780
FXUS63 KDMX 171928
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
228 PM CDT SAT MAY 17 2025
KEY MESSAGES
- CHILLY NIGHT WITH WINDS CONTINUING TO LESSENING; FAR NORTHERN
IOWA LOCALIZED FROST POSSIBLE IN LOW LYING LOCATIONS
- MORE ACTIVE PATTERN SUNDAY NIGHT INTO THE MIDDLE OF NEXT WEEK. MAIN
STORM CHANCES SUNDAY NIGHT INTO EARLY MONDAY AND THEN AGAIN
LATER MONDAY AFTERNOON INTO MONDAY EVENING WITH LARGE HAIL AND
GUSTY WINDS THE MAIN HAZARDS.
- WIDESPREAD RAINFALL WITH RAIN TOTALS BY MIDWEEK RANGING FROM
AROUND 1 INCH OVER NORTHERN IOWA TO 2 TO 3 INCHES OVER
SOUTHERN IOWA. CURRENT EXPECTATION IS WITHIN RIVER AND STREAM
RESPONSES. ANY LOCALIZED FLASH FLOODING FOCUSED OVER URBAN
CENTERS AND WITH THE POTENTIAL HIGHEST MONDAY NIGHT INTO EARLY
TUESDAY.
DISCUSSION
ISSUED AT 228 PM CDT SAT MAY 17 2025
GOES-EAST UPPER LEVEL WATER VAPOR IMAGERY SHOWS THAT THE UPPER
LOW THAT BROUGHT THE STRONG, GUSTY WINDS AND SPOTTY LIGHT
SHOWERS YESTERDAY HAS MOVED OVER THE GREAT LAKES REGION. WITH
THIS PULLING AWAY, THIS HAS ALLOWED THE PRESSURE GRADIENT TO
RELAX SO WHILE IT IS STILL A RATHER BREEZY MAY DAY, CERTAINLY
NOT LIKE YESTERDAY. TO THE WEST, A LOW AMPLITUDE RIDGE IS OVER
THE HIGH PLAINS WHILE A SHORTWAVE TROUGH IS SLIPPING OUT AHEAD
OF A DEEPENING TROUGH OVER THE WEST COAST. IN THE LOWER LEVELS,
WEAK COLD AIR ADVECTION CONTINUES THIS AFTERNOON WITH THE
STRATUS CLOUDS HAVING ERODED LARGELY WITH THE EXCEPTION OF
NORTHEASTERN IOWA. WITH THE SKY BECOMING CLEAR AND WINDS FURTHER
DECREASING TONIGHT AS THE GREAT LAKES UPPER LOW MOVES FARTHER
AWAY, TEMPERATURES WILL DROP INTO THE UPPER 30S OVER FAR
NORTHERN IOWA TO THE LOW AND MIDDLE 40S OVER CENTRAL AND
SOUTHERN IOWA. THE INITIAL FORECAST LOOKS ON TRACK AND IS AROUND
THE 25TH PERCENTILE OF AVAILABLE GUIDANCE. WINDS LOOK TO REMAIN
ABOVE 5 MPH AND TEMPERATURES ARE MARGINAL SO NOT ANTICIPATING
WIDESPREAD FROST. HOWEVER, SHELTERED, LOW LYING AREAS LIKE RIVER
VALLEY OVER NORTHERN IOWA MAY SEE PATCHY FROST IN THOSE FAVORED
LOCATIONS AND HAVE ADDED PATCHY FROST TO THE FORECAST OVER THIS
AREA AS WELL. TO THE SOUTH OF IOWA, THE AFOREMENTIONED LEAD
SHORTWAVE WILL GENERATE SHOWERS AND STORM SOUTH OF THE STATE
TONIGHT INTO SUNDAY MORNING.
AS WE HEAD FURTHER INTO SUNDAY, THE DEEPENING WEST COAST TROUGH WILL
EJECT ONE SHORTWAVE OUT OF ITS BASE WHILE ANOTHER ONE DESCENDS THE
BACKSIDE SUNDAY NIGHT INTO MONDAY WITH THESE TWO WAVES DANCING
AROUND ONE ANOTHER ON MONDAY INTO TUESDAY BEFORE ESSENTIALLY PHASING
INTO ONE UPPER LOW. THESE FEATURES WILL BRING A MORE ACTIVE PATTERN
TO THE STATE WITH THUNDERSTORM CHANCES STARTING SUNDAY NIGHT AS
WAVES OF LOW LEVEL THETA-E ADVECTION MOVE OVER THE REGION THROUGH
TUESDAY NIGHT. WE'LL FIRST DIVE INTO THE SEVERE THUNDERSTORM
POTENTIAL FOLLOWED BY A DISCUSSION OF HYDROLOGY CONCERNS. LOOKING AT
SUNDAY NIGHT, A MESOSCALE CONVECTIVE COMPLEX (MCS) LOOKS TO BLAST
ACROSS MISSOURI SCRAPING PERHAPS SOUTHERN IOWA AS IT MOVES ALONG THE
WARM FRONT. THIS IS SHOWN IN THE 17/0Z C-SHIELD AND MPAS RUNS AS
WELL AS THE LATEST ARW AND FV3 CORE CONVECTIVE ALLOWING MODELS
(CAMS). ADDITIONAL CONVECTION NORTH OF THIS WARM FRONT IS POSSIBLE,
PARTICULARLY OVER SOUTHERN INTO PERHAPS CENTRAL IOWA, THOUGH TO WHAT
EXTENT REMAINS TO BE SEEN. DEEP LAYER SHEAR IS IN AMPLE SUPPLY WHILE
THERE SHOULD BE SUFFICIENT INSTABILITY AND ASSUMING THE MCS STAYS
SOUTH, ANY STORMS OVER IOWA WOULD BE ELEVATED. THIS WOULD THEN MEAN
THAT THE MAIN HAZARDS WOULD BE LARGE HAIL AND PERHAPS GUSTY WINDS IF
THE MCS DOES SCRAPE SOUTHERN IOWA SUNDAY NIGHT/EARLY MONDAY.
AS WE MOVE INTO THE DAYTIME HOURS OF MONDAY, THE UPPER LOW WILL BE
NEAR THE BLACK HILLS REGION WITH A SUB-1000MB SURFACE LOW MOVING
ACROSS KANSAS TOWARDS IOWA THROUGH THE DAY. THINGS WE'LL BE
MONITORING CLOSELY ON MONDAY WILL BE HOW MUCH CLOUD EROSION OCCURS
AND THE LOCATION OF THE WARM FRONT. MUCH OF THE GUIDANCE (6Z/12Z)
WOULD POINT TO THE WARM FRONT STAYING SOUTH OF THE STATE WITH A FEW
SOLUTIONS BRINGING IT CLOSE OR INTO A FEW OF OUR SOUTHERN TIER OF
IOWA COUNTIES. IF THE WARM FRONT DOES STAY SOUTH, THEN TEMPERATURES
MAY NEED TO BE ADJUSTED LOWER AND THERE COULD EVEN BE A BIT OF
DRIZZLE AT TIMES GIVEN THE DEPTH OF LOW LEVEL SATURATION IN THE
SOUNDINGS. FURTHER, BREEZY WINDS ARE ALSO ON TAP FOR THE DAY. WITH
THE WARM FRONT EXPECTED TO STAY SOUTH OF THE STATE GIVEN CURRENT
GUIDANCE, THINKING IS THAT STORMS WILL BE ELEVATED AND LARGELY A
LARGE HAIL RISK AND PERHAPS GUSTY STORM WINDS WITH THESE HAZARDS
FOCUSED OVER SOUTHERN INTO PERHAPS CENTRAL IOWA. TIMING WISE, IT
LOOKS LIKE SEVERE WEATHER POTENTIAL WOULD BE FROM MID TO LATE
AFTERNOON INTO THE MONDAY EVENING HOURS WITH SHOWER AND MORE SO NON-
SEVERE STORM CHANCES CONTINUING OVERNIGHT.
AS THESE STORMS SHIFT AWAY TUESDAY MORNING, THE UPPER LOW WILL
SLOWLY MOVE OVER THE REGION TUESDAY INTO WEDNESDAY. THIS WILL KEEP
CLOUD COVER AND SHOWER AND STORM CHANCES AROUND OVER THE STATE WITH
STORM CHANCES MORE PROBABLE ON TUESDAY COMPARED TO WEDNESDAY. THERE
WILL STILL BE SOME INSTABILITY UP INTO SOUTHEASTERN IOWA ON TUESDAY
AFTERNOON, THOUGH THE SHEAR MAGNITUDE WILL BE LOWER AND THUS ANY
SEVERE RISK LOOKS TO MORE MARGINAL AND IN LINE WITH THE SPC DAY 4
OUTLOOK WITH THE HIGHER PROBABILITIES SOUTHEAST OF THE STATE.
FOR HYDROLOGY CONCERNS, LET'S FIRST SET THE STAGE. THERE HAS BEEN
AROUND OR LESS THAN A HALF AN INCH OF RAINFALL OVER THE LAST 14
DAYS. THIS HAS LED TO 14 DAY NATIONAL WATER MODEL (NWM) STREAMFLOW
ANOMALIES SHOWING BELOW NORMAL TO MUCH BELOW NORMAL OVER SOUTHERN
IOWA WITH A MIXTURE OF BELOW NORMAL AND NORMAL STREAMFLOWS OVER
CENTRAL AND NORTHERN IOWA. THIS IS ALSO VIEWED IN THE USGS AVERAGE
STREAMFLOW COMPARED TO HISTORICAL DATA. FURTHER, NWM SOIL MOISTURE
ANALYSIS SHOWS 50 TO 80% CAPACITY REMAINING WITH THE LATEST NASA
SPORT 0-10CM LAYER RELATIVE SOIL MOISTURE (RSM; THIS LAYER TO ASSESS
RAPID ONSET FLOODING) AND 10-40CM LAYER RSM (THIS LAYER TO ASSESS
RIVER RESPONSE) SHOWING COMPARABLE VALUES TO ABSORB RAINFALL. TO SUM
THIS UP, IT'S BEEN DRY AND THERE IS CAPACITY IN THE SOILS FOR A
DECENT RAINFALL.
WARM CLOUD DEPTHS AND PRECIPITABLE WATER VALUES LOOK SEASONAL
THROUGH MUCH OF THE EVENT, THOUGH ABOVE NORMAL LATER MONDAY INTO
MONDAY NIGHT. THIS IS WHEN THE LATEST NAEFS AND ECMWF 850MB SPECIFIC
HUMIDITY ARE PINGING AT THE 90TH TO 97.5TH PERCENTILE WITH
PRECIPITABLE WATER VALUES ABOVE 1.5 INCHES. ECMWF EXTREME FORECAST
INDEX (EFI) SHOWS 70 TO 80% 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. WHILE THE 6Z RUN
OF THE NAM SHOWED A 1 TO 2 INCH EVENT SUNDAY NIGHT INTO MONDAY
MORNING, MUCH OF THE 12Z GUIDANCE IS POINTING TO AMOUNTS UNDER AN
INCH IF NOT A HALF AN INCH THAT IS FOCUSED SOUTH OF I-80 SUNDAY
NIGHT. THIS IS ALSO SHOWN IN THE ENSEMBLE DATA WITH THE 17/12Z HREF
SHOWING THE HIGHER RAINFALL AMOUNTS CLOSER TO I-70 IN MISSOURI
SUNDAY NIGHT. THIS SHOULD KEEP RIVER RESPONSES MORE MUTED AND FLASH
FLOODING CONCERNS NEAR ZERO GIVEN THE DRY CONDITIONS DISCUSSED
ABOVE. MONDAY NIGHT INTO TUESDAY MORNING LOOKS TO CONTAIN THE HIGHER
RAINFALL AMOUNTS WITH ENSEMBLE MEANS SHOWING 1 TO 2 INCHES OF
ADDITIONAL RAINFALL OVER SOUTHERN INTO CENTRAL IOWA WITH AMOUNTS
LIKELY UNDER AN INCH OVER NORTHERN IOWA. OF COURSE, CONVECTIVE
ELEMENTS WILL HAVE LOCALLY HIGHER RAIN TOTALS. ALL TOLD, SOUTHERN
IOWA HAS ANYWHERE FROM A 50 TO 80% CHANCE OF AT LEAST 2 INCHES OF
RAINFALL FOR THE EVENT TOTAL (SUNDAY NIGHT THROUGH WEDNESDAY) FROM
THE VARIOUS AND GRAND ENSEMBLE MEANS.
GIVEN THE RAINFALL AMOUNTS, WPC HAS ISSUED SLIGHT RISKS OF EXCESSIVE
RAINFALL OUTLOOKS (EROS) OVER SOME PORTION OF IOWA BOTH MONDAY AND
TUESDAY. WITH 1 HOUR FLASH FLOOD GUIDANCE (FFG) VALUES RANGING FROM
2.7 INCHES OVER SOUTHERN IOWA TO 1.75 INCHES OVER NORTHERN IOWA AND
6 HOUR FFG RANGING FROM 4.5 INCHES OVER SOUTHERN IOWA TO 1.75 INCHES
OVER NORTHERN IOWA, THINKING IS THAT DRY SOILS AND THESE HIGH FFG
VALUES WILL LIMIT FLASH FLOODING AND RIVER FLOODING CONCERNS. THE
EXCEPTION WOULD BE URBAN CENTERS, SUCH AS THE DES MOINES METRO, AND
IF THESE AREAS WERE ABLE TO ACCUMULATE 2 OR MORE INCHES IN A SHORT
PERIOD OF TIME THIS COULD LEAD TO FLASH FLOODING AND URBAN STREAM
CONCERNS. OF COURSE, THE RAINFALL WOULD HAVE TO FALL OVER THESE
BASINS AND PLACEMENT ISSUES WITH THE HIGHEST RAINFALL AMOUNTS REMAIN
YET TO BE RESOLVED. FOR RIVERS, LATEST QPF ENSEMBLE HYDROGRAPHS AND
EXPERIMENTAL HYDROLOGIC ENSEMBLE FORECASTING SERVICE (HEFS) SHOW A
HALF A DOZEN TO A DOZEN FORECAST POINTS ABOVE ACTION STAGE WITH MOST
OF THOSE NOT REACHING FLOOD STAGE. THESE ASSUME THE 95TH PERCENTILE
RAINFALL (QPF ENSEMBLE HYDROGRAPH) AND 10% CHANCE OF EXCEEDANCE
(HEFS). WITH THIS GUIDANCE IN MIND AND THE CAPACITY IN THE LARGER
WATERWAYS AND DRY CONDITIONS ABLE TO ABSORB SOME OF THIS RAINFALL
RATHER THAN ROUTE DIRECTLY TO THE STREAMS, MAINLY WITHIN BANK RISES
ARE EXPECTED ON GAGED RIVERS AND STREAMS.
AS RAINFALL ENDS WEDNESDAY, DRIER AIR WILL ARRIVE AS HIGH PRESSURE
PUSHES INTO THE REGION FOR THURSDAY INTO AT LEAST A PART OF FRIDAY.
NORTH AND LATER NORTHWESTERLY FLOW ALOFT WILL KEEP CONDITIONS ON THE
COOLER SIDE THROUGH FRIDAY WITH A LOW CHANCE OF A FEW SHOWERS OR
STORMS BY FRIDAY AS A SHORTWAVE TROUGH MOVES THROUGH THE FLOW.
AVIATION /18Z TAFS THROUGH 18Z SUNDAY/
ISSUED AT 1208 PM CDT SAT MAY 17 2025
OUTSIDE OF MCW WITH HIGHER END MVFR CEILINGS, WHICH SHOULD
IMPROVE LATER THIS AFTERNOON, LOW END VFR CLOUDS PREVAIL ACROSS
CENTRAL IOWA THIS AFTERNOON. CLOUD COVERAGE WILL DIMINISH
THROUGH THE REST OF TODAY WITH VFR CONDITIONS CONTINUING
THROUGH THE PERIOD. GUSTY WINDS FROM THE NORTHWEST WILL DIMINISH
AFTER SUNSET INTO THE OVERNIGHT HOURS BECOMING FROM THE EAST
MONDAY MORNING.
DMX WATCHES/WARNINGS/ADVISORIES
NONE.
UPDATE...BURY
DISCUSSION...ANSORGE
AVIATION...ANSORGE
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