Weather

985 
fxus63 kfsd 290820 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
320 am CDT Friday Aug 29 2008 


Discussion... 
the main weather story over the next few days will be the strong 
southerly winds that develop and the fire weather affects from them. 
Currently...large area of surface high pressure over the central 
portion of the country is expected to shift east as the day 
progresses and return flow is expected to develop in advance of a 
strengthening trough over central Alberta. 


Very mixy conditions are expected today with only a few high clouds 
streaming overhead. Given the dry atmosphere and lack of 
clouds...boundary layer is expected to mix to near 700 mb late this 
afternoon. As boundary layer develops...gusty southwest winds 
should increase ahead of what will serve as a relatively benign wind 
shift that will move through during the afternoon and evening hours. 
Northwest flow behind this boundary will rapidly change back over to 
a southerly direction this evening ahead of intensifying trough to 
the northwest. Once the winds increases tonight...southerly flow 
will remain quite brisk throughout the weekend. Both Saturday and 
Sunday afternoon winds will approach Wind Advisory 
criteria...especially across western County Warning Area where pressure gradient 
will be the most compact. In addition...grassland fire danger across 
Brule and Gregory counties will be in the very high category both 
days with drier fuels and higher winds expected there. On Saturday 
morning...models are suggesting the main axis of elevated warm air 
advection/Theta-E advection will remain just to the west of forecast 
area. With relatively limited moisture return with the Gulf cut off 
by the approaching tropical storm...have kept forecast dry. Models 
are indicating a more widespread low level jet Saturday 
night...drawing residual moisture from the Central Plains 
northward. With this moisture...there could be some stratus which 
will have a significant impact on highs. Have lowered high 
temperatures on Sunday especially in the east with the expectation 
of clouds limiting warming. 


On Sunday night into Monday...strong southerly flow continues 
however...will need to keep an eye on potential for elevated 
thunderstorms overnight. Focus of low level jet appear to be along 
the eastern edge of forecast area and areas further east. With such 
broad focus...elected to keep forecast dry for now. Better chances 
for precipitation Monday night as a few leading shortwaves move through 
ahead of strong cold front that is expected to swing through during 
the afternoon hours on Tuesday. Models in much better agreement with 
this feature than in the previous set of runs...with the GFS 
delaying the progression of the front to a similar timing of the 
European model (ecmwf). 


&& 
Aviation... 
VFR conditions expected most areas through Friday night. Only 
exception is radiation fog around ksux vicinity early this morning. 
Plenty of residual low level moisture from recent heavy rains... 
and strong radiational cooling in center of surface high allowing 
for valley fog to form through parts of the Missouri River channel. 
Proximity of ksux to this valley will result in periods of IFR-LIFR 
visibilities as the patchy fog comes and GOES out of the valley through 
13z. Strong mixing in relatively dry airmass will result in gusty 
southwest to west winds during the afternoon...but gusts should 
diminish quickly as sunset approaches this evening. 


&& 


Fsd watches/warnings/advisories... 


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


&& 


$$ 
Bt/jh