Flooding in Iowa, May 1 to 9, 2001

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
255 PM CDT MON APR 30 2001

SHORT TERM...
MAIN CHALLENGE FOR SHORT TERM IS SEVERE THREAT.  MOIST CONVEYOR
BELT, WITH MODERATELY STEEP LAPSE RATES, CONTINUES SHRA AND MDT TSRA
ALONG IT, KEEPING HEATING TO A MIN OVER OUR CWA.  SOME RESURGENCE IN
CONVECTION OCCURRING IN OUR WESTERN CWA RIGHT NOW.  I AM ALSO
FOCUSING ATTENTION ALONG OUTFLOW BOUNDARY NR SUX-OFK AREA, NICE
COMPACT VORT MAX IN SD AND ASSOCIATED CONVECTION, AND
CONVERGENT/TROUGH LINE FROM OFK-GRI AREA.  CU IS FORMING ALONG THESE
BOUNDARIES ALONG WITH A NEW PRESSURE FALL CENTER.  THE MESOETA
LATCHES ONTO THE MOISTURE CONVERGENCE NR SUX/OFK AND BRINGS
CONVECTION ALONG THE HIGHWAY 20 TO I-80 CORRIDORS THIS EVENING.
CAPES IN GENERATION REGION ARE AROUND 500 J/KG PER LAPS, BUT MAY
INCREASE TO AROUND 1500 J/KG IF TEMPS REACH INTO THE LOWER 80S.  LIS
OF -4 TO -6 IN THIS AREA AS WELL ASSOCIATED WITH STEEPENING LAPSE
RATES AND UPPER COOLING ASSOCIATED WITH SD/NE SHORT WAVE.  ANY
CONVECTION WILL HAVE DIFFICULT TIME REMAINING STRONG OR SEVERE AS IT
HEADS INTO OUR CWA BASED ON RAIN COOLED/MODIFIED AIR AND LACK OF
FRONTOGENESIS.  HIGHEST THREAT OF MARGINAL HAIL/WIND GUSTS IN OUR
WESTERN COUNTIES EARLY THIS EVENING.  I EXPECT LOW TEMPS TO REFLECT
GUIDANCE GIVEN CURRENT DEW POINTS, CLOUDS, AND RAIN COOLED AIR.

LONGER TERM...
ANOTHER MCS IS LIKELY TO DEVELOP OVER THE NORTHERN HIGH PLAINS LATE
TONIGHT AS SOUTHERLY FLOW REBOUNDS, BUT THIS MCS WILL STAY N OF IA
WEDNESDAY.  STRONG 850 MB WARMING OCCURS THROUGH THE DAY, PLACING
THERMAL RIDGE FROM NE-SW ACROSS IA.  MIXING WILL INCREASE THROUGH
THE DAY WITH S/SW SURFACE WINDS GUSTY AT TIMES ONCE AGAIN.  WOULD
NOT BE SURPRISED TO SEE 90 IN WESTERN IA GIVEN LACK OF DEEP MOISTURE
AND RELATIVELY DRY CONDITIONS.  THIS HEATING MAY ENABLE EXPLOSIVE
CONVECTIVE DEVELOPMENT HOWEVER AFTER 21Z ALONG STRENGTHENING SURFACE
FRONT IN NW IA.  WILL KEEP 40-50 POPS (HIGHER THAN GUIDANCE) IN THIS
AREA, WITH STRONG ENOUGH CONVERGENCE TO OVERCOME CIN.  WILL LEAVE
POPS OUT SE OF A ALO-DSM-CSQ LINE WEDNESDAY.  THIS WILL CHANGE
WEDNESDAY EVENING AND OVERNIGHT, WITH STRONG SIGNALS POINTING TO
MULTIPLE MCS EVENT CENTRAL AND SOUTHERN IA.  DIFFERENCES IN THE
MODELS ARE BOUNDARY PLACEMENT, WITH THE META FAVORED IN ITS
SOUTHERLY PUSH OF OUTFLOW.  HOWEVER, AS INTIAL CONVECTIVE
DEVELOPMENT PROPOGATES SE, A SECONDARY SHORTWAVE APPEARS OUT OF
NEBRASKA, CONINCIDING WITH ELEVATED QG FRONTOGENESIS IN NW IA.  DEEP
LAYER FRONTOGENESIS OCCURS ALONG ENTIRE STRETCH OF FRONT IN IA, WITH
50-65KT 850 MB FLOW CREATING STRONG CONVERGENCE AGAINST OPPOSING NE
FLOW N OF FRONT.  SURFACE BASED INSTABILITY MAY WEAKEN AS SURFACE
HEATING DECLINES, SO SEVERE THREAT MAY ALSO DECREASE OVERNIGHT.  BUT
AS NE-SW BOUNDARY STALLS OR SLOWLY MOVES SE, MULTIPLE MCS MAY TRACK
ALONG FRONT INTO WEDNESDAY MORNING AS CONTINUED FRONTOGENESIS IN THE
850-700MB LAYER IS PROGGED.  I FAVOR THE META PUSHING AN OUTFLOW
BOUNDARY OF SORTS INTO MO/KS, WHICH COULD LIMIT AFTERNOON AND
NIGHTTIME CONVECTION WEDNESDAY/WED NIGHT.

THERE IS GOOD AGREEMENT BY AVN/MRF THAT UPPER TROUGH WILL REMAIN
CLOSED OFF OVER UT/AZ THROUGH FRIDAY, WITH BOUNDARY LIFTING BACK N
INTO IA.  ANOTHER ROUND OF MCS'S THAT INITIATES OVER NE THURSDAY
SHOULD TRACK INTO WESTERN AND NORTHERN IA THURSDAY NIGHT INTO
FRIDAY.  UPPER RIDGING WILL BE INCREASING OVER SOUTHERN AND EASTERN
IA, WHICH MAY LIMIT PROGRESS INTO THIS AREA.

EXTENDED...
AS SURFACE AND UPPER RIDGING BUILD W INTO IA/MO FRIDAY INTO
SATURDAY.  PRECIP CHANCES SAT/SUN HINGE ON THE STRENGTH OF THIS
RIDGE, AND MY CONFIDENCE IS LOW THAT THE CLOSED LOW WILL EJECT INTO
OUR REGION.  ENSEMBLE PRECIP PROBS ARE NOT VERY HIGH EITHER THIS
PERIOD, SO WILL STAY WITH CHANCE POPS THIS FAR OUT.  COULD BE
ANOTHER ROUND OF HEAVY RAIN/FLOOD CONCERNS IBF THE UPPER LOW INDEED
MOVES OUT WITH E-W FRONT SLOWLY PROGRESSING N.

.DSM...NONE

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