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Area forecast discussion 
National Weather Service Aberdeen South Dakota 
349 PM CDT Friday Aug 8 2008 


Short term...tonight through Sunday night 


Currently the upper level longwave pattern shows meridional flow with 
upper level ridging over much of the central to western US and a 
longwave trough over the eastern US. At the surface a high is parked 
over the Mississippi River valley...putting US on the unstable 
side of it. A very slow moving warm frontal boundary is situated 
over central to northern South Dakota. Additionally...an upper 
level shortwave trough is riding up over the ridge and is located in 
western North Dakota. The forecast challenges in the short term 
include precipitation chances with the passage of this and other minor 
shortwave troughs through the longwave pattern. 


Both the GFS and the NAM are in good agreement that our 
northeastern County Warning Area corner should see some precipitation as the 
shortwave clips on through this afternoon into the evening. Kept 
probability of precipitation in for eastern County Warning Area as these ridge runner troughs have the 
tendency sometimes to take a more southward dive than anticipated. 
However...significantly reduced probability of precipitation overnight as trough should be 
well into Minnesota by midnight. 


Both 12z and 18z NAM pick up on a surface convergence zone late 
Saturday afternoon into the evening and another upper level 
shortwave trough passing through the flow pattern. Slight chance to 
chance probability of precipitation in the grids reflect anticipated precipitation. 12z GFS 
not picking up on this...but NAM has been more consistent run to run. 


Finally...timing precipitation again Sunday night into Monday with 
the approach of the next surface low poses the final challenge in the 
short term. GFS and NAM both handle the overall surface trough fairly 
consistently with GFS more aggressive on closing the surface low. 






Long term...12z Monday through Friday 
biggest challenge facing the long term deals with precipitation chances 
through the period along with model selection. 


The long range models agree for the most part in the early part of 
the extended. Closed upper low in western Canada drops down to the 
internation border. At the surface...ec/GFS/dgex agree on a closed surface 
feature scooting east along the ND/South Dakota border Monday into Tuesday. 
The GFS shows an increase in 7h and 850 mb frontogenesis along with 
increasing 7h Theta-E forcing. Models are advertising an 
increasing amount of precipitable waters  ahead of the surface feature...with GFS 
showing an area of 1.50 plus inches 00z Tuesday through 12z 
Tuesday. Have pretty much left inherited weather grids intact for the 
12z Monday through 00z Wednesday time period. Did break the grids 
up into 6 hourly pieces to better time the surface trough/low passage. 


Models then begin to diverge greatly in the solutions heading into the 
weekend. The GFS continues to drop the aforementioned 500 mb closed low 
into the northern Continental U.S....although the 12z solution has backed off a bit 
on southern extent versus the 06z solution. The 12z ec on the other 
hand has dropped the 500 mb closed low into northwest Minnesota by 12z 
Friday...reaching NE South Dakota by 00z Saturday. Run to run consistency 
for any model right now seems to be a bit lagging...so am just 
going to leave inherited dry weather grids as is until things shake out 
a bit more. 


Temperature wise...inherited temperature grids also seemed reasonable. If the 
upper low does indeed drop further south into region heading into 
next weekend...then temperatures may need to be dropped a bit as 850 mb 
temperatures approach +10 with the ec showing 7h temperatures around +2 by 00z 
Saturday. 






&& 


Aviation... 
extreme northeast South Dakota and western Minnesota could see 
short periods of MVFR/IFR associated with an area of showers and 
thunderstorms late this afternoon into this evening. Otherwise 
expect good VFR conditions for visibilities and ceilings 
throughout the period. 






&& 


Abr watches/warnings/advisories... 
South Dakota...none. 
Minnesota...none. 
&& 


$$ 
Short term...gionta 
long term...Hintz 
aviation...gionta 


Weather.Gov/Aberdeen