469  
FXUS63 KFSD 241143  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
543 AM CST SAT FEB 24 2018  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 403 AM CST SAT FEB 24 2018  
 
CURRENT UPPER AIR ANALYSIS SHOWS THE STRONG UPPER WAVE MOVING ACROSS  
THE ROCKY MOUNTAINS AT THIS TIME. AT THE SURFACE, A LARGE AREA OF  
HIGH PRESSURE BASED OVER THE GREAT LAKES REGION WILL MOVE EASTWARD  
AND OUT OF THE AREA THROUGH THE DAY. THE UPPER WAVE REMAINS OPEN  
THROUGH THE DAY, AND MOVES TOWARD THE I 29 CORRIDOR BY 00Z, LIFTING  
OUT IN A NEGATIVE TILT EAST OF OUR AREA TONIGHT INTO THE MS VALLEY.  
THE PV IS STILL FAIRLY STRONG ALBEIT NOT QUITE AS STRONG AS PREVIOUS  
RUNS, WITH THE 1.5 PVU SURFACE LOWERING TO ABOUT 550MB SOUTH OF THE  
FORECAST AREA FROM SOUTH CENTRAL NE TO WESTERN IA.  
 
INITIALLY THIS MORNING, A PRETTY STRONG AREA OF LOW TO MID LEVEL  
THETAE ADVECTION LIFTS NORTHWARD THROUGH THE AREA, COUPLED WITH THE  
CWA PLACED IN A RIGHT ENTRANCE REGION OF A JET STREAK IN CENTRAL MN  
AND GREAT LAKES REGION. THESE FEATURES WILL BEGIN THE ENHANCEMENT OF  
LIFT AND SATURATION BEFORE THE SHORT WAVE ARRIVES IN THE MIDDAY AND  
AFTERNOON HOURS. THE SURFACE LOW IS FORECAST TO BE NOT REAL STRONG  
IN THE CENTRAL PLAINS. HOWEVER WHEN THE SHORT WAVE ARRIVES, WE STILL  
HAVE VERY STRONG MID LEVEL FRONTOGENESIS PLACED UNDER INSTABILITY  
ALOFT, WITH VERY UNSTABLE EPV* FROM 700 TO 600MB IN A SATURATED  
LAYER. THE ONE EXCEPTION TO THIS IS IN FAR SOUTHEAST ZONES AROUND  
MIDDAY, WHERE MID LEVEL SATURATION IS A BIT WEAK, NEAR THE STORM  
LAKE VICINITY. THEREFORE WOULD NOT RULE OUT A BIT OF MIDDAY FREEZING  
RAIN IN THAT AREA AS ICE CRYSTAL PRODUCTION COULD BE A LITTLE  
LIMITED. BUT OVERALL, THE VARIOUS MODELS WERE TOO LOW WITH THEIR ICE  
CRYSTAL PRODUCTION SO RAISED THEM TO GET MORE MENTION OF SNOW IN THE  
FORECAST IN OUR FAR EASTERN ZONES. ONE OTHER INTERESTING ITEM  
BESIDES THE STRONG MID LEVEL FRONTOGENESIS, IS OUR EASTERN TWO TIERS  
OF COUNTIES IS SHOWING ENOUGH INSTABILITY ALOFT TO PRODUCE CAPE  
VALUES OF AROUND 70 J/KG IN THE 750-700MB LAYER. THE SOUNDINGS DO  
NOT LOOK QUITE UNSTABLE ENOUGH TO ADD A MENTION OF THUNDER, BUT IT  
COULD BE ENOUGH TO START A CONVECTIVE PROCESS WITH THE SNOWFALL  
ADDING TO INTENSITY OF RATES THIS AFTERNOON. THEREFORE DID NOT TAKE  
THE SECTION OF SOUTHWEST MN OUT OF THE WINTER STORM WARNING DUE TO A  
POTENTIAL TO STILL GET 6 OR MORE INCHES OF SNOW IN THAT LOCATION IN  
A SHORT AMOUNT OF TIME. A LITTLE FURTHER SOUTH OF THERE IN NORTHWEST  
IA, SLIGHTLY DRIER AIR IN THE MID LEVELS SHOULD KEEP THEIR  
ACCUMULATIONS IN ADVISORY LEVEL. INITIALLY THROUGH MID MORNING, A  
FETCH OF DRIER AIR ALOFT WILL FEED INTO SOME OF OUR EASTERN ZONES  
WHICH SHOULD LIMIT SNOWFALL FOR A WHILE. HOWEVER THE VARIOUS CAMS  
THEN BEGIN TO PRODUCE PRECIPITATION RAPIDLY AS SATURATION ALOFT  
STRENGTHENS BY LATE MORNING. BUT DO NOT BE SURPRISED IF THE RADAR  
ECHOES ARE PRETTY LIGHT IN PARTS OF OUR EAST THIS MORNING. SO  
OVERALL, WE ARE LOOKING AT A WIDESPREAD 3 TO 4 INCH SNOW EVENT,  
PERHAPS A BIT LESS IN SOME MO RIVER VALLEY COUNTIES, AND THE TIMING  
STILL LOOKS GOOD ON THE HEADLINE. THE CAMS ARE NOT REALLY PICKING UP  
ON ANY PARTICULAR HEAVY BANDING ANYWHERE, EXCEPT TOWARD THE AREA  
WHERE WE ALREADY HAVE THE WINTER STORM WARNING WHERE A FEW OF THEM  
ARE SHOWING SOME HEAVIER QPF LIKELY DUE TO CONVECTIVE PROCESSES  
MENTIONED ABOVE.  
 
LASTLY, EXTENDED THE DENSE FOG ADVISORY TO 15Z. THE DENSE FOG WILL  
PROBABLY GET A BIT LESS WIDESPREAD, BUT IT MAY NOT TOTALLY SUBSIDE  
UNTIL THE PRECIPITATION BEGINS TO MOVE INTO THE AREA SCOURING IT  
OUT.  
 
THE WAVE EXITS RAPIDLY OVERNIGHT TONIGHT. THE QG FORCING MOVES RIGHT  
OVER OUR FORECAST AREA BUT BECOMES WELL EAST BY 09Z SUNDAY.  
THEREFORE THE PRECIP WILL SHUTOFF PRETTY FAST IN OUR EASTERN ZONES  
AFTER MIDNIGHT TONIGHT. AS THE SURFACE LOW STRENGTHENS TO OUR EAST,  
WIND SPEEDS WILL BECOME RATHER BREEZY EAST OF I 29. CANNOT RULE OUT  
SOME PATCHY BLOWING SNOW ESPECIALLY IN SOUTHWEST MN AFTER MIDNIGHT  
BUT TO KEEP THINGS SIMPLE WITH EVERYTHING GOING ON RIGHT NOW, DID  
NOT INCLUDE THAT IN THE FORECAST YET.  
   
LONG TERM
 
(SUNDAY THROUGH FRIDAY)  
ISSUED AT 403 AM CST SAT FEB 24 2018  
 
DID NOT SPEND A LONG TIME PERUSING OVER THE LONGER RANGE PORTION  
OF THE FORECAST. IN GENERAL THOUGH, TEMPERATURES LOOK NEAR TO A  
BIT BELOW NORMAL. THERE IS A LOT OF MODEL SPREAD WITH THE MID WEEK  
SYSTEM NEXT WEEK. THE GFS AND GEM GLOBAL STILL SHOW A SOUTHERN  
STREAM WAVE IMPACTING OUR FORECAST AREA WITH PRIMARILY SNOWFALL.  
HOWEVER THE ECMWF CUTS OFF THIS AREA WITH THE NORTHERN STREAM. BUT  
CERTAINLY LOW END POPS AS A HEADS UP ARE WARRANTED AT THIS TIME  
IN THE WEDNESDAY AND THURSDAY TIME FRAME.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z SUNDAY MORNING)  
ISSUED AT 542 AM CST SAT FEB 24 2018  
 
STRATUS IS CAUSING IFR TO LIFR CEILINGS ACROSS THE AREA EARLY THIS  
MORNING. THIS WILL BE REPLACED BY A STORM SYSTEM CAUSING SNOWFALL  
TODAY, WHICH WILL LIKELY CAUSE IFR TO LIFR VISIBILITIES AS THE  
SNOW PASSES. THE STORM WILL LIFT EASTWARD THIS EVENING ENDING THE  
SNOW THREAT, AND EVENTUALLY THE STRATUS WILL SCOUR OUT CAUSING VFR  
CONDITIONS BY OVERNIGHT TONIGHT.  
 

 
   
FSD WATCHES/WARNINGS/ADVISORIES
 
 
SD...WINTER WEATHER ADVISORY UNTIL 9 PM CST THIS EVENING FOR SDZ070-  
071.  
 
DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR SDZ038>040-  
053>056-059>062-065>071.  
 
WINTER WEATHER ADVISORY UNTIL 7 PM CST THIS EVENING FOR SDZ038-  
050-052-053-057>060-063>065-068-069.  
 
WINTER WEATHER ADVISORY FROM 9 AM THIS MORNING TO 9 PM CST THIS  
EVENING FOR SDZ039-040-054>056-061-062-066-067.  
 
MN...DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR MNZ071-072-  
080-081-089-090-097-098.  
 
WINTER WEATHER ADVISORY FROM 9 AM THIS MORNING TO 9 PM CST THIS  
EVENING FOR MNZ071-097-098.  
 
WINTER STORM WARNING FROM NOON TODAY TO MIDNIGHT CST TONIGHT FOR  
MNZ072-080-081-089-090.  
 
IA...WINTER WEATHER ADVISORY UNTIL 9 PM CST THIS EVENING FOR IAZ020-  
021-031-032.  
 
DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR IAZ001>003-  
012>014-020>022-031-032.  
 
WINTER WEATHER ADVISORY FROM 9 AM THIS MORNING TO 9 PM CST THIS  
EVENING FOR IAZ001>003-012>014-022.  
 
NE...WINTER WEATHER ADVISORY UNTIL 9 PM CST THIS EVENING FOR NEZ013-  
014.  
 
DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR NEZ013-014.  
 

 
 

 
 
SHORT TERM...MJ  
LONG TERM...MJ  
AVIATION...MJ  
 
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