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Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
410 am CST Sat Nov 21 2009 


Discussion... 
more active pattern setting up across the area with split jet across 
the US. Area of low pressure shifting east into western 
Saskatchewan. As this system shifts east...pressure gradient is 
expected to increase resulting in fairly strong southerly flow 
across the northern plains. Fairly moist air mass poised just south 
of forecast area...with widespread fog across eastern Kansas and 
Iowa. As southerly winds increase...expect moisture to surge 
northward especially east of Interstate 29. With 925 hpa temperature 
warming into the upper single digits...high temperatures today 
should be nearly 20 degrees above normal with highs in the upper 
50s. Could see a few clouds and potential for haze this afternoon 
trickle into northwest Iowa with increase in moisture...but do not 
think this will hamper warming too much. With mild 
temperatures...strong southerly winds and dry fuels...will issue 
range land fire statement for elevated fire danger today. 


Low level air mass becomes saturated fairly quickly this evening 
across Iowa expanding back to the north and west overnight. 
Saturation only extends to around 900 mb or so...but would not be 
surprised to see fog across northwest Iowa expand north and west 
into southwest Minnesota and far southeastern South Dakota 
overnight. There is also a chance of drizzle...but with lack of 
strong lift and strong directional shear above the saturated 
layer....kept probability of precipitation below 15 percent through 12z Sunday. 


Thereafter...during the daytime hours on Sunday...eastern half of 
forecast area becomes positioned in right rear of upper level jet 
streak. While the jet streak is not overly strong...may be enough 
to induce some light drizzle ahead of cold front associated with 
aforementioned low pressure system across central Canada. Flow 
really becomes parallel to the front during the early morning hours 
on Sunday...and front is expected to hang up in the vicinity of the 
James River Valley. With relatively weak flow during the day on 
Sunday...may be a struggle to shake low level cloud cover through 
much of the day...especially ahead of the front. As a result...only 
went slightly above isothermal from 925 mb in the east...while 
expect deeper mixing to the west. 


As upper level trough digs into the Pacific northwest on 
Sunday...expect inverted trough to set up and affect the region 
through Wednesday morning. Models have been all over the map in 
terms of a track with this system...however...00z NAM...00z European model (ecmwf) 
and 00z Canadian in fairly good agreement with digging this system 
west of upper Midwest taking the crux of the wave south of forecast 
area. GFS is not as quick to dig the system...and as a result brings 
the system further to the north. For now..sided closer to the 
ECMWF/NAM/Canadian. Models portend moisture transport Sunday night 
into Monday east of the James River Valley. Not really a great 
focusing mechanism...so for now...just left probability of precipitation in the low chance 
range. 


As the front works through the area Monday night into 
Tuesday...expect a wide variety of temperatures and as a result in 
precipitation types. Appears as though best dynamics may be focused along 
and behind of inverted trough...and have focused highest probability of precipitation there. 
Tried to use temperatures aloft from the NAM in a top down 
forecasting method of precipitation type...however...that resulted in a 
fair amount of freezing rain Monday night into Tuesday. While one 
can not rule out freezing rain with this set up...believe it would 
be more of a transitionary precipitation type as opposed to a predominant 
type and therefore left out of the forecast for now. Agree with 
previous forecasters assessment that best chances for snow appear to 
be across central South Dakota...shifting eastward Tuesday night as 
the system shifts east. /Bt 


&& 
aviation... 
generally...conds will be VFR through today and this evening. The 
exception will be this morning...where areas of MVFR visible will occur through 
15z mostly east of I 29. Watching the more dense fog lift north to 
northeastward early this morning from SW Iowa...it is possible that 
areas around Storm Lake Iowa may get clipped with conds of LIFR early 
today. But again...if those conds occur...they should be confined 
only to the southeast corner of the fsd forecast area. /Mjf 
&& 


Fsd watches/warnings/advisories... 
South Dakota...none. 
Minnesota...none. 
Iowa...none. 
NE...none. 




$$