467  
FXUS63 KFSD 220900  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
300 AM CST MON JAN 22 2018  
   
..HEAVY SNOW AND BLIZZARD CONDITIONS EXPECTED TODAY
 
 
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 257 AM CST MON JAN 22 2018  
 
A VERY IMPACTFUL WINTER STORM CONTINUES TO MOVE TOWARDS THE TRI-  
STATE AREA EARLY THIS MORNING AND A BLIZZARD IS EXPECTED! CURRENTLY,  
AN ELONGATED BAND OF SNOW DEVELOPED BY 700:600 FRONTOGENESIS  
CONTINUES TO PRODUCE FAIRLY HEALTHY SNOWFALL RATES OVER THE  
NORTHWESTERN HALF OF THE CWA. SHORT TERM MODELS HAVE REALLY  
STRUGGLED WITH THIS LAYER, BUT RECENT HRRR/RAP RUNS NOW AT LEAST  
SEEM TO HAVE A BETTER GRASP. AS THE UPPER LOW OVER CENTRAL KANSAS  
BEGINS TO PIVOT NORTHEAST GUIDANCE SUGGESTS THIS BAND SHOULD BEGIN  
TO WEAKEN AS PRECIPITATION BEGINS TO FOCUS ALONG STRONGER 850:750  
FRONTAL ZONE OVER NW IOWA.  
 
ALREADY SEEING RELEASE OF ELEVATED INSTABILITY OVER EASTERN NEBRASKA  
AND WESTERN IOWA WITH SEVERAL LIGHTNING STRIKES IN NEWLY FORMED  
CLUSTERS OF CONVECTION. WITH UPPER DIVERGENCE SPREADING OVERHEAD  
BY DAYBREAK, WE SHOULD BEGIN TO SEE A MASSIVE EXPANSION OF  
PRECIPITATION ON THE RADAR AS A VERY INTENSE FRONTAL BOUNDARY  
BEGINS TO DEVELOP. NEGATIVE EPV ABOVE THE INTENSIFYING  
FRONTOGENESIS BAND BETWEEN 12Z AND 16Z SUGGESTS THE POTENTIAL FOR  
AN ISOLATED LIGHTNING STRIKE AND MAY PUSH SNOWFALL RATES UPWARDS  
OF 2-3" PER HOUR OVER NW IOWA. AS THIS INSTABILITY SLIDES EASTWARD  
BY MID-MORNING, A SIGNIFICANT TROWAL WILL DEVELOP WITH A  
CHANNELED HEAVY SNOWBAND WITHIN CONTINUING WELL INTO THE AFTERNOON.  
 
WINDS ARE STEADILY INCREASING TONIGHT AND WILL CONTINUE TO PUSH THE  
40 MPH MARK AS SNOW FALLS THROUGH THE DAY. INTEGRATING HIGH-RES QPF  
INTO THE BLENDED DATA SUGGESTS POTENTIAL FOR A NARROW CORRIDOR OF 12  
TO 18" OF SNOW ACROSS NORTHWEST IOWA. HONESTLY, GIVEN THE FORCING,  
IT WOULDN'T SURPRISE ME TO SEE A VERY NARROW CHANNEL OF SNOW CLOSER  
TO (OR EVEN ABOVE) THE 18" MARK.  
 
COMBINING THIS SNOW WITH ONGOING WINDS WILL MAKE TRAVEL IMPOSSIBLE  
AND EXTREMELY DANGEROUS OVER NW IOWA AND ADJACENT AREAS OF SD/NE/MN  
TODAY. THE WORST CONDITIONS WILL ALIGN THEMSELVES FROM SIOUX CITY TO  
SPENCER.  
 
THE WESTERN AND NORTHERN EDGE OF THIS TROWAL WILL PRODUCE AN  
EXTREMELY SHARP CUTOFF. HAVE AGAIN TRIED TO NARROW DOWN THE SNOW  
AMOUNTS, BUT GRID RESOLUTION AND UNCERTAINTIES ON WHERE THIS EDGE  
DEVELOPS STILL FORCES SOMEWHAT BROAD ACCUMULATIONS. THERE IS ALSO  
UNCERTAINTY IN THE EXTREME SERN PORTIONS OF THE CWA WHERE SOUNDINGS  
SUGGEST THE LOSS OF ICE CRYSTALS BY MID-AFTERNOON DUE TO THE DRY  
SLOT ROTATING INTO THE STORM.  
 
SNOW WILL ONLY VERY SLOWLY END THIS AFTERNOON AND EVENING IN THE  
EASTERN ZONES.  
 
AS FAR AS ADVISORIES/WARNINGS ARE CONCERNED, NOT ANTICIPATING ANY  
CHANGES AT THIS POINT. AS DAYBREAK ARRIVES AND EARLY CANCELLATION  
MAY BE POSSIBLE OVER THE WESTERN ZONES IN THE MISSOURI RIVER  
VALLEY. HOWEVER, WITH WINDS INCREASING, BLOWING SNOW WILL BE AN  
ISSUE IN MANY AREAS.  
 
   
LONG TERM
 
(TUESDAY THROUGH SUNDAY)  
ISSUED AT 257 AM CST MON JAN 22 2018  
 
MINIMAL CHANGES IN THE MEDIUM AND EXTENDED PORTIONS OF THE FORECAST  
DUE TO THE ONGOING STORM. THE LONGWAVE PATTERN CONTINUES TO SHOW  
FAIRLY QUIET WEATHER CONDITIONS, ABSENT OF BELOW NORMAL  
TEMPERATURES. TEMPERATURES WILL BE DRIVEN BY DEEP SNOWCOVER, BUT  
GIVEN ONGOING CHALLENGES HAVE NOT MADE ANY CHANGES.  
 

 
   
AVIATION
 
(FOR THE 06Z TAFS THROUGH 06Z MONDAY NIGHT)  
ISSUED AT 1115 PM CST SUN JAN 21 2018  
 
MANY AVIATION IMPACTS EXPECTED THROUGHOUT THE TAF PERIOD. LIGHT TO  
MODERATE SNOW WILL CONTINUE LIFTING NORTH FROM NEBRASKA INTO THE  
EARLY MORNING HOURS, BECOMING MODERATE TO HEAVY AT TIMES IN FAR  
SOUTHEAST SD INTO NORTHWEST IA, AND PORTIONS OF SOUTHWEST MN.  
RAIN MAY BRIEFLY MIX WITH THE SNOW IN THE EARLY MORNING HOURS, IN  
WEST CENTRAL IOWA BEFORE CHANGING ENTIRELY TO SNOW BY DAYBREAK  
MONDAY. SNOW WILL CONTINUE ON MONDAY, FINALLY TAPERING OFF FROM  
WEST TO EAST IN THE AFTERNOON OR EVENING. NORTHERLY WINDS WILL  
INCREASE TONIGHT, CAUSING FURTHER VISIBILITY REDUCTIONS IN  
SNOWFALL. EXPECT CONDITIONS TO RAPIDLY DETERIORATE IN SNOWFALL,  
WITH IFR AND LIFR CONDITIONS LIKELY FOR MUCH OF THE TAF PERIOD  
ACROSS THE SOUTHEASTERN HALF OF THE REGION. NEAR ZERO VISIBILITY  
IS POSSIBLE AT TIMES AS WELL, ESPECIALLY FOR THE KSUX TAF SITE.  
CONDITIONS WILL IMPROVE BEHIND THE EXITING SNOWFALL.  
 

 
   
FSD WATCHES/WARNINGS/ADVISORIES
 
 
SD...WINTER STORM WARNING UNTIL 6 PM CST THIS EVENING FOR SDZ065-066-  
068.  
 
BLIZZARD WARNING UNTIL MIDNIGHT CST TONIGHT FOR SDZ067-069>071.  
 
WINTER STORM WARNING UNTIL NOON CST TODAY FOR SDZ050-063-064.  
 
WINTER WEATHER ADVISORY UNTIL 6 PM CST THIS EVENING FOR SDZ040-  
054>056-060>062.  
 
WINTER WEATHER ADVISORY UNTIL 6 AM CST EARLY THIS MORNING FOR  
SDZ052-053-057>059.  
 
MN...BLIZZARD WARNING UNTIL MIDNIGHT CST TONIGHT FOR MNZ081-089-090.  
 
WINTER WEATHER ADVISORY UNTIL 6 PM CST THIS EVENING FOR MNZ071-  
072-097.  
 
WINTER STORM WARNING UNTIL MIDNIGHT CST TONIGHT FOR MNZ080-098.  
 
IA...BLIZZARD WARNING UNTIL MIDNIGHT CST TONIGHT FOR IAZ001>003-  
012>014-020>022-031-032.  
 
NE...BLIZZARD WARNING UNTIL MIDNIGHT CST TONIGHT FOR NEZ013-014.  
 

 
 

 
 
SHORT TERM...DUX  
LONG TERM...DUX  
AVIATION...  
 
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