755  
FXUS63 KFSD 291105  
AFDFSD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SIOUX FALLS SD  
605 AM CDT MON MAY 29 2017  
   
SHORT TERM
 
(TODAY AND TONIGHT)  
ISSUED AT 423 AM CDT MON MAY 29 2017  
 
A BREEZY AND COOL MEMORIAL DAY IS ON TAP FOR THE REGION, AS  
NORTHWEST FLOW USHERS IN COLD AIR ADVECTION ACROSS THE NORTHERN  
PLAINS. MAIN UPPER LOW WILL CONTINUE TO WOBBLE NEAR LAKE SUPERIOR,  
HOWEVER A WEAK SHORTWAVE LOBE AND VORT MAX DROPS SOUTHWARD THROUGH  
MINNESOTA TODAY. STRATOCUMULUS CLOUDS WILL BE ON THE INCREASE,  
LIKELY BLANKETING ALL AREAS ALONG AND EAST OF THE JAMES RIVER BY  
MIDDAY. ISOLATED TO SCATTERED SHOWERS WILL BE POSSIBLE LATE MORNING  
THROUGH THE EARLY EVENING WITHIN THIS DECK, WITH THE BEST CHANCES  
ACROSS SOUTHWEST MINNESOTA AND EAST CENTRAL SOUTH DAKOTA. ACTIVITY  
WILL BE VERY HIT-AND-MISS "POPCORN" SHOWERS. MODEL SOUNDINGS SHOW  
SOME PRETTY WEAK ELEVATED INSTABILITY, BUT SHOULD BE DEEP ENOUGH FOR  
SOME ISOLATED THUNDERSTORM MENTION. NORTHWEST WINDS WILL AGAIN BE  
BREEZY AT THE SURFACE AROUND 15 TO 25 MPH WITH GUSTS TO 35 MPH. WITH  
CONTINUED DRY LOW LEVELS, CANNOT RULE OUT SOME EVEN HIGHER GUSTS IN  
SHOWERS OR THUNDERSTORMS, BUT SEVERE WEATHER IS NOT EXPECTED. HIGHS  
TODAY WILL BE IN THE 60S.  
 
SHOWERS WILL DISSIPATE IN THE EARLY EVENING, WITH CLOUDS DECREASING  
OVERNIGHT. BREEZY WINDS WILL LIKEWISE DECREASE BELOW 10 MPH FROM THE  
WEST NORTHWEST. WITH THE COOL AIRMASS, TEMPERATURES WILL DROP TO  
UNSEASONABLY COOL READINGS IN THE MID TO UPPER 40S.  
   
LONG TERM
 
(TUESDAY THROUGH SUNDAY)  
ISSUED AT 423 AM CDT MON MAY 29 2017  
 
INFLUENCE OF DEEP TROUGH TO THE NORTHEAST WILL PERSIST ON TUESDAY  
WITH A REPEAT OF THE EARLY DAY INCREASE IN NORTHWEST WIND TO REACH A  
GUSTY 15 TO 30 MPH, AN INCREASE IN DIURNAL CLOUDINESS ESPECIALLY  
EAST OF I-29, AND TEMPERATURES LIKELY VERY CLOSE TO THOSE FROM TODAY  
FROM LOWER TO MID 60S IN SOUTHWEST MN TO NEAR 70 IN THE MISSOURI  
RIVER VALLEY. ALTHOUGH ANOTHER SUBTLE LOBE WRAPS AROUND THE VORTEX,  
THE MAIN IMPACT WILL REMAIN EAST OF THE CWA DURING THE AFTERNOON,  
BUT PERHAPS NOT FAR ENOUGH AWAY TO PREVENT JUST A FEW SPRINKLES AT  
TIMES ROUGHLY EAST OF A KTKC TO KSPW LINE.  
 
UNDER A SPRAWLING SURFACE RIDGE ON TUESDAY NIGHT, LIGHT WINDS AND  
MAINLY CLEAR SKIES SHOULD YIELD A CHILLY NIGHT FOR THE END OF MAY,  
WITH PERHAPS A FEW LOCATIONS SLIPPING JUST BELOW 40, BUT MAINLY 40  
TO 45 DEGREES WHICH CONTINUES TO BE ALIGNED WITH THE COLDER SUBSET  
OF GUIDANCE. AS THE RIDGE ALOFT BEGINS TO DISPLACE THE UPPER-LEVEL  
CYCLONIC FLOW SLIGHTLY EASTWARD OVER THE NORTHERN PLAINS ON  
WEDNESDAY, THE SURFACE RIDGE WILL SLIP EAST AND BRING A START TO  
WARMING TEMPERATURES. WHILE WINDS WILL BE LIGHTER, THEY MAY GET A  
BIT STRONGER OUT TOWARD LOWER BRULE AREAS DURING THE AFTERNOON. THE  
LOW-LEVEL JET WILL STRENGTHEN ON WEDNESDAY NIGHT AS START TO SEE  
WARM ADVECTION DEVELOP WITH OVERSPREAD OF STEEPER 700-500 HPA LAPSE  
RATES. MAIN LOWER LEVEL BAROCLINICITY REMAINS TIED UP CLOSER TO I-  
80 AND COULD SEE A SMALLER MCS DEVELOP AROUND CENTRAL NE WHICH  
SPREADS EAST/SOUTHEAST OVERNIGHT AND EARLY THURSDAY. QUESTION  
REMAINS HOW MUCH ACTIVITY WILL DEVELOP NORTH OF THIS, WHERE THERE IS  
A BIT MORE DYNAMICAL FORCING ON EDGE OF STRONGER WESTERLIES. WOULD  
EXPECT TO SEE AT LEAST A BROKEN BAND OF ELEVATED SHOWERS/THUNDER TO  
AT LEAST I-90, AND ESPECIALLY A LIKELY CHANCE THROUGH THE LOWER  
MISSOURI RIVER VALLEY BY LATER NIGHT AS LLJ BEGINS TO VEER MORE WITH  
JET SLIDING TO THE SOUTHEAST OF THE AREA.  
 
UPPER-LEVEL RIDGING WILL CONTINUE TO BUILD INTO THE AREA THROUGH  
THURSDAY, AND MODELS SIMILAR IN SUGGESTING THAT THE LOWER LEVEL  
BOUNDARY ALONG WITH INCREASING INSTABILITY WILL BUILD NORTHWARD  
TOWARD THE AREA THROUGH THE DAY ON THURSDAY AND INTO THURSDAY  
EVENING. LARGER SCALE LIFT FORCING A BIT MORE DIFFICULT TO COME BY,  
BUT IT IS LIKELY THAT A CONTINUED CHANCE FOR SHOWERS AND  
THUNDERSTORMS, ESPECIALLY TOWARD NORTHWEST IOWA, WILL BE IN PLAY AS  
FLOW CONVERGES SOME AROUND LINGERING BOUNDARY FROM THURSDAY MORNING  
ACTIVITY. WHILE FLOW IS NOT TERRIBLY STRONG REACHING INTO THE MID  
LEVELS, THERE WILL BE A STRONG DEGREE OF DIRECTIONAL SHEAR FROM  
SOUTHEAST TO NORTHWEST WHICH REMAINS FAIRLY DISTRIBUTED.  
SHEAR/INSTABILITY PARAMETERS WOULD AT LEAST SUGGEST SOME CAUTION AS  
TO AN ISOLATED SEVERE THREAT TOWARD THE EVENING, BUT LIKELY THAT GFS  
IS GREATLY OVERDONE WITH 3-4KJ/KG INSTABILITY.  
 
BY FRIDAY, MODEL SOLUTIONS ARE ONLY AS GOOD AS THEIR ABILITY TO  
PREDICT THE LOCATION OF PRIOR CONVECTION IN TERMS OF ORGANIZING  
BOUNDARY POSITIONS AND POTENTIAL FOR DESTABILIZATION. THERE  
CONTINUES TO BE SOME FAIRLY SIGNIFICANT DIFFERENCES WHICH RESULT  
FROM SUBTLE DIFFERENCES IN THE LARGER SCALE, WITH ECMWF BARRELING  
ENERGY INTO THE RIDGE ON FRIDAY AND FRIDAY NIGHT, DRIVING A WAVE  
INTO THE NORTHWEST FLOW AND RESULTING IN A MORE SOUTHWARD POSITION  
OF BOUNDARY FOCUSING THE CONVECTIVE THREAT THROUGH SATURDAY - FAIRLY  
NEAR THE HEART OF THE FORECAST AREA. GFS IS A BIT SLOWER WITH  
INITIAL ENERGY, AND LESS INCLINED TO PUNCH THROUGH RIDGE, KEEPING A  
MORE NORTHERN SOLUTION TO BOUNDARY LOCATION THROUGH SATURDAY, AND  
PUSHING A STRONGER COLD FRONT ACROSS FROM WEST TO EAST. THE  
DEVIATIONS ARE EVEN GREATER BY SUNDAY WITH SOLUTIONS VARYING FROM A  
STRONG UPPER RIDGE THROUGH THE DAKOTAS IN THE ECMWF TO ANOTHER  
AMPLIFYING WAVE DIVING THROUGH MINNESOTA IN THE GFS. WILL KEEP THE  
THREAT FOR THUNDERSTORMS AROUND FROM LATER FRIDAY THROUGH SATURDAY  
EVENING, BUT KEEPING OPEN THE POSSIBILITY THAT WARMER TEMPS ALOFT  
COULD ATTEMPT TO CAP OFF DEVELOPMENT ACROSS THE AREA UNTIL AT LEAST  
LATER FRIDAY NIGHT WHEN BETTER DYNAMICAL FORCING WILL BEGIN TO  
ENCROACH ON THE AREA. IN TERMS OF TEMPERATURES, THERE IS CERTAINLY  
POTENTIAL FOR TEMPS TO BE EVEN WARMER THAN EXPECTED ON FRIDAY, WITH  
A GRADUAL TRANSITION BACK TO NEAR NORMAL OVER THE WEEKEND.  
 

 
   
AVIATION
 
(FOR THE 12Z TAFS THROUGH 12Z TUESDAY MORNING)  
ISSUED AT 603 AM CDT MON MAY 29 2017  
 
VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. MID LEVEL  
CLOUDS WILL INCREASE FROM THE NORTH MID MORNING INTO THE  
AFTERNOON, WITH SPOTTY SHOWERS AND THUNDERSTORMS POSSIBLE.  
PRECIPITATION THREAT IS TOO LOW FOR MENTION IN THE TAFS. WINDS  
WILL ONCE AGAIN BE BREEZY FROM THE WEST NORTHWEST AROUND 15 TO 20  
KT WITH HIGHER GUSTS POSSIBLE. WINDS, CLOUDS, AND SHOWERS WILL  
QUITE DIMINISH IN THE EARLY EVENING.  
 

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

 
 

 
 
SHORT TERM...  
LONG TERM...CHAPMAN  
AVIATION...  
 
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