049 
fxus63 kfsd 210213 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
912 PM CDT Sat Mar 20 2010 


Discussion... 
other than a bit of cirrus...clear skies will prevail tonight under 
an area of high pressure. Light winds associated with the ridge axis 
will allow temperatures to cool into the middle teens to lower 20s. So 
far temperatures have held up...but should continue to drop as winds 
remain light...only slightly offset by warming aloft. Could see 
pockets of shallow fog in low lying areas such as river valleys 
towards sunrise...but will not amount to much. 


&& 


Aviation... 
VFR sky clear expected through Sunday as surface high pressure slides southeastward 
across the area. 


&& 


Previous discussion... 
still seeing a scattered stratocu deck through the eastern central 
portions of the County Warning Area this afternoon...but expect these clouds to 
quickly dissipate as the sun sets. This will lead to a quiet night 
across the area as a surface ridge axis becomes situated over the 
region. With clear skies and light winds...stayed on the cool side 
of guidance with lows dropping into the middle teens to lower 20s 
across the County Warning Area. Have coolest readings through our Nebraska zones 
where there is lingering snow cover this afternoon. The snow has 
been slow to melt over this area with less than optimum sunshine 
under a broken cloud deck. If this snow cover would completely come 
off by this evening...may not get quite as cold as currently 
forecasted. /Jm 


Short wave ridging passes across the fsd forecast area Sunday and Sunday night. The 
surface ridge exits ahead of the upper ridging bringing the winds around 
to a SW or south-southwest direction ushering in mild air. It appears that 
mixing potential on Sunday is close to h9...and given the forecast h9 
temperatures off of the NAM and GFS...see no reason not go with the warmer 
guidance values or even a bit warmer. 


On Monday...looks like the models have sped up the wind shift/cold front 
from previous runs. The bulk of the consensus was to bring the leading 
edge of the wind shift about to a Lake Andes to Huron South Dakota line by 00z 
Tuesday. However...the new 12z European model (ecmwf) backed up the 12z NAM solution in 
being a bit quicker than this. On the extremes...the Gem appeared to 
fast and the GFS too slow with the wind shift. At any rate...good 
warming appears on tap...similar to last thursdays warming...of 
temperatures well into the 50s and 60s. 


Then a series of waves moves through. The first is actually on Monday 
which causes the aforementioned wind shift. The second and slightly 
stronger wave impacts this area primarily on Tuesday. The problem is 
dynamics are weak...and much of the moisture is limited to the low 
levels...with dry middle levels. Therefore only small probability of precipitation are 
warranted along and north of the I 90 corridor. In northwest Iowa...the low 
level moisture forecasts were even weaker so left them dry at this time. Maximum 
temperatures are very tricky on Tuesday and probably the most problematic 
element in this entire forecast. If the GFS and NAM are correct in their 
stratus forecast...maximum temperatures are likely to be cooler than what I have in 
there for Tuesday right now. Just do not see that kind of diurnal 
warming with a northerly wind and low clouds. Will re evaluate that again on 
Sunday. 


In the extended Wednesday through Sat...really not a lot going on yet to key 
on anything for precipitation. As HPC said...we are caught in between a large 
upper vortex to the east around the Hudson Bay...and a slow moving 
upper trough moving into and through the Pacific northwest. It is possible by Saturday 
(day 7) that some rain will be realized with the Pacific northwest trough. But the 
model variance was great enough on the timing of this feature to 
leave the forecast dry. Given h925 temperatures...mainly followed off the 
European model (ecmwf)...deviated little from the HPC numbers. /Mjf 


&& 






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


&& 


$$