000 
FXUS63 KFSD 131719
AFDFSD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1119 AM CST TUE JAN 13 2009

.DISCUSSION...
VERY COLD AIR ENTRENCHED OVER THE ERN CWA THIS MORNING STILL 
FIGHTING NEARLY 10 BELOW WITH LIGHT WINDS IN THE E. HOWEVER...CHANGES
UNDERWAY WITH GOOD SNOW BAND DEVELOPING FM CNTRL ND TOWARD THE MID 
JAMES VALLEY ATTM. GOOD CONSENSUS ON DYNAMIC AND THERMODYNAMIC 
STRUCTURE FOR SYSTEM...BUT NOT SO GOOD ON MODEL QPF. STRONG FRONTAL 
BAND GETS FRONTOGENETIC BOOST LATER THIS AFTN AS COMPACT AREA OF DIV 
Q WITH WAVE DIVING SWRD TOWARD MN...WHICH SHOUL

LOTS OF 1/2SM VSBYS WITHIN BAND ACRS ND...AND THIS WITHOUT A GREAT 
DEAL OF WIND. ANALYSIS OF CROSS SECTIONS THRU ISENTROPIC LIFT ZONE
LTR AFTN INTO THE EVENING SUGGEST THAT AREA N/NE OF KFSD WL HAVE
FAVORABLE DEEP DENDRITIC ZONE...AS WELL AS THE BEST PV/FRONTAL 
LINKUP. NICE FRONTOLYTIC OVER FRONTOGENETIC SIGNATURE AND NEAR ZERO 
TO SLIGHTLY NEGATIVE EPV ALSO FAVORS SOME MORE INTENSE BANDING.
ALL THINGS CONSIDERED...AND ENHANCING THE SNOW WATER RATIO GRADIENT 
FROM 12/1 SW TO 18/1 NE...ENDED UP TIPPING 4 INCHES LOCALLY OVER 
TERRAIN OF SW MN. HOISTED A LITTLE WINTER WX ADVY TO COVER THE 3-4 
INCH AREA...BUT FOR NOW DO NOT EXPECT BLOWING SNOW TO BE A BIG ISSUE 
WITH STRONGER WIND SIGNATURES INTO THE NIGHT EXPECTED TO BE TOWARD 
THE WRN/SRN CWA WHERE MUCH LESS SNOW EXPECTED. THEREFORE...WL CUT 
OFF ADVY AROUND MIDNIGHT AFTER WHICH THE DEEPER FORCING LEAVE BNDRY 
BEHIND...AND WL THEN BE ABLE TO FOCUS CLEANLY ON INEVITABLE WIND 
CHILL HIGHLIGHTS WHEN AFTN PACKAGE IS ISSUED.

OVERALL...TRENDS IN POPS/WX IN FORMER GRIDS EXCELLENT IN COMPARISON 
TO EXPECTED DYNAMICS/TIMING WITH BAND DEVELOPMENT...AND MOST CHANGES 
ON THE MINOR SIDE FOR THE SHORT TERM.

&&

.AVIATION...
A BAND OF SNOW WL CONTINUE TO DEVELOP FROM NCNTRL SD THRU ERN SD
THIS AFTERNOON...SPREADING SLOWLY EWRD THIS EVENING. OUTSIDE THE
SNOW BAND...EXPECT VFR CONDITIONS TO BE THE RULE...BUT AS THE SNOW
BEGINS TO FALL...MVFR CEILINGS WILL BE LIKELY WITH IFR CEILINGS
POSSIBLE. HOWEVER...THE MOST LIKELY IFR CIGS/VSBYS WL BE IN THE
BETTER SNOWBAND EXPECTED TO REMAIN MOSTLY NE OF THE KHON/KFSD/KSUX
SITES. OTHERWISE...STRONG NORTH WINDS WILL DEVELOP LATE 
TONIGHT AND CONTINUE INTO WEDNESDAY. 

&&

.PREV DISCUSSION...
CURRENTLY...ARCTIC AIRMASS ACROSS THE AREA RESULTING IN SUBZERO 
READINGS ACROSS THE AREA. WITH WINDS HANGING ON FOR THE NEXT FEW 
HOURS ACROSS FAR EASTERN CWA...WILL EXTEND WIND ADVISORY THROUGH 12Z 
FOR BASICALLY EASTERN TWO TIERS OF COUNTIES. 

SECOND CLIPPER SYSTEM DEVELOPING OVER EASTERN ALBERTA THIS MORNING.  
THIS SYSTEM WILL IMPACT DIURNAL TEMPERATURES OVER THE NEXT 36 HOURS 
OR SO UNTIL THE SECOND SURGE OF ARCTIC AIR MOVES INTO THE NORTHERN 
PLAINS ON THE BACKSIDE OF THE CLIPPER. 

FAIRLY GOOD BAND OF WARM AIR ADVECTION DEVELOPING NEAR HETTINGER ND 
TO MOBRIDGE SD GENERATING LIGHT SNOW. THIS BAND IS EXPECTED TO 
INTENSIFY AS IT DROPS TO THE SOUTHEAST. EXPECT SNOW WITH THIS 
FEATURE TO DEVELOP NEAR KHON AND INCREASE IN INTENSITY AS IT SPREADS 
TO THE EAST. NOT A LOT IN THE WAY OF UPPER LEVEL SUPPORT UNTIL LATE 
IN THE DAY AS UPPER LEVEL WAVE DROPS DOWN IN NORTHWEST FLOW. 
THEREFORE...HAVE MAINLY 1-2 INCHES OF SNOW WITH ISENTROPIC LIFT WEST 
OF I-29 AND 2-3 INCHES OF SNOW EAST OF I-29 AS DYNAMICS INCREASE. 
ENTERTAINED THE IDEA OF ISSUING A WINTER WEATHER ADVISORY FOR 
PORTIONS OF SOUTHWEST MINNESOTA...BUT GIVEN WINDS DO NOT INCREASE 
SIGNIFICANTLY UNTIL SNOW WINDS DOWN...OPTED TO FORGO HEADLINES FOR 
NOW. 

SHOULD SEE TEMPERATURES REMAIN FAIRLY STEADY THIS EVENING PRIOR TO 
ARCTIC FRONT SLICING THROUGH THE AREA. TEMPS WILL CONTINUE TO FALL 
THROUGHOUT THE DAY ON WEDNESDAY AS STRONG NORTHWEST WINDS DRAW THE 
SECOND SHORT OF ARCTIC AIR SOUTHWARD.  REDUCED POPS ON WEDNESDAY 
MORNING WITH FRONT MOVING THROUGH SLIGHTLY FASTER THAN PREVIOUSLY 
ANTICIPATED.  COULD SEE A FEW ICE CRYSTALS FLYING AROUND IN BRISK 
NORTH WINDS...BUT WILL LIKELY BE WILL DIFFICULT TO RESULT IN 
ACCUMULATION DUE TO DRY NATURE OF AIRMASS. WITH BRISK NORTH 
WIND...WILL NEED WIND CHILL ADVISORY FOR AT LEAST NORTHERN HALF OF 
FORECAST AREA DURING THE DAY ON WEDNESDAY. 

AS WINDS FALL OFF WEDNESDAY NIGHT TEMPERATURES WILL FALL TO COOLEST 
LEVELS OF THE YEAR SO FAR.  LOWERED FORECAST LOWS WEDNESDAY NIGHT 
ESPECIALLY IN JAMES RIVER VALLEY WHERE RIDGE AXIS WILL RESULT IN 
NEARLY CALM CONDITIONS.  

DOME OF ARCTIC AIR BEGINS TO SHIFT TO THE THE EAST ON FRIDAY 
RESULTING IN A WARMING TREND HEADED INTO THE WEEKEND.  MODELS 
INDICATING AN UPPER LEVEL WAVE DROPPING DOWN IN NORTHWEST FLOW 
FRIDAY NIGHT INTO SATURDAY MORNING. GFS REMAINS MUCH FURTHER WEST 
THAN ECMWF AND CANADIAN WITH THIS SYSTEM...SO FOR NOW...LEFT SLIGHT 
CHANCE POPS CONFINED TO NORTHEAST CORNER OF FORECAST AREA.    BT

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
MN...WINTER WX ADVY 3 PM TO MIDNIGHT CST FOR MNZ071-072-080-081-090.
NE...NONE.
SD...NONE.
&&

$$

CHAPMAN