Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 240005
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
705 PM CDT SAT MAY 23 2015

.SHORT TERM /TONIGHT/...
ISSUED AT 314 PM CDT SAT MAY 23 2015

THE FORECAST IS FAIRLY STRAIGHT FORWARD IN SOME WAYS WITH HIGH
CONFIDENCE THAT CONVECTION WILL SPREAD NORTH AND EAST INTO IOWA
OVERNIGHT. TOKEN ELEVATED LIGHT SHOWERS IN MOISTURE STARVED
ISENTROPIC LIFT HAVE PASSED THROUGH IA THIS AFTERNOON...BUT BROAD
WEAK TO MODERATE THERMODYNAMIC AND KINEMATIC FORCING IS IN PLACE
UPSTREAM. THIS WILL PERSIST INTO THE NIGHT...AND WHILE THERE IS NOT
A WELL DEFINED BAROCLINIC ZONE...EXPANSIVE WARM AND THETA-E
ADVECTION WILL CONTINUE OVERNIGHT FROM THE CENTRAL PLAINS AND
INCREASE INTO IA. SOUNDINGS SATURATE BY EARLY SUN MORNING WITH MOIST
ADIABATIC LAPSE RATES DOMINATING. THIS WILL PRECLUDE ANY SEVERE
WEATHER THREAT...BUT TURN THE ATTENTION TO INCREASED RAIN
EFFICIENCY. MOST MODEL 300-305K ISENT SURFACES STILL DO NOT DEPICT A
SHARP BAROCLINIC ZONE WITH TIME...MORE OF A GRADUAL LIFT...BUT DO
SHOW SPEED CONVERGENCE DEVELOPING 09-12Z OVER SRN IA. WARM CLOUD
DEPTHS WILL BE PUSHING 12K FT...AND 00Z NAFES SPECIFIC
HUMIDITY...PWAT AND INTEGRATED WATER VAPOR TRANSPORT PERCENTILES ARE
WELL INTO THE 90S AND SOME PUSHING ALL TIME MAXES.  HAVE GENERAL QPF
AMOUNTS UNDER AN INCH THROUGH 12Z...AND THE MOST CONCERN MAY BE INTO
SUN...BUT THE POTENTIAL IS CERTAINLY THERE FOR LOCALLY HIGHER
AMOUNTS TONIGHT AND WILL NEED TO BE MONITORED.

.LONG TERM /SUNDAY THROUGH SATURDAY/...
ISSUED AT 314 PM CDT SAT MAY 23 2015

MAIN FORECAST CONCERNS FOR EXTENDED WILL COME WITH PRECIPITATION
TRENDS FOR THE BEGINNING OF THE PERIOD. EXPECT PRECIPITATION TO BE
ONGOING ACROSS MUCH OF THE CWA BY BEGINNING OF PERIOD AS SURFACE
LOW LIFTS INTO IOWA. PRECIPITATION COULD BE HEAVY IN THE MORNING
HOURS WITH SOUNDINGS SHOWING GOOD SATURATION...DEEP WARM CLOUD
DEPTH AND PWAT VALUES 1.5 INCHES OR HIGHER WHICH IS EXTREMELY HIGH
FOR THIS TIME OF YEAR...WITH CLIMATOLOGY INDICATING 1.6 INCHES IS
NEAR +2SD FOR MAY. HIGH RES MODELS GENERALLY HAVE A LINE OF
SHOWERS AND THUNDERSTORMS PERSISTING ACROSS CENTRAL IOWA THROUGH
THE MORNING HOURS...BEGINNING IN THE WEST AND SHIFTING EAST
THROUGH THE MORNING. AGREE WITH THIS ARE FOR THE HEAVIEST
PRECIPITATION...THOUGH ENOUGH FORCING AND MOISTURE FURTHER NORTH
AND EAST TO WARRANT HIGHER POPS ACROSS THIS AREA. CONCERN WILL
COME WITH AFTERNOON PRECIPITATION CHANCES...WITH MODELS
INDICATING MORE SCATTERED NATURE IN THE AFTERNOON...AND MAY SEE
DRY PERIOD BEFORE THUNDERSTORMS DEVELOP IN THE AFTERNOON. BEST
CHANCES WILL BE GENERAL ALONG AND EAST OF BOUNDARY EXPECTED TO BE
IN CENTRAL IOWA. INSTABILITY AND SHEAR WILL BE GENERALLY
WEAK...THOUGH CANNOT RULE OUT THE POTENTIAL FOR A STRONG TO SEVERE
THUNDERSTORM IN THE LATE AFTERNOON HOURS.

CHANCES FOR RAIN WILL AGAIN INCREASE INTO THE OVERNIGHT HOURS
SUNDAY AS WITH LLJ FEEDING INTO BOUNDARY LIFTING NORTH. AS LOW
PULLS NORTHEAST...COULD SEE LINGERING PRECIPITATION IN THE WEST
MONDAY THROUGH TUESDAY AS BOUNDARY SLOWS SOMEWHAT AS IT PUSHES
SOUTHEAST ACROSS IOWA. HOWEVER...GFS HAS SLOWEST
PROGRESSION...WITH ECMWF/GEM ENDING PRECIPITATION ACROSS THE AREA
QUICKER TUESDAY NIGHT...AND HAVE BEGUN CUTTING BACK ON POPS SOME
FOR MID WEEK. MONDAY WILL BRING THE BEST CHANCE FOR SEVERE
POTENTIAL TO THE AREA WITH CAPE VALUES EXPECTED TO REACH 2000 J/KG
OR HIGHER THROUGH THE CWA. HOWEVER...SOUNDINGS INDICATE WEAK CAP
IN PLACE WITH GENERALLY WEAK SHEAR WILL HELP TO LIMIT SEVERE
POTENTIAL AND FEEL CURRENT SPC MARGINAL RISK IS
REPRESENTATIVE...THOUGH WILL NEED TO BE MONITORED.

BEYOND MID WEEK...GENERALLY SOUTHWESTERLY UPPER LEVEL FLOW WILL
ALLOW FOR PERIODS OF PRECIPITATION THROUGH THE END OF THE PERIOD.
LARGE UPPER LEVEL LOW PRESSURE SYSTEM WILL BEGIN TO DROP SOUTH
INTO CENTRAL CANADA LATE IN THE PERIOD...DROPPING BOUNDARY INTO
NORTHERN PLAINS. THIS WILL BRING BEST CHANCES FOR PRECIPITATION
LATE IN THE PERIOD...GENERALLY LATE THURSDAY THROUGH THE WEEKEND.
TEMPERATURES WILL BE FAIRLY CONSISTENT THROUGH PERIOD WITH HIGHS
IN THE 70S AND 80S...THOUGH COOLER EARL IN THE PERIOD WITH
PRECIPITATION...AND LOWS IN THE 50S AND 60S.

&&

.AVIATION...24/00Z
ISSUED AT 646 PM CDT SAT MAY 23 2015

MVFR CEILINGS HAD MOVED IN SOUTHWEST IOWA AND WILL OVERSPREAD MUCH
OF CENTRAL IOWA TONIGHT ALONG WITH SHOWERS AND A FEW
THUNDERSTORMS. THE LOWER CEILINGS WILL CONTINUE INTO SUNDAY WITH
IFR CONDITIONS FROM TIME TO TIME AS WELL. SHOWERS ACTIVITY WILL
BE WIDESPREAD BY LATE TONIGHT AND PERSIST INTO SUNDAY ALTHOUGH
WITH DECREASED COVERAGE. SURFACE WINDS WILL REMAIN FROM THE
SOUTHEAST FOR MUCH OF THE PERIOD.

&&

.HYDROLOGY...
ISSUED AT 314 PM CDT SAT MAY 23 2015

DETERMINISTIC QPF NOW SUGGESTS WIDESPREAD ONE TO TWO INCH RAINS OVER
THE MEMORIAL DAY WEEKEND...MAINLY THROUGH SUNDAY NIGHT...WITH
LOCALLY HIGHER AMOUNTS CERTAINLY POSSIBLE. THUNDERSTORMS WITH
INTENSE RAINFALL ARE NOT ANTICIPATED...WITH THE EVENT MORE DUE TO A
PROLONGED PERIOD OF MODERATE TO LOCALLY HEAVY RAINS. THIS WILL ALLOW
FOR A BIT MORE GRADUAL RUNOFF AND RIVER FORECAST CENTER MODELING
SUGGESTS THE MOST LIKELY SCENARIO WOULD BE SIGNIFICANT WITHIN BANK
RISES AT ALL FORECAST SITES.

HOWEVER IF THE HIGH END OF ENSEMBLE QPF AMOUNTS WOULD HAPPEN TO
OCCUR IN CERTAIN LOCATIONS...WHICH COULD LOCALLY ADD ANOTHER INCH OR
SO AND THIS IS ENTIRELY POSSIBLE GIVEN THE EFFICIENT NATURE OF
EXPECTED RAINFALL...THE RACCOON RIVER BASIN AND DES MOINES RIVER
BASIN ABOVE SAYLORVILLE LAKE WOULD BE THE MOST SUSCEPTIBLE TO
REACHING OR EXCEEDING FLOOD STAGE. THE RACCOON RIVER WOULD SEEM TO
BE MOST SUSCEPTIBLE WITH MORE EXTREME RISES. THUS NO HYDRO HEADLINES
ARE ANTICIPATED AT THIS TIME BUT THE SITUATION WILL CERTAINLY BE
CLOSELY MONITORED THROUGH THE WEEKEND...ESPECIALLY IF HIGHER
RAINFALL AMOUNTS IMPACT THE RACCOON AND UPPER DES MOINES BASINS.

&&

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

$$

SHORT TERM...SMALL
LONG TERM...AWB
AVIATION...COGIL
HYDROLOGY...SMALL


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