Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 201754
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1254 PM CDT WED AUG 20 2014

.UPDATE...
ISSUED AT 1008 AM CDT WED AUG 20 2014

CURRENT CONVECTION IN EASTERN NEBRASKA CONTINUES TO FILTER
NORTHEASTWARD INTO PARTS OF NORTHWEST IA...EXTREME NORTHEAST NEB
AND FAR SOUTHEAST SD THIS MORNING. THIS ACTIVITY IS ASSOCIATED
WITH A 700MB WAVE...MARKED BY DECENT MID LEVEL FRONTOGENESIS AND
STRONG MID LEVEL SPEED CONVERGENCE IN THAT AREA. SIGNALS BASED OFF
OF THE 12Z NAM CONTINUE TO POINT AT A GRADUAL WEAKENING OF THESE
FEATURES INTO MIDDAY...ALBEIT THERE ARE SOME INDICATIONS THAT THE
MID LEVEL CONVERGENCE MAY MAKE IT ALL THE WAY TOWARD THE IA GREAT
LAKES AREA AND STORM LAKE EARLY THIS AFTERNOON. SO THAT WILL NEED
MONITORED TO SEE IF WE NEED TO INCREASE AFTERNOON POPS IN THAT
AREA.

THIS CONVECTION HAS REALLY FOULED UP THE LOW LEVEL AND SURFACE
BOUNDARIES THIS MORNING. AT THIS TIME...IT APPEARS THAT THE
CURRENT POSITION OF THE MAIN WARM FRONT IS STILL AROUND THE OMAHA
AREA PUSHED SOUTHWARD BY THE RAINFALL. THE VARIOUS SHORTER RANGE
MODELS ARE ALL OVER THE MAP IN HOW THEY LIFT THE FRONT...OR FRONTS...
NORTHWARD THIS AFTERNOON AND EVENING. FURTHERMORE...THE NEW 12Z
NAM HAS THE MAIN SHORT WAVE LIFTING OUT MUCH FURTHER TO THE
NORTHWEST AND NORTH OF OUR FORECAST AREA TONIGHT...WITH A LOBE OF
UPPER QG FORCING ARCING SOUTHWARD INTO OUR AREA THIS EVENING FROM
THE PARENT SHORT WAVE TO OUR NORTH. SO FOR TONIGHT...THE ACTUAL WAVE
MAY NOT BE MUCH OF A PLAYER FOR OUR FORECAST AREA. IN ADDITION...THE
SURFACE WIND SHIFT TO THE WEST DOES NOT TAKE PLACE UNTIL TOMORROW.
SO ANY TORNADIC ACTIVITY WILL LIKELY HAVE TO BE CONFINED TO NEAR
THE NORTHWARD LIFTING WARM FRONT WHICH IS DIFFICULT TO PROG.
FOLLOWING THE 4KM NAM AND HRRR AS GUIDES SINCE THEY ARE VERY CLOSE...
IT APPEARS THAT ONE WARM FRONTAL BOUNDARY SCOOTS NORTHWARD WITH
THE WARM AIR ADVECTION TODAY...EVENTUALLY SETTING UP AND STALLING
GENERALLY FROM CENTRAL ND...TO NORTHEAST SD...AND DOWN TO JUST
NORTH OF MARSHALL MN THIS EVENING. BUT THERE IS A SECOND BOUNDARY
AS WAS SHOWN YESTERDAY GENERALLY EXTENDING FROM LOW PRESSURE IN
WEST CENTRAL SD...TO NEAR CHAMBERLAIN AND SIOUX CITY FROM 21Z TO
03Z TODAY...THEN WASHES OUT LIKELY DUE TO THE SHORT WAVE PASSAGE
TO OUR NORTHWEST. AFTER ALL OF THIS MORNING AND MIDDAY CONVECTION
IN OUR SOUTHERN ZONES...WORRIED THAT THE SOUTHERN PART OF THIS
SECOND WARM FRONT IN OUR SOUTHERN ZONES COULD BE CAPPED OFF INTO
THE EARLY EVENING HOURS. THIS WOULD INCLUDE THE SIOUX CITY...
YANKTON AND STORM LAKE AREAS. BUT FURTHER WEST TOWARD CENTRAL SD
WHERE STRONGER HEATING SHOULD OCCUR...THAT AREA COULD HAVE ENOUGH
LOW LEVEL CURVATURE TO SPAWN ISOLATED TORNADOES THIS EVENING...
AROUND THE CHAMBERLAIN AND MITCHELL AREAS...POSSIBLY EXTENDING
NORTHWARD THEN THROUGH THE EVENING CLOSER TO HIGHWAY 14 IN OUR
SOUTH DAKOTA ZONES. THE AFOREMENTIONED SURFACE LOW TO THE WEST
SHOULD ALSO AID IN LOW LEVEL CURVATURE IN OUR FAR WESTERN ZONES.
SO THIS APPEARS TO BE THE AREA RIGHT NOW FOR THE BEST CHANCE FOR
TORNADIC ACTIVITY THIS EVENING...AT LEAST WITH THE CURRENT
ANALYSIS. A BROADER RISK OF SEVERE WEATHER IS CERTAINLY WARRANTED
THROUGH THE FORECAST AREA INTO THE MIDDLE OF THE NIGHT ALONG THE
UPPER QG FORCING LOBE WHICH WAS MENTIONED ABOVE. WILL CONTINUE TO
MONITOR FOR THE LATEST TRENDS.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 341 AM CDT WED AUG 20 2014

SHOWERS AND THUNDERSTORMS EXPANDING THIS MORNING ALONG AND JUST
SOUTH OF THE MISSOURI RIVER IN AN AREA OF INCREASING ISENTROPIC LIFT
ON THE EDGE OF THE LOW LEVEL JET. THESE STORMS SHOULD GENERALLY
REMAIN SUB SEVERE WITH BRIEF HEAVY RAIN...ALTHOUGH A FEW MARGINALLY
SEVERE HAIL REPORTS ARE POSSIBLE. NOT REALLY A STRAIGHT FORWARD
FORECAST AS WE GO THROUGH THE THE DAY TODAY INTO TONIGHT REGARDING
CONVECTIVE EVOLUTION. THOUGHT IS THE ELEVATED CONVECTION THIS
MORNING WILL GENERALLY BEGIN TO WEAKEN AND DISSIPATE BY MID MORNING
AS THE AREA OF ISENTROPIC LIFT MOVES NORTH INTO DRIER AIR AND WE
HAVE THE DIURNAL WEAKENING OF THE LOW LEVEL JET. THIS IS SUPPORTED
BY A MAJORITY OF THE HI RES GUIDANCE. STILL SHOULD BE QUITE A BIT OF
MID LEVEL CLOUDINESS THAT STICKS AROUND THROUGH THE DAY...AND
POSSIBLY AN ISOLATED ELEVATED SHOWER OR STORM. BUT GENERALLY
THINKING THE DAY ENDS UP MAINLY DRY...UNTIL WE SEE A REGENERATION
THIS EVENING INTO THE OVERNIGHT...WHICH WILL BE DISCUSSED BELOW.
HIGHS TODAY TRICKY AS WELL GIVEN CLOUD COVER UNCERTAINTY. WILL
CONTINUE TO GO MIDDLE GROUND FOR HIGHS...WITH THE CLOUD COVER...HIGH
BOUNDARY LAYER MOISTURE...AND SOUTHEASTERLY WINDS LIMITING MIXING.
STILL SHOULD SEE MID TO UPPER 80S...WHICH COMBINED WITH THE HIGH
DEWPOINTS WILL RESULT IN HEAT INDICES FROM THE UPPER 80S TO MID
90S...HIGHEST ALONG THE MISSOURI RIVER.

MAIN FOCUS OF THIS FORECAST IS ON SEVERE WEATHER CHANCES THIS
AFTERNOON AND TONIGHT. THROUGH THE DAY A WARM FRONT WILL WORK ITS
WAY NORTH...SETTING UP SOMEWHERE ACROSS THE CWA BY THIS EVENING.
THIS WILL ENHANCE LOW LEVEL SHEAR...WITH 0 TO 1 KM SHEAR EXPECTED TO
BE 15 TO 20 KTS. GOOD TURNING OF THE WINDS WITH HEIGHT WILL ALSO
RESULT IN DEEP LAYER SHEAR OF 40 OR MORE KTS...SUFFICIENT FOR
SUPERCELLS. MLCAPE SHOULD BE PLENTIFUL AS WELL WITH MOST LOW LEVELS
AND STEEP LAPSE RATES ALOFT RESULTING IN VALUES OF 2000 TO 3000
J/KG. AM CONCERNED ABOUT LOW LEVEL LAPSE RATES AND STABILITY THOUGH.
MODEL SOUNDINGS SHOW A PRETTY GOOD CAP IN PLACE FOR MUCH OF THE DAY.
LOW LEVEL FORCING DOES NOT SEEM STRONG ENOUGH TO OVERCOME THIS FOR
MOST OF THE DAY...WHICH SHOULD KEEP SURFACE BASED CONVECTION AT BAY.
THE UPPER WAVE WILL PASS BY TO OUR NORTH THIS AFTERNOON AND
EVENING...BUT SHOULD PASS TOO FAR NORTH TO AID TOO MUCH IN
CONVECTIVE DEVELOPMENT...AS 700 AND 500 MB HEIGHT FALLS ARE MINIMAL
TO NON EXISTENT. THUS CONVECTIVE DEVELOPMENT WILL RELY MAINLY ON
FORCING FROM THE WEST TO EAST MOVING COLD FRONT AND THE INCREASING
LOW LEVEL JET TOWARDS 0Z. PREFER TIMING AND PLACEMENT CLOSE TO THE
06Z 4KM NAM...AND 4KM NMMB. THIS WOULD GIVEN CONVECTIVE INITIATION
ACROSS CENTRAL SOUTH DAKOTA...WITH STORMS MOVING TOWARDS CHAMBERLAIN
AND HURON BETWEEN 6PM AND 9PM. GIVEN THE AMPLE CAPE...STORMS SHOULD
BE ABLE TO THEN PROGRESS ACROSS MUCH OF THE CWA...APPROACHING
INTERSTATE 29 AFTER 10 PM AND THE EASTERN CWA AFTER MIDNIGHT. GIVEN
THE GOOD MODEL AGREEMENT WENT WITH LIKELY POPS MOST AREAS...STAYED
SHY OF GOING CATEGORICAL GIVEN THAT THERE IS STILL A CHANCE THE
STORMS PEEL OFF TO THE NORTH CLOSER TO THE BETTER UPPER SUPPORT.

IN GENERAL THE SPC SLIGHT RISK AND PROBABILITIES SEEM GOOD. GIVEN
THE STEEP LAPSE RATES ALOFT AND GOOD SHEAR...LARGE HAIL OF TENNIS
BALL SIZE WILL INITIALLY BE A THREAT. THIS VERY LARGE HAIL THREAT
SHOULD MAINLY BE CLOSER TO WHERE STORMS INITIATE...LIKELY THE
VICINITY OF THE JAMES RIVER OR WEST. AS STORMS MOVE EAST...SOME
LARGE HAIL WILL STILL BE POSSIBLE...BUT THE STRONG WIND THREAT
SHOULD ALSO INCREASE. TORNADO THREAT IS UNCERTAIN GIVEN THE
UNCERTAINTY ON WHETHER STORMS BECOME NEAR SURFACE BASED. BUT GIVEN
THE SHEAR MENTIONED ABOVE...CAN NOT RULE OUT A TORNADO OR
TWO...GREATEST THREAT AGAIN APPEARS TO BE WEST OF INTERSTATE
29...PROBABLY EVEN ALONG OR JUST WEST OF THE JAMES RIVER. HEAVY RAIN
WILL ALSO BE LIKELY IN ANY STORMS...WITH PWATS UP TO 3 STANDARD
DEVIATIONS ABOVE NORMAL. STORMS SHOULD BE MOVING ENOUGH TO PREVENT A
BIG FLASH FLOOD RISK...ALTHOUGH LOCALIZED FLOODING WOULD BE POSSIBLE
IN THE HEAVIER STORMS.

.LONG TERM...(THURSDAY THROUGH TUESDAY)
ISSUED AT 341 AM CDT WED AUG 20 2014

THURSDAY LOOKS WARM...DRY AND HUMID AS UPPER LEVEL RIDGING BUILDS
ACROSS THE REGION. MAIN CONCERN ON THURSDAY WILL BE HEAT INDICES.
WHILE NOT QUITE TO ADVISORY LEVELS COULD SEE SOME LOWER TO MIDDLE
90S FOR HEAT INDICES WHICH WILL FEEL PRETTY HOT GIVEN THE EXPECTED 5
TO 10 MPH WIND.

THURSDAY NIGHT INTO FRIDAY WILL SEE A TROUGH OF LOW PRESSURE SWING
ONTO THE PLAINS WHICH WILL BRING A GRADUALLY INCREASING THREAT FOR
THUNDERSTORMS. JUST A SLIGHT CHANCE THURSDAY NIGHT THEN BECOMING
MUCH MORE LIKELY LATE FRIDAY AFTERNOON AND FRIDAY NIGHT. ISOLATED
SEVERE THUNDERSTORMS AS WELL AS HEAVY RAIN WILL BE A THREAT. FRIDAY
LIKELY TO BE PRETTY WARM AND HUMID...ESPECIALLY THE SOUTHEAST HALF
OF THE CWA. HEAT INDICES LIKELY 90 TO 95 IN THESE AREAS.

IN THE OUTER PERIODS (SATURDAY THROUGH TUESDAY)...A FAIRLY STRONG
TROUGH OF LOW PRESSURE IS FORECAST TO MOVE ACROSS THE NORTHERN
PLAINS SATURDAY NIGHT INTO SUNDAY NIGHT. THE TIMING AND STRENGTH ARE
FAIRLY CONSISTENT IN THE LATEST MODELS SO WILL CONTINUE WITH MID
RANGE POPS DURING THIS TIME. ALSO LOOKS LIKE A DECENT CHANCE FOR
SOME SPOTTY HEAVY RAINFALL AS THE UPPER LEVEL FLOW LOOKS TO POSSIBLY
BECOME PARALLEL TO THE LOW LEVEL BOUNDARY...WHICH COULD SUPPORT SOME
TRAINING OF THUNDERSTORMS. A LITTLE TOO FAR OUT TO REALLY NAIL THIS
DOWN BUT SOMETHING TO WATCH FOR. AS THE WAVE PASSES COOLER AIR WILL
SETTLE INTO THE AREA AND WITH CONTINUED TROUGHING TO THE WEST THE
THREAT FOR SHOWERS AND THUNDERSTORMS...AS WELL AS ADDITIONAL CLOUD
COVER...WILL ALSO CONTINUE. TEMPERATURE GUIDANCE IN THIS TIME FRAME
IS ALREADY BELOW NORMAL AND SEE NO REAL REASON TO STRAY TOO FAR FROM
THAT. STATISTICALLY A GOOD SIGNAL WITH THE GFS MEX GUIDANCE BELOW
CLIMATOLOGICAL NORMALS AND THE ENSEMBLE MEAN OUTPUT COOLER YET WHICH
IS USUALLY A BIG HINT TO STAY BELOW THE COLDER MEN TEMPERATURES.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 1253 PM CDT WED AUG 20 2014

FOR STARTERS...VERY COMPLEX PATTERN IS NOT LENDING A LOT OF
CONFIDENCE TO THE 18Z TAFS. THE PRIMARY WARM FRONT IS STILL WELL
SOUTH OF OUR FORECAST AREA AT THIS TIME...WITH AN ELEVATED WARM
FRONT CONTINUING TO LIFT NORTHWARD. CONVECTION IS CURRENTLY FIRING
ALONG THE ELEVATED WARM FRONT...COUPLED WITH A SHORT WAVE NEAR
700MB ALONG OUR MO RIVER VALLEY ZONES. THOUGHTS ARE THAT THIS
ACTIVITY WILL LIFT NORTHEASTWARD THROUGH THE AFTERNOON. THEREFORE
PUT IN A CHANCE FOR THUNDERSTORMS FOR THE SIOUX FALLS TAF SITE VERY
LATE THIS AFTERNOON AND VERY EARLY IN THE EVENING...BASED ON THE
TIMING OF THE ELEVATED WARM FRONTAL PASSAGE. DYNAMICS SHOULD BE
COMING TO A CLOSE AROUND SIOUX CITY BY MID AFTERNOON AS THE
ELEVATED WARM FRONT LIFTS NORTHWARD. THEREFORE ENDED THE PRECIP
THREAT FOR SIOUX CITY BY THAT TIME. AS THE PRIMARY UPPER SHORT
WAVE LIFTS THROUGH NORTHERN SD AND NORTH DAKOTA THIS EVENING...
CONTINUED TO PLACE THUNDERSTORM CHANCES FOR THE KHON TAF SITE
DURING THE EVENING HOURS. BUT FURTHER SOUTH...THERE COULD BE A
BREAK IN THE KFSD TAF SITE AFTER EARLY EVENING UNTIL THE OVERNIGHT
HOURS. RIGHT DURING THE MIDDLE OF THE NIGHT...AS THE MAIN SHORT
WAVE LIFTS NORTH OF KFSD...THERE COULD BE A PERIOD OF SHOWERS AND
STORMS WHICH WERE MENTIONED IN THE KFSD TAF ASSOCIATED WITH THE
MAIN WAVE IN NORTHERN SD...AS CONVECTION COULD DEVELOP SOUTHWARD
INTO A STOUT LOW LEVEL JET. BUT KSUX MAY BE TOO FAR SOUTH FOR
NOCTURNAL ACTIVITY TONIGHT...OR AT LEAST LESS CONFIDENT OF
CONVECTION...SO LEFT KSUX DRY FOR TONIGHT.

LASTLY...THE LATEST HIRES NAM IS HINTING AT STRATUS DEVELOPMENT
FOR KHON AND KFSD LATE TONIGHT AND EARLY THURSDAY BEHIND THE SHORT
WAVE DEPARTURE. IF RAIN PANS OUT TONIGHT...THIS IS A DISTINCT
POSSIBILITY AS WINDS ARE LIGHT WITH RESIDUAL MOISTURE LINGERING.
FOR NOW...HEDGED MVFR CONDITIONS FOR THOSE TWO LOCATIONS FOR THIS
POSSIBILITY.

&&

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

$$

UPDATE...MJ
SHORT TERM...CHENARD
LONG TERM...08
AVIATION...MJ





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