Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 010453
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1153 PM CDT TUE SEP 30 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 305 PM CDT TUE SEP 30 2014

ACTIVE WEATHER CAN BE EXPECTED TONIGHT AS PLENTY OF FORCING AND
MOISTURE TRANSPORT INTO IOWA WILL LEAD TO NUMEROUS THUNDERSTORMS
ACROSS THE FORECAST AREA.  THUNDERSTORMS HAVE RECENTLY EXPANDED INTO
THE FORECAST AREA THIS AFTERNOON IN AXIS OF LOW LEVEL MOISTURE
TRANSPORT AND FORCING.  THERE IS SOME WEAK INSTABILITY IN THE FAR
SOUTHWEST WHERE SUNSHINE HAS BEEN PREVALENT ALL DAY AND STRONG
THUNDERSTORMS WILL BE POSSIBLE IN THE FAR WEST INTO THE EVENING.
HOWEVER...OVERALL INSTABILITY WILL WEAKEN OVERNIGHT WITH THE THREAT
OF ANY SEVERE CONVECTION REMAINING MINIMAL.  HOWEVER...LOW LEVEL
MOISTURE AXIS WILL SLIP INTO CENTRAL IOWA OVERNIGHT ALONG A 30-40KT
LOW LEVEL JET.  THIS WILL INCREASE PWATS TO 1.50 TO 1.75 INCHES
WHICH IS AROUND THE 99TH PERCENTILE.  WHILE CONVECTION MAY NOT BE
OVERLY STRONG...WARM LAYER DEPTH WILL BE CLOSE TO 11KFT ACROSS THE
WEST AND CENTRAL WITH RELATIVELY EFFICIENT PRODUCERS.
THEREFORE...SOME HEAVY RAINFALL IS EXPECTED WITH THE STORMS
OVERNIGHT AND HAVE INCLUDED IN THE FORECAST WORDING WITH LOCALIZED
AMOUNTS OVER 2 INCHES.  ANTICIPATE THE CONVECTION TO SLIDE GRADUALLY
EAST OVERNIGHT AS THE LLJ AXIS SHIFTS EASTWARD ALONG WITH THE BETTER
FORCING.  FAR EASTERN EDGES OF THE FORECAST AREA WILL LIKELY TO
REMAIN DRY UNTIL CLOSE TO DAYBREAK ON WEDNESDAY. OTHERWISE...
OVERNIGHT LOWS SHOULD REMAIN RELATIVELY WARM GIVEN THE EXPECTED
CLOUDINESS AND DECENT SOUTHEASTERLY FLOW.

.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
ISSUED AT 305 PM CDT TUE SEP 30 2014

LARGE UPPER LEVEL LOW PRESSURE SYSTEM WILL BRING MULTIPLE ROUNDS
OF PRECIPITATION TO THE REGION THROUGH THE END OF THE WORK WEEK.
ANTICIPATE ONGOING PRECIPITATION ACROSS CENTRAL IOWA AS FRONTAL
BOUNDARY CONTINUES TO PUSH EASTWARD. FORCING WILL BE SUFFICIENT TO
ALLOW FOR THUNDER...AND MAIN CONCERN WILL BE WITH HEAVY RAIN
POTENTIAL. PWAT VALUES WILL BE NEAR OR ABOVE 1.5 INCHES THROUGHOUT
MUCH OF THE MORNING...WHICH IS WELL ABOVE NORMAL FOR LATE
SEPTEMBER AND MAY ALLOW FOR LOCALIZED HEAVY RAINFALL. SOME CONCERN
ABOUT POTENTIAL FOR AFTERNOON RAINFALL. ANTICIPATE A BREAK IN
PRECIPITATION CHANCES...ESPECIALLY IN THE WEST AS SYSTEM PUSHES
EAST...THOUGH MODELS ARE A BIT SLOWER WITH PROGRESSION OF
FRONT...AND MAY SEE PROLONGED RAINFALL ACROSS CENTRAL AND EAST AS
BOUNDARY EXISTS SLOWLY. HOWEVER...BEHIND BOUNDARY MODELS INDICATE
DESTABILIZATION WILL OCCUR...WITH CAPE VALUES BETWEEN 1000-2000
J/KG ACROSS WEST IN THE AFTERNOON. THIS MAY ALLOW FOR ADDITIONAL
THUNDERSTORMS IN THE AFTERNOON AND EVENING HOURS...AGAIN WITH
HEAVY RAINFALL POSSIBLE. IN ADDITION...SOME THUNDERSTORMS MAY BE
SEVERE...THOUGH BEST CHANCES ATTM LOOK TO BE WEST OF CWA...THOUGH
WILL BE DEPENDENT ON LOCATION OF BOUNDARY IN AFTERNOON.

SECONDARY SURFACE LOW WILL LIFT NORTH FROM SOUTHERN PLAINS THROUGH
THURSDAY ALLOWING FOR PERSISTENT PRECIPITATION CHANCES THROUGH
THURSDAY. AGAIN...MAY SEE A BIT OF A BREAK BETWEEN WEDNESDAY
EVENING AND OVERNIGHT PRECIPITATION CHANCES BEGINNING...BUT
ANTICIPATE ADDITIONAL LINGERING PRECIPITATION THROUGH THE DAY
THURSDAY. SOME MODELS ARE KEEPING HEAVIEST PRECIPITATION FURTHER
SOUTH INTO MISSOURI...PRIMARILY WEDNESDAY NIGHT. SYSTEM WILL SHIFT
NORTHEAST THROUGH THE DAY THURSDAY. THIS WILL BRING A BREAK IN
PRECIPITATION CHANCES...HOWEVER TEMPERATURES WILL TURN COOLER
AS UPPER LEVEL LOW DIGS SOUTH INTO GREAT LAKES REGION. MODELS ARE
NOT IN GOOD AGREEMENT IN TIMING AND LOCATION OF THIS LOW...WITH
ECMWF SIGNIFICANTLY FASTER AND FURTHER SOUTH. HAVE THEREFORE
TRENDED TOWARD GFS/NAM KEEPING LOW INTO GREAT LAKES REGION. THOUGH
GIVEN STRENGTH OF LOW...CANNOT RULE OUT SHOWERS IN THE NORTH
FRIDAY. STRENGTH OF PUSH OF COLDER AIR WILL ALSO DEPEND ON
LOCATION OF LOW...AND HAVE KEPT TEMPERATURES NEAR GOING...WITH
HIGHS ONLY IN THE 50S FRIDAY AND LOWS A IN THE 30S AND 40S. MAY
SEE FROST DEVELOPMENT...ESPECIALLY IF LOW IS FURTHER SOUTH
OVERNIGHT FRIDAY...THOUGH HAVE KEPT MENTION OUT ATTM AS WINDS WILL
REMAINING STRONG. WINDS WILL BE STRONG FRIDAY WITH GOOD MIXING
AND SUBSIDENCE NEAR LOW...TOP OF MIXED LAYER IS NEAR 30KTS ATTM.

NORTHWESTERLY FLOW WILL THEN PERSIST THROUGH END OF EXTENDED.
SURFACE HIGH WILL HELP TO KEEP CONDITIONS MOSTLY DRY. TEMPERATURES
WILL BE VERY NEAR NORMAL FOR END OF PERIOD.

&&

.AVIATION...01/06Z
ISSUED AT 1153 PM CDT TUE SEP 30 2014

SHRA/TSRA EXPECTED TO EXPAND ACROSS THE AREA OVERNIGHT AND WED
MORNING...ESPECIALLY AFFECTING DSM/OTM...BEFORE MOVING OUT LATER
IN THE DAY ON WED. THE PRECIP WILL BE ACCOMPANIED BY MVFR
CIGS/VSBYS...POSSIBLY BRIEFLY IFR. FROM WED AFTN INTO WED NIGHT
THERE SHOULD BE SCATTERED REDEVELOPMENT BUT OF MORE CONCERN IS THE
POTENTIAL FOR WIDESPREAD FOG/STRATUS DEVELOPMENT. MOST OF THIS
POTENTIAL IS TOWARD OR AFTER THE END OF THE 06Z TAF PERIOD SO THIS
WILL BE ASSESSED IN MORE DETAIL FOR FUTURE TAF ISSUANCES.

&&

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

&&

$$

SHORT TERM...COGIL
LONG TERM...AWB
AVIATION...LEE



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