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fxus63 kfsd 120249 aaa 
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Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
850 PM CST Thursday Mar 11 2010 


Discussion... 
snow band western forecast area has continued to weaken and have cancelled 
Winter Weather Advisory except for the 3 southwestern corner counties from Chamberlain to 
Pickstown. Second rain area moving over eastern forecast area is healthy but 
the best of it should turn southwestward and stay out of the far west. It will 
also weaken a little later. Outside of Winter Weather Advisory area we will see 
occasional to areas of light rain and snow with little snow accumulate. 
Meanwhile...fog seems well enough entrenched southwestern Minnesota to keep advisory 
there esply with the continued very low ceilings. Visibilities have improved a little 
more in northwestern Iowa and looks like enough mixing there to keep widespread 
dense fog from reforming...so have killed dense fog advisory in those 
counties there that had it. Temperatures will continue to hold pretty steady 
and winds look good. 


&& 


Aviation... 
IFR ceilings and generally MVFR visibilities but with occasional IFR visibilities in -rasn 
and fog. The main area of IFR fog should be in southwestern Minnesota until 14z or so. 


&& 


Previous discussion... 


Main precipitation band now west of I-29 as of 20z...as middle level dry slot 
wraps around tightly wound upper low and into eastern half of the 
County Warning Area. Areas beneath the dry slot will be more likely to see drizzle 
and/or fog tonight...though will keep a chance of light rain going 
as some wraparound precipitation working back into the County Warning Area from the 
southeast. Western County Warning Area remains more likely to see just snow as main 
precipitation type tonight...though eastern edges of existing advisory may 
see precipitation taper off by 06z or shortly thereafter as aforementioned 
dry slot pushes into the James Valley. Have trimmed snowfall amounts 
a bit along eastern edge of advisory as a result...though potential 
still there west of the James River for 2 to 5 inches...especially 
in the higher elevations west of the valley. Temperatures will move 
little overnight...though far western areas could dip below freezing 
with more persistent snowfall in that area. /Jh 


Main upper energy will begin to exit the area early tomorrow...but a 
shortwave currently over Illinois will wrap back to the west tonight...and then 
across the County Warning Area tomorrow. NAM and GFS timing differ with the 
SW/v...but both indicate the best chances for measureable 
precipitation will be west of a line from Tracy Minnesota to Yankton South Dakota. 
Precipitation type is again a question tomorrow. As warmer air wraps 
west...chances for snow will almost be nonexistent in the far eastern 
County Warning Area...but chances for snow will exist around i29 ewrd. Accumulations 
are generally not expected...as falling snow will mostly melt on 
contact...but can not rule out an inch or so over the higher 
elevations of Gregory County. The shortwave will move S of the area 
tomorrow night...and as it does so...dry air will infiltrate the middle 
levels...while the low levels will remain saturated. This setup is 
favorable for fog and/or drizzle Sat night right through Sunday. 
Gradient will be stronger in the west...so fog and drizzle will 
likely be muted during the day Sat...but as the gradient relaxes and 
winds decrease...fog and drizzle may become prevalent across the 
area. Overall chances for measurable precipitation are on the low 
side through the weekend...but does not appear there will much if 
any in the way of sunshine. 


Inverted trough is expected to remain stubbornly in place through 
early next week. Models keep slowing the ewrd progression of the 
overall pattern...and with no northern stream wave to scour out the low 
levels...low clouds and/or fog may be around through midweek. GFS 
indicating a couple weak waves moving across the area early to 
midweek. Not confident on timing or placement...but certainly could 
produce some measurable rain or snow. /97 


&& 


Fsd watches/warnings/advisories... 
Iowa...none. 


Minnesota...dense fog advisory until 9 am CST Friday for 
mnz072-080-081-089-090. 


NE...none. 


South Dakota...Winter Weather Advisory until 6 am CST Friday for 
sdz050-057-063. 


&& 


$$ 
Williams