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000
FXUS63 KFSD 191623
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
1123 AM CDT SUN MAY 19 2013

.UPDATE...
ISSUED AT 1123 AM CDT SUN MAY 19 2013

PERSISTENT THETAE ADVECTION CONTINUES TO FILTER INTO OUR AREAS AT
THIS TIME GENERALLY ALONG AND EAST OF I 29. SURFACE OCCLUDED FRONT
PUSHED AS FAR EASTWARD AS A YANKTON TO HURON LINE BEFORE STALLING.
THEREFORE RAINFALL CONTINUES TO FILTER NORTHWARD AHEAD OF THE
OCCLUSION AND ALONG THE THETAE AXIS...AND COULD BE FAIRLY
PERSISTENT THROUGH THE AFTERNOON HOURS. FOR THE AFTERNOON HOURS...
HAVE HIGH POPS GOING IN OUR EASTERN ZONES...TAILING THEM OFF TO A
HIGH SCATTERED POP ALONG I 29...WITH LESS POPS TO THE WEST WHERE
SOME DRY SLOTTING HAS TAKEN HOLD BEHIND THE OCCLUSION. CURRENT
THOUGHTS ARE FOR SEVERE POTENTIAL...IS THAT IT WILL LIKELY BE
CONFINED TO OUR FAR EASTERN ZONES LATE TODAY OR THIS EVENING. THIS
IS DUE TO THE AMOUNT OF RAIN COOLED AIR WHICH WILL OCCUR IN OUR
EAST FOR THE NEXT FEW HOURS. THE FORECAST AREAS SHOT AT SEVERE
WEATHER WILL BE LIMITED IN THAT WHEN CELLS DO DECIDE TO FORM...A
QLCS WILL LIKELY QUICKLY FORM AND RUSH EASTWARD WHERE THE BEST
SHORT WAVE ENERGY IS. THEREFORE BELIEVE THAT THE WINDOW IS
NARROWING FOR A SEVERE WEATHER THREAT.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 404 AM CDT SUN MAY 19 2013

UPPER LEVEL TROUGH DIGGING INTO THE HIGH PLAINS THIS MORNING WILL BE
THE CATALYST FOR ANOTHER ACTIVE WEATHER DAY ACROSS THE NORTHERN AND
CENTRAL PLAINS TODAY. THE TROUGH IS EXPECTED TO NUDGE EASTWARD
AND STRENGTHEN...WITH A CUT OFF FEATURE DEVELOPING OVER WESTERN
SOUTH DAKOTA BY THE AFTERNOON. LATEST ANALYSIS DEPICTS A SURFACE LOW
DEEPENING OVER CENTRAL SOUTH DAKOTA...WHICH IS EXPECTED TO SLOWLY
LIFT NORTH-NORTHEAST IN THE AFTERNOON. GENERALLY SOUTHERLY WINDS
WILL BEGIN TO SHIFT IN OUR WESTERN HALF LATE IN THE PERIOD. WITH
CONTINUED THETA-E ADVECTION THIS MORNING...ISOLATED TO SCATTERED
SHOWERS AND THUNDERSTORMS WILL PERSIST THROUGHOUT THE FORECAST
AREA...HOWEVER BY LATE MORNING AND AFTERNOON...GREATER INSTABILITY
AND DYNAMICS WILL LEAD TO THE DEVELOPMENT OF MORE ORGANIZED
CONVECTION...ESPECIALLY ALONG AND EAST OF THE I-29 CORRIDOR.
DEWPOINTS ARE EXPECTED TO PERSIST IN THE LOW TO MID 60S HERE WHILE
THE AFTERNOON ARRIVAL OF THE BOUNDARY ALONG WITH AN UPPER LEVEL VORT
MAX AND SLIGHTLY BETTER SHEAR PROFILES...WILL CREATE THE BEST
POTENTIAL FOR STRONG TO SEVERE DEVELOPMENT. HOWEVER DESPITE ALL
THIS...THERE REMAINS QUITE A BIT OF UNCERTAINTY IN HOW THE ONGOING
PRECIPITATION ACROSS OUR EASTERN FORECAST AREA WILL AFFECT
CONVECTION LATER ON TODAY. ANY RESIDUAL OUTFLOW BOUNDARIES SETTING
UP ACROSS THE AREA COULD POTENTIALLY BECOME CONVECTION TRIGGERS
TODAY...HOWEVER REMNANT CLOUD COVERAGE IN THIS AREA MAY ALSO HAMPER
DAYTIME HEATING AND DESTABILIZATION A BIT. MAIN SEVERE THREAT WOULD
BE HAIL TO GOLFBALL SIZE AND WIND GUSTS TO 70 MPH...WITH A VERY LOW
TORNADO POTENTIAL MAINLY NEAR THE ADVANCING BOUNDARY.

THE SEVERE STORM THREAT WILL CONTINUE INTO THE EARLY EVENING...BUT
LIKELY IN A SOMEWHAT MORE ISOLATED MANNER...WITH EXCEPTIONS OF FAR
SOUTHEASTERN CWA WHERE ONGOING LINEAR MODE IS LIKELY FORCED BY THE
STRONG AFTERNOON PV ADVECTION. LAST FEW RUNS OF RAP HAVE NOT BEEN
QUITE AS BOISTEROUS WITH INSTABILITY IN PRE CONVECTIVE ENVIRONMENT
EARLY TO MID AFTERNOON... MAINLY 1500-2000 J/KG. SHEAR REMAINS
SOMEWHAT MARGINAL ACROSS THE AREA. DO SEE LOW LEVEL SHEAR VECTORS
SHOULD BE ORIENTED ALONG BOUNDARY BY 00Z...AND SHOULD BE
INCREASINGLY FAVORABLE FOR WIND MODE...WITH SOME HAIL IN STRONGER
UPDRAFTS TO PERHAPS HALF DOLLAR SIZE. WHILE LOW LEVEL JET WILL
BE WORKING TO VEER DURING THE EVENING...AREAS BETWEEN THE PRE
FRONTA/OUTFLOW CONVECTIVE LINE AND THE SYNOPTIC BOUNDARY COULD
STILL FIND AN ISOLATED STORM WITH MARGINALLY SEVERE CHARACTERICS.

.LONG TERM...(MONDAY THROUGH SATURDAY)
ISSUED AT 404 AM CDT SUN MAY 19 2013

PRECIPITATION DISTRIBUTION BECOMES QUITE CHALLENGING AROUND WOBBLY
UPPER LOW WHICH BEGINS ITS JOURNEY ON MONDAY ACROSS NEBRASKA/SOUTH
DAKOTA...AND ENDS UP IN IOWA/MINNESOTA BY WEDNESDAY. VERY LITTLE
AGREEMENT ON TRACK OF CENTER...BETWEEN RUNS OR VARIOUS MODELS...
WHICH IS VERY PREDICTABLE GIVEN THE VARIOUS LOCATIONS OF WAVES
ROTATING AROUND THE FEATURE. WOULD MAKE SENSE FOR PATTERN TO
GRADUALLY BECOME SOMEWHAT MORE DIURNAL TOWARD LATER IN THE PERIOD.
WITH UPPER LOW TO WEST ON MONDAY...SHOULD BE A RELATIVELY QUIET
PERIOD UNTIL AT LEAST LATER IN THE DAY WITH INITIAL LARGER SCALE
FORCING LIFTING NORTH AND EAST EARLY IN THE DAY. LIKELY TO ACTUALLY
BREAK OUT SOMEWHAT ACROSS NORTHWEST IOWA AND ADJACENT LOCATIONS...
AND MODEST MIXING WOULD YIELD 75 TO 80 ACROSS THE AREA...AND MORE
OR LESS PUSHED MOST TEMPS UP AT LEAST A COUPLE DEGREES WITH LESS
WIDESPREAD PRECIPITATION EXPECTED. WILL FINALLY GET A BETTER
COVERAGE OF SHOWERS/ISOLATED THUNDER TOWARD CENTER OF UPPER WAVE
BY LATE AFTERNOON IN THE FAR WEST...AND DEVELOPMENT TOWARD LOWER
MISSOURI VALLEY ON CONVERGENCE AXIS.  LOBE WILL WRAP NORTHWARD...
AND LIKELIHOOD FOR MORE SIGNIFICANT DEVELOPMENT ALONG
OUTFLOW/CONVERGENCE WELL TO THE EAST ON MONDAY NIGHT WILL SPELL
A QUICK DIMINISHMENT TO COVERAGE AGAIN OVERNIGHT...WITH EVEN
WESTERNMOST SHOWERS LIKELY BACKING FOR A TIME WESTWARD.

WHILE PERHAPS NOT QUITE AS MUCH CONCERN ON MONDAY...WITH OUTSIDE
CHANCE THAT AFTERNOON DESTABILIZATION COULD YIELD AN ISOLATED HAIL
OR WIND PRODUCING STORM ACROSS NORTHWEST IOWA...DO STILL HAVE SOME
STRONG CONCERN ABOUT TUESDAY AS POTENTIAL FUNNEL CLOUD/WEAK
TORNADIC SPINUP DAY. LIKELY THAT WILL GET SURFACE REFLECTION OF
UPPER WAVE SETTING UP AN ELONGATED EAST TO WEST LOW LEVEL
VORTICITY AXIS ACROSS THE CWA. SEVERAL FORECAST SOUNDINGS INDICATE
NOT ONLY POTENTIAL FOR 0-2KM SHEAR ON THE ORDER OF 15 KT...BUT
ALSO LIKELIHOOD OF GETTING A PRE CONVECTIVE 0-3KM CAPE OF 50-75
J/KG. WILL LIKELY GET DEEPER CONVECTION GOING GIVEN A 800-1200 J/KG
PRE CONVECTIVE ML CAPE WITH VERY LITTLE CAPPING...SO WILL NOT ONLY
DEVELOP FAIRLY EARLY IN THE DIURNAL CYCLE...BUT WILL LIKELY BE
INITIATED BY LATER MORNING OR MIDDAY WITH ALL MODELS SHOWING NICE
DIV Q WRAPPING UP THE EAST FLANK OF THE CYCLONE.

WITH ACTIVE SHORT TERM WEATHER...ONLY A CURSORY GLANCE INTO THE
LATE WEEK CONDITIONS FOR WEDNESDAY THROUGH SATURDAY. WEDNESDAY
WILL STILL FEEL IMPACT OF UPPER LOW...LIKELY THE COOLEST DAY...
AND ONE WHERE SHOWERS WILL START TO SHOW A DECREASE IN OVERALL
COVERAGE...BUT STRONGER THROUGH THE EAST WITH SOME LOWER LEVEL
FRONTOGENETIC SUPPORT ON BACK SIDE OF DEPARTING WAVE. FINALLY
DRIES OUT BY THURSDAY AND FRIDAY...AND ONLY SIGNIFICANT CHANGE
WAS TO ALLOW FOR SOME DRIER DEW POINTS AND COOLER FRIDAY LOWS
FOR THE EAST/NORTHEAST PORTIONS OF THE AREA WITH VERY DRY
TRAJECTORY OUT OF KEEWATIN HIGH...SOMETHING IMPORTANT TO CONSIDER
FOR FIRE WEATHER CONCERNS...AFTER SEVERAL DAYS OF SOGGY CONDITIONS
FOR PRESCRIBED BURN OPERATIONS EARLY IN THE WEEK. KEPT SOME LOWER
END POPS FOR FRIDAY NIGHT WEST...AND SATURDAY ACROSS THE AREA
WITH DEVELOPMENT OF WARM AIR ADVECTION AS UPPER RIDGE BEGINS TO
SHOW SIGNS OF BREAKING DOWN.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z MONDAY MORNING)
ISSUED AT 637 AM CDT SUN MAY 19 2013

INSTABILITY WILL LEAD TO SCATTERED SHOWERS AND THUNDERSTORMS THROUGH
18Z...WHICH WILL BECOME MORE WIDESPREAD EARLY AFTERNOON THROUGH MID
EVENING. BEST POTENTIAL FOR THUNDERSTORMS WILL BE ALONG AND EAST OF
I-29 THROUGH AROUND MID-EVENING OR 03Z. STRONG TO SEVERE STORMS WILL
BE POSSIBLE...WITH LARGE HAIL AND DAMAGING WIND THREATS.
PRECIPITATION WILL WEAKEN AND BECOME MORE ISOLATED TO SCATTERED AFTER
03Z. OTHERWISE WINDS WILL GENERALLY BE FROM THE SOUTH
SOUTHEAST...HOWEVER OUTFLOW BOUNDARIES FROM OVERNIGHT STORMS HAS
ALLOWED VARIABILITY THIS MORNING IN SOME AREAS. PATCHY LOW CEILINGS
IN THE NORTHEAST ARE EXPECTED TO LIFT WITHIN THE NEXT FEW HOURS.


&&

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

$$

UPDATE...MJF
SHORT TERM...  /CHAPMAN
LONG TERM...CHAPMAN
AVIATION...