613  
FXUS63 KFSD 231720  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
1220 PM CDT SUN APR 23 2017  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 313 AM CDT SUN APR 23 2017  
 
CONCERNS FOR TODAY ARE DEPTH OF MIXING IN INCREASING SOUTH-SOUTHWEST  
FLOW, EFFECTS ON TEMPERATURE/DEW POINT/RH, AND RESULTANT IMPACTS ON  
FIRE WEATHER.  
 
EVEN WITH FAIRLY LIGHT WIND FLOW WITHIN SURFACE RIDGE SATURDAY, WE  
WERE ABLE TO ATTAIN HIGHS IN THE MID-UPPER 60S THROUGHOUT THE AREA.  
FORECAST SOUNDINGS INDICATE POTENTIAL TO MIX ABOVE 800MB, AND LOW  
LEVEL (925-850MB) TEMPERATURES PROGGED TO BE A COUPLE OF DEGREES  
WARMER TODAY. GOING FORECAST ALREADY NEAR THE TOP END OF AVAILABLE  
GUIDANCE FOR HIGHS TODAY, BUT GIVEN RAP TENDENCY TO EXCEL ON WELL-  
MIXED DAYS, HAVE NUDGED HIGHS EVEN A LITTLE HIGHER, ESPECIALLY OVER  
THE WESTERN CWA. DEEP MIXING INTO DRY AIR MASS SHOULD ALSO RESULT IN  
DEW POINTS MIXING LOWER THROUGHOUT THE MID-LATE AFTERNOON, AND THIS  
IDEA SUPPORTED BY LOOKING AT AFTERNOON DEW POINTS IN THE 20S OVER  
MOST OF WESTERN AND CENTRAL NEBRAKSA YESTERDAY AFTERNOON. RAP AND  
ARW SOUNDINGS SUPPORTIVE, AND HAVE LOWERED AFTERNOON DEW POINTS FROM  
PREVIOUS FORECAST AS A RESULT. RAP FORECAST ACTUALLY EVEN LOWER, BUT  
WILL REMAIN SLIGHTLY CONSERVATIVE FOR NOW AND DAY SHIFT CAN MONITOR.  
 
THESE CHANGES TO TEMPERATURE/DEW POINT RESULT IN AFTERNOON MINIMUM  
HUMIDITY LEVELS DROPPING TO 15-25 PERCENT, LOWEST OVER THE SOUTHWEST  
HALF OF THE FORECAST AREA. WITH ANTICIPATED SOUTH-SOUTHWEST WINDS OF  
20-30 MPH, WILL CERTAINLY HAVE TO MONITOR FIRE WEATHER CONDITIONS  
TODAY. HOWEVER, OUR FIRE PARTNERS HAVE INDICATED THAT COOL SEASON  
GRASSES ARE GREENING ENOUGH THAT FIRE STARTS WILL BE LOW, AND THESE  
GRASSES DOMINATE IN THE AREA OF STRONGER WINDS/LOWER HUMIDITY. THE  
WARM SEASON GRASSES WHICH DOMINATE ACROSS THE EAST ARE STILL CURED  
ENOUGH TO CARRY FIRE IF SOMETHING SHOULD START. HOWEVER, WITH WIND  
AND HUMIDITY LEVELS MORE MARGINAL IN THESE AREAS FOR FIRE WEATHER  
HEADLINES, WILL HOLD OFF ISSUANCE FOR NOW AND HIGHLIGHT HIGHER FIRE  
DANGER IN THE HWO.  
 
FOR TONIGHT, BREEZY SOUTH WINDS CONTINUE, ESPECIALLY IN OUR HIGHER  
ELEVATIONS AS SOUTHERLY 925MB WINDS PERSIST AT 30-35KT AND LOW LEVEL  
JET INCREASES TO 40-50KT. THIS WILL RESULT IN A MILD NIGHT WITH LOWS  
IN THE UPPER 40S TO LOWER 50S. MID-HIGH CLOUDS INCREASE AHEAD OF AN  
APPROACHING WAVE, BUT SOUNDINGS REMAIN QUITE DRY BELOW 700MB, SO SEE  
LITTLE IF ANY PRECIPITATION POTENTIAL PRIOR TO 12Z.  
   
LONG TERM
 
(MONDAY THROUGH SATURDAY)  
ISSUED AT 313 AM CDT SUN APR 23 2017  
 
AN UPPER LEVEL SHORTWAVE TRACKS ACROSS THE REGION ON MONDAY, WITH  
THE BETTER UPPER LEVEL DYNAMICS REMAINING TO THE NORTH OF OUR CWA.  
AT THE SURFACE, LOW PRESSURE SLIDES FROM CENTRAL SD IN THE MORNING  
INTO SOUTH CENTRAL MN BY EARLY EVENING, PULLING A FRONTAL BOUNDARY  
ACROSS OUR AREA. WITH THE APPROACH OF THE FRONT, MAY SEE SOME  
SCATTERED SHOWER DEVELOPMENT THROUGH SOUTH CENTRAL SD AND THE  
WESTERN JAMES RIVER VALLEY IN THE MORNING, THEN MAINLY NORTH OF  
INTERSTATE 90 IN THE AFTERNOON, CLOSER TO THE BETTER UPPER LEVEL  
DYNAMICS/SLIGHTLY DEEPER MOISTURE, AS THE BOUNDARY CONTINUES TO  
SLIDE TO THE EAST. CANNOT RULE OUT A COUPLE RUMBLES OF THUNDER IN  
THE AFTERNOON IF ANYTHING DEVELOPS, WITH WEAK INSTABILITY ACROSS THE  
AREA. THERE WILL BE STRONG SOUTHERLY WINDS OUT AHEAD OF THE FRONT,  
AND WITH GOOD MIXING STAYED ON THE HIGH SIDE OF GUIDANCE WITH  
RESPECT TO TEMPERATURES, WITH LOWER TO MID 70S OVER THE MID AND  
LOWER MO RIVER VALLEY/NORTHWESTERN IA TO MID 60S BACK TO THE NORTH.  
 
THERE WILL BE DECENT COLD AIR ADVECTION BEHIND THE FRONT, AND MONDAY  
NIGHT IS LOOKING COOLER WITH LOWS MID 30S NORTH AND WEST TO MID 40S  
THROUGH OUR LOWER IA ZONES. LATE ON MONDAY NIGHT ANOTHER SHORTWAVE  
BEGINS TO LIFT THROUGH EASTERN NEBRASKA WITH UPPER LEVEL SUPPORT  
PROVIDED BY AN UPPER LEVEL JET STREAMING NORTHEASTWARD OUT OF  
COLORADO. WITH THAT, MAY SEE SHOWERS DEVELOP OVER SOUTH CENTRAL SD  
TOWARD TUESDAY MORNING, AND CANNOT RULE OUT SOME SNOW MIXING IN FROM  
CHAMBERLAIN TO HURON IN THE MORNING ALONG THE NORTHERN EXTENT OF THE  
PRECIPITATION SHIELD WHERE TEMPERATURES ALOFT WOULD SUPPORT SNOW.  
 
MODELS IN GOOD AGREEMENT ON LIKELY RAIN CHANCES ON TUESDAY AS THE  
SHORTWAVE TRACKS THROUGH THE REGION AND MIDLEVEL FRONTOGENESIS SETS  
UP ACROSS THE AREA. 850 MB TEMPERATURES CONTINUE TO COOL THROUGH THE  
DAY, AND WITH CLOUDS AND SHOWERS WENT ON THE COOL SIDE OF GUIDANCE  
WITH HIGHS MID TO UPPER 40S, WITH A FEW LOWER 50S IN OUR FAR  
SOUTHEAST.  
 
THE THEME FOR THE REMAINDER OF THE WEEK LOOKS TO BE VERY COOL  
TEMPERATURES FOR THIS TIME OF YEAR WITH HIGHS 40S/50S AND LOWS IN  
THE 30S, WITH POSSIBLE FREEZING TEMPERATURES OVER PARTS OF THE AREA  
ON TUESDAY AND WEDNESDAY NIGHT. MODEL AGREEMENT BECOMES MORE CHAOTIC  
TOWARD THE END OF THE WEEK, SO CONFIDENCE IN PRECIPITATION CHANCES  
IS LOW FOR THAT TIME PERIOD, THOUGH THE HIGHER PROBABILITY OF RAIN  
CHANCES WOULD EXIST SOME TIME IN THE SATURDAY TIME FRAME.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)  
ISSUED AT 1219 PM CDT SUN APR 23 2017  
 
VFR WILL PREVAIL THROUGH THE TAF PERIOD. HOWEVER SOUTH TO  
SOUTHWEST WINDS WILL BE GUSTY, GUSTING UP TO AND AROUND 25 KNOTS.  
IN ADDITION, SOME NON CONVECTIVE LOW LEVEL WIND SHEAR WILL PREVAIL  
TONIGHT.  
 

 
   
FSD WATCHES/WARNINGS/ADVISORIES
 
 
SD...NONE.  
MN...NONE.  
IA...NONE.  
NE...NONE.  

 
 

 
 
SHORT TERM...JH  
LONG TERM...JM  
AVIATION...MJ  
 
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