Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 311815
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
114 PM CDT FRI JUL 31 2015

.SHORT TERM /TODAY/...
ISSUED AT 336 AM CDT FRI JUL 31 2015

COUPLE OF CHALLENGES IN FORECAST FOR TODAY. FIRST...SUBJECTIVE SFC
ANALYSIS SHOWS WEAK TROUGH FROM NORTHEAST IA TO NEAR KOMA AT 06Z
WITH SECONDARY WEAK COOL FRONT STRETCHING FROM NEAR KDLH SSW TO
NEAR ST CLOUD AND ABERDEEN...BOTH MOVING SOUTHEAST WITH TIME
TODAY. WEAK UPPER LEVEL SUPPORT ACROSS NEBRASKA COMBINED WITH H850
MOISTURE PLUME HAS UNSETTLED AIRMASS OVER NORTHEAST NEBRASKA/FAR
WESTERN IA WITH A SMALL BUT INCREASING CLUSTER OF THUNDERSTORMS AS
MODEST WARM AIR ADVECTION AT H850 AND MOISTURE PLUME FROM
SOUTHWEST US TOWARD EASTERN NEBRASKA INTERACT WITH H850 UPSLOPE.
HRRR MODEL CONTINUES TO DEVELOP THE AREA SOUTH SOUTHEAST WITH TIME
THROUGH 12Z WEAKENING IT AS IT DOES. MID TO HIGH LEVEL BLOWOFF
WILL CONTINUE TO ADVECT DOWNSTREAM OVER CENTRAL/SOUTHERN IA
THROUGH MID MORNING. FOR NOW...WILL KEEP TRENDS OF WEAKENING
CONVECTION/-SHRA OVER FAR SOUTHWEST THROUGH 12Z...AND CLOUD DECK
THROUGH MOST OF DAY BUT DRIFTING SOUTH WITH TIME. WHATS LEFT OF
WEAK COOL FRONT WILL END UP NEAR THE IA MO BORDER BY 00Z WITH SOME
ADDITIONAL ISO THUNDER COVERAGE OVER THE FAR SOUTH BY LATE
AFTERNOON. IN REALITY...NO COOLING ALOFT DURING THE DAY SO HIGHS
ONCE AGAIN PUSH INTO THE MID 80S NORTH TO THE UPPER 80S NEAR 90
FROM I80 SOUTH WITH WARMER HEAT ISLAND EFFECTS ACCOUNTING FOR THE
NEAR 90 MARK TODAY. SPC HAS RAISED SVR THREAT TO MARGINAL
SOUTHWEST QUARTER FROM SOUTH OF I80/WEST OF I35. PERHAPS AS FRONT
SETTLES TOWARD THE BORDER THIS EVENING RETURN FLOW FROM 00-06Z MAY
PROVIDE ENOUGH LIFT TO SUPPORT SOME ISO HAILERS AND WIND. WIND
FIELDS ALOFT FAIRLY WEAK SO MOST OF THE SUPPORT SHOULD BE THERMAL
INSTABILITY THROUGH MID EVENING.

.LONG TERM /TONIGHT THROUGH THURSDAY/...
ISSUED AT 336 AM CDT FRI JUL 31 2015

NEXT SEVERE WEATHER AND HEAVY RAIN POTENTIAL WILL BE SATURDAY
NIGHT INTO EARLY SUNDAY. THEN...AN ACTIVE WEATHER PATTERN RETURNS
THROUGHOUT NEXT WEEK WITH SEVERAL SHORTWAVES RIDING THE NORTHWEST
FLOW ALOFT COMBINED WITH A BOUNDARY STALLING ACROSS THE STATE.
MODELS ARE IN DECENT AGREEMENT THROUGH THE WEEKEND BEFORE THE
BIGGER DISCREPANCIES SHOW UP BETWEEN SOLUTIONS SUNDAY NIGHT.

TONIGHT...WEAK THETA-E ADVECTION AND ISENTROPIC LIFT PROVIDE
ENOUGH FORCING TO BRING A FEW ELEVATED SHOWERS AND THUNDERSTORMS
ACROSS FAR SOUTHERN IOWA LATE TONIGHT INTO SATURDAY MORNING.
INTRODUCED POPS PAST 06Z SATURDAY BUT KEPT QPF AMOUNTS LOW ATTM
DUE TO THE ISOLATED NATURE OF THE CONVECTION.

SATURDAY INTO SUNDAY...A STRONGER SHORTWAVE BRINGS THE POTENTIAL
FOR SEVERE WEATHER SATURDAY LATE AFTERNOON INTO THE EVENING. DEEP
LAYER SHEAR IS FAVORABLE TO SUSTAINED THUNDERSTORMS...BUT LOW
LEVEL SHEAR REMAINS WEAK AND THUS LOW CONFIDENCE WITH ANY
TORNADIC ACTIVITY SATURDAY NIGHT. MAIN THREAT APPEARS TO HAIL AND
DAMAGING WINDS WITH DECENT INSTABILITY AND MID-LEVEL LAPSE RATES
IN THE 7.5-8C/KM RANGE. HEAVY RAIN ALSO A THREAT WITH MODELS
DEPICTING STRONG MOISTURE TRANSPORT INTO THE STATE AND WARM LAYER
CLOUD DEPTHS RANGE FROM 3500-3800 METERS. PRECIPITABLE WATER
VALUES IN THE 1.5-1.8 INCH RANGE AND CORFIDI VECTORS SOMEWHAT
CONDUCIVE TO TRAINING STORMS...SO CERTAINLY CANNOT RULE OUT A FEW
LOCALLY HEAVY RAIN PRODUCERS SATURDAY NIGHT. SYSTEM MOVES OUT BY
SUNDAY MORNING WITH THE ECMWF SLIGHTLY SLOWER THAN THE GFS BUT
LOOKS TO BE DRY ACROSS MUCH OF THE CWA BY 18Z SUNDAY AT THE
LATEST.

MONDAY THROUGH THURSDAY...FRONT STALLS ACROSS IOWA FROM WEST-
NORTHWEST TO EAST-SOUTHEAST BY MONDAY AND REMAINS PLANTED OVER THE
STATE INTO AT LEAST TUESDAY AND LIKELY INTO WEDNESDAY. DEPENDING
ON WHICH MODEL...TIMING OF THE SHORTWAVES TO RIDE THE UPPER LEVEL
RIDGE WILL BE PROBLEMATIC. HOWEVER...CONFIDENT ENOUGH TO INCREASE
POPS MONDAY NIGHT AS BOTH ECMWF/GFS ARE AT LEAST IN SOMEWHAT
AGREEMENT WITH WIDESPREAD CONVECTION DEVELOPING ALONG THE STALLED
FRONT AS THE LLJ INCREASES. VERY DEEP MOISTURE...HIGH PRECIPITABLE
WATER VALUES AND NEAR 4000 METERS OF WARM LAYER CLOUD DEPTHS
MONDAY NIGHT SHOULD PROVIDE ANOTHER ROUND OF HEAVY RAIN.
ADDITIONAL SHORTWAVES CONTINUE TO MOVE THROUGH THE REGION FROM
TUESDAY NIGHT INTO THURSDAY BUT AGAIN LOW CONFIDENCE WITH TIMING
AND LOCATION OF THESE SHORTWAVES AND THUS HAVE SMALL CHANCE POPS
GOING DURING THIS TIME FRAME.

&&

.AVIATION...31/18Z
ISSUED AT 114 PM CDT FRI JUL 31 2015

COLD FRONT DROPPING ACROSS TAF LOCATIONS AND WILL BE IN SOUTHERN
IOWA BY 01/00Z.  AN UNSTABLE AIRMASS ACROSS SW IA INVOF FRONTAL
BOUNDARY AND AN UPPER LEVEL DISTURBANCE MOVING ACROSS SW IA MAY
TRIGGER TSRA BUT SHOULD MISS TAF LOCATIONS.  TSRA COVERAGE MAY
EXPAND 00Z THROUGH 04Z UNTIL WAVE PASSES.  A STRONGER SHORTWAVE WILL
APPROACH IA NEAR THE END OF THE TAF FCST PD INCREASING HIGH CLOUDS.
SFC FLOW WILL BE VARIABLE WITH WINDS NRLY AT 10-15KTS BECMG W TO SW
AFT 03Z THEN NRLY AFT 01/15Z.

&&

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

$$

SHORT TERM...REV
LONG TERM...PODRAZIK
AVIATION...FAB


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