Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 230449
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1149 PM CDT FRI AUG 22 2014

.UPDATE...
ISSUED AT 1025 PM CDT FRI AUG 22 2014

HEAVY RAINFALL APPEARS TO STILL BE ON TRACK FOR TONIGHT...WITH
MASS OF CONVECTION ACROSS NEBRASKA WORKING NORTHEASTWARD. EVENING
PRECIPITABLE WATER ON THE ORDER OF 1.5 TO 1.8 INCHES ON RAOB
ANALYSIS...SOMEWHAT LESS THAN THAT OF MODEL ANALYSIS...BUT STILL
FAR ABOVE NORMAL LEVELS. AREA ON EDGE OF DYNAMICS FROM WAVE
LIFTING NORTHWARD THROUGH THE WESTERN PLAINS...WITH LOW LEVEL JET
MODESTLY INCREASING TO 25 TO 30 KNOTS TONIGHT. GOOD LIFTING
BOUNDARY IN PLACE...BUT OVERALL INFLOW DOES NOT APPEAR TO BE
ESPECIALLY PERSISTENT OR STRONG.  RAINFALL RATES IN SLOW MOVING
STORMS TO THE SOUTH HAVE BEEN IMPRESSIVE...WITH 1.83 INCHES IN THE
LAST HOUR AT NORFOLK...AND TORRENTIAL RAINFALL AROUND OMAHA
EARLIER IN THE EVENING.

APPEARS MCV DEVELOPING NOW IN NWRN KS/SWRN NE...AND THIS WILL
FOCUS PRECIPITATION WORKING NORTHEAST THROUGH THE NIGHT AND EARLY
MORNING. MODELS...EVEN SEVERAL HIGH RESOLUTION SOLUTIONS...HAVE
ACTUALLY SLOWED THE APPROACH OF MAIN PRECIPITATION...AND THIS DOES
NOT MATCH UP WELL WITH OBSERVATIONS AND THE CURRENT TIMING OF MAIN
ADVANCING SEGMENT FROM SW. DO EXPECT SYSTEM TO EVOLVE TOWARD A BIT
MORE PROGRESSIVE LIFTING NORTHEAST...BUT TRACK RIGHT ALONG AND NW
OF THE COMMA HEAD COULD GET SOME ENHANCED RAINFALL...WHICH APPEARS
TO BE HEADING TOWARD SOUTHEASTERN SD.   AFTER DISCUSSION WITH
NEIGHBORS...DID NOT HOIST ANY FLASH FLOOD WATCH HIGHLIGHTS THIS
EVENING FOR LATER TONIGHT AND SATURDAY...BUT THIS IS NOT TO SAY
THAT THERE COULD NOT BE ANY FLOODING...AS URBAN AND POOR DRAINAGE
LOCATIONS HAVE AT LEAST A LOWER RISK FOR FLOODING TONIGHT AND
EARLY SATURDAY.

ALSO...STILL DO SEE A THREAT FOR MARGINALLY SEVERE STORMS LATE TONIGHT
APPROACHING THE MISSOURI VALLEY...GIVEN PLUME OF STEEPER MID LEVEL
LAPSE RATES SLIDING IN BEHIND CONVECTION. WILL ALSO HAVE TO WATCH
THREAT FOR SOME WIND WITH DEVELOPMENT OF MCV AND BOWING SEGMENT.
ANY SEVERE WEATHER THREAT LOOKS TO BE SECONDARY TO HEAVY RAINFALL.

&&

.SHORT TERM...(THIS EVENING THROUGH SATURDAY)
ISSUED AT 245 PM CDT FRI AUG 22 2014

SEVERAL FORECAST CONCERNS IN THE SHORT TERM THIS AFTERNOON...FROM
HEAVY RAIN TO SEVERE WEATHER POTENTIAL ON SATURDAY.  A STATIONARY
BOUNDARY HAS TAKEN SHAPE ACROSS PORTIONS OF EASTERN NEBRASKA INTO
NORTH CENTRAL IOWA THIS AFTERNOON.  A LARGELY UNCAPPED ENVIRONMENT
NOW PERSISTS ALONG AND SOUTH OF THIS BOUNDARY...WITH SCATTERED
CONVECTION DEVELOPING ALONG WEAK CONVERGENCE ZONE. GENERALLY FEEL
THIS DIURNALLY BASED ACTIVITY SHOULD REMAIN JUST SOUTH OF THE CWA
THROUGH EVENING...BUT CANNOT RULE OUT A STORM DRIFTING NORTH.
ELSEWHERE...BEGINNING TO SEE SLIGHT UPTICK IN CONVECTION OVER
PORTIONS OF NORTHER NEBRASKA THIS AFTERNOON...LINKED TO A WEAK
SHORTWAVE EMBEDDED IN THE FLOW ON NOSE OF INCREASING 500 MB JET AXIS
BEGINNING TO NOSE INTO THE PLAINS.  THIS ACTIVITY IS SLIGHTLY MORE
TROUBLING AS IT COULD DRIFT INTO THE SOUTHWESTERN CWA BY THIS
EVENING...WITH LIGHT SHOWERS AND ISOLATED THUNDERSTORMS PERSISTING
INTO EARLY EVENING. WILL BUMP UP POPS TO ACCOUNT.

THE MAIN STORY INTO THE OVERNIGHT HOURS IS STILL FOCUSED ON A MUCH
STRONGER SHORTWAVE TROUGH BEGINNING TO PIVOT OUT OF THE FOUR CORNER
REGION ATTM. MODELS CONTINUE TO TREND SLOWER AND MORE MERIDIONAL
WITH THE TRACK OF THIS SHORTWAVE INTO THE REGION AFTER MIDNIGHT.
NEVERTHELESS...ANTICIPATING CONVECTION TO BLOSSOM ACROSS WESTERN
NEBRASKA/KANSAS AS THE WAVE BEGINS TO EJECT INTO THE WESTERN PLAINS
LATER THIS EVENING. AN INCREASED LLJ WILL ALSO BEGIN TO PRODUCE A
LINEAR BAND OF SCATTERED THUNDERSTORMS FURTHER EAST ALONG A RETREATING
BAROCLINIC ZONE THROUGH EASTERN NEBRASKA AND INTO WESTERN IOWA.
SEVERE WEATHER CHANCES CONTINUE TO APPEAR LOW OVERNIGHT...BUT WOULD
NOT DISCOUNT IT GIVEN THE GRADUAL NORTHWARD ADVECTION OF MUCAPE SOME
1000-1500 J/KG TOWARD DAYBREAK AND INCREASED EFFECTIVE SHEAR.  LARGE
HAIL WOULD BE THE MAIN RISK...MAINLY ALONG AND SOUTH OF THE MISSOURI
RIVER.   ALSO QUESTION THE AREAL COVERAGE OF STORMS OVER THE EASTERN
CWA TOWARDS DAYBREAK AS MODELS SUGGEST THE LLJ WILL ACTUALLY BACK
AND FOCUS THE BEST LIFT WEST OF THE CWA.

THE HIGHEST RISK FOR HEAVY RAINFALL LOOKS TO REMAIN WEST OF THE
JAMES RIVER OVERNIGHT.  STRONG LOW-LEVEL MOISTURE TRANSPORT WILL
BRING PWAT VALUES APPROACHING 2 INCHES TO PORTIONS OF CENTRAL AND
WEST CENTRAL SOUTH DAKOTA OVERNIGHT. I WOULD ANTICIPATE ANY STORM TO
BE AN EFFICIENT RAIN PRODUCER...AND WOULDN`T RULE OUT A FLASH FLOOD
RISK ESPECIALLY ALONG THE MISSOURI RIVER. RAIN TOTALS COULD APPROACH
1 TO 3 INCHES.

THE FORECAST BECOME MORE DIFFICULT FOR SATURDAY...AND A CONDITIONAL
SEVERE WEATHER RISK COULD BECOME EVIDENT BY SATURDAY AFTERNOON.
MESOSCALE GUIDANCE CONTINUES TO LIFT A DEVELOPED SURFACE LOW
NORTHWARD INTO NORTH CENTRAL SD BY SUNDAY AFTERNOON...PULLING THE
WARM SECTOR...AND POTENTIAL 90 DEGREE READINGS...WELL NORTH INTO THE
MISSOURI RIVER BASIN.  GRADUAL BACK BUILDING OR UPSCALE GROWTH OF
LINGERING CONVECTION ALONG A FAINT MOISTURE DISCONTINUITY AND
QUASI-WARM FRONT WOULD BE POSSIBLE BY MID-AFTN...WITH MODERATE
INSTABILITY AND STRONGER MID-LVL WINDS SUPPORTING SEVERE WEATHER.
AGAIN...THIS IS HIGHLY CONDITIONAL AND DEPENDENT ON HOW CONVECTION
EVOLVES OVERNIGHT. BUT AGREE WITH THE SENTIMENTS OF THE SWODY2 THAT
CURRENT SLIGHT RISK COULD BE EXPANDED.  ANY CONVECTION THAT DOES
FORM SATURDAY AFTERNOON WILL AGAIN POSE A FLASH FLOOD RISK...MAINLY
OVER THE NORTHERN ZONES AND INTO PORTIONS OF NORTHEAST SD AND WEST
CENTRAL MINNESOTA.

.LONG TERM...(SATURDAY NIGHT THROUGH FRIDAY)
ISSUED AT 245 PM CDT FRI AUG 22 2014

SOMEWHAT ACTIVE PATTERN WILL CONTINUE INTO THE EXTENDED TIME
FRAME. COLD FRONT SHOULD BE IN CENTRAL SOUTH DAKOTA BY SUNDAY
MORNING ALLOWING FOR LINGERING TSRA AHEAD OF IT. FRONT IS SLOW TO
MAKE ITS WAY THROUGH AS THE H5 LOW AND ASSOCIATED SURFACE LOW
MOVES NORTH/NORTHEAST DRAGGING THE FRONT THROUGH BY SUN AFTERNOON
INTO THE EVENING.

NEXT CHANCE OF MORE WIDESPREAD AND POTENTIALLY HEAVY RAIN LOOKS TO
BE MON NIGHT INTO TUES NIGHT. LONGWAVE TROUGH WILL MOVE FROM PACNW
INTO THE INTERMOUNTAIN REGION. IN THE FOREFRONT...A SERIES OF WAVES
PASS THROUGH AS PRECIPITABLE WATER INCREASES TO NEAR 2 INCHES BY
TUES MORNING. LLVL JET INCREASES ENOUGH TO PROVIDE ADDITIONAL
SUPPORT FOR PERIODS OF HEAVY RAIN. MODEL QPF RANGES FROM 1-2
INCHES WITH THE GFS MOST AGGRESSIVE WITH AMOUNTS. DEPENDING ON THE
OUTCOME OF TONIGHT/SATURDAYS RAINFALL...THESE ADDITIONAL AMOUNTS
COULD BE A PROBLEM SO WILL CONTINUE TO MONITOR.

ECMWF/GFS DIFFER ON TIMING OF H5 TROUGH AND SUBSEQUENT ENDING OF
RAIN...ESPECIALLY OVER SOUTHWEST MINNESOTA AND NORTHWEST IOWA.
THE SLOWER ECMWF SEEMS TO MAKE MORE SENSE AS THE STRONG HIGH
ANCHORED OVER THE SOUTHEAST US WOULD TEND TO SLOW THINGS DOWN
UPSTREAM. WITH THAT IN MIND HAVE LINGERED POPS INTO WED EVENING
FOR THE AFOREMENTIONED AREA.

AT THIS TIME FROM WED NIGHT INTO FRIDAY THE AREA WILL REMAIN DRY.

TEMPERATURES THROUGHOUT THE EXTENDED PERIOD WILL REMAIN BELOW
NORMAL IN THE MID TO UPPER 70S.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)
ISSUED AT 1149 PM CDT FRI AUG 22 2014

PRIMARY AVIATION CONCERN THROUGH THIS TAF PERIOD WILL BE POTENTIAL
FOR CONVECTION...AND ASSOCIATED MVFR-IFR CEILINGS/VISIBILITY...
MAINLY EXPECTED TO AFFECT SD PORTIONS OF THE FORECAST AREA. MORE
WIDESPREAD CONVECTION OVER NEBRASKA WILL LIFT INTO SOUTH DAKOTA
THROUGH THE OVERNIGHT HOURS AND INTO SATURDAY MORNING. PERIODIC
RESTRICTIONS IN VISIBILITY AND CEILINGS WILL BE LIKELY AS THIS
ACTIVITY MOVES THROUGH.

PERIOD OF MVFR-LOCALLY IFR STRATUS MAY FOLLOW CONVECTION SATURDAY
MORNING...IMPROVING TO VFR AS WARM FRONT APPROACHES FROM THE SOUTH.
ADDITIONAL CONVECTIVE DEVELOPMENT POSSIBLE WITH THE WARM FRONT
SATURDAY AFTERNOON. NORTHWEST PORTIONS OF THE FORECAST AREA APPEAR
TO BE MORE FAVORED FOR THIS AFTERNOON DEVELOPMENT...AND HAVE ADDED
MENTION INTO KHON FOR THE MID-LATE AFTERNOON. CAPPING INVERSION MAY
LIMIT DEVELOPMENT FARTHER SOUTHEAST AND HAVE KEPT AFTERNOON/EVENING
THUNDER OUT OF KFSD/KSUX AT THIS TIME.

&&

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

$$

UPDATE...CHAPMAN
SHORT TERM...DUX
LONG TERM...SALLY
AVIATION...JH









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