Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 222340
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
640 PM CDT FRI MAY 22 2015

.SHORT TERM /TONIGHT/...
ISSUED AT 417 PM CDT FRI MAY 22 2015

SURFACE PRESSURE RIDGE WILL CONTINUE RETREATING EASTWARD AS THE
BEGINNING OF AN ACTIVE WEATHER WEEKEND APPROACH THE CWA. SHORTWAVE
HELPING TO FORCE THE AREA OF PCPN LIFTING NORTHWARD/NORTHEASTWARD
ACROSS THE MID-MISSOURI RIVER VALLEY WILL WEAKEN TONIGHT. BEST
DYNAMIC AND THERMODYNAMIC FORCING GOES TO THE WEST AND SOUTH OF THE
CWA. THUS HAVE CONTINUED THE TREND OF A RELATIVELY DRY FORECAST AT
LEAST TONIGHT. BEST CHANCES OF PCPN WILL BE ACROSS THE FAR
SOUTHWESTERN AND WESTERN CWA THIS EVENING. SFC WINDS WILL TRANSITION
FROM LIGHT AND VARIABLE TO SOUTHERLY LATER THIS EVENING AS THE
PRESSURE RIDGE CONTINUES ITS RETREAT AND THE PRESSURE GRADIENT
BEGINS TIGHTENING AS THE NEXT SYSTEM DRAWS CLOSER TO THE CWA.

.LONG TERM /SATURDAY THROUGH FRIDAY/...
ISSUED AT 417 PM CDT FRI MAY 22 2015

CURRENT UPPER LEVEL SYSTEM MOVING INTO THE STATE THIS AFTERNOON
WILL SHEAR OUT OVERNIGHT WITH NULL TO WEAK REMNANT FORCING
REMAINING BY 12Z. OVERALL EXPECT SATURDAY TO REMAIN MOSTLY DRY
THOUGH THE POTENTIAL FOR A FEW SHOWERS IS THERE IN THE MORNING
MAINLY FAR WEST THEN AN INCREASING CHANCE OVER THE SOUTHWEST
DURING THE AFTERNOON AS MOISTURE ADVECTION ENHANCES. THE MOISTURE
ADVECTION MAXIMIZES SATURDAY NIGHT INTO SUNDAY AS THE LOW LEVEL
JET REFOCUSES EAST INTO THE STATE AND GULF MOISTURE SURGES
NORTHWARD INTO THE STATE. THE INCREASING MOISTURE WILL SATURATE
PROFILES SATURDAY NIGHT WITH PWATS INCREASING ABOVE 1.5 INCHES BY
12Z. THE SATURATED PROFILES WILL LIMIT OVERALL INSTABILITY WITH
MUCAPES GENERALLY LESS THAN 500 J/KG DURING THE PERIOD. SHOWERS
AND THUNDERSTORMS ARE EXPECTED TO BECOME WIDESPREAD BY LATE
SATURDAY EVENING AND ESPECIALLY AFTER MIDNIGHT AND CONTINUE INTO
SUNDAY MORNING. POSSIBLE A BRIEF PERIOD DURING THE DAY SUNDAY WITH
NOT AS WIDESPREAD ACTIVITY BEFORE CONVECTION BECOMES RENEWED BY
THE AFTERNOON AND EVENING. HEAVY RAINFALL REMAINS POSSIBLE WITH
HIGHLY EFFICIENT RAINFALL RATES DURING THE PERIOD CONSIDERING THE
SATURATED PROFILES...HIGH PWATS AND GOOD WARM CLOUD DEPTHS. WHILE
RAINFALL RATES ARE NOT EXPECTED TO BE EXTREME DUE TO LACK OF
STRONG CONVECTION...THE NUMEROUS ROUNDS OF EFFICIENT RAIN
PRODUCING SHOWERS AND STORMS WILL LIKELY LEAD TO AREAS WITH TOTAL
RAINFALL IN EXCESS OF 3 INCHES. THIS WILL DIMINISH THE FLASH FLOOD
THREAT THOUGH WILL NEED TO CONSIDER THE LACK OF MATURE CROPS WHICH
WILL LEAD TO AN ACCELERATED RUN OFF OF ANY HEAVY PRECIPITATION
THAT OCCURS. REFER TO THE HYDROLOGY SECTION FOR ADDITIONAL DETAILS
REGARDING FLOODING POTENTIAL.

PROFILES WILL DRY OUT SOME BY MONDAY MORNING AND SHOULD HAVE A
PERIOD OF DRIER WEATHER MONDAY MORNING. A GOOD ELEVATED MIXED
LAYER WILL DEVELOP IN RESPONSE THUS SHOULD HAVE SOME CAPPING
THOUGH EARLY AFTERNOON. THE PRIMARY UPPER LEVEL SHORT WILL BE
LIFTING OUT INTO THE AREA SOMETIME MONDAY. THE TIMING OF THE
SYSTEM ARRIVING WITH MID LEVEL COOLING HELPING TO ERODE THE EML
FOR THUNDERSTORM INITIATION. SHEAR PROFILES ARE BETTER SO SOME
ORGANIZED STORMS ARE POSSIBLE. THEREFORE THROUGH THE WEEKEND...THE
BEST SEVERE WEATHER THREAT IS ON MONDAY. AT THIS TIME WILL NOT GET
INTO SPECIFIC DETAILS REGARDING THE SEVERE WEATHER THREAT WITH THE
CONVECTION THROUGH SUNDAY NIGHT POSSIBLY HAVING AN IMPACT ON THE
EVOLUTION INTO MONDAY.

THE ACTIVE FLOW WILL CONTINUE THE THE EXTENDED WITH SEVERAL
CHANCES FOR THUNDERSTORMS. AS MENTIONED IN PREVIOUS
DISCUSSIONS...THE PERIOD WILL BE MORE DRY THAN WET BUT CHANCES ARE
THERE REGARDLESS. HIGH TEMPERATURES WILL BE IN THE 70S AND 80S
THROUGH THE EXTENDED.

&&

.AVIATION...23/00Z
ISSUED AT 630 PM CDT FRI MAY 22 2015

FIRST WAVE OF FORCING PUSHING INTO CENTRAL IOWA THIS EVENING BUT
DRY LOW LEVELS KEEPING PRECIPITATION AT BAY. ANTICIPATE A
RELATIVELY DRY FORECAST PERIOD AS BULK OF PRECIPITATION WILL NOT
ARRIVE UNTIL SATURDAY NIGHT. CONDITIONS WILL REMAIN VFR WITH
CEILINGS AT OR ABOVE 8KFT AND UNRESTRICTED VISIBILITIES. SURFACE
WINDS WILL BE LIGHT AND VARIABLE TONIGHT WITH INCREASING SOUTHEAST
WINDS ON SATURDAY.

&&

.HYDROLOGY...
ISSUED AT 417 PM CDT FRI MAY 22 2015

WET NEXT FEW DAYS IS EXPECTED TO AN INITIAL RISK FOR FLASH FLOODING
SATURDAY NIGHT INTO SUNDAY NIGHT...AND THEN A SUBSEQUENT RISK OF
AT LEAST ADDITIONAL RISES ON AREA RIVERS...AND POSSIBLY RIVER
FLOODING...BEGINNING AS EARLY AS SUNDAY NIGHT AND CONTINUING INTO
THE EARLY PARTS OF NEXT WEEK. THE MIDDLE AND LOWER PORTIONS OF THE
DES MOINES AND RACCOON BASINS...AS WELL AS MISSOURI RIVER DRAINAGE
BASINS IN THE SOUTHWEST CWA...ARE OF MOST CONCERN AT THIS TIME.

SEE THE LONG TERM SECTION ABOVE FOR FURTHER DISCUSSION ON FLASH
FLOODING. IN TERMS OF RIVER FLOODING...THE 12Z HYDRO MODEL 72 HR QPF
ENSEMBLES ARE SHOWING A WIDE SPREAD BETWEEN THE EXPECTED AND 95
PERCENT MAX QPF TRACES...INDICATING RELATIVELY HIGH SENSITIVITY IN
THE HYDROLOGIC MODEL TO QPF. FOR EXAMPLE...THE 72 EXPECTED QPF
TRACES SLOW THE FALLS OF MOST STREAMS...WITH SOME SLIGHT TEMPORARY
INCREASES...WHILE THE 72 HR 95 PERCENT MAX QPF TRACES SHOW
SIGNIFICANT RISES...WITH MULTIPLE RIVER FORECAST POINTS RISING TO
NEAR OR ABOVE FLOOD STAGE.

IT SHOULD ALSO BE NOTED THAT THE 12Z QPF ENSEMBLES USED IN THE 12Z
HYDRO MODEL APPEAR TO BE UNDERDONE...WITH TOTAL EXPECTED 72-HR QPF
POSSIBLY FALLING CLOSER TO THE 95 PERCENT MAX QPF ENSEMBLE MEMBER
THAN THE 50 PERCENT QPF ENSEMBLE MEMBER.

GIVEN THE HYDROLOGIC MODEL SENSITIVITY TO THE QPF...AS WELL AS THE
POSSIBILITY OF ENSEMBLE QPF BEING UNDERDONE...UNCERTAINTY IN THE
RIVER RESPONSES IS HIGHER THAN AVERAGE FOR THIS PARTICULAR EVENT.
THIS HIGHER THAN AVERAGE UNCERTAINTY WILL NECESSITATE CLOSE
MONITORING OF QPF TRENDS...AND SUBSEQUENT IMPACTS ON THE HYDRO MODEL
GOING FORWARD.

&&

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

$$

SHORT TERM...ZOGG
LONG TERM...DONAVON
AVIATION...COGIL
HYDROLOGY...ZOGG



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