Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 301142
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
642 AM CDT TUE SEP 30 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 347 AM CDT TUE SEP 30 2014

STILL LOOKING AT A VERY STRONG SYSTEM MOVING NORTH TO NORTHEAST
ACROSS THE NORTHERN PLAINS TODAY AND THIS EVENING. SYNOPTICALLY...
WATER VAPOR IMAGERY SHOWS THE MID AND UPPER LEVEL LOW CURRENTLY
SPINNING OVER THE NEB PANHANDLE. THE LOW WILL THEN TRACK ACROSS WEST
CENTRAL SD TODAY...BEFORE ENTERING CENTRAL ND THIS EVENING. THE
SYSTEM IS STACKED SO THE SURFACE LOW IS FOLLOWING THE PLACEMENT OF
THE UPPER LOW IN CLOSE PROXIMITY...WITH A WEAK WARM FRONT JUTTING TO
THE SOUTHEAST INTO SOUTH CENTRAL SD.

THE DRY SLOT IS CURRENTLY VISIBLE IN FAR WESTERN KS AND SOUTHWEST
NEB...AND IT WILL MOVE NORTHWARD IMPACTING OUR WESTERN ZONES AS THE
DAY PROGRESSES. AHEAD OF IT...EXPECTING A BAND OF PRECIPITATION
ELONGATED FROM NORTH TO SOUTH TO MOVE ACROSS THE FORECAST AREA
ALONG THE FIRST SHORT WAVE. THE MAIN CONCERN IS THE SEVERE WEATHER
THREAT. THE AREA TO FOCUS ON IS WILL BE JUST AHEAD OF THE DRY
SLOT...AND THE DAY 1 SPC OUTLOOK LOOKS LIKE IT HAS IT NAILED
ENCOMPASSING THE JAMES AND VERMILLION RIVER VALLEYS... WHICH IS ALSO
WHAT WE WERE THINKING YESTERDAY. THE DIFFICULT PART OF THE FORECAST
IS EXACT TIMING WHEN A FEW SEVERE STORMS MAY OCCUR. NOT AT ALL
CONVINCED THAT THE INITIAL BAND OF RAIN WILL PRODUCE SEVERE WEATHER.
THIS IS BECAUSE THE CONVECTION WILL LIKELY BE ELEVATED...NOT TAPPING
INTO THE STRONG 0-1KM SHEAR. OVERALL...CAPES THROUGH MUCH OF THE DAY
ARE ALSO THIN. BUT JUST AHEAD OF THE DRY SLOT...WHEN THE UPPER LOW
IS IN WEST CENTRAL SD...A SECOND SHORT WAVE LIFTS NORTHEASTWARD THIS
AFTERNOON ACROSS THE WESTERN PORTIONS OF OUR FORECAST AREA. THIS IS
THE WAVE TO KEEP AN EYE ON FOR SEVERE WEATHER CHANCES...AS BY 21Z
WITH THE DRY SLOT IN PLACE IN SOUTH CENTRAL SD...THE SLIGHT RISK
AREA COULD TAP INTO THE 0-1KM WIND SHEAR WHICH IS EXTREMELY STRONG.
IN FACT 0-1KM HELICITY VALUES ARE OVER 250 IN THOSE AREAS...
JUXTAPOSED WITH SOME MODERATE INSTABILITY OF ELEVATED MU CAPE IN THE
925-850MB LAYER OF 1000 TO 1500 J/KG...WHICH FINALLY NOSES
NORTHWARD. IF LOW TOPPED SUPERCELLS CAN DEVELOP...AN ISOLATED
TORNADO CANNOT BE RULED OUT AROUND THE JAMES VALLEY AREA FOR A BRIEF
TIME IN THE MID AND LATE AFTERNOON. THE OTHER SEVERE WEATHER THREAT
FOR THE VERMILLION RIVER VALLEY WOULD LIKELY BE WINDS AROUND 60 TO
65 MPH AND HAIL UP TO QUARTERS. EVEN AT JUST 925MB...WIND SPEEDS ARE
AVERAGING 30 KNOTS TODAY...COUPLED WITH A JET STREAK OF 65 TO 75
KNOTS NUDGING UP INTO OUR WESTERN ZONES BY MIDDAY AND EARLY THIS
AFTERNOON. SO IT WOULD NOT TAKE A LOT TO PUSH SEVERE WIND DOWN TO
THE SURFACE JUST AHEAD OF THE DRY SLOT.

AND SPEAKING OF STOUT WINDS...IT WILL BE CLOSE TO WIND ADVISORY
CRITERIA IN OUR FAR WESTERN ZONES TODAY NEAR THE DRY SLOT. THE ONLY
LIMITING FACTOR APPEARS TO BE MIXING POTENTIAL. BUT IF THAT AREA
MIXES HIGHER THAN EXPECTED...WIND ADVISORY TYPE WINDS COULD BE
OBTAINED IN AND CLOSE TO THE LOWER BRULE AREA. HIGHS TODAY ARE
PROBLEMATIC...TOTALLY DEPENDENT ON DEGREE OF LATE DAY CLOUD COVER.
LOOKS LIKE THE NORTHEAST QUAD OF OUR FORECAST AREA MAY STAY IN THE
60S WHERE CLOUDS SHOULD LINGER THE LONGEST. MID 70S ARE LIKELY IN
THE DRY SLOT. FOR TONIGHT...WINDS WILL SLACKEN OFF FAST WITH LOWS
STILL PRETTY MILD FOR THIS TIME OF YEAR. THERE REALLY NEVER IS A
WIND SHIFT FOR OUR FORECAST AREA USHERING IN COOLER AIR AS THE WAVE
LIFTS TO THE WEST OF OUR REGION. BUT RAIN COULD LINGER IN OUR
SOUTHEAST ZONES THROUGH PARTS OF NORTHWEST IA AS A WEAK SHORT WAVE
MOVES ACROSS NEBRASKA.

.LONG TERM...(WEDNESDAY THROUGH MONDAY)
ISSUED AT 347 AM CDT TUE SEP 30 2014

VERY ACTIVE FORECAST IN THE MID RANGE AS A SERIES OF FRONTS MOVE
THROUGH THE AREA.  ON WEDNESDAY...APPEARS AS THOUGH THERE WILL BE
THE POTENTIAL FOR STRATUS ALONG AND AHEAD OF A SLOW MOVING COLD
FRONT MAINLY ALONG AND EAST OF I-29.  MODELS SUGGEST THAT STRATUS
WILL MIX OUT BY MID MORNING...THOUGH THAT MAY BE A LITTLE FAST.
OVERALL...THINK THE STRATUS WILL EVENTUALLY MIX OUT...AND AIRMASS
AHEAD OF THE FRONT WILL BECOME UNSTABLE.  INTERESTING SET UP BECAUSE
THERE IS NOT A LOT OF UPPER LEVEL SUPPORT WITH THE FRONT...AND
CONVERGENCE IS NOT THAT STRONG WITH VERY WEAK SOUTHERLY FLOW AHEAD
OF THE FRONT
...BUT COULD BE STRONG ENOUGH FOR A STORM OR TO TO DEVELOP ACROSS
THE EASTERN CWA. CONFIDENCE IN CONVECTIVE DEVELOPMENT IS
LOW...ESPECIALLY WITH BETTER CONVECTIVE SET UP FURTHER TO THE
SOUTH...AND HAVE REDUCED POPS TO SLIGHT CHANCE.

THROUGH THE OVERNIGHT WEDNESDAY NIGHT INTO THURSDAY...SHORT WAVE
MOVES ACROSS NEBRASKA AND COULD LEAD TO SOME THUNDERSTORMS ALONG THE
MISSOURI RIVER VALLEY INTO NORTHWEST IOWA AFTER MIDNIGHT.

MUCH STRONGER PUSH OF COOLER DRIER AIR WORKS INTO THE REGION ON
THURSDAY AS SECONDARY COLD FRONT MOVES THROUGH THE AREA. MUCH
STRONGER WINDS BEHIND THIS FRONT...AND WITH COLD AIR ADVECTION WILL
LIKELY BECOME GUSTY THROUGH THE AFTERNOON HOURS ON THURSDAY.

FRIDAY WILL BE THE DOWNRIGHT WINDS DAY THOUGH AS 925 HPA WINDS RUN
IN THE NEIGHBORHOOD OF 30-40 KNOTS.  HAVE INCREASED WINDS ABOVE
GUIDANCE VALUES AS WIND ADVISORY LOOKS LIKELY.  HAVE ALSO INCREASED
CLOUD COVER AS A VEIL OF MID TO HIGH CLOUDS DROPS DOWN IN NORTHWEST
FLOW ALOFT.  DESPITE THE MIXY CONDITIONS AND THE UNIDIRECTIONAL
FLOW...GUIDANCE VALUES STILL LOOKED TO WARM BASED ON TEMPERATURES
ALOFT...AND COOLED HIGHS FOR FRIDAY COMPARED TO GUIDANCE.

MORE TRANQUIL CONDITIONS ARE EXPECTED THROUGH THE WEEKEND.  A FEW
SUBTLE SHORTWAVES MAY DROP DOWN IN THE NORTHWEST FLOW ALOFT...BUT
WITH LITTLE MOISTURE TO WORK WITH...KEPT THE FORECAST DRY.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z WEDNESDAY MORNING)
ISSUED AT 641 AM CDT TUE SEP 30 2014

CONFIDENCE IS FAIRLY HIGH WITH THE OVERALL WEATHER TREND FOR THE
KHON...KFSD AND KSUX TAF SITES. BUT CONVERSELY...CONFIDENCE IS
SHAKY AT BEST WITH THE SMALL SCALE DETAILS INVOLVING EXACT
THUNDERSTORM CHANCES AND STRATUS POTENTIAL. CURRENTLY...MANY OF
THE MODELS INCLUDING THE NAM...RAP AND GFS HAVE ABUNDANT LOW LEVEL
MOISTURE BASED JUST ABOVE THE SURFACE WHERE NO STRATUS EXISTS.
THAT SAID...OBSERVATIONS SHOW A TON OF STRATUS IN THE IFR AND LIFR
CATEGORIES INCHING INTO OUR FORECAST AREA FROM THE EAST ALONG THE
EASTERLY FLOW OF AIR. EVEN THOUGH THE WINDS MAY SHIFT A BIT TO THE
SOUTHEAST...LIFR STRATUS IS ALSO ABUNDANT IN SOUTHERN IOWA SO ONE
WOULD THINK THAT SOONER OR LATER...THIS WOULD ENCROACH INTO THE
KFSD AND KSUX TAF SITES. MIXING THIS MORNING MAY NUDGE THE STRATUS
UP INTO THE VERY LOW END OF THE MVFR CATEGORY...BUT IT WILL NEED
TO BE WATCHED AND KFSD AND KSUX TAFS UPDATED WHEN NEEDED.
OTHERWISE THE PRECIP TREND FOR KFSD AND KSUX IS SIMILAR BECAUSE
THE TIMING OF THE RAIN/THUNDER BAND IS SIMILAR FOR THE TWO SITES.
TONIGHT...CURRENT INDICATIONS ARE THAT THE STRATUS MAY HANG ON FOR
KFSD IN THE MVFR CATEGORY...BUT COULD BREAK OUT AT KSUX BEING
FURTHER SOUTH.

KHON IS ALSO TRICKY. CURRENT INDICATIONS ARE THAT MVFR STRATUS...
BORDERLINE IFR...WILL DEVELOP FOR KHON BY AROUND 15Z. SPOTTY
STRATUS DOES EXIST CURRENTLY IN EAST CENTRAL SD BUT IS HARD TO SEE
DUE TO THE THICK HIGH CLOUDS ON SATELLITE IMAGERY. MOST CONCERNING
FOR KHON HOWEVER IS THE POSSIBILITY FOR STRONG TO SEVERE STORMS
LATE THIS AFTERNOON ALONG A SECONDARY SHORT WAVE AND DRY SLOT.
THIS WAS INCLUDED IN THE TAF FOR THE 21Z TO 23Z TIME FRAME.

&&

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

$$

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


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