River Flood Warning 177, NWS Des Moines, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1207 AM CDT TUE JUL 20 2010

...UPDATED FOR 06Z AVIATION DISCUSSION...

.SHORT TERM /TONIGHT/...
PREVIOUS CONVECTION AND OUTFLOW CONTINUE TO COMPLICATE THE FORECAST.
NUMEROUS SYSTEMS PASSING THROUGH THE STATE TODAY HAVE KEPT THE
BOUNDARY ACRS NE KS INTO NORTHERN MISSOURI. HAVE BEGUN TO SEE
RECOVERY IN WESTERN IA BY MID AFTERNOON AS SUNSHINE HAS BROKEN OUT
FROM CLOUDINESS BUT WILL BE DIFFICULT TO GET TOO UNSTABLE PRIOR TO
SUNSET. SPC MESOANALYSIS WOULD INDICATE THE SOUTHERN TWO TIER OR SO
UNDER THE BEST THREAT OF ANY SEVERE CONVECTION THIS EVENING SHOULD
ANYTHING DEVELOP. FARTHER NORTH...AIR IS QUITE WORKED OVER AND WOULD
LIMIT THE STRENGTH OF ANY CONVECTION. IT WOULD BE QUITE ELEVATED AS
WELL SINCE THE BOUNDARY REMAINS MUCH FARTHER SOUTH. THIS WOULD ALSO
LIMIT THE HEAVY RAIN POTENTIAL THIS EVENING TO OUR FAR.
HOWEVER...ANOTHER SHORTWAVE APPROACHES LATER WHICH WILL INCREASE
FORCING IN THE WEST AND IN COMBINATION WITH THE LLJ...IS EXPECTED TO
FIRE CONVECTION BEFORE SUNRISE IN OUR FAR WEST. HAVE DECIDED TO HOLD
ON TO THE FLASH FLOOD WATCH AT THIS POINT GIVEN THE SATURATED
CONDITIONS AND ADDITIONAL THREAT OF CONVECTION THROUGH TONIGHT.

.LONG TERM.../TUESDAY THROUGH MONDAY/
VERY LITTLE CHANGE IN THINKING FOR GENERAL FORECAST SCENARIO OVER
THE NEXT SEVEN DAYS. WE WILL REMAIN IN A WET AND ACTIVE PATTERN FOR
MOST OF THE WORK WEEK WITH FAIRLY BRISK WEST NORTHWEST 500 MB FLOW
AND LIGHT SURFACE FLOW AS A DECAYED FRONTAL BOUNDARY WOBBLES AROUND
NEARBY. AS SHORTWAVE IMPULSES MOVE THROUGH THE MID-LEVEL
FLOW...AND/OR NOCTURNAL WAA/LLJ ENHANCEMENTS KICK IN...WE WILL
LIKELY SEE SEVERAL ROUNDS OF THUNDERSTORMS AT VARIOUS TIMES AND
LOCATIONS AROUND THE CWA. THE FIRST WILL LIKELY BE ONGOING TUESDAY
MORNING ACROSS OUR CENTRAL OR SOUTHERN COUNTIES...THEN SINK
SOUTHWARD THROUGH THE DAY CLOSER TO THE NEAR-SURFACE FRONTAL
BOUNDARY WITH A WEAK SURFACE HIGH MOVING INTO THE NORTH. A RELATIVE
LACK OF INSTABILITY SHOULD LIMIT THE SEVERE WEATHER THREAT TO OUR
FAR SOUTH...WITH A BRIEF REPRIEVE ACROSS ABOUT THE NORTHERN HALF.
THIS PATTERN WILL LIKELY PERSIST THROUGH TUESDAY NIGHT...THEN BY
WEDNESDAY EVENING WEAK 500 MB RIDGING WILL MOVE OVERHEAD AND THE
SURFACE HIGH WILL MOVE OFF TO THE EAST...PUTTING US IN SOUTHEASTERLY
SURFACE FLOW. ON WEDNESDAY NIGHT THERE IS UNANIMOUS AND CONSISTENT
AGREEMENT AMONG THE PROGNOSTIC MODELS THAT A SHORTWAVE WILL RIDE
OVER THE RIDGE ACROSS NORTHERN IOWA...COINCIDENT WITH AN INCREASE IN
WARM AIR/MOISTURE ADVECTION AS OUR LOW-LEVEL FLOW TURNS TO SOUTH AND
A LLJ KICKS IN AFTER SUNSET. GIVEN THE PERSISTENT STRENGTH OF THIS
SIGNAL AS WELL AS ITS CLIMATOLOGICAL REASONABILITY HAVE INCREASES
POPS TO LIKELY ACROSS ABOUT OUR NORTHERN HALF WEDNESDAY NIGHT. THIS
PRECIP WILL MOVE AWAY WITH THE SHORTWAVE THURSDAY...THEN IT APPEARS
YET ANOTHER IMPULSE WILL PUSH A WEAK COLD FRONT THROUGH THE AREA
SOMETIME AROUND FRIDAY OR FRIDAY NIGHT WHICH MAY OR MAY NOT BRING A
BREAK FROM THE RAIN FOR THIS WEEKEND...BUT UNCERTAINTY IS HIGHER IN
THOSE PERIODS AND IT REMAINS TO BE SEEN WHETHER SAID WEAK FRONT
WOULD CLEAR OUR AREA OR NOT. NEVERTHELESS HAVE KEPT THE FORECAST DRY
FROM SATURDAY OUT FOR NOW.

&&

.AVIATION...20/06Z
LITTLE CHANGE FROM PREVIOUS FORECAST WITH SCT THUNDERSTORMS
REMAINING LIKELY OVER THE SOUTH INCLUDING KDSM AND KOTM OVERNIGHT
AND INTO THE MORNING. POSSIBLE MAY CLIP INTO KFOD AND KALO.
CONDITIONS ARE BECOMING MORE FAVORABLE FOR LOW STRATUS AND FOG OVER
THE NORTH WHERE IFR CONDITIONS MAY OCCUR AND LOCALIZED LIFR
ESPECIALLY NEAR KMCW. THUNDERSTORM CHANCES CONTINUE OVER THE SOUTH
THROUGH 00Z THEN BETTER CHANCE FOR A BREAK LATE.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH THROUGH TUESDAY MORNING FOR ADAIR-ADAMS-APPANOOSE-
AUDUBON-BOONE-CARROLL-CASS-CLARKE-CRAWFORD-DALLAS-DAVIS-DECATUR-
GREENE- GUTHRIE-JASPER-LUCAS-MADISON-MAHASKA-MARION-MONROE-POLK-
POWESHIEK-RINGGOLD-STORY-TAYLOR-UNION-WAPELLO-WARREN-WAYNE.

&&

SHORT TERM...COGIL
LONG TERM...LEE
AVIATION...DONAVON

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
107 AM CDT TUE JUL 20 2010

IAZ057-058-070>074-081>086-092>097-200815-
AUDUBON-GUTHRIE-CASS-ADAIR-MADISON-WARREN-MARION-ADAMS-UNION-
CLARKE-LUCAS-MONROE-WAPELLO-TAYLOR-RINGGOLD-DECATUR-WAYNE-
APPANOOSE-DAVIS-
INCLUDING THE CITIES OF...AUDUBON...GUTHRIE CENTER...ATLANTIC...
GREENFIELD...WINTERSET...INDIANOLA...KNOXVILLE...CORNING...
CRESTON...OSCEOLA...CHARITON...ALBIA...OTTUMWA...BEDFORD...
MOUNT AYR...LEON...CORYDON...CENTERVILLE...BLOOMFIELD
107 AM CDT TUE JUL 20 2010

...HEAVY RAIN POTENTIAL INCREASING ACROSS THE AREA...

THUNDERSTORMS ARE LINING UP OVER SOUTH CENTRAL AND SOUTHWEST IOWA.
THE THUNDERSTORMS ARE EXPECTED TO TRAIN ACROSS THE AREA AND
PRODUCE A SWATH OF VERY HEAVY RAINFALL. A BAND OF 2 TO 5 INCHES OF
RAIN IS LIKELY ACROSS PORTIONS OF THE REGION. HIGHER AMOUNTS OF 4
TO 7 INCHES MAY OCCUR OVER WAYNE AND APPANOOSE COUNTIES. THESE
RAINFALL AMOUNTS COULD PRODUCE SIGNIFICANT PONDING OF WATER IN
ADDITION TO FLASH FLOODING THROUGH THE REMAINDER OF THE NIGHT.

DONAVON

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
149 AM CDT TUE JUL 20 2010

IAZ057-058-070>073-081>086-092>097-200900-
AUDUBON-GUTHRIE-CASS-ADAIR-MADISON-WARREN-ADAMS-UNION-CLARKE-
LUCAS-MONROE-WAPELLO-TAYLOR-RINGGOLD-DECATUR-WAYNE-APPANOOSE-
DAVIS-
INCLUDING THE CITIES OF...AUDUBON...GUTHRIE CENTER...ATLANTIC...
GREENFIELD...WINTERSET...INDIANOLA...CORNING...CRESTON...
OSCEOLA...CHARITON...ALBIA...OTTUMWA...BEDFORD...MOUNT AYR...
LEON...CORYDON...CENTERVILLE...BLOOMFIELD
149 AM CDT TUE JUL 20 2010

...STRONG TO SEVERE THUNDERSTORMS ARE EXPECTED EARLY THIS MORNING...

SCATTERED THUNDERSTORMS CONTINUE TO AFFECT PORTIONS OF CENTRAL AND
SOUTHERN IOWA...MAINLY ALONG AND SOUTH OF AN AUDUBON...TO
INDIANOLA...TO OTTUMWA LINE. OTHER THUNDERSTORMS OVER EASTERN
NEBRASKA WILL ENTER THESE SAME AREAS EARLY THIS MORNING RESULTING
IN A PROLONGED THREAT OF THUNDERSTORMS.  SOME OF THE STRONGER
STORMS MAY PRODUCE WINDS TO 60 MPH AND UP TO QUARTER SIZED HAIL.

TORRENTIAL...POTENTIALLY FLOODING...RAINS MAY ALSO ACCOMPANY THESE
STORMS. TWO TO FIVE INCHES OF RAIN MAY OCCUR WITH ISOLATED AMOUNTS
UP TO SEVEN INCHES. A FLASH FLOOD WATCH REMAINS IN EFFECT THROUGH
700 AM.

SMALL

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