Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 281738
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1238 PM CDT TUE JUL 28 2015

.SHORT TERM /TODAY/...
ISSUED AT 358 AM CDT TUE JUL 28 2015

A VERY COMPLEX FORECAST FOR TODAY WITH MANY CONSIDERATIONS. FIRST
AND FOREMOST...OUR MESSY CONVECTIVE PATTERN CONTINUES AS WE
REMAIN WITHIN THE SAME SOUPY AIRMASS THAT HAS PERSISTED THIS
WEEKEND. REGIONAL RADAR CURRENTLY DEPICTS A LONG SQUALL LINE
STRETCHING DOWN THE EASTERN DAKOTAS AND INTO NORTHEASTERN
NEBRASKA...MOVING STEADILY EASTWARD TOWARD NORTHWESTERN IOWA.
AHEAD OF THIS LINE...SCATTERED THUNDERSTORMS ARE DEVELOPING ACROSS
ABOUT THE NORTHERN HALF OF IOWA WITHIN A BROAD WARM AIR ADVECTION
REGIME. RADAR TRENDS AND HIGH RESOLUTION MODEL FORECASTS SUPPORT A
CONTINUATION OF THESE SCATTERED STORMS UNTIL THE SQUALL LINE MOVES
ACROSS OUR NORTHERN COUNTIES LATER THIS MORNING. THERE IS LOWER
CONFIDENCE IN THE SOUTHERN HALF OR SO OF THE FORECAST AREA BEING
AFFECTED. IF THE STORMS DIVE DOWN THE INSTABILITY GRADIENT
DEPICTED BY MESOANALYSIS THEN THEY MAY REACH SOUTHERN
IOWA...HOWEVER WARMER AIR ALOFT IS WORKING IN FROM THE SOUTHWEST
AND SATELLITE IMAGERY SHOWS A DISTINCT LACK OF CONVECTIVE CLOUD
DEVELOPMENT IN THAT AREA. IT IS MORE LIKELY THAT THE SQUALL LINE
WILL AFFECT NORTHERN IOWA...LEAVING IN ITS WAKE SOME DEBRIS CLOUDS
AND REMNANT OUTFLOW BOUNDARIES AS WELL AS A GRADUALLY MODIFYING
COLD POOL BY AROUND MIDDAY.

IN THE SOUTH AND SOUTHEAST...IF CONVECTION REALLY DOES REMAIN
CONFINED TO OUR NORTH...TEMPERATURES WILL WARM INTO THE LOWER 90S
BY THIS AFTERNOON WITH DEWPOINTS IN THE MID 70S OR SO. THIS WILL
PRODUCE HEAT INDEX VALUES NEAR 105 DEGREES...AND JUSTIFIES THE
ISSUANCE OF A SMALL HEAT ADVISORY AGAINST THOSE ALREADY ISSUED BY
NEIGHBORING OFFICES AGAINST OUR SOUTHEASTERN CORNER. HOWEVER...IF
THE MORNING STORMS MAKE IT FURTHER SOUTH THAN EXPECTED THE
ADVISORY MAY NOT PAN OUT...AND CONVERSELY IF MORNING CONVECTIVE
DEBRIS REMAINS CONFINED TO NORTH OF HIGHWAY 30 THEN THE ADVISORY
MAY NEED TO BE EXPANDED NORTHWARD A BIT DURING THE DAY.

BY LATE THIS AFTERNOON AND EARLY EVENING AS WE REACH PEAK DIURNAL
HEATING TIME THE AFOREMENTIONED RESIDUAL OUTFLOW BOUNDARIES SHOULD
PROVIDE FOCI FOR ADDITIONAL THUNDERSTORM DEVELOPMENT WITHIN THE
VERY UNSTABLE AIRMASS PRECEDING OUR WELL ADVERTISED COOL FRONT
APPROACHING FROM SOUTH DAKOTA AS DISCUSSED IN THE LONG TERM
SECTION BELOW. THERE IS SOME UNCERTAINTY REGARDING LOCATION OF
THIS REDEVELOPMENT CONTINGENT ON THE PATH MORNING STORMS
TAKE...BUT ANY STORMS THAT DO DEVELOP LATE IN THE DAY WILL
CERTAINLY BE WITHIN A FAVORABLE SEVERE WEATHER ENVIRONMENT AS
OUTLINED BY SPC PRODUCTS. HAVE MAINTAINED LIKELY POPS AFTER ABOUT
22Z TO ACCOUNT FOR THIS.

.LONG TERM /TONIGHT THROUGH MONDAY/...
ISSUED AT 358 AM CDT TUE JUL 28 2015

MAIN FORECAST CONCERN WAS FOCUSED ON COLD FRONT TO SWEEP ACROSS
THE STATE TONIGHT AND THE CONVECTION IT WILL BRING TO THE FORECAST
AREA. MODELS ARE IN OKAY AGREEMENT WITH THE TIMING OF THE COLD
FRONT TONIGHT WITH THE 28.00Z NAM THE SLOWEST MODEL AND NOT
INITIALIZING WELL THIS MORNING...THUS KEPT IT AS THE OUTLIER
TONIGHT INTO TOMORROW AND LEANED TOWARD THE GFS/ECMWF.

TONIGHT THROUGH WEDNESDAY...CONVECTION WILL BE ONGOING THIS
EVENING WITH THE PROGRESSION OF THE COLD FRONT ENTERING THE
WESTERN TO NORTHWESTERN PORTION OF THE CWA. DEEP LAYER SHEAR AHEAD
OF THE FRONT IS MAINLY UNIDIRECTIONAL BUT STRONG ENOUGH TO
SUSTAIN A COLD POOL. INSTABILITY IS PLENTIFUL WITH MUCAPE VALUES
RANGING FROM 1500-3500J/KG PER GFS/ECMWF. MCS MAINTENANCE
PROBABILITY VIA THE ECMWF/GFS ARE OVER 40 PERCENT WITH GFS AS
HIGH AS 90 THIS EVENING. CONFIDENCE STORMS WILL BE MOVING ACROSS
CENTRAL DURING MUCH OF THE EVENING BEFORE CLEARING THE FORECAST
AREA B/T 06-09Z WEDNESDAY. LOWER CONFIDENCE IS WIDESPREAD SEVERE
WEATHER...BUT STILL CANNOT RULE OUT A FEW STORMS PRODUCING STRONG
WINDS/LARGE HAIL EARLY IN THE EVENING WITH THE SHEAR AND
INSTABILITY AVAILABLE. THE OTHER THREAT IS HEAVY RAIN WITH STRONG
MOISTURE TRANSPORT VECTORS AND THETA-E ADVECTION INTO THE STATE
AHEAD OF THE FRONT COMBINED WITH THE DEEP WARM LAYER CLOUD DEPTHS
AND PRECIPITABLE WATER VALUES FROM 2 TO 2.4 INCHES. ISOLATED AREAS
OF FLASH FLOODING COULD POSE AN ISSUE TONIGHT IF ANY TRAINING
OCCURS ON TOP OF THE AREAS THAT WERE HIT WITH HEAVY RAIN MONDAY
AND OVER THE WEEKEND.

COLD FRONT WILL BRING SOME MUCH NEEDED COOLER AND DRIER AIR INTO
THE STATE OVERNIGHT TONIGHT THROUGHOUT THE DAY WEDNESDAY.
WEDNESDAY LOOKS TO BE A LITTLE BREEZY...ESPECIALLY ACROSS THE
NORTH WHERE PRESSURE GRADIENT AND SLIGHTLY DEEPER MIXING IS
ANTICIPATED. HOWEVER...SUB-ADVISORY ATTM.

THURSDAY THROUGH MONDAY...SURFACE HIGH PRESSURE AND A LARGE UPPER
RIDGE BUILD INTO THE REGION THROUGH THE END OF THE WORK WEEK. SOME
MODERATING TEMPERATURES IN THE 80S TO LOWER 90S BEGIN ON FRIDAY
AND CONTINUE THROUGH THE WEEKEND. WEAK SHORTWAVE CUTS ACROSS THE
STATE FRIDAY BUT WITH LIMITED MOISTURE AND FORCING...KEPT FORECAST
DRY FOR NOW. LATE IN THE WEEKEND...THE GFS AND ECMWF HINTING ON A
DECENT COLD FRONT AND UPPER LEVEL SHORTWAVE TO MOVE ACROSS THE
REGION LATE SATURDAY NIGHT INTO SUNDAY...THEN ANOTHER SHORTWAVE
THROUGH THE FORECAST AREA BY EARLY MONDAY.

&&

.AVIATION...28/18Z
ISSUED AT 1234 PM CDT TUE JUL 28 2015

WEAK BOUNDARY WITH ONGOING TSRA MAY AFFECT KFOD AND KDSM EARLY IN
THE PERIOD. APPROACHING FRONT WILL BRING BEST CHANCES FOR TSRA IN
LATER AFTERNOON AND NEAR 00Z TO SITES. HAVE INCLUDED VCTS MENTION
FOR NOW WITH UNCERTAINTIES IN TIMING AND LOCATION OF TSRA. TSRA
AT SITES MAY CAUSE MVFR OR LOWER CEILINGS AND VISIBILITIES...ALONG
WITH STRONG WINDS. BEHIND BOUNDARY WINDS WILL SHIFT TO THE
NORTH...AND WILL INCREASE BEYOND 12Z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
HEAT ADVISORY UNTIL 7 PM CDT THIS EVENING FOR APPANOOSE-DAVIS-
MAHASKA-MONROE-WAPELLO-WAYNE.

&&

$$

SHORT TERM...LEE
LONG TERM...PODRAZIK
AVIATION...AWB


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