869 
fxus63 kfsd 220331 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
930 PM CST Friday Nov 21 2008 


Discussion... 
tight upper low moving southeastward over County Warning Area tonight with main snow in wraparound 
band within and just north and west of upper low. Warmer air aloft around 
850 mb bringing a brief period of freezing rain-S- from the James Valley to near 
Missouri valleys before evap cooling and a fairly wet snow takes 
over. Snow has basically ended behind upper low from northwest to north central 
South Dakota...and this should move southeastward into northern and western County Warning Area later tonight... 
but the snow should pick up in southeastern half County Warning Area and will beef up probability of precipitation 
and amounts some to 1 to 2 inches into a good part of northwestern Iowa and 
parts of southwestern Minnesota. A few topographically favored areas could get 3 
inches or so. Overall previous forecast in the ballpark...with the fairly 
minor adjustments. 


Rsr 


&& 


Aviation... 
main second band of precipitation poised to move east-southeastward into The Heart of County Warning Area 
next few hours and overnight. This will produce rapid lwrng of ceilings 
and visibilities to MVFR and areas of IFR Hon/mhe 03z-06z and fsd 07z-10z. 
This will continue into northwestern Iowa and even probably as far S as sux after 09z 
if earlier models runs are right...New Run to be seen shortly. Far northestern 
and southwestern corners of forecast area may escape the IFR...esply the area SW 
of MO river where precipitation may stay spotty and light and partly rain or 
-fzra for a while. Eventually widespread increasing low level moisture will send 
ceilings gnly below 1 thousand feet except possibly in that southwestern corner...even as precipitation 
decreases from the northwest after 09z. Some 3-5sm br lingering after precipitation into 
the 15z-18z period. Lingering low ceilings should improve to MVFR after 15z 
but very slowly...in weak low level flow behind this system it will be very 
hard to mix out the stratified low level moisture...therefore have been 
keeping those low clouds gnly Thursday falid taf forecast period. 


Williams 


&& 


Previous discussion... 
strong upper wave acsr northwestern South Dakota into southeastern Montana will dominate the early 
period weather across the region. Wave appears to be undergoing 
transition to increased curvature on latest few WV images... 
and this should help to focus lift forcing even more within 
strong left exit region and broaden along length of low to middle level 
baroclinic zone through the night. The battle initially will be the mass 
of cold and dry air in place to the east. Lots of virga from khon to 
toward kykn in developing warm air advection Wing early afternoon...and expect that to 
continue to work on saturation shifting ewrd into the early evening 
hours. With as much wet bulb potential...ptype should easily start 
as snow...even with some late afternoon/early evening low level temperatures a touch 
above freezing. Will likely be a couple areas of more concentrated 
precipitation...the first being along the stronger warm air advection zone which contains 
concentrated isentropic lift due to strong cold dome to east. This precipitation 
will feature a very rapid degradation of visibilities heading through the evening 
hours...mainly along a khon to kfsd axis...enhancing this evening 
with the prior strong jet forcing coexisting with the strong frontal 
forcing. This area will shift to the east with time...and may start to 
weaken in the face of drier air to the east...while a second core of 
precipitation slides along a track a bit S of the initial area and is 
associated with the trailing circulation seen on WV in S central Montana. 
The intensity in this band will likely be a bit lower than the warm air advection band 
with a more shallow frontal circulation in place. Still indications 
of a little warm wedge sneaking in to the south of the low level low 
through the western MO river corridor...which could threaten a little light 
freezing rain or even rain if temperatures warm at the surface more than 
expected tonight. However...by far most of the precipitation should favor snow 
tonight with moisture remaining deep enough to keep the ice 
processes in play until perhaps very late in the far W/NW. 
End result should be a band averaging 1 to 2 inches snowfall from 
khon through kfsd toward kslb/kspw area. 


Upper wave exits the far southeast corner of the forecast area early Saturday 
morning and do not expect precipitation to linger too long into the morning 
hours. Could see another 0.5 to 1.0 inch over northwest Iowa after 
12z...but expect most areas to see precipitation ending by 15z or so. Low 
clouds will likely persist across the northeast half of the County Warning Area... 
slowly eroding from the southwest as low level winds switch to a 
westerly direction. This will hold temperatures down below freezing in our 
northeast areas most of the day...possibly pushing up just above 
freezing late in the day. Southwest areas should see more sunshine 
for the better part of the afternoon...helping warm readings into 
the 40s there. 


Another quick moving wave slides southeast across the region Sunday 
into Sunday evening. Moisture remains sparse on both GFS and NAM 
below 700mb as this wave moves through so will stick with our dry 
forecast...though will see an increase in middle-high clouds and shift 
to breezier northwest surface winds marking the passage of the wave. 
Decent mixing potential will help temperatures warm into the upper 30s to 
middle 40s...with cooler temperatures again in place for Monday...yet 
really still close to normal for this late in November. 


Much of next week looks quiet with upper ridge moving east across 
the region Tuesday through early Thanksgiving day. This should 
provide dry weather with near or above normal temperatures through 
the Thanksgiving Holiday. Upper trough begins to make its way into 
the Central Plains later Thursday and moves into the middle-Mississippi 
Valley by Friday evening. At this time...most model solutions keep 
precipitation shield associated with this wave south of our area...with 
21/00z GFS the only exception. Thus will keep our forecast area precipitation-free for 
the end of the week...though will have to keep an eye on this time 
frame as any shift to the north could affect Holiday travel. 


&& 


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


&& 


$$ 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)