431 
fxus63 kfsd 071640 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
1037 am CST Wednesday Jan 7 2009 


Discussion... 
couple bands of flurries moving through the area this morning. First 
with better middle level moisture across mainly South Dakota/NE/far western Iowa 
counties. Second area in far northeast County Warning Area where stratus deck in 
colder air is squeezing out a few flakes. Neither area will amount 
to much and expect most flurries to diminish through the afternoon 
as drier low level air works into the forecast area. Tweaked temperatures a couple of 
degrees cooler earlier this morning as most areas having trouble 
recovering in continued weak cold advection. Updated grids have been 
sent and will get new pfm/zone forecast product out shortly. 


&& 


Aviation... 
drier low level air working into the area from the north this 
morning scouring out much of the stratus outside a couple of narrow 
bands. First will continue to push south through Missouri Valley 
this morning...briefly lowering these areas including ksux to MVFR 
range ceilings/visibilities in flurries. Latest RUC and NAM show this area of 
MVFR ceilings continuing to push south and dry out some through the 
afternoon...lingering just over the far southwest corner of the forecast area 
into the evening before making a slow push back to the east toward 
James Valley after 12z Thursday. Second band of stratus moving 
through northeast sections of the forecast area will affect mainly southwest 
Minnesota/Iowa Great Lakes area into the early-middle afternoon... 
though satellite trends show possibility of MVFR ceilings pushing back 
into khon/kfsd for a few hours before layer dries out later in the 
afternoon. After 21z-23z...VFR conditions expected to dominate east 
of the James River. 


&& 


Previous discussion... 
northern plains continues to reside in location just past large scale 
broad western ridge...meaning digging systems that tend to pull in just 
a bit more cold air than expected. One such system extends through Minnesota 
and Iowa...with 140 knots jet punching into the plains. Several weak 
disturbances noted on WV riding the ridge back into the epac...and 
timing the systems along with their effects on the baroclinic zone 
expected to swing back and forth for several days with the various 
systems. 


For today...cold air continues to build southward behind exiting wave... 
with temperatures cold enough for occasional flurries in the transition. Clouds 
certainly a big challenge with broad hole now opened up across eastern 
South Dakota...but plenty of upstream stratus across central/eastern ND...and little 
push of middle clouds coming down toward western County Warning Area with weak reinforcing 
wave sliding quickly southeast. NAM seems overdone with pulse of 
lift/snowfall indicated with this system and its uptake in middle level 
frontogenesis...as radars have been fairly quiet...and no reports of 
actual snowfall below skiddish echoes across western ND. Likely will end up more 
with the flurry end of the spectrum with temperatures nicely dendritic...and 
will keep across mainly western 1/2 of County Warning Area where baroclinic zone remains. 
Likely that band of stratus will remain through the far western County Warning Area within this 
thermal gradient...while overall weak subsidence and advection of 
drier air erodes away mass of clouds heading southward. As a result...temperatures 
not likely to have much Luck rebounding today...esply where the better 
fall is just getting going across the S. May very well be as warm at 
forecast issuance time than will have a chance to recover to this afternoon 
after fall last into at least the middle morning hours. 


Tonight...surface ridge completes nosing into The Heart of the County Warning Area. 
Expect that areas from near i29 and ewrd will have potential to have at 
least partial clearing coexist with the diminished gradient... 
and consensus of -14 to -16c airmass at 925 hpa would yield lows 
likely a shade below zero in the more snowcovered north...to single digits 
above S. Temperatures out in the baroclinic zone and cloudier west will be 
greatly restrained by reradiative effects from falling more than 
6-9 degrees from highs...with temperatures aloft starting to take off toward 
morning with developing southeast flow. Did make chance to substantially 
lower probability of precipitation through western County Warning Area back to lower chance. Real problem is there is 
not a bit of indication of any deep support for precipitation within this 
baroclinic zone. Best of isentropic lift remains skirting the far 
western County Warning Area...and is mainly after midnight. 


Concern is that does not take a whole lot of cross frontal flow to 
get precipitation development with such a boundary...as long as the large scale 
remains neutral to lift. Much more shallow support to lift forcing 
overall...so gradually tapered off probability of precipitation in band heading slowly ewrd 
early Thursday. Temperatures will warm within this zone as well...and make the 
whole precipitation process less efficient overall. 


However...by very late day...flow looks to enhance the cross frontal 
flow with frontogenetic forcing looking to increase. Lift looks to 
organize with retreating baroclinic zone Thursday evening from northestern South Dakota to southwestern 
Minnesota. Soundings suggest mostly snow during evening through much of area... 
but warming temperatures aloft will start to poke in the threat for light 
freezing rain toward the back edge of the lift zone...which 
continues to focus more favorably to the northeast through the night. 
Raised probability of precipitation across northestern County Warning Area...even back toward kfsd in the early evening. 
After the better middle level forcing shifts away after midnight...temperatures 
warming aloft and drying out above inversion will likely yield a better chance 
for freezing drizzle development. At this time...appears best threat 
should be along/north of i90 and down toward Iowa Great Lakes. Temperatures likely to 
be rising for many areas on Thursday night. 


Freezing drizzle will remain a concern into Friday morning...mainly 
for locations ahead of the inverted trough which should be through the 
western County Warning Area around daybreak...and kept area going ahead of this feature 
into the Erly afternoon. Temperatures will continue to slowly warm...with temperatures 
getting to around freezing near i90 across SW Minnesota. Main energy with 
troughing really seems to split around the immediate area...north 
along the main baroclinic zone...and an appreciable piece of the 
upper level system digging toward the Central Plains. As the main 
system slides by to the northeast...County Warning Area will get into the classic SW 
subsident quadrant with strong cold air advection and 40-50 kts at the top of the 
developing mixed layer. Therefore...winds will become a significant 
issue. Model timing all over the board...with operational GFS way 
out in the lead of the wind shift and cold punch...but at odds with 
even its own ensemble set. Therefore...went closer to timing of 
Canadian/European model (ecmwf) blend...not quite as slow as NAM. This should push the 
main wind development toward the afternoon hours for a bulk of the area. 
Would be very concerned with some blowing snow in areas which 
received the most snow last weekend from Chamberlain to Huron to 
Marshall...but there will be some threat to top this snow with 
freezing drizzle in the meantime. Deep forcing again very much 
lacking...but there will be a window as temperatures crash for shallow 
frontogenetic forcing behind the front for a chance of light 
snowfall. Will have to monitor for a better wraparound potential to 
affect SW Minnesota areas later afternoon/Erly evening per latest NAM...but for the 
time...moved out precipitation quickly by evening with only a few wind blown 
flurries remaining. 


Colder air again surges southward Friday night and Sat...with forecast hinging 
on a somewhat slower large scale progression to cold air per 
Canadian. Temperatures may rise Sat night as flow turns around to southerly. 
Sunday features some of the better tropopause forcing to come across 
area...but with main baroclinicity and low level wave again to the 
north...will keep low levels likely too dry to get precipitation down to the County Warning Area. 


More very cold air looms near the end of the extended range. Winter 
reigns on. 


&& 


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


&& 


$$ 
Jh 

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