000
FXUS63 KFSD 182053
AFDFSD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SIOUX FALLS SD
353 PM CDT SAT MAY 18 2013

.SHORT TERM...(THIS EVENING THROUGH SUNDAY)
ISSUED AT 320 PM CDT SAT MAY 18 2013

AT THIS TIME...SURFACE TROUGH EXTENDS FROM NORTH CENTRAL SD TO WEST
CENTRAL NE. ALOFT...LARGE TROUGH EXISTS THROUGHOUT THE INTERMOUNTAIN
WEST WITH WATER VAPOR SHOWING A SHORT WAVE NEAR THE FOUR CORNERS
AREA. THE SHORT WAVE WILL LIFT EASTWARD TONIGHT INTO THE
SOUTHWESTERN PLAINS AREA. AT THE SURFACE...THERE ARE SOME
INDICATIONS THAT A SURFACE LOW WILL GENERATE IN SOUTH CENTRAL SOUTH
DAKOTA AS THE NIGHT PROGRESSES. THE LOW LEVEL JET INCREASES MODESTLY
MOSTLY INTO THE 25 TO 30 KNOT RANGE AT 850MB. PERSISTENT LOW LEVEL
THETAE ADVECTION EXISTS. BECAUSE OF ITS PERSISTENCE...IT IS
DIFFICULT TO ASCERTAIN GOOD DETAILS ON EXACTLY WHAT THE CONVECTION
MAY DO AFTER IT FIRES UP THIS EVENING. IN THE NEAR TERM...WOULD NOT
BE SURPRISED TO SEE SOME SCATTERED SHOWERS AND AN ISOLATED
THUNDERSTORM IN OUR SOUTHEASTERN ZONES LATE THIS AFTERNOON AND EARLY
EVENING WHERE CUMULUS HAS DEVELOPED. BUT OVERALL...TRYING TO FIGURE
OUT WHEN THE MORE BONAFIDE CONVECTION WILL DEVELOP AND WHEN IS A
LITTLE MORE CHALLENGING. PER UPPER QG FORCING...THE UPPER WAVE
ACTUALLY STAYS TO THE WEST OF OUR AREA. SO EXAMINING DETAILS LOWER...
700-500MB QG DOES SHOW A SHORT WAVE MOVING ACROSS OUR FORECAST AREA
TONIGHT EXITING TO THE EAST ON SUNDAY MORNING. CONVECTION SHOULD
FIRST FIRE NEAR THE SURFACE TROUGH CLOSE TO THE SHORT WAVE EARLY
THIS EVENING...POSSIBLY IMPACTING SOME OF OUR WESTERN ZONES EARLY
THIS EVENING. IT SHOULD THEN MOVE EAST TO NORTHEAST ALONG THE MID
LEVEL FLOW. A SEVERE POTENTIAL CERTAINLY EXISTS. BUT SEEING THE
AMOUNT OF CLOUD COVER WHICH HELD ON TODAY...ML CAPES ARE LIKELY
OVERDONE A BIT OFF OF THE NAM MODEL. IN ADDITION...NAM SURFACE DEW
POINTS ARE A LITTLE HIGH. WIND WISE...DECENT SHEAR EXISTS IN OUR
WESTERN ZONES THIS EVENING BEFORE WANING IN THAT AREA...WITH THE
SHEAR SHIFTING EASTWARD TOWARD THE I 29 CORRIDOR OVERNIGHT AND LATE
TONIGHT. HAVE SEEN BETTER SHEAR FOR THIS TIME OF YEAR...BUT STRONG
WIND IS CERTAINLY A THREAT WITH ANY QLCS DEVELOPMENT ALONG WITH
BOWING SEGMENTS...AND THE SHEAR IS PERPENDICULAR TO ANTICIPATED
CONVECTIVE MOVEMENT. LARGE HAIL COULD ALSO BE A THREAT IN OUR
WESTERN ZONES THIS EVENING WITH ANY DISCREET CELL DEVELOPMENT. HIGH
POPS ARE CERTAINLY WARRANTED...ESPECIALLY ALONG AND NORTH OF I 90
WITH LESS...SCATTERED TYPE POPS FROM SIOUX CITY IA TO STORM LAKE IA.
BUT THEY COULD SEE SOME DECENT RAIN ALSO...JUST LIKELY NOT AS
WIDESPREAD.

ON SUNDAY...SURFACE WIND FIELDS SHOW THE LIKELIHOOD OF A TROUGH
THROUGH OUR SOUTHWEST ZONES BY AFTERNOON. ACTUALLY PREFER THE
ORIENTATION OF THE GFS AND ECMWF WITH THE PRECIPITATION FIELDS BY
SUNDAY AFTERNOON...WITH A DEVELOPING LINE JUST AHEAD OF THE WIND
SHIFT NEAR THE I 29 CORRIDOR. DEPENDING ON CLOUD COVER DEBRIS...
THIS ACTIVITY COULD ALSO BE SEVERE. FOR MUCH OF THE DAY...AM A BIT
CONCERNED THAT SOME OF OUR POPS COULD BE OVERDONE...ESPECIALLY IN
OUR WESTERN ZONES. BUT WARM AIR ADVECTION DOES PERSIST ALOFT THROUGH
THE DAY.


.LONG TERM...(SUNDAY NIGHT THROUGH SATURDAY)
ISSUED AT 320 PM CDT SAT MAY 18 2013

ANY CONVECTION THAT DEVELOPS THROUGH THE CENTRAL PORTIONS OF THE CWA
WILL PUSH OFF TO THE EAST THROUGH THE EVENING...WITH SOME SEVERE
STORMS STILL POSSIBLE DURING THE EVENING BEFORE THE INSTABILITY
WANES LATER IN THE EVENING. THINK THERE WILL BE A GENERAL DECREASE
IN COVERAGE OF CONVECTION LATER AT NIGHT WITH DRYING MIDLEVELS. DO
BEGIN TO SEE THERMAL PROFILES COOL JUST A BIT...WITH LOWS RANGING
FROM THE LOWER 60S EAST TO MID 50S WEST.

MESSY PATTERN CONTINUES THROUGH AT LEAST MIDWEEK AS AN UPPER LEVEL
LOW SPINS OVER THE THE CENTRAL PORTIONS OF THE NATION FOR MONDAY
AND TUESDAY BEFORE PUSHING INTO THE NORTHERN MISSISSIPPI
VALLEY/WESTERN GREAT LAKES REGION ON WEDNESDAY. WITH THIS
FEATURE...WILL HAVE PERIODIC SCATTERED SHOWERS AND THUNDERSTORMS
ACROSS THE AREA DURING THIS TIME FRAME. OF COURSE HARD TO
PINPOINT CONVECTIVE TIMING/DEVELOPMENT IN THIS KIND OF SET
UP...WITH VARIOUS SHORTWAVES/VORT MAXES ROTATING AROUND THE UPPER
LEVEL LOW THROUGH THE PERIOD. HOWEVER...AT THIS POINT THE BETTER
CHANCE OF PRECIPITATION ON MONDAY LOOKS TO BE OVER THE WEST CLOSER
TO THE PARENT LOW/WIND SHIFT LINE...THEN SHIFTING EASTWARD ON
TUESDAY AND WEDNESDAY AS THE DYNAMICS/BEST LIFT PULLS OFF TO THE
EAST. WOULD NOT REALLY EXPECT MUCH IN THE WAY OF SEVERE STORMS
THROUGH THIS PERIOD WITH AN INCREASINGLY STABLE
ATMOSPHERE...THOUGH AS MENTIONED IN PREVIOUS DISCUSSION CANNOT
RULE BRIEF FUNNEL CLOUDS/SPIN UPS ON MONDAY OR TUESDAY AFTERNOON
WITH THE COLD CORE LOW OVERHEAD...BUT WOULD DEPEND ON
PRECIPITATION TRENDS/BREAKS IN CLOUDS/AND RESULTANT LOW LEVEL
LAPSE RATES ETC.  WITH THE LOW OVERHEAD...WILL SEE A GENERAL
COOLING TREND THROUGH MID WEEK...WITH HIGHS TRENDING TOWARD BELOW
NORMAL LEVELS...INTO THE 50S AND 60S FOR TUESDAY AND WEDNESDAY.

FARTHER OUT IN THE EXTENDED...THE UPPER LEVEL LOW CONTINUES TO FILL
AND PULL OFF TO THE EAST FOR THE END OF THE WEEK...WITH UPPER
LEVEL RIDGING INTO THE PLANS STATES FOR THURSDAY THROUGH SATURDAY.
WITH THIS COMES A WARMING TREND...BACK TO NORMAL...AND MOSTLY DRY
CONDITIONS THROUGH THE PERIOD.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1236 PM CDT SAT MAY 18 2013

LOW PRESSURE WILL BE MEANDERING AROUND NORTH CENTRAL NEBRASKA AND
CENTRAL SOUTH DAKOTA THROUGH THE TAF PERIOD. THUNDERSTORM
DEVELOPMENT WILL BE LIKELY...WITH SEVERE WEATHER A THREAT...AS THE
EVENING PROGRESSES AND LASTING INTO LATE TONIGHT AND EARLY SUNDAY.
HOWEVER CONFIDENCE IS VERY LOW ON EXACT DETAILS AND TIMING ON HOW
THE CONVECTION WILL EVOLVE OVER TIME. FOR THE TAFS...TOOK A MODEL
BLEND OF THE NAM12...GEM REGIONAL AND GFS MODELS TO COME UP WITH
SOME TIMING OF THE CONVECTION IMPACTING THE AIRPORT SITES. BUT
AGAIN THIS WILL HAVE TO MONITORED CLOSELY. THE THUNDERSTORMS WILL
HAVE THE STRONG POTENTIAL TO SEND CONDITIONS DOWN INTO THE MVFR
RANGE...AND PROBABLY EVEN SOME SHORTER TERM IFR CONDITIONS WHERE
THE HEAVIER RAIN CELLS ARE. FURTHERMORE...THERE MAY EVEN BE SOME
IFR TO MVFR STRATUS SUNDAY MORNING IN THE RAIN COOLED AIR MASS...
ESPECIALLY EAST OF THE JAMES RIVER VALLEY. THE NAM HUMIDITY TIME
SECTIONS ARE STRONGLY HINTING AT THIS SCENARIO. BACKING UP CLOSER
TO THE NEAR TERM...WOULD NOT BE SURPRISED TO SEE SOME SHOWERS AND
ISOLATED THUNDERSTORMS IN NORTHWEST IOWA LATE THIS AFTERNOON AND
EARLY EVENING UNDER CUMULUS DEVELOPMENT.


&&

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

$$

SHORT TERM...MJF
LONG TERM...JM
AVIATION...MJF