Weather

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Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
955 PM CDT Wednesday Aug 27 2008 


Discussion... 


As expected main rain showers/thunderstorms and rain developing with main convergence along developing frontal 
boundary from southwestern Minnesota to northwestern Iowa and northestern NE at this time...as main short wave 
moves eastward over the Dakotas. So precipitation chances in South Dakota rapidly dmnshg rest of 
night and will have to fine tune the precipitation grids some more. Overall 
previous forecast working out well but may have to speed things up some. 


Rsr 


&& 


Aviation... 


MVFR ceilings in SC deck and MVFR visibilities in scattered thunderstorms and rain flwd by areas of 
br...mainly east of i29 through 09z. Potential for local IFR ceilings and 
visibilities vicinity of northwestern Iowa...mainly in areas of fog from 09z to 14z. 


&& 


Previous discussion... 
not much going on as of 19z with fnt stalled across southeast South Dakota and SW Minnesota. 
Capping still in place with some warm air advection at 850 mb. Cloud cover also 
preventing good mixing near fnt. So will have to wait until this evening 
when short wave gets closer to County Warning Area. This should start to produce some scattered 
elevated storms near the fnt across SW Minnesota and then build southwestward along 
the fnt into southeast South Dakota and northwest Iowa. Llm is also increasing and as storms 
begin to develop...should be able to tap into some of the llm as 
cooler air aloft with short wave breaks cap. Not too sure that we will see 
a lot of severe weather...but could see locally heavy rains as storm motion will 
be from west to east along stalled boundary. Will go with highest probability of precipitation from late 
evening through about 06z or 07z and then cut back probability of precipitation again as short wave 
pulls away. GFS has precipitation hanging back across southern County Warning Area through 12z 
Thursday...so will need to watch for that espec for heavy rains. Further 
west...pbly will not see any precipitation as drier air in place...but cannot 
rule out a stray storm this evening as far west as the James Valley. 


By Thursday morning...boundary will be located across our 
northwestern Iowa zones...and could see a continuation of convection 
over this area through 15z with lingering Theta-E 
advection/instability interacting with last impulse rounding the 
main upper level trough. Precipitation should wind down pretty 
quickly by late morning as the main energy finally swings through to 
our north...pulling dynamics to the east as drier air works into the 
region. Will become increasingly breezy behind the system with 
ensuing lower/midlevel cold air advection and surface pressure rises allowing 
higher winds from aloft to mix down. Temperatures should be close to 
normal...in the upper 70s to lower 80s. Thursday looks like a cooler 
night with dying winds and clear skies. 


On Friday...southerly flow begins to return to the region as surface 
high pressure axis shifts to our southeast. This will result in 
warmer temperatures in the afternoon...with a few middle clouds over 
our far north with low level convergence zone/weak shortwave 
residing from northern Minnesota through northern South Dakota. 


Models in good agreement on the pattern for the weekend...as upper 
level ridging builds into the region. Under a strong southerly 
flow...temperatures will climb to 4 to 8 degrees above normal for 
this time of year...as 850 mb readings get into the upper teens to 
lower 20s c. Sunday will be the warmer day...in the upper 80s to 
lower 90s across the County Warning Area. Did stay below warmest guidance however 
with surface winds out of the southeast. Will be pretty breezy both 
days...especially in our western areas...as a tight pressure 
gradient sets up across the region in response to a developing 
surface trough over the northern rockies/western High Plains. 


Models then begin to diverge somewhat by the beginning of next week 
with respect to the progression of an upper level trough swinging 
through the western U.S. Late Sunday into Monday. Even so...all 
point toward the upper system and surface reflection getting into 
our area in the Tuesday to early Wednesday time frame. With that in 
mind...did keep small probability of precipitation in the forecast toward the end of the 
extended period. While temperatures will begin to cool down for the 
beginning of next week...some uncertainty with respect to onset of 
cooling in consideration of above mentioned model discrepancies. 


Jm 


&& 


Fsd watches/warnings/advisories... 


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


&& 


$$