Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 301148
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
648 AM CDT SAT AUG 30 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 437 AM CDT SAT AUG 30 2014

ISOLATED SHOWERS AHEAD OF WEAK NORTHERN STREAM WAVE IN WEST CENTRAL
MN HAVE COME TO AN END ACROSS SW MN EARLY THIS MORNING...AND NOW
WATCHING MASS OF STRATUS SPILLING SOUTHWARD ACROSS THE NORTHERN
PLAINS.  THROUGH AT LEAST MIDDAY...THIS STRATUS SHOULD BE QUITE
PERSISTENT OVER MUCH OF THE CWA...ERODING IN THE VICINITY OF THE LOW
LEVEL RIDGE AXIS NEAR THE JAMES VALLEY. CLOUDS MAY BE SOMEWHAT MORE
PERSISTENT EAST...STYMIED BY FAIRLY WEAK FLOW TOWARD I29 AND
SOUTHWEST MINNESOTA...BUT SHOULD SEE A BETTER DECREASING TREND BY
AFTERNOON. THIS WILL IMPACT POTENTIAL HIGHS TODAY...WHICH WILL
LIKELY RANGE FROM MID TO UPPER 70S EAST...TO MID 80S WEST.

HEADING THROUGH THE NIGHT...ENERGY ACROSS THE PACIFIC NORTHWEST WILL
BEGIN TO NUDGE OUT INTO THE NORTHERN PLAINS. FALLING PRESSURES TO
THE WEST WILL INCREASE SOUTHERLY WIND FLOW...ESPECIALLY OFF SURFACE
AS A 40 TO 50 KNOT LOW LEVEL JET DEVELOPS BY LATE EVENING...VEERING
TOWARD THE FSD CWA BY THE FINAL HALF OF THE NIGHT. AIR AT THIS LEVEL
REMAINS FAIRLY DRY...WITH MOISTURE SOURCE COMING MORE ALOFT NEAR OR
ABOVE 700 HPA. ALL PRECIP TONIGHT IS DEPENDENT ON HIGH BASED SHOWERS
AND THUNDERSTORMS DEVELOPING IN WARM AIR ADVECTION ON NOSE OF
STRONGER ELEVATED MIXED LAYER ACTUALLY BEING SIGNIFICANT ENOUGH TO
BREAK THROUGH THE DRIER LAYER BELOW. AS INITIAL PV LOBE LIFTS OUT
ACROSS THE AREA AFTER 06Z...COULD SEE AT LEAST A LOWER COVERAGE OF
PRECIPITATION...BUT CERTAINLY DOES NOT WARRANT MORE THAN A LOW
CHANCE AT THIS POINT WITH THE DEGREE OF DRY AIR AT LOWER
LEVELS...AND WITH BETTER CONVERGENCE OF THE LOW LEVEL JET TO THE
NORTH. SHADED THE HIGHEST CHANCE BY LATER TONIGHT ACROSS THE HIGHWAY
14 CORRIDOR.

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 437 AM CDT SAT AUG 30 2014

FOCUS OF THIS FORECAST IS CERTAINLY ON THUNDERSTORM POTENTIAL AND
ASSOCIATED SEVERE WEATHER RISK SUNDAY...DURING THE MIDDLE OF THE
HOLIDAY WEEKEND. NAM SEEMED TO BE AN OUTLIER IN PATTERN TIMING
AND/OR LOCATION, SO HAVE LARGELY FAVORED A GFS/ECMWF BLEND. WITH
THIS...ANTICIPATE ANY EARLY MORNING ELEVATED CONVECTION TO LIFT
NORTH OF THE AREA/DIMINISH THROUGH MID MORNING...WITH PRIMARY
FOCUS THEN SHIFTING TO SOUTHEAST PORTIONS OF THE CWA AS ONE OF
MANY WAVES SLIDES THROUGH THE MID LEVEL TROUGH MOVING INTO THE
NORTHERN PLAINS. LOCATION OF LOW LEVEL BOUNDARIES MAY BE IMPACTED
BY ANY MORNING CONVECTION...BUT ON BROAD SCALE SHOULD SEE COLD
FRONT INTO WESTERN CWA BY MIDDAY...WITH WARM FRONT LOCATION A
LITTLE MORE UNCERTAIN. PREFERRED MODELS POINT TO STRONGEST LOW
LEVEL WARM ADVECTION ALONG 850MB WARM FRONT EXTENDING INTO EASTERN
NEBRASKA AT MIDDAY...THEN LIFTING NORTHWARD INTO NORTHWEST IOWA
WITH THE MID LEVEL WAVE BY LATE AFTERNOON/EVENING. THIS WAVE
ACCOMPANIED BY STRONG DEEP LAYER SHEAR OF 40+KT AND MUCAPE VALUES
IN EXCESS OF 2500J/KG. FORECAST SOUNDINGS SHOW A WEAKENING BUT
STILL EXISTENT CAPPING INVERSION ABOVE 850MB AT KSUX AND OTHER
POINTS IN OUR CWA AT 21Z...BUT THE APPROACHING WAVE SHOULD HELP
OVERCOME THIS...WITH THE CAP ALLOWING FOR MORE DISCRETE INITIAL
CELL DEVELOPMENT IN THE LATE AFTERNOON TO EARLY EVENING. THIS
SHOULD BE TIME OF GREATEST SEVERE POTENTIAL... WITH LARGE
HAIL...DAMAGING WINDS AND ISOLATED TORNADOS ALL POSSIBLE.

THEREAFTER SYSTEM SHOULD CONGEAL INTO PROGRESSIVE CLUSTER OF STORMS
THROUGH THE EVENING...WHICH THEN SHIFTS EAST OF OUR FORECAST AREA BY
EARLY MONDAY MORNING. DUE TO EXPECTED PROGRESSIVENESS OF SYSTEM...DO
NOT ANTICIPATE WIDESPREAD FLOODING THREAT FROM TRAINING STORMS...BUT
ISOLATED MAINLY URBAN FLASH FLOODING POSSIBLE WITH DEEP MOISTURE
PROVIDING POTENTIALLY HEAVY RAINFALL RATES.

LABOR DAY SHOULD BE QUIET AND COOLER WITH HIGHS BACK INTO THE 70S.
ECMWF/GEM BOTH SHOWING ANOTHER WEAK WAVE TRACKING THROUGH THE REGION
AND BRINGING A CHANCE OF STORMS TO THE AREA MONDAY NIGHT.

LONGER RANGE TUESDAY THROUGH FRIDAY LOOKS TO START OFF AS A FAIRLY
QUIET PERIOD...WITH COOLER HIGHS IN THE 70S TUESDAY WARMING BACK
INTO THE 80S WEDNESDAY/THURSDAY. FAIR CONSENSUS BETWEEN THE GFS AND
ECMWF SHOWING AN INCREASED PRECIP POTENTIAL BY THE END OF THE WEEK.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z SUNDAY MORNING)
ISSUED AT 647 AM CDT SAT AUG 30 2014

IFR TO LIFR CEILINGS WITH STRATUS ENCOMPASSING TAF SITES EARLY
THIS MORNING...AS WELL AS AREAS OF IFR TO MVFR VISIBILITY MAINLY
ACROSS SOUTHWEST MINNESOTA AND NORTHWEST IOWA. WEAKENING LOW LEVEL
FLOW WILL MAKE DECREASE IN STRATUS SOMEWHAT A DIFFICULT PROSPECT...
AND HAVE GONE A BIT SLOWER DECREASE THAN SLOWEST MODEL FOR KFSD...AND
CLOSER TO SLOWER RAP FOR KHON AND KSUX NEARER THE EDGE OF THE FIELD.

&&

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

$$

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


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