919 
fxus63 karx 180833 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
328 am CDT Friday Jul 18 2008 


Short term...today through Sunday 


Immediate concern will be rain showers/ts chances across the south 
today...then increasing chances across the rest of the area 
tonight/Saturday. Heavy rain potential still there later tonight 
through Saturday...especially across the south. 


Surface map early this morning had a weak area of low pressure over 
Western Lake Superior with a weak cold front extending from the low 
through northwest WI into southern Minnesota. Slowly diminishing complex of 
thunderstorms from Thursday convection that dove southeast through 
the area now extended from southeast WI through southeastern Iowa. 
Upper Midwest vwp network at 925-850mb level showing west to 
northwest drying wind component owing to diminishing trend in the 
convection. Otherwise...a very weak pressure gradient was in place 
across the area. Resulting light winds combined with very moist 
lower boundary layer was producing areas of fog...with worst 
visibilities in the 2-2.5sm range. Otherwise...temperatures across 
the area were in the middle 60s to around 70. 


Short term models showing weak high pressure building into the area 
today...pushing the frontal boundary into far southern WI/northeast 
Iowa...and then hanging up there. Should be a fairly sharp delineation 
of where rain showers/ts chances will exist today. Have included a very 
small chance of across the south this morning with the dwindling 
thunderstorm complex skimming across eastern Iowa/northern Illinois. 
However...NAM/GFS showing 0-3km MLCAPE increasing into the 
2000-3000j/kg range across far southern portions of the area by this 
afternoon as the front moves into that area. The local 17.21z rucwrf 
shows a band of thunderstorms erupting along the front by 21z. 0-3km 
shear rather weak though...so would just expected pulse-type storm 
activity with some heavier downpours as precipitable water values hover around 2 
inches. Based on these signals will shift probability of precipitation further northward 
going into the afternoon hours. 


For Friday night through Saturday evening...models still showing middle-level 
shortwave trough ejecting out of the High Plains into the upper 
Midwest. This will aid moderate amount of 925-850mb moisture 
transport into/over the nearly stationary boundary hung up from 
central Iowa into southern WI for showers and thunderstorm 
development. With precipitable waters still hovering in the 1.5-2 
inch range...convective precipitation could get heavy with some 
localized flooding possible. Best chances for the 
convection/flooding potential will be across the south where best 
moisture transport impinges on the stationary frontal boundary. Will 
have to keep an eye on this scenario for possible Flash Flood Watch 
if favorable parameters hold together. Will hit heavy rain wording 
in severe weather potential statement and may issue an esf for flooding potential. 


Rain showers/ts chances will continue Sunday as a middle-level trough drops 
southeast across the area from Canada. 


Bottom line...appears to be a soggy pattern across the area through 
period with perhaps some localized flooding possible across the 
south tonight through Saturday evening. Will pass concern for a 
possible Flash Flood Watch onto the dayshift for further 
assessment. 


Long term...Sunday night through Thursday 


18.00z medium range models continuing to show some differences 
through the extended portion of the forecast. Both start off in 
fairly good agreement Monday through Tuesday with a deep middle-level 
shortwave trough moving southeast through the area into the Great 
Lakes region. Rain showers/S chances are warranted on Monday with cold 
pool/steep lapse rates overhead. Difference then arise in the 
strength of the ridge building east into the area from the plains 
Wednesday through Thursday. The GFS shows much stronger ridging 
while the European model (ecmwf) wants to bring a trough/convection over the ridge 
from the northern plains into the area Wednesday though Thursday. 
Current extended grids reflect this uncertainty with small rain showers/ts 
chances in for the Wednesday/Thursday time frame. No changes were 
made. 


&& 


Aviation...today and tonight 


Appears taf sites will get a break from convection for much of 
today/tonight. Weak high pressure and light northwest winds will 
build into the area today...pushing the active frontal boundary and 
deeper moisture further south of the area. Some drier air will also 
spread into the area today...helping to limit rain showers/thunderstorms and rain today/early 
tonight. However...light winds and recent rains have produced a 
saturated boundary layer with minimal mixing early this morning. 
Areas of MVFR/local IFR visibilities in fog will be common across much of 
western WI/southeast Minnesota through about 14z this morning. Will monitor 
this closely early this morning...as extent of fog will be impacted 
by amount of clearing of middle/high clouds. Rain showers/thunderstorms and rain chances increase 
for later tonight/Sat as deeper moisture return and lift/convergence 
spread back into the area. 


&& 


Arx watches/warnings/advisories... 


WI...none. 
Minnesota...none. 
Iowa...none. 


&& 


$$ 


Short/long term...das 
aviation...rrs