Severe Thunderstorm Warning 196, NWS Aberdeen, SD

AREA FORECAST DISCUSSION FOR CENTRAL/N-EAST SD AND W-CENTRAL MN
NATIONAL WEATHER SERVICE ABERDEEN SD
335 PM CDT SAT AUG 31 2002

MAIN FORECAST ISSUE IS THUNDERSTORM CHANCES FOR TONIGHT INTO SUNDAY
MORNING.  ETA/AVN/UKMET IN GOOD AGREEMENT WITH TIMING ISSUES OF
SURFACE BOUNDARY COMING THROUGH AND UPPER LEVEL FEATURES.  MODEL
INITIALIZATIONS NOT IN TERRIBLE DISAGREEMENT ALTHOUGH ETA 700 HPA
THERMAL FIELD SEEMS TO HAVE A BETTER HANDLE.  HAVE TRENDED TOWARD
THE ETA FOR THIS FORECAST.

19Z SURFACE ANALYSIS CONTINUES TO SHOW DRYLINE FEATURE EXTENDING
FROM SOUTH CENTRAL NORTH DAKOTA INTO SOUTHWEST SOUTH DAKOTA.
RADAR DATA AND SATELLITE IMAGERY HAS DEPICTED SOME MAINLY MID LEVEL
CLOUDS DEVELOPING IN THIS AREA BUT CAP STRENGTH HAS PREVENTED ANY
GOOD VERTICAL DEVELOPMENT UP TO THIS POINT.  WITH STRONG SURFACE
HEATING THROUGHOUT AFTERNOON...RUC SOUNDINGS INDICATING CONVECTIVE
INHIBITION HAS WEAKENED ACROSS WESTERN CWA.  HOWEVER...DEW POINTS
DEPRESSIONS IN THE LOW AND MID LEVELS REMAIN FAIRLY HIGH ACROSS
WESTERN CWA AND LOW LEVEL CONVERGENCE ALSO WEAK.  STILL FEEL THAT
THERE IS POSSIBILITY OF ISOLATED TSRA DEVELOPING IN NEXT FEW HOURS
IN WESTERN CWA.

MOST OF CONCERN WILL BE LATER IN THE EVENING HOURS AS UPPER LEVEL
DISTURBANCE OVER CENTRAL MONTANA APPROACHES DAKOTAS.  MODELS
IN GOOD AGREEMENT IN TAKING ASSOCIATED LOW LEVEL TROUGH AXIS TO
CENTRAL SOUTH DAKOTA BY 06Z.  AXIS OF FRONTOGENETIC FORCING AND
STRENGTHENING LOW LEVEL FLOW/WAA DOWNSTREAM OF TROUGH AXIS SHOULD
AID IN BETTER THUNDERSTORM COVERAGE BY LATE THIS EVENING...MAINLY
ACROSS NORTH CENTRAL AND EASTERN CWA WHERE LOW LEVEL MOIST TONGUE
WILL DEVELOP.  WEAK MID AND UPPER LEVEL JET STREAK ALSO WILL PULL
OUT INTO THE NORTHERN PLAINS TONIGHT IN ADVANCE OF THIS WAVE...WITH
BEST UPPER DIVERGENCE ALSO SETTING UP ACROSS EASTERN CWA.  PWATS
ACROSS THE EAST WILL BE IN THE 1.3 TO 1.5 INCH RANGE...BUT WILL NOT
INCLUDE HEAVY RAIN MENTION WITH THE HOPE THAT PROGRESSIVE NATURE OF
THIS WAVE WILL CUT OFF HEAVY RAIN POTENTIAL.  A FEW SEVERE STORMS
ARE POSSIBLE GIVEN VERY STEEP MID LEVEL LAPSE RATES BASED ABOVE LLJ
AND SUFFICIENT DEEP LAYER SHEAR WITH APPROACHING SPEED MAX.  GIVEN
THIS...WILL KEEP HIGHEST CHANCE POPS IN NORTH CENTRAL AND EASTERN
CWA...WITH 30 POPS ACROSS SOUTHWEST CWA WHICH WILL NOT EXPERIENCE
MOISTURE/WAA ADVECTION WITH LLJ TO A GREAT EXTENT.

SUNDAY SHOULD FEATURE LINGERING -TSRA ACROSS FAR EAST BEFORE ENTIRE
CWA BECOMES MOSTLY SUNNY AS SHORT WAVE EXITS REGION.  DEW POINTS
SHOULD ALSO BE SIGNIFICANTLY LOWER.  MODELS HAVE TRENDED TO A MORE
AMPLIFIED SOLUTION WITH NEXT SHORT WAVE MOVING ACROSS SOUTH CENTRAL
CANADA.  ETA/UKMET MORE AMPLIFIED THAN AVN SOLUTION AND WILL LEAN
TOWARD ETA/UKMET GIVEN STRENGTH OF EASTERN PACIFIC JET STREAK.  MORE
AMPLIFIED SOLUTION WILL ALLOW FOR SOME MODEST LOW LEVEL MOISTURE
ADVECTION LATE SUNDAY NIGHT ACROSS EASTERN CWA...AND WITH ASSOCIATED
SURFACE TROUGH PASSAGE WILL INCLUDE SLIGHT MENTION.  MONDAY AND
TUESDAY LOOK TO BE DRY WITH UPPER RIDGING SETTING INTO AREA.

.EXTENDED DISCUSSION...
MDLS IN GOOD AGREEMENT IN THE WED-SAT PERIOD WITH DEVELOPING A LARGE
50H LOW/TROUGH ACROSS THE WESTERN UNITED STATES AND A 50H RIDGE
ACROSS THE EASTERN UNITED STATES BY THE END OF THE WEEK. THIS WILL
BE A GOOD SETUP FOR ABOVE NORMAL TEMPS ACROSS THE ABR CWA FOR THE
WED-FRI PERIOD. THE EXTENDED AVN SHOWS A SIGNIFICANT PIECE OF S/W
ENERGY KICKING NE ACROSS THE UPPER MIDWEST ALONG WITH A SFC COLD
FRONT MOVING THROUGH FRI AND FRI NIGHT. THIS LOOKS LIKE THE BEST CHC
FOR TSRAS OVER THE ABR CWA. SAT SHOULD BE MUCH COOLER ACROSS THE CWA
AS COOL CANADIAN AIR BUILDS IN. WENT HIGHER THAN THE MEX TEMPS
THROUGH THE FORECAST...ESPECIALLY ON FRI BEFORE THE FRONT COMES
THROUGH.

.ABR...NONE.

MARSILI/MOHR