641 
fxus63 kfsd 212221 
afdfsd 


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


Discussion... 
low level moisture continues to stream north into the region on 
Stout southerly flow. Patches of stratocu pushing north a bit faster 
than previously anticipated...though some of the leading stratocu 
may still diminish with sunset with more widespread/persistent 
stratus working into far southern County Warning Area by 03z-04z. Areas drizzle/fog 
still seem a decent possibility over eastern County Warning Area late tonight into 
Sunday as low level moisture deepens beneath layer of weak lift/warm 
advection. With surface dew points in the lower to middle 40s over the 
Central Plains expanding north into the area...think temperatures 
will remain on the mild side ahead of the cold front moving into the 
James Valley by 12z...and have bumped lows up closer to met guidance 
values. /Jh 


Weakening cold front moves slowly southeastward into County Warning Area sun with south-southwesterly flow of 
relatively mild moist air ahead of it...so kept the lower clouds and 
small chance of -ra/dz in the southeastern half of County Warning Area into the afternoon. Strong upper 
trough digs into rockies Monday driven by strong jet on backside...and will 
probably be stronger and slower than GFS...closer to European model (ecmwf) and 
Canadian. GFS also too far north with upper level cyclogenesis through Monday 
night and Tuesday. Low level moisture will back up to the northwest through Monday as 
inverted surface trough develops from eastern South Dakota to eastern NE through Monday night. Chance of -ra 
mainly northestern half County Warning Area later Sun night through Monday morning with warm advection 
and moisture return. Main rain should develop west of inverted trough in 
western County Warning Area Monday afternoon and spread east at night. Dynamic cooling from 
aloft will likely change rain to wet snow James Valley west Monday night 
spreading east as far as i29...then moving eastward through Tuesday with upper low. 
Speeded up timing of this changeover based on faster models. 
Potential there for some heavy wet snow especially from lower James Valley 
northeastward through southeastern South Dakota to higher elevations of southwestern Minnesota through Tuesday and will 
have to be watched. HPC seems to have a lit bit of a east bias at this time 
as main wraparound and cooling still appears to be west. Another 
wave with bring a chance -shsn main eastern County Warning Area Wednesday with further cold 
advection...so kept it on the cold side Wednesday through Thursday. Nighttime lows 
should be pretty cold western County Warning Area Wednesday night and northern and eastern County Warning Area Thursday night 
with optimum radiational cooling near approaching cold and dry surface 
ridge. Air mass should modify with return southerly flow rest of extend./ Rsr 


&& 


Aviation... 
patches of MVFR stratocu moving thorugh south/east portions of the 
County Warning Area this afternoon/evening. Deeper low level moisture will continue 
to work into the area on south winds through tonight...with more 
widespread MVFR-locally IFR ceilings/visibilities in br expected into ksux area 
after 02z/03z...spreading north along and east of Interstate 29 
through 09z. MVFR ceilings likely to persist over eastern portions of 
the forecast area...generally east of kmml-kfsd-kykn line through 
00z Monday...with areas MVFR-locally IFR visibilities in -dz/br. /Jh 


&& 


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


$$