557 
fxus63 karx 212054 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
245 PM CST Friday Nov 21 2008 


Short term...tonight through Monday night 


Main forecast challenge will be precipitation chances late tonight 
through Saturday...and again Sunday through Monday night. 


Surface map this afternoon had a ridge of high pressure from the 
southern Mississippi River valley...extending northward through the 
upper Midwest. Sunny skies were in full force across the area under 
the ridge. Otherwise...despite full sunshine the temperatures have 
struggled to make it to 20 across the north...and the middle/upper 20s 
across the south. Meanwhile...water imagery this afternoon showed a 
very compact circulation rotating southeast through the Dakotas. 
Surface observations under this middle-level shortwave revealed some 
decent snow falling across portions of eastern Montana/southwest South Dakota. 


Latest short range models remain in good agreement lending to 
generally higher forecast confidence. 


For tonight...the shortwave over the northern plains will continue 
to move slowly southeast to near kfsd by Sat 12z. Models also 
continue to focus best isentropic lift/Omega and frontogenetic 
forcing just southwest of the forecast area after midnight. Fairly 
strong dynamic associated with the wave will have to battle a very 
dry airmass for total saturation of the column. For this 
reason...have opted to continue smaller end -sn chances south of a 
line from Austin Minnesota to Richland Center WI from after midnight 
tonight through 18z Saturday. In addition...used a non-diurnal 
temperature curve tonight with the increase in clouds and south 
winds overnight. Have temperatures bottoming out by midnight...then 
becoming steady through the remainder of the night. The wave dampens 
out as it progresses southeast into Illinois Saturday afternoon. This in 
turn diminishes frontogenetic forcing/isentropic lift into the 
southern portion of the forecast area...thereby shutting off snow 
chances. Based on thicker cloud cover have lowered highs on Saturday 
a few degrees. 


Warm air advection takes hold across the area Saturday night ahead 
of low pressure tracking east across southern Canada. Will have to 
watch for stratus/zl potential late Saturday night. Both GFS and NAM 
showing an increase in low level relative humidity...but feel stratus will not be 
deep enough for collision/coalescence process for zl. However...if 
stratus gets thick enough...the potential will be there. Will keep 
mention out for now...but will definitely have to be monitored by 
later shifts. 


Low pressure moving across southern Canada will drag a cold front 
into the area Sunday for a chance of rain or snow showers in the 
afternoon. Cold air advection/cyclonic flow aloft should produce 
some snow showers Sunday night. 


The Canadian low becomes vertically stacked/closed off and tracks 
southeast across the area on Monday. Cold cyclonic flow along with 
steep surface-850mb lapse rates around 9c/km should continue to 
produce scattered snow showers through the day...eventually tapering 
off Monday evening. Appears latter half of Monday night will be dry 
as the low tracks further east into the Great Lakes region. 


Long term...Tuesday through Friday 


21.12z GFS and 21.00z European model (ecmwf) in reasonably good agreement through the 
extended period with respect to overall evolution of synoptic 
pattern. For Tuesday through Thanksgiving day...both models showing 
deep troughing along the west and east coasts with the middle-section 
of the Continental U.S. In general ridging. By Friday...it appears there will 
be a split flow developing with southern stream energy passing to 
our south while the forecast area remains under general ridging. Bottom 
line...appears the extended period will be dry with temperatures 
right around seasonal normals. 


&& 


Aviation... 


Surface ridge axis passing through today as winds begin backing to 
more southerly or return flow. In addition short wave trough is 
dropping through the plains but airmass remains quite dry. Forcing 
from approaching wave will likely saturate relatively high so expect 
mainly high level clouds to gradually work in from the west next 24 
hours remaining in VFR conditions. 


&& 


Arx watches/warnings/advisories... 
WI...none. 
Minnesota...none. 
Iowa...none. 
&& 


$$ 


Short/long term...das 
aviation..........Shea