038 
fxus63 kfsd 030256 
afdfsd 


Area forecast discussion for southeast South Dakota/southwest Minnesota/northwest Iowa 
National Weather Service Sioux Falls South Dakota 
857 PM CST Tuesday Dec 2 2008 


Discussion... 
decided to expand our Winter Weather Advisory to cover our northern two tiers of 
counties based on latest radar trends...and also the latest nam40 
and ruc40 analyses. At this time...area 88d radars showing a decent band of 
snow from east central South Dakota...to southwestern South Dakota. Airmass is saturating rapidly from 
the top downward. Initially...level dew point temperatures are pretty dry 
providing a pretty large surface dew pt depression. With rapid wet 
bulbing downward due to saturation...decided to eliminate most...if 
not all our mention of -ra ahead of the snow. Latest nam40 really 
GOES to town with its quantitative precipitation forecast output giving a 12 hour total amount of near 
a half inch from SW Minnesota...back toward the Mitchell South Dakota area. Looked at 
the nam40 dynamics...and the low to middle level frontogenesis is 
strong...maximized in the 700 mb-h6 layer. However it is progressive not 
suggestive of a lingering event which would give these kinds of 
amounts. There is some weakly stable air after midnight in this maximum 
quantitative precipitation forecast area. However by that time...the strongest frontogenesis has 
shifted into northwest Iowa where a bit more drier airmass is evident...thus 
the lighter quantitative precipitation forecast amounts. So...will play the progressive solution 
which is closer to the RUC output of holding some amounts down... 
and keeping the heaviest amounts north of I 90. 


&& 


Aviation... 
falling snow will create visibilities as low as 1/2 statue mile at 
times and ceilings in the 500 to 1000 feet range. The bulk of the snow 
will fall along and north of Interstate 29 tonight...with little if 
any snow into sux. P6sm by sunrise tomorrow as snow shifts east of 
the area...however IFR to MVFR ceilings are likely to stick around 
through Wednesday afternoon. Northwest surface winds of 15 to 25 kts through 
the taf period at all local taf sites. 


&& 


Previous discussion... 
immediate focus on quick moving snow event for tonight. Decent upper 
wave crossing northern rockies this afternoon...will move into the 
northern plains tonight and early Wednesday. NAM/GFS agree in moving 
strong middle level front southeast across the County Warning Area in advance of the 
wave overnight...and could see 3-4 hour period of heavier snowfall 
rates in many areas as band of maximum lift along the frontal zone 
interacts with favorable snow growth/dendritic temperature range. 
Still initially dealing with warm and dry airmass...but strong lift 
with approaching wave should quickly overcome the dry air and help 
cool the column to switch precipitation over to snow...especially in area 
of greatest lift. 


Both models similar in placement of best dynamic support for heavier 
snow band...beginning in our northwest counties early this evening. 
Band then quickly shifts southeast and expands east...from south 
central South Dakota into kfsd-kpqn during the middle-late evening hours...then 
continues southeast into far southeast South Dakota/northwest Iowa after 06z. 
Generally looking at widespread 1-3 inches over most of the County Warning Area. 
However...areas where deep and strong lift intersects dendritic zone 
will likely see a period of 1 inch/hour snowfall rates. While these 
rates will be relatively short-lived...will still allow snow to pile 
up pretty quickly in a short amount of time. Most likely area for 
prolonged period...3-4 hours...of these higher snowfall rates looks 
to be in a fairly narrow corridor along and about a County or so 
south of I-90...and have issued a Winter Weather Advisory for 2-4 
inches from our south central counties...east through kfsd and into 
Jackson and Dickinson counties. Lift within the dendritic temperature range 
decreases as the band shifts farther southeast through northwest 
Iowa after 09z...so snowfall rates southeast of a korc-kspw line 
should diminish and keep snowfall amounts below advisory criteria in 
that area. 


Any lingering snowfall within the band will be very light and on the 
way out by daybreak...and kept a low chance probability of precipitation in the far southeastern County Warning Area. 
Subsidence on cold side of frontal band will end precipitation threat for a 
short time...perhaps breaking lower clouds for a bit...and keeping 
northwest to north winds up into the afternoon. Gradually...a widespread SC deck 
will be the main feature on Wednesday. With temperatures in the fairly thin 
cloud layer residing in the warmer end of favorable dendritic 
range...expect there will be flurries. Not looking for much more 
despite the area of strong divergence q sliding by during morning...as 
there is also a very strong stable layer which will essentially 
disconnect upper forcing until some middle level cooling can occur. 
Temperatures likely to go a whole lot of nowhere with anticipated new 
snowcover. 


The middle level cooling will come in on Wednesday night and early Thursday...as 
strong trailing impulse slides east-southeast across area. Low level frontal 
forcing is much more communicative with upper wave...and a deeper 
saturated layer within dendritic zone corresponding with maximum lift 
should easily produce some light snowfall. The forcing moves into 
the middle James Valley toward late evening...and slides into southwestern 
Minnesota/northwest Iowa late night. Accumulations should be an inch or less...mainly 
north of a khon to kfsd to kspw line. 


Cyclonic cold core on Thursday...and after initial concentration of lift 
leaves eastern County Warning Area early...should quickly end up with SC streets featuring 
short scattered snow showers through the day. Again trended maximum temperatures 
downward in consideration of snow cover and second pulse of 
cold air advection...fairly close in line with shallow mixing. 


Took a chance and drastically undercut guidance on Thursday night as surface 
ridge sinks into the Central Plains...allowing a classic decoupling SW 
wind to set up during the night. Should keep winds up somewhat across 
southwestern Minnesota...and likely to find coldest spots in lower elevations. Warm air advection 
starts into dry atmosphere on Friday...but snowcover and increasing high 
clouds will probably not help the inversion work out very quickly...so 
again sided toward cold side of guidance. 


In the extended range /Sat through Tuesday/ pattern looks to become more 
active but still northern stream dominant. Several slns are trending 
toward a slower/more diggy/slightly phased sln by later Monday into Tuesday 
which really slows up the precipitation threat and allows for a potentially 
more significant system by later Monday into Tuesday. These slns for the time 
appear on the fringe...and prefer as per HPC to remain closer to 00z 
GFS...realizing that this run is on the fast side of the ensemble 
envelope. Cold push early Sat prognosticated in a variety of strengths... 
and kept things toward the colder side based on long wave position. 
Next backdoor front will settle southward through the day. Low level ridge should 
dry things out enough to limit warm air advection precipitation threat later Sat night...with 
warm air advection zone developing Sunday into Sunday night. Hard to ignore a 00z 
GFS ensemble 70 to 90 percent of members producing 0.05 inch quantitative precipitation forecast in a 
12h period from 06z-18z Monday...and other members of the superensemble 
suggestive of decent lift forcing. Given the uncertainty in 
timing/evolution as well as the unusually low operational probability of precipitation given 
the various dynamic support...will settle for an extended period of 
chance range probability of precipitation from Sunday through Monday...holding onto the probability of precipitation for 
the last day or so mainly for the southeast half to cover potential 
slowdown or phasing threat. 


&& 


Fsd watches/warnings/advisories... 
Iowa...Winter Weather Advisory until 6 am CST Wednesday for iaz001>003. 
Minnesota...Winter Weather Advisory until 6 am CST Wednesday for mnz071-072- 
080-081-089-090-097-098. 
South Dakota...Winter Weather Advisory until 6 am CST Wednesday for sdz038>040- 
050-052>067. 
NE...none. 


&& 


$$ 


Liebl/mjf 

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