690 
fxus63 kfsd 182050 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
350 PM CDT Friday Jul 18 2008 


Discussion... 
currently...our ts complex which managed to move eastward along the Nebraska 
and South Dakota border today...is finally meeting its demise near Yankton. 
However...isolated rain showers has recently developed as expected in the southern 
portions of northwest Iowa right along the east to west thte ridge axis. Really 
not much in the way of any boundaries in that area...but there is a 
bit of h925 convergence. A lot of this is likely heating 
driven...and would not be surprised to hear of a few funnel repeats as 
oax and dmx have reported. But again...those will rapidly subside by 
evening. The instability axis then shifts a bit westward overnight and 
late tonight. Short wave energy is noted in southwestern Montana at this time...with more 
convection in eastern Montana. I believe it is the bonafide short wave near 
Yellowstone which we will have to worry about late tonight. Both the 
GFS and NAM take the initial stages of the complex into northwest South Dakota after 
evening. But then the NAM moves it eastward across northern South Dakota which was not 
followed. What was followed was more of a GFS solution which begins 
to move the mesoscale convective system southeastward into a more unstable and moist airmass. It just 
does not make sense at this time why the complex would move straight east 
into the crux of the stable low level high pressure system which is to 
the north and NE of this area. Therefore chose to move the thunderstorms 
around the periphery of the high along the instability and thte 
ridge axis. Agree with Storm Prediction Center that any severe ts late tonight would 
probably be confined to our far west with the upslope flow. 
Otherwise...altered low temperatures very little tonight. Am also a bit 
concerned about areas of stratus developing through the eastern half of the 
forecast area with a fairly moist Ely fetch of air continuing. 


Ongoing complex the main issue on Saturday. Plenty of dynamic 
support to keep organization to convection with distinct wave 
pushing through region...with best divergence q slight along northern border. 
However...continue to prefer gradient of instability axis as well 
as developing cold pool edge to keep development going through morning 
and into early afternoon in deferrence to deeper dynamics. Behind 
wave subsidence and veering/weakening of low level jet should diminish coverage by 
early to middle afternoon...as main core of precipitation spreads through southern Minnesota. 
However...surface boundary and weak convergence zone will slide into southeastern South Dakota 
late in day. Overall severe threat certainly does not look 
widespread...but best threat may be along this outflow/frontal 
convergence axis setting up from southwestern Minnesota to northestern Nebraska by late 
day. Still a bit too far off better westerlies to get tremendous shear... 
but more so multicell to brief supercell structures would be 
expected with 30-40 kts deep layer shear. Either debris...pcpn...or 
sheer cold pool should restrain temperatures well below guidance esply North/East in 
County Warning Area...with late day clearing through west allowing some better recovery. 


This precipitation should linger through southeastern County Warning Area into the evening hours before 
sinking southward with lower based instability and overall large scale 
subsidence forcing. However...by late night...middle level boundary not 
going far with weak ridging...and another weak middle level wave could 
utilize the gradient and slight increase in cross boundary flow to 
tap steep lapse rates on edge of capping inversion by early sun 
spreading into S central South Dakota. Introduced small chance probability of precipitation through Sun 
morning. Here is where NAM and GFS/Canadian/ec really start to 
deviate. NAM turns this minor wrinkle into the main player using to 
blow up large mesoscale convective system Sun afternoon/night from southeastern South Dakota through northern Iowa...while 
others keep this a fairly inconsequential feature with spotty quantitative precipitation forecast... 
and use boundary to interact with stronger wave Sun night into Monday 
with mesoscale convective system farther nwrd...I 90 or north. Latter makes a lot more sense 
considering strength of middle level cap building nwrd with steep lapse 
rates through the day Sunday. Will keep acc around through day with threat 
spreading slowly newrd...which will only help differential heating across 
boundary. Likely to become exceedingly unstable but capped near boundary 
around MO river lt Sunday...and some question as to whether it will 
try to initiate more surface based toward evening in northestern Nebraska on 
low level boundary...or farther north toward dynamics and elevated 
instability axis in central South Dakota. For now...kept things in the chance range 
from Sunday night into Monday...but will have to be fine tuned as 
effects from tonights show play out on the boundary locations. 


While limited Monday night to small chance through southern 1/2 of County Warning Area...this too 
may change with need to increase the eastern half of the area as trough 
really trending toward digging/slowing across Minnesota in several slns. 
Considering all the intevening probability of precipitation...will let things really shake 
out before splashing yet more precipitation in to the forecast. 


In the extended range /Tue through Friday/ there is absolutely no 
agreement on the larger scale between various slns. Several GFS runs 
are much more ridgy aloft midweek...with support from Canadian... 
while ec crashes a midweek strough through ridge. Ensemble mean fans 
should take note that in the 6-18h period from last nights 00z GFS 
ensemble...there were no members that produced any precipitation in 
sctnrl South Dakota...with a zero percent chance of 0.05 inches. Not exactly 
a confidence maker for a sln with very little spread. The GFS 
ensemble also features fairly little spread in the extended... 
with the greatest uncertainty around timing of a wave moving through 
the Canadian prairie provinces /the one European model (ecmwf) digs aggressively through 
the northern plains/ and the depth of the southeastern Canadian trough. 
Operational GFS looks like a warm outlier overall. To make a long 
story short...very little confidence in where any more significant 
systems might affect northern plains. While it is almost certain that 
there will be some convection within the extended period...there is 
no sense in plastering low probability of precipitation in every period per guidance. 
Therefore...a much drier extended forecast is being presented than 
previous forecast. Will have some very small probability of precipitation in the far SW to account 
for more favored periods of High Plains drifters. Some hints that by 
Thursday night and Friday may be more favored...but for day 7 will let things 
shake out more. Overall...a fairly seasonable week looks in store. 


&& 


Aviation... 
this area into the evening will be VFR...except for some lingering 
MVFR in parts of SW Minnesota and northwest Iowa. It is this high degree of low 
level moisture which has ME worried about areas of stratus 
developing later tonight...especially along I 29 and areas eastward. This is 
because the low level Ely fetch of air continues. Therefore opted to 
hedge at this time...and include some MVFR ceiling/visible for the kfsd and ksux taf 
sites late tonight early Sat. In fact...would not be surprised to 
see ksux go IFR for a brief time around sunrise Saturday. Confidence 
in any of this happening is only moderate...but something to watch 
for. What is a bit higher confidence is a ts complex moving eastward late 
tonight and Sat morning across the area. And of course in any 
thunderstorms...winds can be erratic with brief IFR to MVFR conds. && 


Fsd watches/warnings/advisories... 


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


&& 


$$ 


Mjf/Chapman 

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