555 
fxus63 karx 231001 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
401 am CST Monday Nov 23 2009 


Short term...today through Wednesday 


At 3 am...a band of rain was moving north across parts of 
northeast Iowa...southeast Minnesota...and west central Wisconsin. 
This rain was associated with moisture transport and frontogenesis 
ahead of a short wave trough moving northeast across eastern South 
Dakota...southern Minnesota...Iowa...and northern Illinois. While 
the courser 23.00z models are showing that this rain will continue 
through 23... 4 km Storm Prediction Center NAM WRF indicates that this 
rain will continue through at least 23.15z and then it moves 
quickly north through southeast Minnesota and west central 
Wisconsin between between 23.15z and 23.18z. Since this model is 
doing so well this morning...the forecast for today was trended 
toward it. 


Like the past couple of days...it continues to look like the best 
forcing and moisture transport will remain west of the forecast 
area this afternoon and evening. As a result...these forecast 
periods were left dry. 


From late tonight through Tuesday night...the models have 
continued to trend a bit further south with the low pressure area. 
The NAM/WRF is by far the farthest south with this system. It is 
actually an outlier when compared with the GFS...Gem...and European model (ecmwf). 
Due to this...the NAM/WRF was not used for these time periods. 
There was an attempt to lower the precipitation chances along and 
north of Interstate 94 on Tuesday...but there was still enough concern 
that this precipitation may be a bit further north that there was 
little change to the precipitation chances in these areas. While 
there was no change to this...there was some timing for the rain 
added to the grids on Tuesday. 


On Wednesday...it continues to look like cold air advection will 
move into the forecast area during the afternoon. Due to this a 
non diurnal temperature curve was used. In addition...the snow 
chances were removed across western Wisconsin...and delayed until 
the middle to late afternoon in northwest Iowa and southeast Minnesota. 


Long term...Wednesday night through Sunday 


The 23.00z GFS...Gem...and European model (ecmwf) continues to show that a 
secondary short wave trough will drop into the long wave trough 
located across northern and Central Plains...middle and upper 
Mississippi River valleys...Great Lakes and Ohio Valley on 
Wednesday night. This greatly enhances the frontogenesis across 
Wisconsin...eastern Minnesota...and northern Iowa. In addition... 
the surface low slowly deepens as it moves northeast across 
southern and eastern Wisconsin. As this occurs...the GFS indicates 
that warm air aloft /possibly related to a trowal/ is brought back 
westward into the forecast area. Meanwhile the European model (ecmwf) keeps this 
warm air further east and the precipitation type remains in the 
form of snow. With some uncertainty in the precipitation type 
across Wisconsin...a mix of rain and snow was introduced into the 
weather grids for our Wisconsin counties for Wednesday night and 
Thanksgiving. Meanwhile further west across southeast Minnesota 
and northeast Iowa...it looks like the precipitation type will be 
mainly snow. 


A majority of the GFS mref plumes show less than 2 inches of snow 
with this system. However there are two of them which have 
snowfall totals around 6 inches. While there will likely be a mix 
of rain and snow in La Crosse...the precipitation type plumes 
still show some light snow accumulations /less than an inch/. 
Meanwhile with the precipitation staying in the form of snow in 
the emf...the snowfall amounts across the forecast area range 
from 4 to 9 inches. Needless to say...this system still bears 
some watching. 


&& 


Aviation...today and tonight 


Band of -ra continued to extend from near dlh south across rst and 
into southern Iowa. Rain was associated with some frontogenetic 
forcing...isentropic upglide...and in an area of 850mb moisture 
transport. Latest ruc13 and nam12 shifts this forcing west/northwest 
late this morning...and so should the light rain. Other mesoscale models 
//spcwrf...etc// also support this. Latest fog/stratus satellite 
imagery...coupled with surface observation...placed ceilings across the local 
area from 6 to 10 kft. Satellite trends were shifting this ac deck to 
the north/northwest with wind profilers and VAD winds showing 
backing to the southeast in the 925-700 mb layer over the past few 
hours. Just to the east of this cloud mass the skies were 
clear...which was resulting in some fog and stratus formation over 
northern ill/southeast WI. Will have to monitor trends before 12z 
issuance to see if some of these lower ceilings/fog could impact the 
klse. Believe krst will be too far west. Regardless....3-5sm MVFR br 
should persist through the morning. 


Upper level trough will be moving over the Southern Plains by 12z 
Tuesday...and there looks to be a decent surge of moisture northward 
ahead of this system. Ceilings will gradually lower...potentially 
IFR conditions by Tuesday night. However...if the current middle clouds 
stay mostly west/north of krst/klse tonight...some fog and stratus 
development could occur by 12z Tuesday. Numerical guidance points to this 
possibility. Confidence not particularly high at the moment...but 
trends will have to be monitored. Will hint at this possibility... 
with later updates refining the potential. 


&& 


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


$$ 


Short/long term...boyne 
aviation...rieck