000
FXUS63 KFSD 251739
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1239 PM CDT SAT MAY 25 2013

.UPDATE...
ISSUED AT 1239 PM CDT SAT MAY 25 2013

BASED ON LATEST MESOANALYSIS...ADDED A SLIGHT CHANCE THIS
AFTERNOON FOR WESTERN CWA IN SOUTH CENTRAL SOUTH DAKOTA. LATEST
SURFACE PLOT SHOWS THE NORTHWARD MOVING WARM FRONT NOW LOCATED
FROM NORTH OF RAPID CITY...TO NORTH OF ALLIANCE/O`NEILL
NEBRASKA...AND SOUTHEAST THROUGH SE NEBRASKA. CU ALSO DEVELOPING
BETWEEN KIEN/KVTN WITH SURFACE HEATING. LATEST SPC MESOANALYSIS
SHOWS AXIS OF LITTLE CIN AND SBCAPE OF 1500+ J/KG STRETCHED ACROSS
SOUTH CENTRAL SOUTH DAKOTA. WITH DAYTIME HEATING AND WEAK VORT
MAX IN FLOW ALOFT APPROACHING...ISOLATED CELL INITIATION IS
POSSIBLE THIS AFTERNOON...ALTHOUGH MUCH OF THE INITIATION IS
EXPECTED OUT IN WESTERN SODAK WITH STRONGER SHORT WAVE AND HIGHER
INSTABILITY.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 430 AM CDT SAT MAY 25 2013

THUNDERSTORM COMPLEX OVER NEBRASKA OVERNIGHT CONTINUES EARLY THIS
MORNING. SEEING SHOWERS AND STORMS BEGINNING TO EXPAND ACROSS
NORTHWEST IOWA AS WELL AS LOW LEVEL THETAE ADVECTION INCREASES. HIGH
RES MODEL SUITE LOOK TO HAVE A PRETTY GOOD HANDLE ON THIS ACTIVITY.
EXPECT IT TO CONTINUE THIS MORNING AND POSSIBLY EXPAND A BIT FURTHER
NORTH INTO PORTIONS OF SOUTHWEST MINNESOTA. LOWERED POPS ACROSS
SOUTHEAST SOUTH DAKOTA...AS ALL INDICATIONS ARE THAT THESE STORMS
WILL NOT DEVELOP NORTHWEST OF THEIR CURRENT LOCATION. MOST OF THE
STORMS SHOULD BE EAST OF THE CWA BY 16Z. MAIN IMPACTS WILL BE
BRIEFLY HEAVY RAIN AND POSSIBLY SMALL HAIL WITH THESE CELLS...AS
EFFECTIVE SHEAR AND CAPE ARE TOO LOW FOR ANY SEVERE THREAT. MUCH OF
THE AFTERNOON SHOULD THEN BE DRY ACROSS THE CWA WITH AN AREA OF
SUBSIDENCE LIKELY BEHIND THIS MORNINGS SHORTWAVE. WARM FRONTAL
BOUNDARY WILL POSITION ITSELF SOUTH OF THE MISSOURI RIVER THIS
AFTERNOON. HRRR AND RAP TRY TO BREAK THE CAP THEIR THIS
AFTERNOON...BUT BOTH THESE MODELS SEEM A BIT OVERDONE WITH DEWPOINTS
AND INSTABILITY. FEEL MORE LIKELY SCENARIO IS ACTIVITY WAITS FOR THE
LOW LEVEL JET TO KICK IN AFTER 0Z BEFORE IT GETS GOING.

OTHER ISSUE TODAY IS CLOUD COVER AND HIGH TEMPERATURES.
INTERESTINGLY THE HRRR AND RAP ARE COMPLETE OPPOSITE OF EACH
OTHER...GIVING AROUND A 20 DEGREE DIFFERENCE IN HIGHS...WITH THE RAP
HAVING MORE STRATUS AND COOLER TEMPERATURES. THE GEM AND ECMWF SIDE
CLOSER TO THE RAP...WITH THE NAM IN BETWEEN. SINCE THE AREAL
COVERAGE OF LOW STRATUS IS CURRENTLY LESS THAN EXPECTED...DID GO
AHEAD AND RAISE HIGHS SEVERAL DEGREES FOR TODAY IN OUR CENTRAL AND
WEST. ALSO THE HRRR HAS A GOOD HANDLE ON THE CURRENT SITUATION...SO
ONE WOULD THINK ITS SHORT RANGE FORECAST SHOULD BE SOMEWHAT
RELIABLE...AND IT HAS CONSISTENTLY BEEN WARM FOR TODAY. LOWERED
HIGHS A FEW IN THE EAST WHERE IT IS RAINING...AND WHERE STRATUS
SHOULD INDEED HOLD IN FOR MUCH OF THE DAY. AGAIN THIS IS A PRETTY
LOW CONFIDENCE TEMPERATURE FORECAST FOR ANYWHERE EAST OF THE JAMES
RIVER. BUT FEEL GOING FORECAST IS A DECENT STARTING POINT...WITH THE
DAY SHIFT LIKELY HAVING TO ADJUST UP OR DOWN DEPENDING ON CLOUD
COVER TRENDS.

QUIET SHORT TERM CONDITIONS FROM THE AFTERNOON HOURS SHOULD TURN
QUICKLY TO MORE STORMY CONDITIONS OVERNIGHT. GAVE A FAIR AMOUNT
OF CREDIT TO THE CONSISTENCY OF THE 00Z HIGH RESOLUTION NAMDNG5
AND WRF-ARW SOLUTIONS...WHICH FIT THE OVERALL CONCEPTUAL MODEL FOR
CONVECTIVE INITIATION TONIGHT. BOUNDARY FROM CURRENT NIGHT MCS
WILL BE FESTERING AND DRIFTING NORTHWARD...WITH LIKELY 1000 TO
1500 J/KG ELEVATED CAPE LIFTED FROM 1 TO 1.5 KM AGL. AS 35 TO 40
KNOT LOW LEVEL JET DEVELOPS TONIGHT THROUGH THE CENTRAL PLAINS...
SHOULD BECOME ENOUGH TO INITIATE ELEVATED THUNDERSTORMS NEAR/NORTH
OF THE BOUNDARY WHICH WILL BE DRIFTING NORTHWARD OF INTERSTATE 80
DURING THE EVENING HOURS. EFFECTIVE SHEAR IN AREAS NORTH OF THE
BOUNDARY WILL BE VERY CONCENTRATED IN THE 1-3KM LAYER WITH STRONGEST
WINDS IN LOW LEVELS AND WEAKENING MID LEVEL WINDS ROUGHLY 20-30
KNOTS...WITH LESSER DEEP LAYER SHEAR CONTRIBUTIONS FROM ABOVE. ALL
SUGGEST AT LEAST A MINIMAL THREAT FOR ISOLATED LARGER HAIL OR
DAMAGING WINDS THROUGH THE MISSOURI RIVER CORRIDOR AND LIFTING
SOMEWHAT NORTHWARD THROUGH 066-08Z. OTHER SIGNIFICANT CONVECTIVE
INITIATION IS LIKELY TO OCCUR BACK THROUGH THE WESTERN HIGH PLAINS
WHERE STRONGER SHEAR EXISTS WITH A BIT STRONGER MID LEVEL WINDS
TO AROUND 25 KNOTS...AS WELL AS THE DEEP UPSLOPE TRANSPORT OF MOISTURE
TO RESULT IN STRONGER INSTABILITY. EVENTUALLY...WHEN SUPERCELL MOTIONS
WHICH WILL TAKE ACTION MORE SOUTHWARD THAN EAST START TO EASE...AND
LOW LEVEL JET VEERS EASTWARD...WILL FIND A MORE LINEAR MCS LIKELY
TAKING SHAPE. MANY HI RES MODELS SUGGEST THIS MCS WILL ZIPPER DOWN
THE LOCATION OF THE WARM ADVECTION WING SPREADING NORTHWARD...MOVING
TOWARD EAST CENTRAL SD AND SOUTHWEST MN LATE NIGHT. SEVERE THREAT
WITH THIS FEATURE WOULD SEEM TO BE SOMEWHAT LOWER...BUT LIKELY TO
GET SOME GUSTY WINDS AND AREAS OF HEAVY RAINFALL.

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 430 AM CDT SAT MAY 25 2013

MCS EXPECTED TO WEAKEN AND SHIFT EAST ON SUNDAY...AND WITH MID
LEVEL FORCING SHOWING TENDENCY TO FORCE SUBSIDENCE AROUND
MIDDAY...AGAIN EXPECT THAT MUCH OF THE DAYTIME PERIOD WILL
PROBABLY END UP PRECIPITATION FREE. HOWEVER...CLOUDS LIKELY TO
PERSIST WITH STRATUS DEVELOPING IN WAKE OF SYSTEM. MOST LIKELY TO
GET BREAKS THROUGH THE WESTERN AND SOUTHERN CWA DURING THE
AFTERNOON...AND HAVE PUSHED EXPECTED TEMPS TO NEAR 80 THERE...
WHILE MORE MID TO UPPER 60S LINGER IN THE FAR EAST. RESULT OF THIS
WILL BE GENERATION OF DECENT POTENTIAL INSTABILITY OF 1500-2000
J/KG ACROSS THE AREA...WITH WEAKEST OF THE CAPPING INVERSION NEAR
THE MISSOURI RIVER CORRIDOR. SHEAR BECOMES A BIT MORE DISTRIBUTED
THROUGH THE EFFECTIVE LAYER...AND FAIRLY CLOSE TO THOSE VALUES
FROM THIS EVENING FROM 30 TO 35 KNOTS. A FEW MORE SEVERE STORMS
WOULD BE POSSIBLE MAINLY THROUGH THE SOUTHWEST HALF OF THE AREA
DURING THE LATER AFTERNOON AND EVENING HOURS...AS INITIATION
LIKELY TO BE A RESULT OF WEAK IMPULSE SLIDING UP BACK SIDE OF
RIDGE IN THE LATER AFTERNOON AND EARLY EVENING.

THE FURTHER ONE GOES INTO THE FORECAST...THE LESS RELIABLE ANY
GUIDANCE BECOMES GIVEN THE ABUNDANCE OF CONVECTION...AND THE
INCREASINGLY GREAT UNCERTAINTY AS TO WHERE CRITICAL BOUNDARIES MAY
LIE. ONE THING FOR SURE...GIVEN THE CLOSED LOW ACROSS THE WESTERN
U.S. AND THE SOUTHWEST FLOW ALOFT...DIFFICULT TO PINPOINT ANY TIME
FROM MONDAY ON THAT ONE COULD SAY THERE WAS ABSOLUTELY NO CHANCE
FOR PRECIPITATION...BUT ON THE SAME FRONT IT CERTAINLY WILL NOT BE
AN ALL TIME WASHOUT WITH MANY BREAKS DEFINED ON THE MESOSCALE.
MEMORIAL DAY AND MONDAY NIGHT WILL PLACE THE AREA ON THE SOUTHERN
FLANK OF A COUPLE OF WAVES MOVING OVER THE RIDGE...SO LIKELY TO
HAVE ENHANCEMENT OF PRECIPITATION IN THE NOCTURNALLY FAVORED
PERIODS. WHILE A FEW SEVERE STORMS ARE AGAIN WELL WITHIN
REASON...HIGH PRECIPITABLE WATER AND FAIRLY SLOW MOVING WIND
PROFILES BRING THE CONCERN OF AREAS OF HEAVY RAINFALL...AT THIS
RANGE HARD TO DEFINE. AT THE MOMENT...THE PROSPECT FOR MORE
SIGNIFICANT CONVECTION AND SEVERE WEATHER WOULD APPEAR TO BE LATE
DAY WEDNESDAY INTO THURSDAY...AS DEEP WESTERN CLOSED LOW KICKS
STRONGER JET ENERGY INTO THE PLAINS. MOISTURE THROUGH LOWER LEVELS
/THUS INSTABILITY/ WILL CONTINUE TO BENEFIT FROM PERSISTENT
SOUTHERLY FLOW. TEMPERATURES WILL LIKELY GRAVITATE TO SOMEWHAT
ABOVE NORMAL READINGS THROUGH THE UPCOMING WEEK.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1239 PM CDT SAT MAY 25 2013

MVFR CIGS ACROSS SW MN AND NW IA DID ADVECT INTO THE FSD/SUX TAF
SITES THIS MORNING WITH ESE LOW LEVEL FLOW IN PLACE. EXPECT MVFR
CIGS TO CONTINUE THROUGH THIS AFTERNOON WITH THE EDGE RIGHT AT FSD
POSSIBLY WAVERING...ALTHOUGH ADDITIONAL EXPANSION OF THIS STRATUS
IS NOT EXPECTED TOWARD HON UNTIL LATER THIS EVENING. EXPECTING
CONVECTION TO DEVELOP THIS EVENING ALONG THE WARM FRONT EXTENDING
ACROSS SW SD AND NE NEBRASKA THEN PUSH EAST AND NORTHEAST ACROSS
THE AREA LATE EVENING AND OVERNIGHT. IN ADDITION...LOW LEVEL
MOISTURE WILL CONTINUE TO INCREASE WITH ...RESULTING IN IFR CIGS
WITH SOME AREAS OF HAZE LIKELY. BY EARLY MORNING /2Z/ CONVECTION
SHOULD BE EXITING THE AREA...ALTHOUGH IFR/MVFR CIGS ARE EXPECTED
TO PERSIST THROUGH MID MORNING.

&&

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

$$

UPDATE...
SHORT TERM...CHAPMAN/CHENARD
LONG TERM...CHAPMAN
AVIATION...