Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 251103
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
603 AM CDT MON AUG 25 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 413 AM CDT MON AUG 25 2014

SOME PATCHY AREAS OF ADVECTION FOG HAVE DEVELOPED BEHIND THE SURFACE
FRONT IN NORTHWEST IOWA THIS MORNING.  EXPECT THE FOG TO REMAIN
RELATIVELY PATCHY...AND BURN OFF QUITE QUICKLY THIS MORNING.
NORTHWEST FLOW DRAWING DRIER AIR INTO THE REGION...HOWEVER THE FRONT
TO THE SOUTH OF THE FORECAST AREA IS EXPECTED TO STALL AND LIFT
NORTH TONIGHT AS LOW LEVEL JET INCREASES AND CONVECTION DEVELOPS.

AS OF 08Z...AREA OF DISORGANIZED CONVECTION ASSOCIATED WITH A WEAK
SHORTWAVE ACROSS NORTHERN COLORADO INTO SOUTHERN WYOMING IS EXPECTED
TO LIFT NORTHEAST THROUGHOUT THE DAY.  THIS CONVECTION IS MOVING
INTO A MORE STABLE ENVIRONMENT...AND THEREFORE BACKED OFF ON THE
POPS REMOVING THEM ALL TOGETHER THROUGH THE MORNING HOURS...AND ONLY
HAVE A SLIGHT CHANCE ACROSS SOUTH CENTRAL SOUTH DAKOTA THIS
AFTERNOON.  THEREAFTER...THE FOCUS SHIFT SOUTH WITH STALLED
BOUNDARY.  INITIALLY...EXPECT BOUNDARY AND ASSOCIATED CONVECTION TO
DEVELOP SOUTH OF THE FORECAST AREA...BUT WITH TIME ALL THE MODELS
SHOW THE BOUNDARY INCHING NORTH THROUGH THE OVERNIGHT HOURS.  THE
PRECISE PLACEMENT OF THE FRONT IS STILL UP FOR DEBATE...WITH THE NAM
FAIRLY PROGRESSIVE WHILE THE GFS...ECMWF AND GEM MUCH SLOWER TO
BRING THE FRONT TO THE NORTH.  WITH THE FORCING BOTTLED WELL TO THE
SOUTH...CONTINUED TO TRIM ON THE NORTHERN EXTENT OF THE POPS
ESPECIALLY NORTH OF I-90.  STILL KEPT A FAIRLY TIGHT POP GRADIENT
ACROSS NORTHWEST IOWA...WITH HIGH POPS IN THE SIOUX CITY TO STORM
LAKE CORRIDOR. THE KEY IN TIMING WILL BE TO WATCH MOISTURE RETURN IN
THE 700-800 MB LAYER...FOR WITHOUT MOISTURE IN THIS LAYER CONVECTION
WILL FAIL TO DEVELOP.  SEVERE THREAT IS GENERALLY LOW WITH VERY
ELEVATED STORMS AND LINEAR HODOGRAPHS...BUT CAN/T RULE OUT AN
ISOLATED SEVERE STORM WITH ELEVATED CAPE OF 1500 J/KG OR SO.

.LONG TERM...(TUESDAY THROUGH SUNDAY)
ISSUED AT 413 AM CDT MON AUG 25 2014

VERY PERSISTENTLY WET PATTERN SETS UP OVER THE FORECAST AREA IN THE
MID AND LONG RANGE...WITH THE SOUTHERN FORECAST AREA - PARTICULARLY
ACROSS THE US HIGHWAY 20 CORRIDOR OF NORTHWEST IOWA AND NORTHEAST
NEBRASKA.

LARGE AND SLOW MOVING UPPER LOW NEAR THE NORTH DAKOTA AND CANADIAN
BORDER GRADUALLY LIFTS NORTHEAST WHILE TROUGHING ALONG THE PACIFIC
COAST MEANDERS ACROSS THE INTERMOUNTAIN WEST THROUGH THURSDAY. THIS
LOCKS THE CENTRAL AND NORTHERN PLAINS IN A MOIST SOUTHWEST TO WEST
UPPER LEVEL FLOW FOR MUCH OF THE WEEK AHEAD. THE MID LEVEL THETA-E
GRADIENT NEAR THE NEBRASKA BORDER INTO NORTHWEST IOWA TUESDAY
MORNING WILL WEAKEN AND SHIFT SLIGHTLY SOUTHWARD BY TUESDAY
AFTERNOON...AS SOME MINOR MID LEVEL SUBSIDENCE NOSES INTO OUR
SOUTHEASTERN CORNER. THIS SHOULD ALLOW SHOWERS AND THUNDERSTORM
CHANCES TO DWINDLE SLIGHTLY IN AFTERNOON...BUT CLOUDS WILL LIKELY
STICK AROUND FOR MUCH OF THE DAY. HAVE LOWERED AFTERNOON POPS TO
SLIGHTS AND CHANCES. THIS BREAK WILL BE SHORT LIVED ONCE THE NEXT
IMPULSE RIDES THE SOUTHWEST FLOW INTO THE CENTRAL PLAINS AND
INTERACTS WITH THE STALLED MID LEVEL BOUNDARY NEAR OUR SOUTHERN
BORDER. THIS FEATURE WILL SPREAD ANOTHER ROUND OF SHOWERS AND
THUNDERSTORMS INTO OUR SOUTHERN FORECAST AREA TUESDAY NIGHT.
PRECIPITABLE WATER VALUES IN THE SIOUX CITY AREA REACH VERY HIGH
VALUES AROUND 2 INCHES...WHICH WILL MEAN THAT THUNDERSTORMS THAT DO
DEVELOP WILL BE VERY HEAVY RAIN PRODUCERS WITH HIGH RAINFALL RATES.
AGAIN...THE MAIN FOCUS FOR STORMS WILL BE ALONG THE US 20 CORRIDOR
IN OUR FORECAST AREA WHILE CHANCES TAPER OFF VERY QUICKLY MOVING
NORTH OF INTERSTATE 90.

MODELS BEGIN DIVERGING A BIT ON WEDNESDAY...WITH THE GFS AND NAM
IMMEDIATELY RUNNING THE NEXT WAVE INTO THE AREA ON WEDNESDAY AND
MOVING IT SLOWLY ACROSS NORTHERN NEBRASKA AND NORTHWEST IOWA INTO
WEDNESDAY NIGHT. THE ECMWF AND GEM ARE ABOUT 12 HOURS SLOWER IN
INTRODUCING THIS FEATURE. AGAIN...THE MAIN FOCUS REMAINS NEAR OUR
SOUTHERN BORDER WHERE ANOTHER ROUND OF VERY HEAVY RAINFALL WILL BE
POSSIBLE. SIDED WITH THE GFS AND NAM SOLUTION FOR NOW...WHICH KEEPS
A STEADY STREAM OF PRECIPITATION INTO WEDNESDAY NIGHT.

THE GFS...ECMWF AND GEM ALL HAVE MOVED TO A SLOWER SOLUTION IN
DRYING OUT THE REGION LATE WEEK...AS THE TROUGH OPENS UP OVER THE
HIGH PLAINS AND MAKES VERY LITTLE EASTWARD PROGRESSION. MOISTURE
CONTINUES TO STREAM INTO THE PLAINS THURSDAY AND THURSDAY
NIGHT...HOWEVER THERE IS SOME INDICATION THAT THE MID LEVEL BOUNDARY
DOES SLOWLY SHIFT SOUTHEAST OF THE FORECAST AREA. THE ECMWF REMAINS
THE SLOWEST...HOWEVER THE OTHER MODELS DO KEEP THINGS ACTIVE IN OUR
SOUTHEASTERN THIRD THROUGH AT LEAST THURSDAY NIGHT OR FRIDAY.

STORM TOTAL RAINFALL OF 2 TO 4 INCHES...AND LOCALLY EVEN HIGHER
AMOUNTS WILL BE LIKELY. WITH PERSISTENT RAINFALL IN THE SAME AREA
FOR MULTIPLE DAYS...THERE IS THE CONCERN FOR FLASH FLOODING AND
RISING STREAMS AND RIVERS. WITH PLENTY OF CLOUDS AND RAIN IN THE
FORECAST...HIGH TEMPERATURES WILL BE BELOW NORMAL THROUGH THE
PERIOD...IN THE UPPER 60S AND 70S.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z TUESDAY MORNING)
ISSUED AT 600 AM CDT MON AUG 25 2014

VFR CONDITIONS WILL PREVAIL THROUGH THE DAY MONDAY...WITH WEST TO
NORTHWEST WINDS BELOW 15KT. CHANCE OF LATE AFTERNOON CONVECTION
RETURNS TO SOUTHERN PARTS OF THE FORECAST AREA AFTER 22Z...WITH
CHANCES INCREASING AFTER 26/03Z. MVFR OR LOWER VISIBILITY TEMPORARILY POSSIBLE
WITH ANY TSRA THAT DEVELOP...MOST LIKELY TO AFFECT KSUX DURING THE
OVERNIGHT HOURS MONDAY THAN TAFS FARTHER TO THE NORTH.

&&

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

$$

SHORT TERM...
LONG TERM...
AVIATION...





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