Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 311745
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1245 PM CDT SUN AUG 31 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 435 AM CDT SUN AUG 31 2014

GREATEST CONCERN TODAY IS WITH THE SEVERE WEATHER POTENTIAL LATER
TODAY.

LEADING UPPER WAVE LIFTING THROUGH THE NORTHERN PLAINS AT PRESENT.
ENERGY WITH SYSTEM IS PULLING MOSTLY TOWARD NORTH DAKOTA...AND AS AS
RESULT HAVE SEEN EARLIER STRONGER ACTIVITY ACROSS WESTERN SOUTH
DAKOTA REALLY WEAKEN HEADING INTO SOUTHEAST SOUTH DAKOTA. STRONG LOW
LEVEL JET EXHIBITING MUCH BETTER CONVERGENCE TO THE NORTH...AND ON
VEERING NOSE ARE STARTING TO SEE EXTENSION OF MID CLOUDS SOUTHWARD
ACROSS SW MN AND NW IA...WHICH VERY WELL COULD NOT HAVE A CHANCE TO
ACTIVATE TO A BETTER CONVECTIVE BAND BEFORE EXITING THE FSD CWA...
PER SEVERAL RECENT HIGH RESOLUTION MODEL RUNS. WHAT REMAINS IN THE
AREA OF STEEPER LAPSE RATES BEHIND THIS INITIAL GRADIENT...AND
WITHIN AXIS OF MID LEVEL MOISTURE ADVECTION AND TAIL OF PV ADVECTION
JUST A FEW SHOWERS AND THUNDERSTORMS. THIS BAND WILL LIKELY NEVER
HAVE MUCH FOR COVERAGE...AND WILL LIMP EASTWARD THROUGH THE REST OF
THE NIGHT AND MORNING HOURS. HARD TO JUSTIFY MUCH MORE THAN A LOW
CHANCE/SCATTERED LEVEL POP THROUGH THE FIRST HALF OF THE DAY...AS
WILL SOON LOSE SUPPORT OF WEAK PV ADVECTION...AND THERE WILL BE A
FAIRLY SUBSTANTIAL WAIT FOR THE MAIN UPPER ENERGY SEEN PUSHING INTO
SOUTHWESTERN WYOMING AND COLORADO ON EARLY MORNING WATER VAPOR
IMAGERY.

AS PRESSURE FALL IN RESPONSE TO BETTER UPPER JET TO THE SOUTH...AS
WELL AS THE SLOWER PROGRESSION... WILL PROBABLY GET A BIT BETTER
BACKING TO LOW LEVEL WINDS AHEAD OF BOUNDARY ACROSS THE LOWER
MISSOURI VALLEY INTO PARTS OF NW IA BY LATE DAY COMPARED TO
LOCATIONS FURTHER NORTHWARD. BOTH THE DEEP LAYER SHEAR AND THE 0-1KM
SHEAR APPEAR AS IF THEY COULD END UP BETTER THAN OPERATIONAL MODELS
INDICATE...ESPECIALLY IF PRE FRONTAL TROUGH ENDS UP A BIT LESS SHARP
ACROSS SOUTHWEST MINNESOTA AND NORTHWEST IOWA. ANOTHER FACTOR TOWARD
SEVERE INTENSITY TODAY IS THE POTENTIAL DESTABILIZATION. AT THIS
TIME...DOES NOT APPEAR AS IF THERE WILL BE A GREAT DEAL OF
DISTURBANCE TO WARMING BY EARLY DAY CONVECTION...OTHER THAN HAVING
AXIS OF MID AND HIGH CLOUDS WITH A MORE SPOTTY COVERAGE TO
PRECIPITATION TO WORK PAST THROUGH MIDDAY. SEEMS AS IF THERE SHOULD
BE A WINDOW TO BOTH GET A LITTLE DIABATIC HEATING...AS WELL AS A
GRADUAL RETURN OF DEEPER MOISTURE AHEAD OF ADVANCING COLD FRONTAL
BOUNDARY. ONE THING TO WATCH IS GROWING AREA OF CONVECTION TO THE
SOUTH ACROSS NORTHERN KS...WHICH MAY SPREAD VERY CLOSE TO THE
SOUTHEASTERN CWA TOWARD MIDDAY.

HOWEVER...AS MAIN PV ADVECTION INCREASES VERY LATE THIS AFTERNOON...
EXPECT THE POTENTIAL SEVERE ACTIVITY TO FINALLY START TO DEVELOP AND
INCREASE RAPIDLY IN COVERAGE AFTER 22Z...WITH MAIN INITIATION CLOSER
TO ENHANCED CONVERGENCE NEAR THE LOWER MISSOURI VALLEY.
INITIALLY... SHEAR VECTORS WOULD SUGGEST DISCRETE DEVELOPMENT...IN
AN ENVIRONMENT WITH PERHAPS 1500-2000 J/KG MLCAPE. ALL MODES OF
SEVERE THREAT WOULD BE IN PLAY...INCLUDING ISOLATED TORNADOES AND
LARGER SIZE HAIL ACROSS NORTHWEST IOWA AND ADJACENT LOCATIONS VERY
LATE AFTERNOON AND EARLY EVENING. THE STRONG LINEAR FORCING ALONG
THE FRONT...AND GRADUAL SHIFT IN SHEAR COMPONENT AWAY FROM CROSS
GRADIENT SHOULD SPELL A CHANGE TOWARD LINEAR STRUCTURES WHICH WILL
FAVOR MORE IN THE WAY OF WIND. OVERALL...LOOKING AT THE MOST
SIGNIFICANT THREAT FOR SEVERE...INCLUDING TORNADOES...ACROSS
NORTHWEST IOWA...EASTERN PORTIONS OF SW MN...AND EXTREME NORTHEAST
NEBRASKA...WITH LESSER SEVERE TYPES BACK ACROSS FAR EASTERN SOUTH
DAKOTA AND INTO ADJACENT SW MN. RAINFALL RATES ARE A CONCERN FOR
POTENTIAL FLASH FLOODING... ESPECIALLY FOR URBAN AREAS...AND THOSE
LOCATIONS OF IDA/WOODBURY COUNTY WHICH RECEIVED VERY HEAVY
RAINFALL THE LAST COUPLE OF DAYS.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 435 AM CDT SUN AUG 31 2014

LABOR DAY HOLIDAY SHOULD BY DRY AND COOLER WITH COLD FRONT AND
ASSOCIATED PRECIP EXPECTED TO EXIT OUR FAR SOUTHEAST EARLY IN THE
DAY. WEAK UPPER WAVE COULD BRING SPOTTY SHOWERS AND STORMS TO
WESTERN PORTIONS OF THE AREA LATE IN THE DAY...BUT BETTER CHANCE
LOOKS TO HOLD OFF UNTIL AFTER SUNSET...THOUGH EVEN THIS EXPECTED TO
BE SCATTERED AT BEST WITH MOISTURE/LIFT MAINLY FOCUSED ABOVE 750MB.
HIGHS FOR THE HOLIDAY SHOULD ONLY BE IN THE MID TO UPPER 70S BEHIND
THE EXITING COLD FRONT...WITH TEMPERATURES MONDAY NIGHT COOLING INTO
THE 50S WITH LIGHT WINDS AND CLEARING SKIES LATE AS THE WAVE SHIFTS
EAST. SUNSHINE AND SLIGHTLY WARMER TUESDAY...WITH STRONGER PUSH OF
WARM AIR MOVING THROUGH TUESDAY NIGHT AND WEDNESDAY. CANNOT RULE OUT
SPOTTY PRECIP DEVELOPMENT WITH THE WARM ADVECTION TUESDAY NIGHT...
BUT FORECAST SOUNDINGS LACKING MOISTURE AND WILL LEAVE DRY FOR NOW.
MARGINALLY BETTER PRECIP CHANCE IN OUR NORTHEAST WEDNESDAY NIGHT AS
STRONGER LOW LEVEL JET DEVELOPS ACROSS THE PLAINS...BUT AREAS TO OUR
NORTHEAST APPEAR TO BE MORE FAVORED AREA AT THIS TIME.

ANOTHER STRONGER WAVE AND COOL FRONT EXPECTED TO PUSH THROUGH THE
REGION LATE IN THE WORK WEEK...WITH PRECIP CHANCES RAMPING UP
THURSDAY NIGHT AND FRIDAY. TIMING DIFFERS AMONG THE MODELS...WITH
ECMWF/GEM MOVING THE FRONT AND ASSOCIATED PRECIP SOUTH OF OUR AREA
MORE QUICKLY THAN GFS. WILL HANG ONTO LOW CHANCES...MAINLY SOUTH...
INTO FRIDAY NIGHT TO ACCOUNT FOR SLOWER GFS. LATE WEEK TEMPERATURES
SHOULD STILL BE FAIRLY MILD...WITH HIGHS IN THE UPPER 70S AND 80S
THURSDAY. FASTER ECMWF/GEM ALSO HAVE A STRONGER COLD PUSH...AND IF
THEY DO VERIFY...HIGHS FRIDAY/SATURDAY COULD STRUGGLE TO REACH 70.
HOWEVER GIVEN UNCERTAINTY IN TIMING AT THIS RANGE...WILL STICK CLOSE
TO MODEL CONSENSUS IN THE LOWER TO MID 70S FOR NOW.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)
ISSUED AT 1238 PM CDT SUN AUG 31 2014
CONVECTION IS THE PRIMARY AVIATION CONCERN FOR THE 18Z TAF
ISSUANCE. MODEL TRENDS HAVE BEEN TO ACCELERATE THE FRONT
SLIGHTLY...SO HAVE SHIFTED THE FOCUS FOR DEEP COMVECTION LATE THIS
AFTERNOON TO MAINLY NORTHWEST IOWA ALONG AND AHEAD OF A COLD
FRONT PUSHING THROUGH THE REGION. THESE STORMS WILL BE CAPABLE OF
PRODUCING LARGE HAIL...DAMAGING WINDS AND AN ISOLATED TORNADO OR
TWO. SIOUX CITY APPEARS TO BE ON THE WESTERN EDGE OF THE MAIN
THREAT AREA.

THERE REMAINS AN OUTSIDE POTENTIAL OF MVFR CEILING/VISIBILITY
THREAT WORKING SOUTHWARD BEHIND MAIN UPPER TROUGH AFTER 06Z INTO
KHON AND AFTER 09Z AT KFSD...BUT HAVE LEFT CLEAR AS APPEARS LOW
PROBABILITY FOR NOW.


&&

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

$$

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



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