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fxus63 karx 201927 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
225 PM CDT Sat Mar 20 2010 


Short term...tonight through Tuesday night 


Short term forecast focus is on the precipitation chances for early next 
week. 


Somewhat benign weather pattern for the local area...as the polar 
jet hugs southern Canada...and the sub tropical holds along the Gulf 
states. A closed upper level low over the Southern Plains will move 
due east across the lower Mississippi River valley over the next 
couple days as a result...while farther north...the trough that 
brought the areas of snow to Iowa/ill/southwest WI last night will 
continue to sink south and east...while another...stronger upper 
level shortwave trough drives across Canada. This leaves an area of 
high pressure at the surface to be the main weather maker for the upper 
Mississippi River valley through Monday night. 


On Tuesday...both the 20.12z GFS and NAM spin a shortwave trough 
east across the northern plains and into the region. Decent swath of 
850-700 mb warm air advection ahead of this feature...with some 
isentropic upglide on the 285-300 k sfcs...best in the 06-18z Tuesday 
time frame. Some qg response in the 1000-700 mb layer also during 
this time. However...moisture could be limited. A west-east running 
time/height x-section is rather dry prior to 18z Tuesday...and even 
after...the saturation is not overly impressive. The 
GFS...however...is more robust with its moisture by 18z...and also 
has a stronger upper level shortwave. Both solutions suggest that 
the low level thermodynamics would go into some saturation rather 
than precipitation...with the GFS then producing some quantitative precipitation forecast with the shortwave 
itself. The NAM stays dry while the ec favors precipitation chances staying 
farther to the west. The Gem would suggest some light precipitation 
possibilities Tuesday night with the 20.09z sref keeping its better 
chances well north. Overall...a relatively weak system without any 
overtly strong signals...or model consensus. Have small probability of precipitation in 
current forecast...and will hold onto these for now. Forecast 
confidence not high enough to refine timing/chances at this moment. 


Long term...Wednesday through Saturday 


GFS and European model (ecmwf) continue in decent agreement with a split upper level 
flow regime for the region into the upcoming weekend...with the 
polar jet holding over southern Canada and the sub tropical across 
the far south. The initial impact would be to keep the brunt of an 
upper level shortwave trough across southern Canada/northern Great 
Lakes for Tuesday night/Wednesday night...while another in the southern stream 
tracks east across the Southern Plains into the Tennessee River valley 
Thu-Sat. Both models then favor spinning a shortwave across the 
upper Mississippi River valley in the Friday-Sat time frame...with the 
GFS a bit quicker and stronger with this feature compared to the ec. 
Like the potential system for Tuesday...forecast confidence is not high 
in positioning/timing of this upper level shortwave. So...will opt 
to stay with a dry forecast for the moment. Temperatures would favor 
a return to more seasonable conditions toward the later part of next 
week. 


&& 


Aviation... 


VFR conditions will be experienced through Sunday as high pressure 
builds over the area. Soundings and model relative humidity fields indicating a dry 
airmass overhead so expecting only some cirrus through this period. 
With the influence of the high...winds will be light...which in some 
circumstances would cause concern for fog. However...moisture at the 
surface/boundary layer looks to be the limiting factor. Dewpoints in 
the teens overnight with low temperatures in the 20s should keep the 
temperature/dewpoint spread large enough to keep any mention of fog 
or decreased visibilities out of the tafs...although following 
shifts should continue to watch this for further issuances. 


&& 


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


$$ 


Short/long term... rieck 
aviation.......... Brooks