Weather

396 
fxus63 karx 070811 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
311 am CDT sun Sep 7 2008 


Short term...today through Tuesday night 


Biggest forecast concern is on precipitation chances today through 
Monday night along with temperatures. 


Water vapor imagery depicts several shortwaves within the mean long 
wave trough across the northern Continental U.S.. one across eastern 
Minnesota/western Wisconsin and another one across the Dakotas. At 
the surface a weak cold front was seen from northwest Minnesota... 
back into South Dakota. Radar showing some scattered showers across 
the northern zones being driven by the lead shortwave. 


07.00z models continue to show longwave trough to persist across the 
upper Mississippi River valley region into Tuesday before flow 
becomes more zonal by Wednesday. Several shortwaves expected to 
move through the trough bringing periodic rain chances to the 
forecast area. GFS and NAM in decent agreement with the wave to 
impact the area today...but differ in timing for the stronger wave 
Monday and Monday night. GFS faster in pushing the system through 
the area versus the slower NAM. Have used a blend of the two for 
the gridded database. 


In the near term...expect shortwave to move from the Dakotas across 
central Wisconsin this afternoon. Weak isentropic upglide will 
result in some showers today...mainly across the northern 
sections of the forecast area. 21z sref probabilities for measurable 
rainfall has probabilities in the likely category which is 
consistent with the current gridded database. Only made a few 
changes to the southern areas based on track of wave. Cool 
thermodynamic profiles...along with steep 850-700mb lapse rates of 
7 to 8 degrees celsius will result in some isolated thunderstorms 
ahead of the wave/surface trough. Wave pushes east of the area 
tonight but will linger low probability of precipitation during the evening hours. 


Attention then turns to a second...more potent wave to track from 
Nebraska...across Iowa and southern Wisconsin Monday/Monday 
evening. Isentropic upglide on th 300k surface increases as attendant 
surface low tracks south of the area. Model cross section shows 
decent fn convergence across northeast Iowa and southwest Wisconsin 
Monday afternoon and evening. Favorable left exit region of upper 
jet with aid in lift across the area. Question will be how far 
north precipitation shield gets with drier air being advected in 
from the north as high pressure builds across the plains. Signal 
consistent enough to raise probability of precipitation into th categorical range across 
northeast Iowa and southwest Wisconsin...then taper them northward 
to just slight chances in Taylor County. Wave to push east of the 
area Monday night with lingering probability of precipitation across the southeast during 
the evening hours. High pressure then settles in for Tuesday and 
Tuesday night with quite chilly temperatures. 


With the clouds and rain expected on Monday have undercut guidance 
temperatures...although mav numbers look in Ball Park. Colder air 
then filters in Monday night. Depending on how much clearing occurs 
will likely see lower 40s in most areas with some 30s in the 
favorable colder spots. 


Long term...Wednesday through Saturday 


07.00z GFS and European model (ecmwf) consistent in pushing cold front across the 
forecast area Wednesday night and Thursday with high pressure 
building across the area for the end of the week. Trimmed back a 
little on probability of precipitation across the eastern areas on Wednesday...as front looks 
to remain across the plains/western Minnesota. Warm advection showers 
more likely in this regime across southeast Minnesota and northeast 
Iowa. Otherwise remainder of forecast looks on track and no other 
changes made. 


&& 


Aviation...today and tonight 


Currently an upper level disturbance was moving east across eastern 
Minnesota/western WI while another disturbance was dropping southeast into 
the eastern Dakotas. Widely scattered light rain showers activity from the 
first disturbance mostly avoiding the krst/klse taf sites at this 
time ... the bulk of the activity moving east of the taf 
sites by 10-12z. Second disturbance from the Dakotas will approach 
the area by this afternoon...producing increase in cumulus/scattered 
rain showers activity once again by 18z. There is a question of how much 
instability will be available for possible ts activity. Right now it 
appears best instability/chances of ts activity will remain north of 
the krst/klse taf sites. Due to the anticipated scattered nature of 
the convection...will continue to carry vcsh in the krst/klse taf 
site from 18-23z. For 12z taf issuance...will revisit latest model 
trends for instability values for possible inclusion of cumulonimbus/ts 
mention into the tafs for this afternoon. This disturbance moves 
east of the area this evening...ending rain showers threat. Approach of yet 
another upper level disturbance/surface low out of Colorado later 
Sunday night will keep middle/hi cloud cover across the area. 
Otherwise...appears VFR conditions will prevail through the period. 


&& 


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


&& 


$$ 


Short/long term...raberding 
aviation..........das