Weather

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Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
330 PM CDT Sat Oct 11 2008 




Discussion... 


Stationary front generally along the i29 corridor this afternoon 
will slowly drift ewrd this evening. The front will stall out around 
midnight and then rapidly shift back to the west. Temperatures to 
rise overnight as the front moves wwrd. Radar and water vapor 
indicate a plume of moisture streaming nwrd out of central Nebraska. 
Will see continuing chances showers and thunderstorms through the 
overnight as moisture migrates into the region. Expecting shower and 
thunderstorm activity to diminish in the eastern County Warning Area this evening...but 
increase in coverage into the western areas...where stronger Theta-E 
advection and vorticity advection will be in place. Do not anticipate 
severe storms...but could see some pea sized hail and brief heavy 
rain. 


Boundary moves only very slowly on Sunday...remaining just along our 
western counties through much of the day. With prodigious Gulf 
moisture being advected northward on a 50 knots jet...which will be 
parallel to the boundary...will have the potential for heavy rain 
along the front as it begins to push very slowly eastward in the 
late afternoon. With a tightening pressure gradient through the 
early part of the day...looks to be windy...though winds will taper 
off some in the afternoon as the surface trough in the west pushes 
eastward and gradient slackens. Once again...temperatures are going 
to be tricky. As stated before...front will remain in our far west 
through much of the day...though once it begins to push east...feel 
that temperatures will fall off pretty quickly behind the system 
considering the kind of temperatures we are looking at today over 
western South Dakota. Front does not really move into the James 
River Valley until around 21z...so did not really start to drop 
temperatures off until after that in our west...and will probably 
drop off quicker than I have it...but rather hard to pin down at 
this point. 


Models in fairly decent agreement with timing of front on Sunday 
night...though the GFS pushes it across the area a little quicker 
than the NAM/ECMWF...and went with the slightly slower solution. 
Again will be the potential for heavy rain for Sunday night and 
Monday...and with slowness of system...some places may accumulate 
close to 2 and half to 3 inches by Monday evening. May have to 
entertain the thought of some kind of flood statement at some 
point...but will hold off for now. With the slower solution...front 
does not exit our County Warning Area until Monday evening. Temperatures may 
actually fall in our eastern areas on Monday afternoon with the 
passage of the front...but did not really incorporate that into the 
forecast with it being 2 days out and some uncertainty on exact 
timing of frontal passage. 


Models seem to be still hinting at secondary energy over The Four 
Corners region following the exit of the first upper low on 
Tuesday...which could interact with a frontal boundary dropping 
southward to bring some precipitation to our area on Tuesday night 
into Wednesday. Even so...kept it out for now with models 
inconsistent with this feature from run to run. By the middle of 
next week...upper level pattern transitions to a more zonal type 
flow...then models begin to really diverge on upper level pattern 
toward the end of the week...so kept it dry. Temperatures look to 
remain close to normal through the period. 


&& 


Aviation... 


Stationary cold front will slowly drift east this evening and then 
rapidly shift west into central South Dakota by sunrise. West of the front... 
widespread MVFR to IFR ceilings...as well as...some reduced 
visibilities from fog. East of the front...strong S to southeast winds will 
develop tonight. Showers and thunderstorms will become more numerous 
along and west of the Interstate tonight. 


&& 


Fsd watches/warnings/advisories... 


South Dakota...none. 
Minnesota...none. 
Iowa...none. 
NE...none. 


&& 


$$