503
FXUS63 KDMX 190439
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1139 PM CDT SUN MAY 18 2025
...UPDATED FOR THE 06Z AVIATION DISCUSSION...
KEY MESSAGES
- SHOWER AND STORM CHANCES INCREASE THIS EVENING FROM SOUTHWEST TO
NORTHEAST OVER CENTRAL IOWA WITH CHANCES LOWERING INTO THE DAYLIGHT
HOURS OF MONDAY. A FEW SEVERE STORMS ARE POSSIBLE TONIGHT, MAINLY
OVER SOUTHWESTERN IOWA WITH LARGE HAIL THE MAIN HAZARD AND GUSTY
WINDS A SECONDARY HAZARD.
- STRONG TO SEVERE STORMS ARE FORECAST LATE MONDAY AFTERNOON INTO
MONDAY EVENING WITH THE HIGHEST RISK OVER SOUTHWEST INTO SOUTH
CENTRAL IOWA. THE MAIN HAZARD AGAIN IS LARGE HAIL WITH GUSTY WINDS A
SECONDARY HAZARD.
- WIDESPREAD RAINFALL FORECAST WITH TOTALS FROM TONIGHT THROUGH
WEDNESDAY RANGING FROM 1 INCH OVER NORTHERN IOWA TO 3 INCHES OVER
PARTS OF SOUTHERN IOWA. LOCALLY HIGHER AMOUNTS ARE EXPECTED,
PARTICULARLY SOUTH OF I-80. MAINLY WITHIN RIVER BANK RISES ARE
EXPECTED. LOCALIZED FLASH FLOODING POTENTIAL HIGHEST OVER URBAN
AREAS FROM MONDAY NIGHT INTO EARLY TUESDAY.
- BREEZY TO WINDY TONIGHT INTO TUESDAY
DISCUSSION
ISSUED AT 257 PM CDT SUN MAY 18 2025
EARLY AFTERNOON GOES-EAST VISIBLE SATELLITE IMAGERY SHOWS JUST AREAS
OF HIGH CLOUDS OVER THE STATE AS WEAK RIDGING PUSHES INTO THE
REGION. THE REST OF THE DAYTIME WILL BE BENIGN AS HIGH CLOUDS
CONTINUE TO ARRIVE, BUT A MORE ACTIVE PATTERN IS STILL FORECAST TO
TAKE SHAPE FROM TONIGHT INTO THE FIRST HALF OF THE UPCOMING WEEK.
THIS MORE ACTIVE PATTERN WILL BE DUE TO A MORE PRONOUNCED TROUGH
AXIS OVER THE WESTERN US THAT IS NEARING THE ROCKIES THIS AFTERNOON.
ONE SHORTWAVE TROUGH IS LIFTING OUT OF THE BASE OF THE TROUGH OVER
THE FOUR CORNERS REGION WHILE ANOTHER SHORTWAVE IS COMING DOWN EAST
OF THE SPINE OF THE CASCADES. THESE TWO SHORTWAVES STILL LOOK TO
PIVOT AROUND ONE ANOTHER BEFORE PHASING INTO ONE UPPER LOW TOWARD
TUESDAY BEFORE SLOWLY SLIDING ACROSS THE REGION AT MIDWEEK. AS I DID
YESTERDAY AFTERNOON, LET'S FIRST DISCUSS THE CONVECTIVE CONCERNS
FOLLOWED BY THE HYDROLOGY HAZARDS.
CLOUDS WILL THICKEN INTO TONIGHT AND LOWER AS THE FIRST SHORTWAVE
LIFTS TOWARDS THE REGION WITH A BROAD SURFACE LOW PRESSURE AROUND
THE MIDDLE OR UPPER 990MBS MOVING OVER CENTRAL NEBRASKA AND KANSAS.
A STRONG SLUG OF LOW LEVEL THETA-E ADVECTION AND QG CONVERGENCE WILL
MOVE OVER IOWA AND NORTH OF A WARM FRONT STRETCHED FROM
KANSAS/NEBRASKA INTO MISSOURI TONIGHT. MUCH OF TODAY'S GUIDANCE (THE
LATEST OF EITHER THE 0Z, 6Z, OR 12Z CONVECTIVE ALLOWING MODELS
(CAMS)) CONTINUES TO FOCUS THE STRONGEST CONVECTION TONIGHT OVER
MISSOURI. THAT'S NOT TO SAY WE WON'T SEE ANY SEVERE RISK OR
RAINFALL, BUT THE HIGHER CHANCE OF THE SEVERE HAZARDS LOOKS TO BE
OVER MISSOURI AS A MESOSCALE CONVECTIVE SYSTEM (MCS) MOVES THROUGH
SOME PORTION OF THAT STATE. MODELS DO SHOW VARYING DEGREES OF SHOWER
AND STORM COVERAGE OVER IOWA STARTING CLOSER TO MID-EVENING OVER OUR
SOUTHWESTERN FORECAST AREA (ATLANTIC, CRESTON, BEDFORD) THAT SPREADS
NORTHEASTWARD OVER ALL OF CENTRAL IOWA OVERNIGHT AND IS LARGELY EAST
OF THE FORECAST AREA (WATERLOO TO OTTUMWA) BY DAYBREAK TUESDAY. WITH
THE WARM FRONT REMAINING SOUTH OF THE STATE, STORMS ARE EXPECTED TO
REMAIN ELEVATED WITH A GOOD AMOUNT OF MUCAPE ABOVE THE INVERSION.
WHILE DEEP LAYER SHEAR LOOKS IMPRESSIVE, THE EFFECTIVE LAYER IS LESS
SO AND THUS THE FOCUS FOR SEVERE WEATHER LATER THIS EVENING INTO THE
EARLY MORNING HOURS OF MONDAY WILL BE A LARGE HAIL RISK OVER
SOUTHWEST INTO PORTIONS OF SOUTH CENTRAL IOWA. THE OUTLIER, UNLIKELY
CASE WOULD BE IF THE 6Z/12Z NAM CARRIES THE MCS INTO SOME PORTION OF
IOWA, WHICH WOULD BRING THE POTENTIAL OF STRONG WINDS INTO THE
SOUTHERN PORTION OF THE STATE. BUT AGAIN, THIS SEEMS UNLIKELY AT
THIS TIME.
AS THESE STORMS MOVE EAST EARLY IN THE DAYLIGHT HOURS OF MONDAY,
THERE SHOULD BE A RELATIVE LULL IN THE HEAVIER, STRONGER ACTIVITY.
HOWEVER, FORECAST SOUNDINGS AND HREF LOW CLOUD WOULD POINT TOWARD
LIMITED SUNSHINE AND THERE MAY BE ENOUGH DEPTH OF SATURATION FOR
DRIZZLE OR MIST. HOWEVER, FOR SIMPLICITY HAVE NOT INCLUDED THAT IN
THE FORECAST. AS MENTIONED YESTERDAY, WILL BE WATCHING THE WARM
FRONT'S POSITION, WHICH IS EXPECTED TO BE OVER NORTHERN MISSOURI AND
COULD SNEAK CLOSE TO THE IOWA STATE LINE. THIS WILL DEPEND ON THE
OVERNIGHT TONIGHT'S CONVECTION AND WHERE THE EFFECTIVE WARM FRONT
ENDS UP. THERE ARE A FEW MODELS THAT SHOW THE WARM FRONT SNEAKING
INTO SOUTHWEST OR SOUTH CENTRAL IOWA WITH THE MOST NOTABLE BEING THE
18/0Z C-SHIELD, WHICH THEN PROJECTS A MORE ACTIVE AFTERNOON/EVENING
OVER IOWA. HOWEVER, THIS IS AN OUTLIER AND WITH THE WARM FRONT
EXPECTED TO STAY LARGELY SOUTH OF THE MISSOURI BORDER, SURFACE BASED
HAZARDS ARE A MUCH LESSER CONCERN. DEPENDING ON THE CLOSENESS OF THE
WARM FRONT, WE MAY GET STORMS THAT FORM ON THE WARM FRONT OVER
NORTHERN MISSOURI THAT CROSS INTO OUR SOUTHERN ROW OF COUNTIES WITH
LARGE HAIL AND PERHAPS GUSTY WINDS. MUCAPE VALUES WILL BE HIGHEST
OVER OUR SOUTHWESTERN INTO PARTS OF OUR SOUTH CENTRAL FORECAST AREA
(ROUGHLY A DENISON TO LAMONI LINE), THOUGH EFFECTIVE SHEAR IS MUCH
LESS THAN THE DEEP LAYER SHEAR AGAIN SO THINKING IS LARGE HAIL WILL
BE THE PRIMARY RISK HERE WITH PERHAPS GUSTY WINDS A SECONDARY
THREAT. THE MAIN TIME AND AREA OF CONCERN FOR THESE STORMS WOULD BE
FROM LATE MONDAY AFTERNOON INTO MONDAY EVENING WITH THE SEVERE RISK
WANING AFTER MIDNIGHT. HOWEVER, SHOWERS AND STORMS WILL PERSIST
OVERNIGHT MONDAY INTO THE DAY TUESDAY.
AS FOR HYDROLOGY HAZARDS, NOTHING HAS CHANGED WITH ANTECEDENT
CONDITIONS SINCE THIS TIME YESTERDAY SO I REFER THE INTERESTED
READER TO LOOK AT THE FORECAST DISCUSSION FROM 228PM SATURDAY FOR
THOSE DETAILS. PRECIPITABLE WATER VALUES WILL TREND UP INTO TONIGHT
AND THEN REMAIN BETWEEN 1.25 AND 1.5 INCHES OVER SOUTHERN IOWA AND 1
INCH OVER NORTHERN IOWA FROM MONDAY THROUGH EARLY TUESDAY. THESE
VALUES ARE SEASONAL TO SEASONALLY HIGH AND CONFIRMED BY THE
CONTINUED 90TH TO 97.5TH PERCENTILE SHOWN IN THE NAEFS AND ECMWF
850MB SPECIFIC HUMIDITY FIELD. THE WARM CLOUD DEPTHS DON'T LOOK TOO
NOTABLE UNTIL MONDAY INTO MONDAY NIGHT WHEN THEY EXCEED 3000 TO
3500M OVER SOUTHERN IOWA. SO FOR TONIGHT INTO EARLY MONDAY, OUTSIDE
OF THE 6Z/12Z NAM THAT BRING THE MCS AND ITS HIGHER QPF AMOUNTS INTO
SOME PORTION OF OUR FORECAST AREA, RAINFALL AMOUNTS WILL GENERALLY
BE UNDER A HALF AN INCH OVER WESTERN INTO SOUTH CENTRAL IOWA.
THEREFORE, FLASH FLOODING AND RIVER FLOODING RISK REMAINS NEAR ZERO
FOR THIS TIME PERIOD.
THIS RISK WILL CHANGE AS WE HEAD INTO MONDAY NIGHT INTO EARLY
TUESDAY WHEN RATES ARE EXPECTED TO BE HIGHER AND STORM MOTIONS
SHOULD SLOW AS THE UPPER LOW BEGINS TO PUSH TOWARD THE STATE. THE
LATEST ECMWF EXTREME FORECAST INDEX HAS INCREASED TO 80 TO 90% OF
ITS ENSEMBLE MEMBERS POINTING TO AN ANOMALOUS, BUT NOT EXTREME EVENT
RELATIVE TO ITS MODEL CLIMATE AND THAT THE EVENT WILL OUTPERFORM THE
ENSEMBLE MEAN. FURTHER, THE OVERNIGHT'S FORECAST AS WELL AS INITIAL
NATIONAL BLEND OF MODELS (NBM) QPF IS NEARING DAILY RECORD VALUES IN
A FEW PLACES. DURING THIS PERIOD, AN ADDITIONAL THREE QUARTER OF AN
INCH TO AN INCH OVER NORTHERN IOWA TO AN ADDITIONAL 2 TO 3 INCHES OF
RAIN OVER SOUTHERN IOWA ARE FORECAST. DETERMINISTIC MODELS AND
LATEST HREF LOCALIZED PROBABILITY MATCHED MEAN DO SHOW BULLSEYES OF
HIGHER VALUES, LARGELY IN VARYING PLACES OVER SOUTHERN IOWA, WITH
THE MAGNITUDE BEING 3 TO 5 INCHES. WPC HAS MAINTAINED THEIR SLIGHT
RISKS OF EXCESSIVE RAINFALL OVER SOME PORTION OF THE STATE FROM
TONIGHT THROUGH EARLY TUESDAY. WHILE FLASH FLOOD GUIDANCE REMAINS
HIGH, LOCALIZED FLASH FLOODING OVER AN URBAN CENTER CANNOT BE RULED
OUT. HOWEVER, THERE ARE CONCERNS THAT THE QPF MAY BE OVERDONE IF THE
CONVECTION STAYS LARGELY SOUTH OF THE STATE WITH RATES AND AMOUNTS
PERHAPS LESSER ON THE COOL SIDE OF THE WARM FRONT.
TODAY'S LATEST QPF ENSEMBLE HYDROGRAPHS AND EXPERIMENTAL HYDROLOGIC
ENSEMBLE FORECASTING SERVICE (HEFS) SHOW AN INCREASE IN THE NUMBER
OF FORECAST POINTS THAT WOULD REACH OR EXCEED ACTION STAGE. HOWEVER,
THE QPF ENSEMBLE HYDROGRAPHS ASSUME 95TH PERCENTILE RAINFALL;
HOWEVER, WHEN EXAMINING THE EXPECTED RAINFALL ONLY ONE SITE REACHES
ACTION STAGE. THE SAME IS TRUE WITH THE HEFS AS YOU NEED TO REACH
THE 10% CHANCE OF EXCEEDANCE TO GET TO ACTION OR MINOR FLOOD STAGE.
THIS COUPLED WITH NO HIGHER THAN NORMAL STREAMFLOW AND LARGELY BELOW
NORMAL STREAMFLOW WHERE THE HIGHEST RAINFALL IS FORECAST TO FALL,
THERE IS CAPACITY WITHIN THE GAGES RIVERS AND STREAMS TO HOLD THIS
RAINFALL TO WITHIN BANK RISES.
SHOWERS ARE FORECAST TO BECOME MORE SCATTERED ON WEDNESDAY AS
LINGERING LIFT FROM THE UPPER LOW SLIDES OVER THE AREA. IT WILL ALSO
BE COOL ON WEDNESDAY WITH HIGHS MAINLY IN THE 50S AND WHILE IT DOES
'WARM' INTO THE 60S ON THURSDAY, THIS TOO IS COOL FOR THIS TIME OF
YEAR. WHILE THE INITIAL NBM HAS DRIED OUT THE FORECAST ON THURSDAY,
THERE IS LINGERING MID-LEVEL QG CONVERGENCE THAT KEEPS THE SCATTERED
SHOWERS OVER PORTIONS OF THE STATE IN THE DETERMINISTIC MODELS SO
WILL HAVE TO ASSESS THOSE TRENDS IN THE COMING DAYS. A SHORTWAVE
TROUGH WILL APPROACH THE AREA BY LATER FRIDAY AND COULD BRING
ADDITIONAL SHOWER AND STORM CHANCES BACK TO IOWA BY FRIDAY NIGHT
INTO SATURDAY.
AVIATION /06Z TAFS THROUGH 06Z TUESDAY/
ISSUED AT 1134 PM CDT SUN MAY 18 2025
VFR CONDITIONS TO START THE PERIOD WILL DEGRADE THROUGH THE
NIGHT AS SHOWERS/STORMS MOVE INTO THE AREA OVERNIGHT. LIGHTNING
TRENDS ARE DIMINISHING SO HAVE KEPT WITH SHOWER MENTIONS
OVERNIGHT BUT UPDATES MAY BE NEEDED SHOULD STORMS HOLD TOGETHER
AS THEY NEAR A TAF SITE. IN ADDITION TO THE SHOWERS/STORMS, CIGS
WILL CONTINUE TO LOWER WITH VISIBILITY REDUCTIONS AT TIMES AS
WELL DURING HEAVIER SHOWERS. CONFIDENCE IN TIMING THESE GREATER
IMPACTS OUT TO BE WITHIN 5SM OF TAF SITES REMAINS LOWER,
ESPECIALLY PAST THE FIRST OVERNIGHT ROUND, SO UPDATES MAY BE
NEEDED. WILL ALSO NEED TO ADDRESS MORE OF THE 2ND ROUND OF
RAIN/STORMS MONDAY AFTERNOON/EVENING IN FUTURE ISSUANCES
FOLLOWING FIRST ROUND OVERNIGHT INTO MONDAY MORNING AND
LINGERING SHOWERS/DRIZZLE/MIST THROUGH THE REST OF MONDAY
MORNING. WINDS FROM THE EAST TO SOUTHEAST WILL INCREASE TONIGHT
INTO MONDAY WITH GUSTS OF 25-35 KNOTS BECOMING COMMON, HIGHER
GUSTS POSSIBLE, AS IS SEVERE WEATHER, ESPECIALLY SOUTH, AS NOTED
IN THE REGULAR DISCUSSION ABOVE.
DMX WATCHES/WARNINGS/ADVISORIES
NONE.
DISCUSSION...ANSORGE
AVIATION...05
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