Flash Flooding and Hail in Iowa, May 20 to 21, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
633 AM CDT TUE MAY 20 2014

.SHORT TERM /TODAY/...
ISSUED AT 332 AM CDT TUE MAY 20 2014

ISOLATED CENTRAL IA THUNDERSTORM APPROACHING SEVERE LIMITS
DISSIPATED QUICKLY LAST HOUR. RAP SOUNDINGS SEEMED TO REFLECT THE
ENVIRONMENT QUITE WELL WITH POTENTIAL FOR STORMS ROOTED NEAR THE
SURFACE SHORTLY AFTER MIDNIGHT...BUT MLCINH INCREASING QUICKLY AS
BOUNDARY LAYER COOLS FURTHER AND DECOUPLING FINALLY TAKING PLACE.
WITH THIS PROGRESSION DO NOT REALLY EXPECT ANY ADDITIONAL
DEVELOPMENT UNTIL PEAK HEATING OF THE AFTERNOON...WHICH IS THE
PRIMARY FORECAST CONCERN.

SCENARIO FOR TODAY HAS CHANGED LITTLE OVER THE PAST FEW DAYS WITH
SEVERE WEATHER POTENTIAL STILL CONDITIONAL. MODELS ALL DEPICT
SIMILAR THERMODYNAMIC ENVIRONMENT WITH MLCINH ERODING DURING THE
AFTERNOON AND MLCAPES TO AROUND 1500 J/KG. HOWEVER WHILE THE CINH
HAS DIMINISHED SUFFICIENTLY FOR CONVECTION TO FIRE...LIMITING
FACTORS INCLUDE WEAK CONVERGENCE AND THE FACT THAT WARM 1-3KM
TEMPS WILL KEEP PARCELS FROM REALLY ACCELERATING UNTIL REACHING
3KM OR HIGHER. THERE IS A QUESTION WHETHER THE WEAK CONVERGENCE
WILL OVERCOME THIS. LARGE SCALE FORCING MAY HELP LIFT AND
DESTABILIZATION OF 1-3KM LAYER TOWARD 00Z HOWEVER AS CURRENT
DAKOTAS MN JET SEGMENT MOVES EASTWARD. THUS STILL HAVE SCATTERED
THUNDERSTORMS DEVELOPING IN THE EASTERN HIGHWAY 30/I80/HIGHWAY 92
CORRIDORS. MAIN SEVERE THREATS STILL APPEAR TO BE LARGE HAIL OR
WIND. LOW LEVEL SHEAR FAIRLY WEAK WITH HIGHER LCLS. STORM MOTION
WILL BE FAIRLY SLOW TOO BASED ON MEAN WIND AND BUNKERS
MOTION...LIKELY UNDER 25KTS.

.LONG TERM /TONIGHT THROUGH SUNDAY/...
ISSUED AT 332 AM CDT TUE MAY 20 2014

THE SURFACE TROUGH SERVING AS A FOCUS FOR THUNDERSTORM DEVELOPMENT
THIS AFTERNOON...AS DISCUSSED IN THE SHORT TERM SECTION
ABOVE...WILL MOVE SOUTH OUT OF THE AREA TONIGHT WITH DECREASING
POPS OVERNIGHT. THEREAFTER...THE BOUNDARY WILL REMAIN OVER
MISSOURI WITH A LARGE SURFACE HIGH PRESSURE AREA BUILDING OVER
IOWA AND THE UPPER MIDWEST FOR SEVERAL DAYS. THIS WILL KEEP
TEMPERATURES RELATIVELY COOL TOWARD THURSDAY AND FRIDAY WITH RAIN
CHANCES REMAINING OVER MISSOURI OR POSSIBLY AFFECTING THE FAR
SOUTHERN FRINGE OF IOWA AT TIMES.

MEANWHILE...AT 500 MB A LARGE CUTOFF LOW WILL DEVELOP OVER THE
FAR SOUTHWEST U.S. LATE THIS WEEK AND RESIDE SOMEWHERE AROUND THE
CALIFORNIA/ARIZONA BORDER BY FRIDAY MORNING. THIS WILL SQUEEZE A
500 MB RIDGE DOWNSTREAM ACROSS THE CENTRAL U.S. AND UP TOWARD
IOWA. AS THE CUTOFF LOW SLOWLY MOVES EASTWARD THIS CENTRAL U.S.
RIDGE WILL STEADILY ERODE...ALLOWING THE UPPER MIDWEST SURFACE
RIDGE TO SLIDE EASTWARD WITH A RETURN OF SOUTHEAST SURFACE FLOW
OVER IOWA AND THE OLD FRONTAL BOUNDARY MOVING BACK OVERHEAD. THIS
WILL RESULT IN RAIN CHANCES RETURNING TO THE FORECAST AREA THIS
WEEKEND AND INTO EARLY NEXT WEEK...PERSISTING FOR MULTIPLE DAYS AS
THE 500 MB LOW MOVES INTO THE AREA THROUGH THE END OF THE SEVEN
DAY FORECAST PERIOD. CONSEQUENTLY FROM SATURDAY ONWARD GENERALLY
30 TO 40 POPS ARE CARRIED IN EVERY PERIOD WITH GRADUALLY
INCREASING TEMPERATURES.

&&

.AVIATION...20/12Z
ISSUED AT 633 AM CDT TUE MAY 20 2014

LIFR/IFR FOG CONDITIONS ARE IN PLACE ACROSS PORTIONS OF NRN IA
EARLY THIS MORNING BUT HAVE YET TO AFFECT TAFS SITES OUTSIDE OF
MVFR BRUSHING KFOD/KMCW. AFTER MORNING FOG DISSIPATES ATTENTION
WILL TURN TOWARD CONVECTIVE DEVELOPMENT ALONG CURRENT WEAK FRONTAL
BOUNDARY NOTED FROM KOMA AREA TO NEAR KMCW AT 12Z. FRONT WILL
LIKELY TRIGGER THUNDERSTORMS WITH LIMITED COVERAGE ACROSS CENTRAL
IA 21Z-00Z AS IT DRIFTS SOUTH THEN LASTING INTO THE EARLY EVENING
BEFORE DISSIPATING WITH LOSS OF SUN. LIMITED COVERAGE WILL MAKE
TIMING AND LOCATIONS UNCERTAIN ENOUGH THAT HAVE STARTED WITH VCSH
WORDING AND NO THUNDER AS OF YET...ALTHOUGH IT WOULD BE LIKELY AT
ANY SITES THAT DO SEE PRECIP. MAINLY VFR OVERNIGHT ALTHOUGH PATCHY
FOG POSSIBLE...ESPECIALLY SOUTH /KOTM/ IN CLOSER PROXIMITY TO
DIFFUSE FRONT.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

SHORT TERM...SMALL
LONG TERM...LEE
AVIATION...SMALL

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
358 AM CDT TUE MAY 20 2014

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-211100-
EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO-
POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN-
WEBSTER-HAMILTON-HARDIN-GRUNDY-BLACK HAWK-CRAWFORD-CARROLL-GREENE-
BOONE-STORY-MARSHALL-TAMA-AUDUBON-GUTHRIE-DALLAS-POLK-JASPER-
POWESHIEK-CASS-ADAIR-MADISON-WARREN-MARION-MAHASKA-ADAMS-UNION-
CLARKE-LUCAS-MONROE-WAPELLO-TAYLOR-RINGGOLD-DECATUR-WAYNE-
APPANOOSE-DAVIS-
358 AM CDT TUE MAY 20 2014

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA.

.DAY ONE...TODAY AND TONIGHT

SCATTERED THUNDERSTORMS WILL DEVELOP THIS AFTERNOON AND
EVENING...MAINLY SOUTH OF HIGHWAY 20. SOME STORMS COULD BE
SEVERE...WITH LARGE HAIL AND STRONG WINDS THE PRIMARY THREATS.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

NON SEVERE THUNDERSTORMS ARE FORECAST ACROSS SOUTHWEST IOWA AT
TIMES FROM EARLY THURSDAY MORNING THROUGH FRIDAY NIGHT.
THUNDERSTORM CHANCES WILL THEN SPREAD ACROSS THE REMAINDER OF THE
AREA THIS WEEKEND...WITH A LOW THREAT OF SEVERE WEATHER DURING
THAT TIME.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS POSSIBLE LATE TODAY.

LEE