Weather

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Area forecast discussion 
National Weather Service La Crosse WI 
325 PM CDT Sat Oct 11 2008 


Short term...through Tuesday night 


Forecast concerns were primarily focused on rain chances Sunday 
afternoon through Tuesday night. 


Afternoon surface analysis had nearly stationary front extending 
from northeast to southwest Minnesota...then across eastern Nebraska to 
low pressure organizing over the central rockies. Warm sector ahead 
of the front had few clouds...which has allowed temperatures to 
reach into the 70s to lower 80s across the upper Mississippi Valley. 
Radar composite indicated cluster of convection advancing from 
northwest Iowa into southern Minnesota...which was associated with short 
wave seen in water vapor moving across northern plains. 


Water vapor imagery also indicated large closed middle level circulation 
situated over the southwest part of the nation...with several short 
waves rippling through flow. Of note...plume of middle and high level 
moisture from Hurricane Norbert near baha peninsula becoming 
entrained in southwesterly flow aloft from Texas into Central Plains. 


Deterministic model trends have settled upon a slower progression of 
surface front across the upper Mississippi Valley. 11.12z 
deterministic NCEP and European models had good clustering of upper 
level and surface features. This lends high confidence the forecast 
area should receive rain during the first part of the week...with the 
front moving through. 


Sunday and Sunday night...based on forcing tied to proximity of 
surface front to the west...maintained rain chances generally over 
north and west sections of the forecast area. In addition...high 
clouds expected to be prevalent...as shown by time/height cross 
sections. Strong south flow favors unseasonably warm temperatures... 
with 850mb readings around 15c. We believe sufficient insolation on 
Sunday should allow maximum values to reach middle 70s to lower 80s. 


Monday...front should push into the forecast area...as middle 
level and surface wave tracks into southern Canada. Sufficient 
low Theta-E convergence combined with onset of upper level 
divergence associated with 300mb jet maximum supports rain chances 
areawide...with highest probabilities over north and west sections of 
the forecast area. Of note...remnants of Hurricane Norbert could 
enhance available moisture...which models already indicated 
precipitable water values of 1.5-1.75 inches. 


Monday night...low level Theta-E convergence...along with upper level 
divergence associated with right entrance region of 300mb jet 
maximum expected to enhance rain shield in the Post frontal 
environment. Therefore...opted to increase rain probabilities into 
likely category across the entire forecast area. In fact...would 
expect subsequent forecasts to warrant further increase in rain 
probabilities. 


Tuesday into Tuesday night...deterministic 11.12z NCEP and European 
models supported surface front scouring out clouds Tuesday and thus 
ending rain chances. Therefore...rain probabilities were confined to 
morning hours...before next surface and upper level wave brings rain 
chances back in Tuesday night. Biggest question in this time frame 
was timing of precipitation. At this time...utilized a broad-brush 
technique for the entire forecast area...as in current data base. 
Would not be surprised if rain chances need to be increased in the 
Tuesday night time frame. 


Long term...Wednesday through Saturday 


Primary concerns were associated with rain chance Wednesday again 
late in the week. 


Deterministic GFS and European model (ecmwf) have been at odds with each other in 
the extended outlook...which was no different with todays 11.12z data 
suite. Both models indicated another surface wave should move along 
the surface front at midweek...which by Wednesday will be positioned 
across the middle Mississippi Valley. Timing of the wave was still in 
question and thus forecast confidence was low in details. At this 
time...data base was weighted toward a dry forecast on Wednesday. 


Looking ahead...European model (ecmwf) and GFS have suggested another upper level and 
surface wave should affect the forecast area late in the week. 
However...model differences in timing and strength of this system 
have varied greatly. As a result of uncertainty...forecast confidence 
was low and chose not to make any changes to the current extended 
outlook. Therefore...low-end rain chances remain in the current data 
base for Thursday night...which appears reasonable at this time. 


&& 


Aviation...late this afternoon through Sunday 


The surface pressure gradient...between a strengthening low pressure 
system over the Central Plains and a high pressure system over the 
Middle Atlantic States...will gradually tighten through Sunday. The 
winds will likely gust into the 17 to 23 knot range through 11... 
and then will quickly decouple. The wind gusts will likely return 
between 12.14z and 12.17z as the nocturnal inversion breaks. The 
last places to see the winds gusts will likely be in the river 
valleys. With stronger winds aloft on Sunday...wind gusts will 
likely range between 23 and 30 knots in most locations. Due to the 
Wisconsin River Valley being more sheltered and climatology showing 
lower wind gusts at Boscobel /kovs/...wind gusts at this location 
will likely only get into the 15 to 20 knot range. 


Scattered to broken middle clouds will be seen through late this 
afternoon as a 500 mb short wave moves to our west and north. For 
the remainder of the period...there will likely be a broken 
cirrostratus deck between 20k and 25k. 


&& 


Climatology...tonight 


With southerly winds keeping the boundary layer well mixed and 
surface dew points around 60 degrees...the minimum temperatures will 
remain unseasonably warm for tonight. Minimum temperatures for 
tonight will be mainly in the lower 60s. This is close or warmer 
than some of the record warmest minimum temperatures for this date. 
The Table below contains the warmest minimum temperature records for 
October 12th. 


October 12 warmest low temperatures... 


Charles City Iowa...59 in 1984 
Decorah Iowa...64 in 1997 
Oelwein Iowa...60 in 1984 
Austin Minnesota...62 in 1984 
Preston Minnesota...63 in 1997 
Rochester Minnesota...61 in 1997 and 1930 
Winona Minnesota...69 in 1924 
La Crosse WI...65 in 1886 
Medford WI...56 in 1938 
Prairie Du Chien...63 in 1899 
Sparta...61 in 1997 


&& 


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


$$ 


Short/long term...Thompson 
aviation/climate..boyne 

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