Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 250920
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
420 AM CDT SAT JUL 25 2015

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 420 AM CDT SAT JUL 25 2015

CONVECTIVE TRENDS OVER THE NEXT 12 TO 24 HOURS OF GREAT CHALLENGE.
EARLY THIS MORNING...DEALING ONCE AGAIN WITH A MASSIVE CONVECTIVE
DEVELOPMENT ACROSS SOUTHERN NEBRASKA AND IOWA...NEAR TO THE LOCATION
OF THE LOW LEVEL BOUNDARY. OTHER CONVECTION HAS DEVELOPED IN WEAK
COUPLED JET REGION BETWEEN ZONAL JET DOWNSTREAM OF FLAT RIDGE AXIS
OVER MINNESOTA AND EASTERN NORTH DAKOTA...AND OTHER JET STREAK
PUSHING NORTHEAST ACROSS WYOMING. PUSH OF STEEPER MID LEVEL LAPSE
RATES INTO SOUTHWEST SOUTH DAKOTA...AND WARM ADVECTIVE WING OF
CONVECTION SPREADING INTO CENTRAL SD. THERE IS A NOTABLE DIFFERENCE
IN AIRMASS QUALITY SPREADING TOWARD EASTERN SOUTH DAKOTA...WITH MUCH
LESS INSTABILITY AND LESS SUPPORT OF LOWER TO MID LEVEL INFLOW
SPREADING EAST OF THE JAMES RIVER. THEREFORE...THE MOST SHORT TERM
DURING THE PREDAWN HOURS...WOULD EXPECT A LIMITED COVERAGE OF ANY
PRECIPITATION SPREADING EAST/SOUTHEAST TOWARD THE SIOUX FALLS AREA...
AND OVERALL TREND SHOULD BE LESSER.

GOING INTO TODAY...NOT ENTHRALLED WITH HOW SHORT RANGE MODELS...
ESPECIALLY THE CAMS... ARE DEALING WITH THE WESTERN CONVECTION...AND
THIS LENDS FAIRLY LOW CONFIDENCE AS TO EVOLUTION.  06Z NAM PROBABLY
THE CLOSEST TO SIMULATING SHORT TERM REALITY...AND THIS SOLUTION
WOULD SUGGEST A MUCH HIGHER THREAT FOR CONVECTION THROUGH THE
DAYTIME HOURS ALONG A NORTHWEST TO SOUTHEAST AXIS THAN HAVE GIVEN
CREDIT. HAVE MAINTAINED SOME MIDDLE CHANCE LEVEL POPS ALONG MID
LEVEL GRADIENT THROUGH MIDDAY...THEN ALLOWED TO DROP OFF A BIT FOR
PARTS OF THE AFTERNOON...BUT NOT FULLY DROP OFF WITH CONTINUED
APPROACH OF UPPER WAVE CURRENTLY ACROSS WESTERN WYOMING...AND
PREEXISTING MID LEVEL BOUNDARY. OVERALL...CHALLENGE WILL BE TO
INCREASE INSTABILITY IN FACE OF DEBRIS FROM MORNING CONVECTION AND
THE MODIFIED AIRMASS TO THE SOUTH. HOWEVER...EXPECT TO SEE AIRMASS
MODIFY AS LOW LEVEL WINDS TURN MORE SOUTHERLY AND INCREASE MOISTURE
TRANSPORT. WHILE CLOUDS WILL BE AN ISSUE...THERE SHOULD GET TO BE A
LARGE ENOUGH AREA OF CLEARING TO THE SOUTHWEST...SO LIKELY THAT
COMBINATION OF MOISTURE AND SOME DIURNAL HEATING WILL INCREASE
MLCAPE TO 1500-2000 J/KG ACROSS THE SOUTHWESTERN CWA...LESSER TO THE
NORTHEAST. LIKELIHOOD OF NEAR SURFACE BOUNDARY REMAINING WELL SOUTH
OF THE AREA IS HIGH...SO NOT LIKELY TO SEE A SIGNIFICANT 0-1KM SHEAR
COMPONENT BEYOND LOCAL STORM SCALE. HOWEVER...DEEPER SHEAR IS FAIRLY
STRONG AT 35 TO 45 KNOTS...AND WOULD SUPPORT STORMS ORGANIZING INTO
CLUSTERS AND LINEAR SEGMENTS GOING THROUGH THE EVENING. SUCH STORMS
WOULD CERTAINLY BE CAPABLE OF LARGER HAIL UP TO GOLF BALLS AND
DAMAGING WINDS TO 60-70 MPH. WITH TIMING OF STRONGEST QG FORCING
TO ARRIVE DURING THE LATER EVENING HOURS...WOULD EXPECT EARLIER
SCATTERED ACTIVITY TO INCREASE THROUGH LATER EVENING...THEN
FOLLOWING A VEERING BUT MODERATELY WEAK LOW LEVEL JET TOWARD THE
EAST OVERNIGHT. PRIMARY SEVERE THREAT LOOKS TO BE THE SOUTHWESTERN
HALF TO TWO THIRDS OF THE CWA...AND DURING THE EVENING HOURS.

CAVEAT TO THIS SCENARIO IS WILD CARD OF THE INITIAL MORNING
CONVECTION COMING OUT OF WESTERN SD. HIGH RES SOLUTIONS LARGELY
IGNORE THESE STORMS. HOWEVER...MODEST AMOUNT OF ELEVATED
INSTABILITY...AND HAVE THUS FAR FORMED IN AREA OF BETTER LOW LEVEL
JET TO THE WEST. OVERALL...THERE IS ENOUGH SHEAR TO BE CONCERNED
THAT IF STORMS DO NOT FOLLOW A TYPICAL DIURNAL WEAKENING /AND SUCH
MAY BE THE CASE GIVEN THE EARLIER NOTED DYNAMICAL TIE/ THERE COULD BE
A WIND/HAIL THREAT WITH EARLIER DAY STORMS FROM MID TO LATE MORNING
ALONG AND NORTH OF THE I 90 CORRIDOR IN THE WESTERN CWA...DIVING
MORE SOUTHEASTERLY WITH TIME TOWARD THE MISSOURI RIVER.

.LONG TERM...(SUNDAY THROUGH FRIDAY)
ISSUED AT 420 AM CDT SAT JUL 25 2015

SUNDAY SHOULD SEE ISOLATED SHOWERS AND THUNDERSTORMS IN THE EASTERN
THIRD OF THE CWA AS WEAK MID LEVEL FORCING DISSIPATES THROUGH THE
DAY. ONE CONCERN BESIDES THE THUNDERSTORMS WILL BE THE POTENTIAL FOR
STRATUS. THE GFS...ECMWF AND NAM ALL SUGGEST THAT AT LEAST THE
MORNING WILL SEE STRATUS SO WILL GO AHEAD AND BUMP UP CLOUD COVER
AND THEN DECREASE HIGHS A BIT IN THE EASTERN CWA WHERE CLOUDS WOULD
BE LAST TO DISSIPATE. HIGHS SHOULD MAINLY BE IN THE 80S.

SUNDAY NIGHT INTO MONDAY WILL SEE AN INCREASING SOUTHERLY RETURN
FLOW AS LOW PRESSURE DEEPENS OVER THE WESTERN HIGH PLAINS. JUST A
SMALL CHANCE FOR THUNDERSTORMS DURING THIS TIME AS THE AIR STILL
ONLY MARGINALLY UNSTABLE. WITH THE INCREASING SOUTHERLY WINDS
OVERNIGHT LOWS WILL BE PRETTY WARM...MAINLY UPPER 60S TO LOWER 70S.
HIGHS MONDAY WILL BE IN THE 80S WITH SOME 90S IN CENTRAL SD.

THE CHANCE FOR THUNDERSTORMS WILL INCREASE ON MONDAY NIGHT...BUT
STILL IN THE SLIGHT CHANCE TO LOW END CHANCE CATEGORY. THE AREA IS
SOMEWHAT DEEP INTO THE SOUTHERLY FLOW SO NOT MUCH CONVERGENCE. THERE
ARE SOME HINTS OF A 925MB TO 850MB FRONT IN THE NORTHEAST CWA BUT AT
THIS TIME LOOKING PRETTY DRY IN THAT LAYER. LOWS VERY WARM MONDAY
NIGHT AND MAINLY 70 TO 75.

IN THE OUTER PERIODS(TUESDAY THROUGH FRIDAY)...A FAIRLY STRONG WAVE
WILL BRING A COLD FRONT INTO THE AREA ON TUESDAY WITH VERY WARM
READINGS EXPECTED AHEAD OF THE FRONT. THE MODELS HAVE BEEN WOBBLING A
BIT ON THE TIMING OF THE FRONT WHICH WILL PLAY A BIG ROLE IN NOT
JUST HOW HOT AND HUMID IT WILL BE BUT HOW HIGH TO GO ON POPS. WILL
CONTINUE THE IDEA THAT NORTHWEST IOWA WILL GET PRETTY HOT AND HUMID
TUESDAY WHILE THE FRONT SHOULD WORK THROUGH MOST OF SOUTH DAKOTA AND
PART OF SOUTHWEST MINNESOTA. IN FACT COULD EASILY SEE SOME MID TO
UPPER 90S IN NORTHWEST IOWA IF THE FRONT IS SLOW ENOUGH BUT
CONFIDENCE NOT QUITE HIGH ENOUGH TO GO THAT WARM. PLENTY OF
INSTABILITY...CAPES LIKELY AROUND 3000 J/KG...AS WELL AS DECENT LOW
LEVEL SHEAR AND DEEPER LAYER SHEAR. SO WILL HAVE TO WATCH FOR THE
POTENTIAL FOR A FEW SEVERE STORMS.

AFTER THIS WEAK TO MODERATE NORTHWEST FLOW ALOFT DEVELOPS AND MAINLY
DRY CONDITIONS ARE EXPECTED WEDNESDAY INTO FRIDAY. WILL HAVE A SMALL
POP IN ON FRIDAY AS A WEAK BOUNDARY COULD PRODUCE A FEW
THUNDERSTORMS. TEMPERATURES ON WEDNESDAY LIKELY A LITTLE BELOW
NORMAL WHILE THEY SHOULD BE NEAR NORMAL ON THURSDAY AND A BIT ABOVE
NORMAL FRIDAY. WILL HAVE TO WATCH FRIDAY AS THAT COULD BE A DAY
WHERE TEMPERATURES CAN GET A LITTLE WARMER THAN FORECAST.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z SATURDAY NIGHT)
ISSUED AT 1029 PM CDT FRI JUL 24 2015

MAINLY VFR CONDITIONS EXPECTED THROUGH THE TAF PERIOD. ISOLATED
THUNDERSTORMS WILL BE POSSIBLE THROUGH THE MISSOURI RIVER VALLEY
LATE TONIGHT INTO SATURDAY MORNING...THOUGH EXPECT ANY ACTIVITY
TO REMAIN WEST OF TAF SITES. THUNDERSTORMS MAY REDEVELOP ON
SATURDAY EVENING...WITH BETTER CHANCES AROUND KHON AFTER 02Z.

&&

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

$$

SHORT TERM...CHAPMAN
LONG TERM...08
AVIATION...JM


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