Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 261133
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
633 AM CDT WED AUG 26 2015

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 406 AM CDT WED AUG 26 2015

SURFACE HIGH PRESSURE EXPANDS OVER THE UPPER MISSISSIPPI VALLEY
TODAY WHILE UPPER LEVEL RIDGING CENTERED OVER THE HIGH PLAINS NUDGES
EASTWARD. SOUTHEASTERLY WINDS WILL INCREASE THROUGH MIDDAY...
BECOMING BREEZY WEST OF I29 IN THE AFTERNOON. THIS WILL ALSO ALLOW
MOISTURE TO GRADUALLY INCREASE THROUGH THE DAY AS DEWPOINTS CREEP UP
TO AROUND 60 BUT A STRONGLY CAPPED ENVIRONMENT WILL HOLD OFF ANY
PRECIPITATION. SHALLOW VALLEY FOG THIS MORNING WILL QUICKLY
DISSIPATE. OTHERWISE SKIES WILL BE MOSTLY SUNNY WITH HIGHS RANGING
FROM THE MID AND UPPER 70S EAST OF I29 TO THE UPPER 80S IN SOUTH
CENTRAL SD.

MID AND HIGH LEVEL CLOUDS WILL BEGIN TO BUILD INTO THE AREA FROM THE
WEST AHEAD OF A POTENT UPPER LEVEL WAVE PUSHING INTO WYOMING THIS
EVENING. MID LEVEL THERMAL BOUNDARY WILL BEGIN NOSING INTO SOUTH
CENTRAL SD IN THE EVENING AND NUDGE INTO SOUTH CENTRAL SD LATE
TONIGHT. WHILE THERE COULD BE A FEW ACCAS SHOWERS POPPING UP IN THE
EVENING IN SOUTH CENTRAL...THE MAIN PRECIPITATION THREAT WILL LIKELY
HOLD OFF UNTIL AROUND MIDNIGHT WHEN THE ARRIVAL OF THE LOW LEVEL JET
MAY HELP TRIGGER CONVECTIVE DEVELOPMENT. A DECENT AMOUNT OF ELEVATED
CAPE...MODEST EFFECT SHEAR AND STEEP MID LEVEL LAPSE RATES WILL
ALLOW FOR A MARGINAL SEVERE WEATHER RISK MAINLY WEST OF THE JAMES
RIVER...WITH HAIL TO QUARTER SIZE AND GUSTS TO 65 MPH THE MAIN
THREATS. AREAS ALONG AND EAST OF I29 SHOULD SEE STORMS HOLD OFF
THROUGH SUNRISE THURSDAY.

.LONG TERM...(THURSDAY THROUGH TUESDAY)
ISSUED AT 406 AM CDT WED AUG 26 2015

MAIN ATTENTION IN THE LONGER RANGE OF THE FORECAST IS VERY FRONT
WEIGHTED...DEALING WITH UPPER WAVE MOVING ACROSS THE AREA FROM
THURSDAY THROUGH FRIDAY. MOST MODELS INDICATE AN AMPLIFYING TROUGH
PRESSING EASTWARD...WITH A FORMIDABLE DEEPENING OF THE WAVE AS IT
SLIDES THROUGH. SYSTEM APPEARS TO ORIGINATE WITHIN THE SUBTROPICAL
SIDE OF THE MAIN WESTERLIES...CURRENTLY SOUTH OF THE GREAT SALT
BASIN. WAVE WILL CROSS THE MEAN RIDGE ON THURSDAY...STARING A SLOW
EAST SOUTHEAST TREK ACROSS THE AREA LATER THURSDAY INTO EARLY
FRIDAY. MOST SOLUTIONS HAVE ALSO SLOWED A BIT ON PROGRESSION WITH
THE GREATER DEEPENING OF THE LOWER TO MID LEVEL CYCLONE...AND HAVE
SOME FEAR THAT THIS RESULTS FROM CONVECTIVE FEEDBACK. INTERPRETATION
COULD ALSO YIELD POTENTIAL FOR RESIDUAL MCV WHICH REACTIVATES LATER
AFTERNOON AND EVENING AS IT DIGS EAST/SOUTHEAST THROUGH THE HEART OF
THE FSD CWA...OR JUST A SHADE SOUTHWARD...DEPENDING ON SOLUTION.

NEVERTHELESS...LOOKS AS IF A SOGGY SCENARIO SHOULD EVOLVE ACROSS THE
AREA...ESPECIALLY THURSDAY NIGHT INTO EARLY FRIDAY. SCATTERED
CONVECTION IS LIKELY ON THURSDAY MORNING ARCING ALONG THE NOSE OF
THE PROGRESS OF THE ELEVATED MIXED LAYER/CONVERGENCE OF 850-700 HPA
FLOW...BUT COVERAGE STILL IN QUESTION ENOUGH TO SEE POPS REMAIN
TOWARD MID TO HIGH CHANCE THROUGH THE MORNING. IN FACT...MUCH OF THE
DAY...ESPECIALLY HEADING THROUGH THE AFTERNOON...WILL BE IN AN
ENVIRONMENT OF INCREASING DIV Q...AND WILL BE HARD TO SAY THERE MAY
NOT BE A HIGHER BASED RANDOM SHOWER OR STORM ABOUT ANYTIME THROUGH
THE DAY...BUT ESPECIALLY SO AGAIN BY MID TO LATE AFTERNOON AS
STRONGER LIFT FORCING BEGINS TO ENCROACH INTO THE WESTERN CWA. VERY
FAVORABLE ORIENTATION OF LOW LEVEL JET GOING THROUGH THURSDAY NIGHT
AS IT SLOWLY VEERS AHEAD OF THE UPPER WAVE...BUT ALSO DEVELOPS A
WEAK TROWAL LIKE STRUCTURE TO THE PRECIPITATION AS WRAP AROUND
FLOW/FRONTAL FORCING INTENSIFIES TO THE NORTH OF THE MID LEVEL WAVE.
THE PROBLEM WITH HAVING A FAR NORTHWARD SOLUTION SUCH AS THE
SIGNIFICANT SHIFT IN THE ECMWF IS LOCATION OF THE SURFACE-925 HPA
BOUNDARY...WHICH IN ALL RIGHTS SHOULD REMAIN SOUTH TOWARD I80
THROUGH THURSDAY...LIFTING NORTH TOWARD NORTHWEST IOWA ON THURSDAY
NIGHT.  IF ACCEPTING THE BULK OF MORE SOUTHWARD DYNAMICAL SOLUTIONS
THIS WOULD BRING FAIRLY HIGH CONFIDENCE TO SOME HEAVY PRECIPITATION
POTENTIAL ACROSS AREAS NEAR INTERSTATE 90 AND SOUTHWARD...ESPECIALLY
ACROSS NORTHWEST IOWA. PRECONVECTIVE VALUES OF PRECIPITABLE WATER
AROUND 1.5 TO 1.6 INCHES...MORE THAN SUFFICIENT. OVERALL...THE
INSTABILITY IS NOT TREMENDOUS...AND AFTER INITIAL DEVELOPMENT...
WILL LIKELY SEE MUCH MORE PRECIPITATION COVERAGE THAN THUNDER...
ESPECIALLY DURING THE LATTER OVERNIGHT AND INTO FRIDAY. HAVE SLOWED
UP THE EXIT OF THE PRECIPITATION /AND CLOUDS/ ON FRIDAY AS WRAPPED
UP LOW WANDERS EASTWARD. BY THE TIME RAINFALL THREAT DWINDLES IN THE
AFTERNOON ON FRIDAY...WILL LIKELY SEE A GOOD AREA OF 1 TO 2 INCHES
OF RAINFALL...WITH SOME ACROSS NORTHWEST IOWA APPROACHING 3 INCHES.

SEVERE WEATHER RISK WOULD APPEAR TO BE QUITE MINIMAL THROUGH
THURSDAY NIGHT...BUT WILL HAVE TO MONITOR TRENDS FOR HYDROLOGICAL
ISSUES...AS SECTIONS OF NORTHWEST IOWA HAVE BEEN QUITE MOIST WITHIN
THE LAST WEEK OR TWO.

THURSDAY TEMPS ARE SOMEWHAT DIFFICULT GIVEN THE POTENTIAL SCATTERED
NATURE OF THE PRECIP. PLENTY OF CLOUDS SHOULD BE AROUND...BUT DID
NOT DESIRE TO GO TOO COOL GIVEN THE SLIGHT WARMING TO TEMPS ALOFT.
SHOULD BE MAINLY 70S FOR TEMPS...A SHADE COOLER THAN TODAY FOR
MOST...BUT PERHAPS A FEW LOWER 80S TOWARD THE MISSOURI VALLEY.
HIGHER MOISTURE CONTENT SHOULD DRIVE LOWS CLOSER TO A WET BULB ON
THURSDAY NIGHT..UPPER 50S TO LOWER 60S...WARMEST IN THE SOUTH/WEST.
FRIDAY WILL FEATURE A STUBBORN BUNCH OF LOWER CLOUDS ALONG WITH
LINGERING PRECIPITATION...AND MAY TAKE TOO LONG TO EVEN ALLOW TEMPS
MUCH PAST 70 DEGREES IN THE EASTERN CWA...WHILE SOME LATER DAY
CLEARING AND A LESS SOGGY TRAJECTORY FOR THE WEST COULD SEE SOME
READINGS RETURN TO AROUND 80.

BEHIND THIS SYSTEM...UPPER LEVEL RIDGE IS FAIRLY QUICK TO BUILD...
AND LOW LEVEL FLOW WILL QUICKLY RETURN TOWARD A MORE SOUTHERLY
PREFERRED DIRECTION. THIS SHOULD SPELL A RETURN TO A QUIET WEATHER
PERIOD AS WESTERLIES RETREAT NORTHWARD...AND CAPPING TEMPERATURES
BUILD ALOFT.  NOT LIKELY TO SEE ANY MEANINGFUL PRECIPITATION THREAT
FROM SATURDAY ONWARD...WITH POTENTIAL EXCEPTION OF HAVING A WEAK
BOUNDARY PROGRESS SOUTHWARD INTO THE AREA AROUND TUESDAY OR TUESDAY
NIGHT. EVEN THEN...WILL BE LITTLE MORE THAN A FOCUS FOR A VERY SMALL
CHANCE FOR PRECIPITATION. TEMPERATURES ARE LIKELY TO RETURN TO
NORMAL TO SOMEWHAT ABOVE NORMAL READINGS THROUGH THE REMAINDER OF
THE LONGER RANGE SATURDAY AND BEYOND...MAINLY 80S FOR HIGHS AND 60S
FOR LOWS.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z THURSDAY MORNING)
ISSUED AT 625 AM CDT WED AUG 26 2015

VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. SOME SHALLOW
VALLEY FOG NEAR RIVERS EARLY THIS MORNING WILL LIFT WITHIN THE
NEXT HOUR AND IS NOT EXPECTED TO IMPACT TAF SITES. OTHERWISE...
LOOK FOR SOUTHEAST WINDS TO INCREASE TODAY AROUND 10 TO 15 KT.
ISOLATED HIGH BASED SHOWERS OR THUNDERSTORMS WILL BE POSSIBLE
MAINLY NORTH OF I90 IN EAST CENTRAL SD THROUGH 15Z. AFTER 06Z
TONIGHT...SCATTERED THUNDERSTORMS WILL BE POSSIBLE ALONG AND WEST
OF THE JAMES RIVER...BUT COVERAGE IS TOO LOW FOR MENTION IN THE
KHON TAF.

&&

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

$$

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


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