Severe Thunderstorm Warning 20, NWS Sioux Falls, SD

AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA
NATIONAL WEATHER SERVICE SIOUX FALLS SD
350 PM CDT FRI APR 20 2001

DECENT BUBBLE HIGH NOSED INTO CWA THROUGH MIDDAY...BUT STARTING TO
GIVE UP TO WAA PRESSURE FALLS IN THE WESTERN PLAINS.  SFC FRONT HAS
COME TO A HALT ERLR ACROSS FAR SOUTHERN IOWA INTO SOUTHEAST NEBRASKA
AND NORTHERN KANSAS...INTERSECTING WITH A DRY LINE ACROSS CENTRAL
KANSAS...BUT OBS IN WRN IA AND ERN NEBRASKA HINT A NWRD MOVEMENT
TO BNDRY AS FBY PROFILER SHOWING START OF INCREASE TO LLJ.  18Z
SOUNDINGS WITH A FORMIBLE MIXED LYR OF WARM AIR ROUGHLY 850-500HPA.

ONLY ETA YDA PICKED UP ON THE IMPRESSIVE SWRD SURGE OF FRONT DUE
TO NRN STREAM WAVE...AND NOT ENOUGH AT THAT. SEE NO REASON WHY
APPARENTLY STRONGER WAVE TONIGHT PASSING TO N WILL NOT HAVE AT LEAST
AS MUCH EFFECT...SO...FOR TDA... WL SIDE WITH A SLIGHTLY SLOWED AND
TEMPERED ETA FCST...WHICH WILL RESULT IN SOME CONSIDERABLE DAY TO DAY
CHANGES IN THE FORECAST WITH RESPECT TO TMPS...AND PRECIP TYPE W.

TONIGHT...WATCHING THE STRONG WAVE WHICH IS OBVIOUSLY UNDERDONE
MOVING FROM WY ACRS SD. CONVECTION HAS BEEN ONGOING ALL DAY OVR
MTS...AND THIS IS SIGN OF FORMIDIBLE DYNAMICS WITHOUT ANY MDL DATA.
LOOK FOR LTL IN WAY OF LT AFTN INITIATION ALG WMFNT DUE TO STG
INHIBIITION /SW FLOW ALFT USUALLY NOT GOOD AROUND THESE PARTS/ BUT
RATHER WKNG WITH STG DYNAMIC LIFT HEADING OUT FM PLAINS OF CO DURG
EVNG. TWO AREAS APPEAR MOST FAVORABLE FOR PCPN...NWRN AND SERN
CWA...WITH ONLY SLIGHT LOWER THREAT THRU HEART.  FOR NOW...WL LIMIT
SVR THREAT MENTION TO SERN ZNS...SIMILAR TO AREA OF SPC DAY 1
UPDTS.  MAY SEE WIND THREAT CLOSER TO FRONTAL POSITION WITH HAIL
THREAT LTL TO N.  SOME LCL NUMBERS WORKED UP BY MESOANALYST SUPPORT
SMALL RISK OF ELEVATED SUPERCELLS.  WITH SFC LOW EXPECTED TO SLICE
ACRS CWA...WND FCST IS DIFFICULT AND USED MAIN PV ADVECTION TO TRACK
LKLY POSN.  WILL HIT THE CLRG IN SUBSIDENT QUADRANT OF LOW MOST IN
S...AND SHIFT WINDS TO W OR SW.

ON SATURDAY...SFC LOW PULLS AWAY...AND FRONT IS AIDED SWRD ONCE AGAIN
FROM UPR DYNAMIC INTERACTIONS.  CLOUDS SHOULD DVLP SWRD...MAINLY ASSD
WITH WINDS SHIFTING FM W TO N.  AREAS OF RAIN IN DEFORMATION BAND
SHOULD PERSIST...AND SAG GRADUALLY SWRD TO NEAR OR ALG I90 DURG AFTN
WITH COVERAGE WANING WITH TIME AS TROUGH SHEARS OUT. TMPS LIKLY NRLY
STEADY N...WITH NICE WARMING AHEAD OF CD PUSH IN SERN CWA BEFORE THE
FALL.

MOST IMPRESSIVE WAVE ALG THE NRN CA COAST NOW...AND THIS WL BE THE
SECOND PLAYER IN OUR WEATHER GAME LATER IN THE WEEKEND. DEVELOPING
FRONTOGENESIS AND WAA WL BRING PRECIP THREAT CLOSE TO SWRN FRINGE OF
CWA LT IN NIGHT...WITH REMAINDER PROTECTED BY RIDGE. MAY BREAK SOME
CLOUDS...SO WL HEDGE TMPS DOWN A BIT...BUT NOT AS EXTREME AS
GUIDANCE.

SUNDAY BECOMES INTERESTING AS THE MAIN ENERGY BOTTOMS OUT AND STARTS
OUT INTO THE CNTRL PLAINS. PRECIP SHUD BECOME WDSPD AS STG AND DEEP
FRONTOGENESIS MOVES INTO THE FOREFRONT...IN A COUPLED JET FOCUSSED
IN NWRD CWA...WITH EVER MORE MARKED SFC BNDRY FM SERN NEBRASKA ACRS
IA.  ONLY HINT OF INSTABILITY BRUSH SERN CWA...AND THUNDER LIMITED
TO THOSE AREAS.  TYPE ISSUE TO WEST.  SOURCE OF CD AIR IS IN PLACE
ON RAOBS THRU CNTRL CANADA...SO ASSUME IT WILL BE ON WAY SWRD AS
PROGGED...BUT ETA JUST TOO AGRESSIVE FOR MY TASTE. ETA 850-700HPA
THICKNESS WOULD SUGGEST RAIN/SNOW LINE TO KMML-KFSD-KYKN BY 00Z
MON...BUT WL HOLD CHGOVER BACK TO W IN JAMES VLY ATTM. 700HPA TMPS
LTL TOO WARM FOR SNOW TOWARD KFSD.

TRANSITIONS CONTINUE IN EXTENDED.  AVN CONTINUES TO BE SLOWEST AT
SHORT RANGE WITH PROGRESSION OF CLOSED LOW THRU PLAINS...BUT NOT
AS FAST AS CANADIAN...NOR AS SLOW AS MRF.  WL END PCPN THREAT AFTER
MON IN LIFTING/SHEARING TROUGH AS BEST JET LEAVES BEHIND. WL MENTION
RAIN/SNOW IN WRN AREAS AND ONLY RAIN E. TRAILING TROUGH MAY BRING A
FEW SHOWERS TO ALL BUT FAR SWRN CWA LT TUE/ERLY |WED...WHICH MAY
REINFORCE COOLER AIR ESPLY E FOR A DAY...BUT LOOK FOR GUD RIDGE AND
A S TO SW FLOW TO ALLOW WARMING THROUGH THE END OF THE WEEK.

.FSD...NONE
CHAPMAN