Special Weather Statement, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1235 PM CDT SUN APR 27 2014

.SHORT TERM /TODAY/...
ISSUED AT 359 AM CDT SUN APR 27 2014

SEVERE WEATHER POTENTIAL WILL BE PRIMARY CONCERN TODAY WITH
CONDITIONALITY OF EVENT SEEMING TO DIMINISH WITH TIME. CURRENT
NARROW RIBBON OF ELEVATED CONVECTION ROOTED 2-3KM AND DRIVEN BY
STRONG WARM/THETA-E ADVECTION ZONE WILL CONTINUE TO LIFT NORTH AND
EAST OUT OF THE AREA EARLY THIS MORNING. 3HR MUCAPE CHANGES
SUGGEST THAT AIRMASS ALOFT IS BECOMING INCREASINGLY UNSTABLE AS
INSTABILITY SURGES NORTHWARD SO...WHILE STORMS HAVE REMAINED BELOW
SEVERE LIMITS OVERNIGHT AND EFFECTIVE SHEAR IS MINIMAL...WOULD NOT
BE SURPRISED TO SEE A FEW NUDGE ABOVE SEVERE BEFORE EXITING
FORECAST AREA.

ATTENTION THEN TURNS TO THE SOUTH AND WEST TOWARD APPROACHING PARENT
LONG WAVE TROUGH AND ASSOCIATED POTENT SHORT WAVE/PV ANOMALY.  BROAD
WEAK/MODERATE QG FORCING AND UPPER LEVEL DIFFLUENCE IS ALREADY IN
PLACE ACROSS THE MO VALLEY SO COULD SEE PRECURSOR CONVECTION DEVELOP
AHEAD OF MAIN EVENT AT ANY TIME BEHIND EXITING THERMODYNAMIC ZONE.
PRIMARY FOCUS WILL BE ON THE AFTERNOON HOURS. PV ANOMALY SHOULD BE
OVER KS BY 18Z WITH DRY SLOT AND ATTENDANT LOBE OF FORCING LIFTING
INTO SWRN IA. THIS MECHANICAL FORCING SHOULD HAVE NO PROBLEM
DEVELOPING CONVECTION WITH NAM/GFS BOTH SHOWING LITTLE TO NO CINH
AFTER 18Z. MLCAPES AROUND OR JUST SHY OF 1000 J/KG IN BOTH
INSTANCES.

EXAMINATION OF CONVECTIVE MODE POINTS SQUARELY AT LOW TOPPED
SUPERCELLS AND TORNADO POTENTIAL. 0-6KM SHEAR IS 30-50KTS DEPENDING
ON MODEL...0-3KM CAPES ARE 100 J/KG OR MORE WITH LOW LCLS...20-30KTS
OF 1KM SHEAR AND 100-300 M2/S2 SRH. MUCH LIKE EXPECTED YESTERDAY THE
SW THIRD OR SO OF IA APPEARS TO BE HIGHLIGHTED AND WOULD NOT BE
SURPRISED AT ALL TO SEE FUTURE SPC DAY1 OUTLOOKS LIFT MOD RISK INTO
PORTIONS OF SRN IA. HAVE ADDED SEVERE WORDING TO FORECAST IN
THESE AREAS. INTERESTINGLY TWO OF THE TOP EIGHT CIPS ANALOG EVENTS
ARE ANDOVER KS /26 APR 91/ AND THE 1995 IA MEMORIAL DAY WEEKEND
TORNADO OUTBREAK /27 MAY 95/.

SECONDARY CONCERN WILL BE STRONG GRADIENT WINDS IN POTENT ELY FLOW
AHEAD OF STEADY OR SLIGHTLY DEEPENING LOW PRESSURE AREA. SEVERAL
SITES ARE CURRENTLY AT ADVISORY CRITERIA...AND WHILE THERE MAY BE
A MIDDAY LULL...EXPECT WINDS TO INCREASE ONCE AGAIN BY LATE
AFTERNOON. ADVISORY WILL BE IN EFFECT THROUGH 02Z ALONG AND NORTH
OF HIGHWAY 20.

YET ANOTHER CONCERN WILL BE HEAVY RAIN POTENTIAL. PWS WILL BE 1.25
INCHES OR BETTER WITH VERY STRONG 1000-850MB MOISTURE TRANSPORT
FEEDING MCS. PWS ARE PLUS 2-3SD AND LOW LCLS WILL INCREASE WARM
CLOUD DEPTHS AND EFFICIENCY. PRECIP AMOUNTS WILL NOT BE TOO
EXTREME THROUGH 00Z WITH CONVECTION FAIRLY TRANSIENT...BUT WOULD
NOT BE SURPRISED TO SEE LOCAL AMOUNTS EXCEED AN INCH.

HIGH TEMPS WILL VARY QUITE A BIT WITH 08Z SURFACE ANALYSIS SHOWING
WHAT APPEARS TO BE A DUAL WARM FRONTAL STRUCTURE...ONE FROM NE INTO
KS/MO AND ANOTHER THROUGH IA INTO IL. FORECAST SOUNDINGS SHOW
SOME MINIMAL MIXING RESULTING IN AROUND A 15-20F REBOUND NORTH TO
SOUTH...CLOSER TO THE WARMER END OF MOS GUIDANCE.

.LONG TERM /TONIGHT THROUGH SATURDAY/...
ISSUED AT 359 AM CDT SUN APR 27 2014

AFTERNOON AND EVENING STORMS...WITH SOME SEVERE WEATHER THREAT IN
THE EVENING AS DISCUSSED IN THE SHORT TERM DISCUSSION ABOVE...WILL
LIFT NORTHEAST ACROSS THE AREA THROUGH MIDNIGHT TONIGHT WITH
SLOWLY DIMINISHING INTENSITY. WE SHOULD THEN SEE A DECREASE IN
COVERAGE OF SHOWERS AND THUNDERSTORMS LATE TONIGHT AND MONDAY
MORNING. HOWEVER...THE SURFACE WARM FRONT WILL SHARPEN SOMEWHERE
ACROSS CENTRAL IOWA IN ADVANCE OF THE PARENT LOW PRESSURE SYSTEM
APPROACHING FROM THE WEST...AND ON MONDAY AFTERNOON DIURNAL
HEATING WILL ALLOW A ZONE OF LOW LEVEL INSTABILITY AND VORTICITY
TO DEVELOP ALONG/NEAR THE FRONT. THIS IS REFLECTED BY NUMEROUS
NUMERICAL MODELS AS WELL AS CLIMATOLOGY IN SUCH PATTERNS AND
REFLECTS A THREAT FOR SEVERE WEATHER WITH LANDSPOUT TYPE
TORNADOES POSSIBLE. A MORE PRECISE LOCATION OF THE WARM FRONT AND
SEVERE WEATHER THREAT DURING THIS TIME WILL BE DETERMINED AFTER
THE SUNDAY/SUNDAY NIGHT CONVECTION...BUT THIS SITUATION WILL NEED
TO BE MONITORED AS WE HEAD INTO MONDAY.

ON MONDAY NIGHT AS THE LARGE STACKED GYRE MOVES OVER CENTRAL IOWA
ANY SEVERE WEATHER THREAT WILL QUICKLY DIMINISH AND SEVERAL DAYS
OF COLD RAIN WILL BEGIN...PERSISTING AT LEAST THROUGH THURSDAY
WITH FREQUENT BUT GENERALLY LIGHT RAIN...OVERNIGHT LOWS IN THE
UPPER 30S TO MID 40S...AND DAYTIME HIGHS ONLY IN THE 40S AND 50S
TO USHER IN THE MONTH OF MAY. NEAR THE END OF THE WORK WEEK RAIN
CHANCES WILL DIMINISH...HOWEVER THE 500 MB FLOW WILL REMAIN
CYCLONIC WITH NO SIGNIFICANT WARMING FORECAST THROUGH NEXT
WEEKEND.

&&

.AVIATION...27/18Z
ISSUED AT 1234 PM CDT SUN APR 27 2014

PRIMARY CONCERN THIS AFTERNOON/EVENING WILL BE SEVERE STORMS
MOVING ACROSS THE AREA AS WELL AS STRONG EASTERLY WINDS. ATTEMPTED
TO TIME OUT PROGRESSION OF LINE MOVING EAST NORTHEAST WITH TIME
TODAY...AND WILL UPDATE AS NEEDED. LINE SHOULD BE MOVING EAST OF
REGION BY 23-00Z WITH SHOWERS EXPECTED TO CONTINUE. POTENTIAL
FOR HAIL...WIND AND POTENTIAL FOR TORNADOES SOUTH MAINLY BUT HAIL
THREAT MAY EXTEND NORTH SOMEWHAT WITH A FEW STORMS. EASTERLY WINDS
CONTINUE TO BE A PROBLEM FOR THE EVENING HOURS WITH STRONG LOW TO
WEST. MONDAY WILL SEE ANOTHER CHANCE FOR STORMS DURING THE
AFTERNOON ALONG THE BOUNDARY AND WILL NEED TO MONITOR FOR
POTENTIAL SEVERE IN THE AFTERNOON HOURS./REV

&&

.HYDROLOGY...
ISSUED AT 359 AM CDT SUN APR 27 2014

EVENT TOTAL QPF IS EXPECTED TO BE WIDESPREAD ONE TO THREE INCH
RAINS...MAINLY TODAY INTO MONDAY...WITH HIGHEST AMOUNTS OVER THE
NORTHEAST HALF OF THE FORECAST AREA. THIS AREA ALSO HAS THE
RELATIVELY WETTEST SOILS...REMOVED FROM THE D1-D2 DROUGHT FARTHER
SOUTH AND WEST. ENSEMBLE AND DETERMINISTIC QPF AND RESULTANT RIVER
MODEL OUTPUT SUGGESTS IOWA AND CEDAR RIVER BASINS WOULD BE FAVORED
TO SEE SIGNIFICANT WITHIN BANK RISES TO MINOR FLOODING. HIGHER END
ENSEMBLE SOLUTIONS WOULD ALSO SUGGEST MODERATE FLOODING IN A FEW
LOCATIONS.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY UNTIL 9 PM CDT THIS EVENING FOR BLACK HAWK-BREMER-
BUTLER-CALHOUN-CERRO GORDO-EMMET-FRANKLIN-GRUNDY-HAMILTON-
HANCOCK-HARDIN-HUMBOLDT-KOSSUTH-PALO ALTO-POCAHONTAS-SAC-WEBSTER-
WINNEBAGO-WORTH-WRIGHT.

&&

SHORT TERM...SMALL
LONG TERM...LEE
AVIATION...REV
HYDROLOGY...SMALL

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
412 AM CDT SUN APR 27 2014

IAZ044>050-057>062-070>075-081>086-092>097-280915-
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-
412 AM CDT SUN APR 27 2014

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA.

.DAY ONE...TODAY AND TONIGHT

ISOLATED TO SCATTERED THUNDERSTORMS ARE EXPECTED THIS MORNING WITH
A LOW THREAT OF SEVERE WEATHER. THUNDERSTORMS WILL BECOME MORE
NUMEROUS WITH SIGNIFICANTLY HIGHER SEVERE WEATHER CHANCES THIS
AFTERNOON AND EVENING WHEN MULTIPLE TORNADOES...LARGE
HAIL...DAMAGING WIND AND LOCALLY HEAVY RAIN WILL ALL BE POSSIBLE.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

THERE WILL BE A CHANCE FOR THUNDERSTORMS MONDAY AND MONDAY
NIGHT...MAINLY TOWARD THE INTERSTATE 80 AND HIGHWAY 30 CORRIDORS.
SOME STORMS COULD BE SEVERE ON MONDAY AFTERNOON.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS POSSIBLE THIS AFTERNOON AND EVENING...AND
AGAIN MONDAY AFTERNOON.

IAZ004>007-015>017-023>028-033>039-280915-
EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO-
POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN-
WEBSTER-HAMILTON-HARDIN-GRUNDY-BLACK HAWK-
412 AM CDT SUN APR 27 2014

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA.

.DAY ONE...TODAY AND TONIGHT

A WIND ADVISORY IS IN EFFECT FOR THE AREA. PLEASE REFER TO THE
NATIONAL WEATHER SERVICE WEBSITE AT WEATHER.GOV/DESMOINES FOR
FURTHER DETAILS.

ISOLATED TO SCATTERED THUNDERSTORMS ARE FORECAST FOR MUCH OF
TODAY...BECOMING MORE WIDESPREAD FROM LATE THIS AFTERNOON THROUGH
TONIGHT. SEVERE WEATHER AND LOCALLY HEAVY RAIN WILL BE POSSIBLE
LATE THIS AFTERNOON AND EVENING.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

THUNDERSTORMS ARE FORECAST AGAIN ON MONDAY...WITH A FEW SEVERE
STORMS POSSIBLE TOWARD HIGHWAY 20 IN THE AFTERNOON. ISOLATED NON
SEVERE THUNDERSTORMS ARE THEN EXPECTED ON MONDAY NIGHT.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS POSSIBLE LATE THIS AFTERNOON AND EVENING.

LEE

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