Severe Thunderstorm Warning 112, NWS Sioux Falls, SD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
447 AM CDT SAT JUN 26 2010

.DISCUSSION...
CURRENTLY...SOME ISOLATED THUNDERTORMS HAVE JUST REDEVELOPED NEAR
THE SPENCER IA VCNTY. THIS ACTIVITY SHOULD CONT TO WANE DURING THE
PRE DAWN HOURS AS THE LOW LEVEL JET IN NEBRASKA DECREASES CAUSING
LESS SPEED CONVERGENCE IN OUR AREA. ELSEWHERE...AREAS OF DENSE FOG
HAVE DEVELOPED BEHIND THE DEPARTING MCS IN SW MN. THIS FOG IS
PROBABLY FAIRLY SHALLOW AND SHOULD BURN OFF BY AT LEAST MID MORNING.
THEREFORE CHOSE TO FORGO ANY DENSE FOG ADVY ATTM. SOME OF THIS FOG
MAY ADVECT INTO THE BROOKINGS SD AREA...AND OTHER PARTS OF E CENTRAL
SD BY SUNRISE. BUT AGAIN IT SHOULD NOT GET TOO OUT OF CONTROL.

OTHERWISE WE ARE LOOKING AT A QUASI STALLED SFC BOUNDARY PROJECTED
TO SETUP RIGHT ALONG I 90 AS THE DAY PROGRESSES. ONE CAN ALREADY
KIND OF SEE THE WIND SHEAR...WITH S OR SE SFC WINDS IN NW IA...
CHANGING TO MORE OF A LIGHT ELY FETCH ALONG I 90 AND NWD. THIS
BOUNDARY...COUPLED WITH ANOTHER VERY STRONG SHORT WAVE MOVG EWD LATE
TODAY AND TONIGHT...WL BE THE FOCUS FOR STRONG TO SEVERE CONVECTION
ONCE AGAIN. ML CAPES ARE ONCE AGAIN AROUND 3000 TO 3500 J/KG SO
INSTABILITY ABOUNDS. DIRECTIONAL 0-6KM SHEAR IS VERY STOUT JUST
AHEAD OF THE ADVANCING CDFNT FM THE WEST...WITH BACKING LOW LEVEL
WINDS JUST AHEAD OF THE EWD MOVG CDFNT WHICH IS BOTHERSOME. 0-6KM
SHEAR FOR THIS TIME OF YEAR IS FAIRLY STRONG AT 30 KTS FOR MUCH OF
OUR FA EARLY THIS EVENING. BESIDES THE USUAL SVR WX POTENTIAL THIS
EVENING...ONE HAS TO BE CONCERNED ABOUT MORE TORNADIC DEVELOPMENT
BEFORE 06Z AHEAD OF THE ADVANCING CDFNT...AND ALONG AND NORTH OF THE
WRMFNT DRAPPED ALONG I 90. THIS BASICALLY PLACES A BOX OF THE
STRONGEST SVR WX WITH HEAVY RAINFALL FROM HUTCHINSON COUNTY TO
KINGSBURY COUNTY ON THE WEST END...THEN EXTENDING STRAIGHT EWD FM
THERE TO INCLUDE SW MN AND EXTREME NW IA. WL HIGHLIGHT THIS IN THE
HWO. CONCERNING WATER...THE SHORTWAVE IS MOVG PRETTY FAST...SO NOT
EXPECTING A TON OF RAIN IN ANY GIVEN LOCATION. BUT ONE TO THREE
INCHES IS CERTAINLY POSSIBLE...ESP IN THE AFOREMENTIONED BOX...SO
THEREFORE OPTED TO ISSUE A FLASH FLOOD WATCH KNOWING THAT THE GROUND
IS ALREADY SATURATED IN THIS AREA FOR MANY LOCATIONS.

THOUGHTS ARE FOR TIMING...THAT SOME CONVECTION COULD MOVE INTO OUR
WRN ZONES AS EARLY AS LATE MORNING NOTING THE COMPLEX OF TSRA IN
NWRN SD RIGHT NOW. THE ACTUAL SHORT WAVE IS CURRENTLY SPARKING SOME
TSRA NEAR BILLINGS ATTM. THEN IN THE EARLY AFTERNOON...CHCS OF
CONVECTION ARE A THREAT ALONG AND NORTH OF THE WRMFNT. THEN ONCE THE
SHORTWAVE INFRINGES ON THIS FA LATE TODAY...CONVECTION SHOULD FIRE
IN EARNEST ACROSS OUR WRN ZONES LATE THIS AFTERNOON...MOVG EWD THIS
EVENING. THINGS SHOULD THEN QUIET DOWN AFTER 06Z. ONE THING THAT WE
REALLY DO NOT HAVE WORKING FOR US LATE TODAY AND TONIGHT THAT WE HAD
YESTERDAY AND OVERNIGHT A FEW HOURS AGO...IS SO FAR THE MODELS ARE
PROGGING NOT A VERY STRONG LOW LEVEL JET SOUTH OF THIS FA LIMITING
SPEED CONVERGENCE. BUT STILL...THE MID LEVEL WINDS ARE STRONG AND
THE AFOREMENTIONED DIRECTIONAL SHEAR AHEAD OF THE CDFNT AND NEAR THE
WRMFNT IS ALSO STRONG.

ANOTHER WARM DAY IS IN STORE...AND HEAT INDICE VALUES ARE
APPROACHING HEAT INDEX CRITERIA IN EXTREME SERN SD AND NWRN IA. THIS
MAY HAVE TO BE MONITORED BY THE DAY SHIFT. SO FAR...I AM JUST BLO
CRITERIA OF 105 DEGS.

OTHERWISE THE CDFNT TONIGHT SWEEPS EVERYTHING THROUGH...SO WE ARE
LOOKING AT A MUCH LESS HUMID DAY ON SUNDAY. SUNDAY AFTERNOON POPCORN
CONVECTION COULD OCCUR ALONG AND EAST OF A LINE FROM KINGSBURY
COUNTY TO THE SPENCER IA AREA. ML CAPES ARE AROUND 300 TO 500 J/KG
WITH CHILLY AIR ALOFT IN THAT AREA. BUT RIGHT NOW IT LOOKS JUST A
BIT TOO DRY SO LEFT OUT FOR NOW. THEN A SHOT OF COOL AIR COMES DOWN
WITH A SUBTLE SFC FROPA SUNDAY NIGHT. THEREFORE MONDAY LOOKS ABOUT A
CATEGORY COOLER THAN SUNDAY FOR MAX TEMPS. NOT SURE WHY GUIDANCE
HIGHS ARE SO WARM ON MONDAY AND UNDERCUT THEM. SUNDAY LOOKS TO BE
THE WARMER OF THE TWO DAYS.    /MJF

&&

.AVIATION...
VFR CONDS EXPECTED ACROSS THE AREA THRU 18Z TODAY WITH A NOTABLE
EXCEPTION. LIFR TO VLIFR CONDS IN FOG HAVE DEVELOPED OVER PARTS OF
SW MN BEHIND THE DEPARTING COMPLEX OF THUNDERSTORMS. THE FOG IS
LIKELY RELATIVELY SHALLOW AND SHOULD BEGIN TO BURN OFF BY MID
MORNING. SOME OF THE FOG MAY LEAK INTO THE BROOKINGS SD VCNTY BEFORE
DISSIPATING. OTHERWISE SCATTERED SHOWERS AND THUNDERSTORMS ARE
EXPECTED TO DEVELOP ALONG A FRONTAL BOUNDARY OVER SOUTHEASTERN SOUTH
DAKOTA...MAINLY AFTER 20Z...PUSHING TO THE SOUTH AND EAST THROUGH
THE EVENING. MVFR CIGS AND VISBYS WILL BE POSSIBLE IN ANY
THUNDERSTORMS ACTIVITY THAT OCCURS.    /JM

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
IA...FLASH FLOOD WATCH FROM THIS EVENING THROUGH LATE TONIGHT FOR
     IAZ001>003-012>014.

MN...FLASH FLOOD WATCH FROM THIS EVENING THROUGH LATE TONIGHT FOR
     MNZ071-072-080-081-089-090-097-098.

NE...NONE.
SD...FLASH FLOOD WATCH FROM THIS EVENING THROUGH LATE TONIGHT
     FOR SDZ039-040-054>056-060>062-065>067.

&&

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