258
FXUS63 KDMX 242358
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
658 PM CDT THU APR 24 2025
...UPDATED FOR THE 00Z AVIATION DISCUSSION...
KEY MESSAGES
- ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS FROM THIS EVENING
INTO FRIDAY MORNING. LOW RISK OF SEVERE WEATHER IN THE FORM OF
HAIL AND GUSTY WINDS. LOCALLY HEAVY RAIN ALSO POSSIBLE.
- MORE THUNDERSTORM CHANCES FROM SUNDAY INTO EARLY NEXT WEEK.
HIGHER POTENTIAL FOR SEVERE WEATHER ON MONDAY, BUT DETAILS
UNCLEAR AT THIS RANGE.
DISCUSSION
ISSUED AT 247 PM CDT THU APR 24 2025
SCATTERED THUNDERSTORMS DEVELOPED ACROSS IOWA EARLIER THIS MORNING,
MAINLY THE SOUTHERN HALF WITHIN OUR AREA, BUT HAVE DIMINISHED AND
MOSTLY MOVED INTO NORTHERN IOWA SINCE THEN. MEANWHILE, A LARGER
AREA OF SHOWERS AND STORMS THAT MOVED FROM KANSAS INTO EASTERN
NEBRASKA THIS MORNING IS APPROACHING THE MISSOURI RIVER AND WILL
CROSS IOWA THIS EVENING AND TONIGHT. NEAR- SURFACE MOISTURE IS
POOLING JUST SOUTH OF A FRONTAL BOUNDARY STRETCHING FROM
SOUTHWESTERN/WESTERN IOWA INTO CENTRAL/NORTH CENTRAL IOWA,
ROUGHLY FROM AROUND SHENANDOAH AND ATLANTIC UP AND OVER TO
MARSHALLTOWN AND NEWTON. DEWPOINTS WITHIN THIS SWATH ARE AROUND
60 DEGREES AT THIS TIME, AND AS DIURNAL HEATING CONTINUES MODEST
INSTABILITY WILL BE REALIZED LATE THIS AFTERNOON AND EARLY THIS
EVENING WHEN SURFACE-BASED CAPE SHOULD PEAK AROUND 1500 J/KG.
DEEP- LAYER SHEAR WILL ALSO INCREASE WITH EFFECTIVE BULK HEAR OF
25-30 KT DURING THAT TIME, SUPPORTING SOME DEGREE OF UPDRAFT
ORGANIZATION. A FEW STRONGER STORMS WITHIN THIS CORRIDOR MAY
THUS BE CAPABLE OF PRODUCING HAIL AND GUSTY WINDS, BUT OVERALL
THE SEVERE WEATHER THREAT IS LIMITED BY THE RELATIVE LACK OF
FLOW/SHEAR AND A MARGINAL RISK IS HIGHLIGHTED BY SPC. FARTHER TO
THE NORTH AND SOUTH, LOW- LEVEL MOISTURE APPEARS TOO LIMITED TO
SUPPORT INSTABILITY SUFFICIENT FOR EVEN THAT THREAT, THOUGH
FORECAST SOUNDINGS AND MODEST ELEVATED INSTABILITY STILL SUPPORT
GENERAL THUNDER CHANCES. ALSO WORTHY OF NOTE IS THAT PORTIONS
OF OUR SERVICE AREA HAVE RECEIVED GENEROUS RAINFALL IN RECENT
DAYS, INCLUDING AROUND WATERLOO WHERE OVER 2" OF RAIN FELL LAST
NIGHT AND NEARLY 5" HAS FALLEN IN THE LAST WEEK. WITH STORMS
PERHAPS TRAINING THROUGH THE AFOREMENTIONED CORRIDOR OF
INSTABILITY THIS EVENING AND TONIGHT, LARGELY OVER AREAS THAT
ARE NEARING SATURATION FROM RECENT RAINS, WE COULD REALIZE SOME
THREAT OF FLOODING AS ELABORATED IN THE HYDROLOGY SECTION BELOW.
A HIGH PRESSURE AREA BUILDING DOWN FROM THE NORTHWEST WILL PUSH INTO
IOWA BEHIND THE RAIN TONIGHT, FORCING IT EASTWARD OUT OF OUR AREA BY
FRIDAY MORNING AND LEADING TO A COUPLE DAYS OF DRIER AND QUIETER
WEATHER. SKIES WILL BE PARTLY TO MOSTLY SUNNY FROM FRIDAY AFTERNOON
THROUGH SATURDAY, WITH TEMPERATURES RELATIVELY COOL DUE TO THE HIGH
PRESSURE AREA AND HIGHS GENERALLY IN THE 60S ON BOTH DAYS. BY SUNDAY
MORNING, A LARGE 500 MB LOW PRESSURE CENTER WILL BE LOCATED ROUGHLY
OVER NEVADA, WITH A REGION OF STRONG SOUTHWESTERLY FLOW AND
DIFFLUENCE ALOFT SPREADING ACROSS THE FOUR CORNERS AREA. A
CORRESPONDING BELT OF ENHANCED SOUTHWESTERLY 850 MB FLOW WILL
DEVELOP OVER THE HIGH PLAINS AND INTO IOWA DURING THE DAY, PROMOTING
SOMEWHAT WARMER TEMPERATURES AND STEADY MOISTURE ADVECTION, ALONG
WITH A RETURN OF RAIN AND THUNDERSTORM CHANCES. THE LARGE PARENT
STORM SYSTEM WILL THEN FINALLY APPROACH AND MOVE OVER IOWA MONDAY
AND MONDAY NIGHT. TEMPERATURES WILL BE NOTABLY WARMER ON MONDAY,
PUSHING UP INTO THE 80S ACROSS MOST OF OUR FORECAST AREA WHILE
DEWPOINTS SURGE INTO THE 60S. THE ROBUST WIND FIELDS AHEAD OF THE
APPROACHING CYCLONE, ALONG WITH STRONG INSTABILITY RESULTING FROM
THE LOW- LEVEL WARM AIR/MOISTURE ADVECTION, WILL CREATE A FAVORABLE
PARAMETER SPACE FOR ORGANIZED THUNDERSTORMS AND POTENTIALLY
SUPERCELLS WITHIN THIS BROAD REGION. HOWEVER, MANY OF THE GLOBAL
MODELS AND ENSEMBLES HAVE BEEN STRUGGLING TO DEPICT A CORRESPONDING
COVERAGE OR INTENSITY OF CONVECTION. QUESTIONS REMAIN AS TO WHERE
THE HIGHEST THREAT WILL MATERIALIZE, HOW MUCH CONVECTION WILL OCCUR
IN THE WARM SECTOR VERSUS ON THE ADVANCING/TRAILING COLD FRONT, AND
HOW THE DETAILS OF THIS SCENARIO WILL SHAKE OUT. SPC HAS INDICATED
AN ENHANCED SEVERE WEATHER RISK ACROSS IOWA ON MONDAY AND WE WILL BE
CLOSELY MONITORING THIS POTENTIAL OVER THE NEXT SEVERAL DAYS.
AVIATION /00Z TAFS THROUGH 00Z SATURDAY/
ISSUED AT 639 PM CDT THU APR 24 2025
SHOWERS AND THUNDERSTORMS WILL EXPAND ACROSS IOWA THIS EVENING
AND OVERNIGHT. CIGS WILL LOWER AS WELL AND EVENTUALLY WILL
DECREASE TO IFR/LIFR AS STRATUS SETTLES IN LATER TONIGHT. GUSTY
WINDS AND SMALL HAIL ARE POSSIBLE THROUGH MID EVENING. CIGS
WILL THEN GRADUALLY IMPROVE ON FRIDAY. GENERALLY NORTHEAST WIND
TO BEGIN THE PERIOD WILL BECOME NORTHWEST WITH FROPA OVERNIGHT.
HYDROLOGY
ISSUED AT 247 PM CDT THU APR 24 2025
IN THE SHORT TERM, CONCERNS ABOUT FLASH FLOODING LATER TODAY INTO
TONIGHT HAVE DECREASED. LATEST MODEL RUNS HAVE TRENDED DOWNWARD ON
AVERAGE IN BOTH QPF AMOUNTS AND RATES. IN URBAN AREAS, THE MOST
LIKELY SCENARIO WOULD BE SOME TEMPORARY PONDING ESPECIALLY IN FLOOD
PRONE AREAS. IN TERMS OF SMALLER STREAMS, THE LATEST NATIONAL WATER
MODEL 12-HR AND 18-HR FORECAST OUTPUT SUGGESTS THE MOST LIKELY
SCENARIO WILL BE WITHIN BANK RISES WITH ISOLATED MINOR FLOODING
POSSIBLE. THE AREA OF MOST CONCERN IS WEST OF I-35, NORTH OF I-80
AND SOUTH OF US 20.
IN TERMS OF RIVER FLOODING POTENTIAL, THE RISK HAS DECREASED ON
AVERAGE AS WELL DUE ALSO TO THE AVERAGE DOWNWARD TREND IN QPF. HEFS
OUTPUT BRINGS SOME OF OUR FORECAST POINTS TO ACTION STAGE AT THE 30%
AND 50% LEVELS, AND INTRODUCES SOME MINOR FLOODING AT THE 10% LEVEL.
THE GFS AND NBM-FORCED NWM BOTH SUGGEST SIGNIFICANT WITHIN BANK
RISES ON MANY STREAMS MAINLY FROM WESTERN INTO NORTHEASTERN IOWA,
WITH SOME ISOLATED MINOR FLOODING POSSIBLE. THE GREATEST CONCERN IS
THE CEDAR RIVER BASIN AND TRIBUTARIES. PRESENTLY WE DO HAVE A FLOOD
WATCH FOR THE BLACK HAWK CREEK AT HUDSON DUE TO STREAM FLOODING
POTENTIAL THERE. IF RAINFALL AMOUNTS COME IN HIGHER THAN EXPECTED,
HOWEVER, THEN RIVER FLOODING WILL BE A GREATER RISK.
DMX WATCHES/WARNINGS/ADVISORIES
NONE.
DISCUSSION...LEE
AVIATION...DONAVON
HYDROLOGY...ZOGG
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