longitude further east than the European model solution. This will 
have a significant impact on the westward progression of the Arctic 
airmass that is forecast to drop southward into the north-central 
United States at this time. The European model solution would allow 
500 dm to 510 dm 1000-500 mb thickness values to be present across 
southeast Wyoming and the western Nebraska Panhandle...while the GFS 
continues to hold onto much warmer temperatures with 1000-500 mb 
thickness values remaining around 530 dm to 540 dm. The last 00z 
ensemble guidance from the GFS model shows at best one ensemble 
member indicating a colder solution more similar to the European 
model. Significant spread between high temperature forecast values 
from the GFS for the Nebraska Panhandle for Tuesday through Thursday 
offer little confidence. GFS ensemble forecast spread is resulting 
in guidance temperatures ranging from the upper 20s to the middle 50s 
for highs Tuesday through Thursday in the Nebraska Panhandle. The 
European model solution would result in even colder temperatures 
than the colder GFS ensemble solutions...especially on Thursday. 
Both models show a 180-190 knot nearly zonal jet streak across the 
North Pacific. The GFS indicates an eastward shift in the 
subtropical high Wednesday into Thursday which will translate into 
the jet splitting a bit farther east and the eastward shift of the 
upper ridge along the Pacific coast. There does not appear to be 
much support for this eastward shift...so at this time will trend 
more toward the lower end of the available guidance. This is similar 
to the idea supported in the day 3-7 guidance from HPC. 


&& 


Aviation...tonight...strong west winds will be subsiding through 
the evening hours. Strong cold frontal passage will occur late this 
evening and shortly after midnight with winds shifting to northwest. 
VFR this evening. Scattered to numerous snow showers will produce 
IFR and MVFR west of Interstate 25 after midnight. 


Friday...strong northwest winds will produce mountain turbulence and 
Lee eddy effects with the strongest winds across western Nebraska. 
VFR with localized MVFR and IFR in snow showers across the Snowy 
Range and northern Laramie range. 


Friday night...MVFR and IFR in the evening across the Snowy Range in 
snow showers...VFR elsewhere. Rubin 


&& 


Cys watches/warnings/advisories... 
Wyoming...High Wind Warning until midnight Friday for wyz059-wyz061-wyz062- 
wyz063-wyz064-wyz065-wyz066-wyz067-wyz069. 
NE...high wind watch from 8 am Friday to 6 PM Friday for nez001- 
nez002-nez003-nez019-nez020-nez021-nez054-nez055. 


&& 


$$ 


Short term/aviation...Rubin 
long term...CArpenter