Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 291159
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
659 AM CDT FRI AUG 29 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 417 AM CDT FRI AUG 29 2014

SEEING SPOTTY ACTIVITY...MAINLY SHOWERS...IN BROAD LIFT AHEAD OF
UPPER TROUGH EDGING INTO EASTERN SOUTH DAKOTA EARLY THIS MORNING.
WITH EXTENDED FETCH OF MOISTURE THROUGH THE CENTRAL PLAINS AHEAD OF
THIS TROUGH...SEE NO REASON WHY POTENTIAL FOR SCATTERED SHOWERS AND
ISOLATED THUNDER WOULD DIMINISH AND WILL HOLD ONTO MID RANGE POPS
NEAR/AHEAD OF WEAK SURFACE BOUNDARY WHICH WILL PUSH ONLY SLIGHTLY
EASTWARD FROM NEAR JAMES VALLEY...TO NEAR THE HIGHWAY 60 CORRIDOR
THROUGH NORTHWEST IOWA/SOUTHWEST MINNESOTA BY AFTERNOON. WITH DEEP
LIFT AND MOISTURE...WOULD EXPECT PRECIP COVERAGE TO INCREASE EAST
OF THE FRONT THROUGH THE EARLY-MID AFTERNOON. EXTENSIVE CLOUD COVER
WILL LIMIT WARMING ACROSS THE EAST...AND SHEAR IS FAIRLY WEAK...SO
WOULD NOT EXPECT MUCH OF A SEVERE THREAT. HOWEVER...BREAKS IN THE
CLOUD COVER COULD LEAD TO POCKETS OF GREATER INSTABILITY AND
ISOLATED STRONGER STORMS.

FARTHER TO THE NORTHWEST...EXPECT AT LEAST PARTIAL CLEARING BEHIND
THE BOUNDARY TO ALLOW FOR GREATER HEATING. SUBTLE IMPULSE SLIDING
THROUGH THE WESTERN DAKOTAS MAY BE ENOUGH TO TRIGGER A FEW STORMS IN
OUR NORTHWEST AREAS IN THE LATE AFTERNOON/EVENING. SLIGHTLY STEEPER
MID LEVEL LAPSE RATES COULD ALLOW FOR SOME STRONGER STORMS IN OUR
FAR WEST DURING PEAK HEATING...BUT STILL DO NOT SEE A WIDESPREAD
SEVERE THREAT DEVELOPING AT THIS TIME GIVEN MEAGER DEEP LAYER SHEAR.
ANTICIPATE HIGHS TODAY RANGING FROM MID TO UPPER 70S EAST...TO THE
LOWER TO MID 80S WEST WHERE POTENTIAL TO BREAK OUT OF CLOUDS IS
GREATER.

NORTHWEST ACTIVITY COULD LAST A LITTLE PAST SUNSET AS IT DRIFTS
THROUGH NORTHERN PARTS OF THE CWA...BUT OVERALL EXPECT DOWNWARD
TREND IN CONVECTION THROUGH 06Z. QUITE A BIT OF LINGERING MOISTURE
WILL NOT ALLOW FOR TOTAL CLEARING...AND WITH PERSISTENT NEAR SURFACE
MOISTURE IN WEAK WIND FIELD...POTENTIAL FOR PATCHY FOG DEVELOPMENT
THROUGH THE EARLY MORNING HOURS.

.LONG TERM...(SATURDAY THROUGH THURSDAY)
ISSUED AT 417 AM CDT FRI AUG 29 2014

SHORT WAVE RIDGING WILL HAVE FULL HOLD ON SATURDAY BEHIND EXITING
SHORT WAVE. INITIALLY ARE LIKELY TO SEE SOME PATCHY FOG/LOWER
CLOUDS ACROSS THE AREA...AS WEAK BOUNDARY SETTLES SOUTH TO STALL
EARLY IN THE DAY ACROSS THE SOUTHERN CWA...OR MORE LIKELY JUST
SOUTH. WEAK FLOW OVERALL AND MOIST GROUND WILL LIMIT WARMING MOST
AREAS...BUT MORE LIKELY TO GET A LITTLE WARMER IN THE WEST WITH
BETTER POTENTIAL FOR A GREATER AMOUNT OF MIDDAY AND AFTERNOON
SUNSHINE. HIGHS SHOULD REACH UPPER 70S EAST TO MID 80S WEST.

DELAYED ONSET OF PRECIPITATION SATURDAY NIGHT AS OVERALL WAVE
PUSHING THROUGH THE A BIT SLOWER... AND WILL HAVE SOME DRIER AIR
TO DISPLACE WORKING EAST INTO THE PLAINS. DIV Q ASSOCIATED WITH
THE STRONG JET STREAK INTO THE NORTHERN ROCKIES WILL ONLY APPROACH
THE AREA AFTER 06Z...AND APPEARS THAT ANY MEANINGFUL CONVERGENCE
WITH THE LOW LEVEL JET WILL BE WELL NORTH OF THE AREA FOR MUCH OF
THE NIGHT. HAVE MAINTAINED SOME LOWER CHANCE POPS LATE NIGHT
ROUGHLY WEST OF A MARSHALL TO YANKTON LINE.

IF PRECIPITATION IS NOT TOO WIDESPREAD ON EARLY SUNDAY...THIS
BECOMES A GREATER CONCERN ON SUNDAY AFTERNOON AND EVENING IN TERMS
OF A GREATER SEVERE WEATHER THREAT. STRONG WAVE ACROSS THE
NORTHERN ROCKIES WILL SHOVE OUT INTO THE PLAINS...KNOCKING DOWN
THE WARMER LAYER ALOFT. WILL HAVE A VERY DISTINCT FRONTAL BOUNDARY
PUSHING EASTWARD...WITH QUESTION OF TIMING ALONG WITH DIURNAL
CYCLE AND APPROACHING LARGE SCALE DYNAMIC LIFT. AS EXPECTED...NAM
AND GFS HAVE TRADED ROLES SOMEWHAT ON FRONTAL TIMING...WITH NAM
NOW THE SLOWEST...GFS AMONG QUICKEST...ALTHOUGH ADMITTEDLY THE
SPREAD HAS DIMINISHED...AND UPPER PATTERNS ARE NOT DISSIMILAR IF
SLOWING UP GFS ABOUT 3 HOURS. LOOKING AT POTENTIAL PARAMETERS NEAR
THE BOUNDARY ON SUNDAY AFTERNOON AND EVENING...THE LOW LEVEL JET
IN PLACE WILL MEAN A SIGNIFICANTLY ENHANCED 0-1KM SHEAR...AS MUCH
AS 35 TO 40 KNOTS NEAR BOUNDARY AND INTO THE WARM SECTOR. A LOOK
BEFORE MODELS CONVECT...WOULD ALLOW PERHAPS 1500-2000 J/KG MLCAPE...
AND THIS MAY END UP EVEN A BIT HIGHER IF CAN KEEP CLOUD COVERAGE
DOWN A BIT MORE AND PRECIP FROM SPREADING EAST TOO QUICKLY.
ADMITTEDLY...SOMEWHAT BETTER DEEP LAYER SHEAR WILL OCCUR FOR
SOUTHERN CWA...AS UPPER JET PUSHES MORE STRONGLY TOWARD CENTRAL
PLAINS...AND WILL ALSO BE LOCATION IN BETTER PROXIMITY TO
POTENTIALLY RICHER INSTABILITY. STRONG PV ADVECTION WILL PUSH OUT
TOWARD THE FRONT TOWARD 00Z...SO WOULD EXPECT A FESTERING BOUNDARY
TO DEMONSTRATE A SIGNIFICANT UPTICK IN CONVECTIVE INTENSITY AND
COVERAGE TOWARD 21-23Z. DEEP SHEAR WILL KEEP ALL STORM MODES IN
PLAY...PERHAPS SOMEWHAT MORE TOWARD AN EVENTUAL LINEAR STRUCTURE
GIVEN FORCING. HAIL... DAMAGING WINDS...AND EVEN A TORNADO THREAT
FROM MID AFTERNOON THROUGH AT LEAST MID EVENING...PRIMARILY FROM
NEAR I 29 AND EASTWARD...ESPECIALLY THE LOWER MISSOURI VALLEY
THROUGH NORTHWEST IOWA. INTENSE RAIN RATES ALSO LIKELY WITH STORMS
GIVEN RICH MOISTURE/WARM PROFILES...BUT EVENTUALLY EXPECT LINEAR
MODE TO KEEP TOTALS FROM BEING TOO EXCESSIVE. WHAT FALLS WILL DO
SO QUITE QUICKLY...HENCE AT LEAST A MENTION OF FLASH FLOOD
POTENTIAL IN THE HWO.

FRONTAL PRECIPITATION WILL HAVE EXITED EVEN THE EASTERN CWA BY
DAYBREAK MONDAY...WITH FAIRLY STRONG SUBSIDENCE FORCING SIGNATURE
ACROSS THE AREA. HAVE SIDED MUCH MORE STRONGLY TOWARD THE RAW
MODEL DEWPOINTS...WITH GFS/CANADIAN AND ECMWF IN UNUSUALLY GOOD
AGREEMENT WITH LOW LEVEL DEWPOINTS IN MIXED ENVIRONMENT. DID NOT
GO AS LOW AS MIGHT HAVE GIVEN THE RECENT WET PERIOD. QUICK MOVING
WAVE ON MONDAY NIGHT WITHIN CYCLONIC ZONAL PATTERN...KICKING UP
MID LEVEL CONVERGENCE A TOUCH...AND WITH MID LEVEL THERMAL RIBBON
IN PLACE SUGGESTS PERHAPS A COUPLE SHOWERS/THUNDERSTORMS SPEEDING
THROUGH THE VICINITY.

FAIRLY QUIET IN THE TUESDAY THROUGH THURSDAY WINDOW AS ANY
LINGERING ACTIVITY FROM QUICK MOVING WAVE MONDAY NIGHT SHOULD BE
PAST...WITH ZONAL TO SLIGHTLY RIDGY FLOW ALOFT STARTING TO DEVELOP
ACROSS THE PLAINS AS LONGER WAVE TROUGH REPOSITIONS TOWARD THE
WEST ONCE AGAIN. LOOKS LIKE A SURGE OF BETTER ELEVATED MIXED LAYER
WILL MOVE IN ALONG WITH ANOTHER ZONAL DISTURBANCE ON WEDNESDAY AND
WEDNESDAY NIGHT...AND COULD BE ENOUGH FOR AN ISOLATED TO SCATTERED
THREAT FOR ELEVATED THUNDER...AT MINIMUM A PERIOD OF MID LEVEL
CLOUDS. SOME MODELS INDICATE THAT SURFACE WARM FRONT TO THE SOUTH
WILL START TO POOL SOME LOWER LEVEL MOISTURE UNDER THE VERY WARM
LAYER ALOFT BY WEDNESDAY AFTERNOON AND ESPECIALLY WEDNESDAY
NIGHT...AND COULD SEE AN INCREASING THREAT FOR STRATUS/FOG. ECMWF
INDICATES SLIGHTLY STRONGER WAVE CROSSING RIDGE ON THURSDAY AND
MAY BE SLIGHTLY AGGRESSIVE DRIVING SHARP FRONTAL BOUNDARY
SOUTHWARD...WHEN COMPARING TO OTHER SOLUTIONS AND ENSEMBLES...
SO HAVE KEPT THURSDAY DRY FOR NOW...AS PRECIPITATION WOULD EVEN
TEND TO FAVOR NORTH OF A SYNOPTIC LOW LEVEL BOUNDARY DUE TO THE
VERY WARM TEMPS ALOFT.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z SATURDAY MORNING)
ISSUED AT 659 AM CDT FRI AUG 29 2014

A VARIETY OF AVIATION CONCERNS TODAY...INITIALLY WITH AREAS OF
IFR-LIFR STRATUS ALONG AND WEST OF A WEAK SURFACE TROUGH AXIS NEAR
THE JAMES RIVER VALLEY INCLUDING KHON AT THE START OF THE PERIOD.
THE TROUGH AXIS WILL SLOWLY WORK EASTWARD TOWARD SWRN MN/NWRN IA
THROUGH THE MORNING...AND THE LOWER MVFR-IFR CEILINGS COULD IMPACT
KFSD/KSUX FOR A FEW HOURS IN THE MID-LATE MORNING. HOWEVER GENERAL
IMPROVEMENT INTO AT LEAST MVFR RANGE IS EXPECTED BY MIDDAY. LATE
MORNING INTO THE AFTERNOON WILL BRING AN INCREASING CHANCE OF
SHOWERS AND SOME THUNDERSTORMS TO LOCATIONS ALONG AND EAST OF THE
WEAK TROUGH AXIS. THIS ACTIVITY WOULD MOST LIKELY IMPACT KSUX...
BUT MAY ALSO BRUSH KFSD FOR A FEW HOURS. CONVECTION EXPECTED TO
DIMINISH AFTER SUNSET...BUT LINGERING LIGHT WINDS AND ABUNDANT LOW
LEVEL MOISTURE COULD LEAD TO REDEVELOPMENT OF MVFR-LOCALLY IFR
VISIBILITY IN BR EARLY SATURDAY MORNING.

&&

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

$$

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


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