Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 221743
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1242 PM CDT FRI AUG 22 2014

.SHORT TERM /TODAY/...
ISSUED AT 409 AM CDT FRI AUG 22 2014

A CLUSTER OF THUNDERSTORMS DEVELOPED ACROSS SOUTHWEST IOWA OVERNIGHT
AND IS CURRENTLY WEAKENING AS IT PROGRESSES EASTWARD ACROSS ABOUT
THE SOUTHERN HALF OF THE FORECAST AREA. OVERALL INTENSITY TREND
SHOULD CONTINUE DOWNWARD WITH THIS ACTIVITY EARLY THIS MORNING.
MEANWHILE FOG AND PATCHY STRATUS HAVE DEVELOPED ACROSS THE NORTH IN
AN AIRMASS CHARACTERIZED BY NEARLY CALM WINDS AND DEWPOINTS IN THE
LOWER 70S...HOWEVER VSBYS HAVE SO FAR NOT DROPPED BELOW A MILE AND
BELIEVE ANY DENSE FOG WILL BE ISOLATED THUS NO ADVISORY IS BEING
ISSUED WITH THE MORNING PACKAGE BUT THE SITUATION WILL BE CLOSELY
MONITORED.

THE AFOREMENTIONED THUNDERSTORMS AND FOG SHOULD BOTH DISSIPATE
WITHIN A COUPLE HOURS AFTER SUNRISE. THEREAFTER...IT WILL BE ANOTHER
VERY HUMID DAY WITH LIGHT WINDS AND GENERALLY VERY SWEATY
CONDITIONS. THERE IS A HIGH DEGREE OF UNCERTAINTY REGARDING
ADDITIONAL SHOWER/THUNDERSTORM DEVELOPMENT TODAY. MOST OF THE LARGE
SCALE AND EVEN SOME OF THE HI RES MODELS ARE PAINTING LIGHT QPF
ACROSS CENTRAL IOWA THROUGH MIDDAY AND THE
AFTERNOON...HOWEVER...UPSTREAM RADAR BELIES THIS WITH NO DEVELOPMENT
OF ANY KIND AND FORECAST SOUNDINGS SHOW A MODEST BUT EFFECTIVE
CAPPING INVERSION FOR MOST OF THE DAY...PARTICULARLY GIVEN THE
LIKELIHOOD OF CONVECTIVE DEBRIS CLOUDS EARLY IN THE DAY LIMITING
INSOLATION. THERE IS ALSO NO REAL FOCUS MECHANISM IN TERMS OF
FORCING FOR MOST OF THE DAY. CONVECTION SEEMS SOMEWHAT MORE PROBABLE
LATE IN THE DAY TOWARD THIS EVENING AS SURFACE HEATING ALLOWS THE
CAP TO BREAK AND WE WILL LIKELY SEE TYPICAL LATE SUMMER POPCORN
STORMS TOWARD SUNSET...WHICH IS COVERED BY 30 TO 40 POPS AND
SCATTERED WORDING IN THE FORECAST. TIED INTO THE DIFFICULT PRECIP
FORECAST FOR TODAY IS YET ANOTHER LOW CONFIDENCE TEMPERATURE
FORECAST. THE DEGREE OF SUNSHINE VERSUS CLOUDS IS PARTICULARLY
PROBLEMATIC...BUT EVEN ON THURSDAY WHEN THERE WAS AMPLE SUNSHINE
TEMPERATURES HAD A DIFFICULT TIME WARMING GIVEN THE VERY HUMID
AIRMASS IN PLACE. WILL CONTINUE THE TREND OF THE LAST COUPLE OF DAYS
IN LIMITING HIGH TEMPERATURES IN THE UPPER 80S AS IT APPEARS THE
REAL WARM UP WILL WAIT FOR THIS WEEKEND.

.LONG TERM /TONIGHT THROUGH THURSDAY/...
ISSUED AT 409 AM CDT FRI AUG 22 2014

ALTHOUGH THERE WILL CERTAINLY BE SOME BREAKS...THE PATTERN WILL
REMAIN ACTIVE INTO NEXT WEEK BEFORE CALMING DOWN TO END THE PERIOD.
EXPECT MORE CONVECTION AGAIN TONIGHT BUT FROM A DIFFERENT SCENARIO
THAN RECENTLY. ATTENTION WILL TURN UPSTREAM LOOKING AT EVOLUTION OF
AZ SHORT WAVE. REMNANTS SHOULD REACH THE PLAINS TONIGHT WITH LARGE
SCALE FORCING INCREASING INTO THE UPPER/MID MO VALLEY. THIS WILL
ALSO MATURE THERMODYNAMIC FORCING WITH THETA-E ADVECTION AND
BAROCLINIC ZONE STRENGTHENING AS IT LIFTS NORTHWARD. ALTHOUGH BEST
SUPPORT LOOKS TO REMAIN NORTH AND WEST OF IA...THIS SHOULD STILL BE
SUFFICIENT FOR MCS WITH AIRMASS STILL VERY FAVORABLE FOR CONVECTION.
CONVECTIVE POTENTIAL SHOULD DIMINISH SOUTH TO NORTH SAT WITH
BAROCLINIC ZONE EXITING IA TO THE NORTH...AND WARM FRONT LIFTING
FROM SRN IA TO OR BEYOND THE MN BORDER BY 00Z.  TEMPERATURES HAVE
BEEN PROBLEMATIC RECENTLY...OVER FORECAST...WITH MIXING DIFFICULT TO
GRASP DUE TO LINGERING CONVECTION AND LIGHTER WINDS. MOS HAS NOT
DONE TOO POORLY CAPTURING THIS HOWEVER SO STAYED CLOSE TO THOSE
NUMBERS FOR MAXES SAT.

FORCING ASSOCIATED WITH NRN ROCKIES/NRN PLAINS LONG WAVE TROUGH WILL
CONTINUE TO BRUSH WRN IA THROUGH THE WEEKEND...AND ALTHOUGH
THERMODYNAMIC FORCING WILL BE WEAKER...IT MAY STILL BE SUFFICIENT FOR
A FEW SURGES OF CONVECTION KEEPING LOW POPS GOING NORTH AND WEST SAT
NIGHT.  IA SHOULD BE THOROUGHLY INTO WARM SECTOR SUN WITH MORE
QUESTIONS ON THE DEGREE OF MIXING.  NAM IS VERY STRONG IN THIS
REGARD WITH RAW TEMPS SUGGESTING HIGHS FURTHER INTO THE 90S.
DEWPOINTS WILL BE A TAD LOWER...BUT STILL 70 PLUS...SO HAVE
FINALLY NUDGED HIGHS JUST OVER SATURDAYS /ONLY LOW 90S/ WITH
LITTLE PRECIP EXPECTED. DEGREE OF MIXING WILL NEED TO BE WATCHED
HOWEVER TO SEE IF NAM SUGGESTIONS CAN OVERCOME HIGH LOW LEVEL
MOISTURE CONTENT.

SURFACE FRONT MAY JUST REACH WRN SECTIONS LATE SUN WITH SURFACE
BASED POTENTIAL TOWARD THE MO RIVER. THERE SHOULD BE SUFFICIENT
FORCING FOR AT LEAST WEAK SCATTERED CONVECTION INTO THE NIGHT.
MODELS BEGIN TO DIVERGE INTO THE MIDDLE OF NEXT WEEK WITH
DIFFERENCES IN STRENGTH OF NRN STREAM SHORT WAVE AND LINGERING
ROCKIES SHORT WAVE ENERGY...BUT MOST SOLUTIONS SUGGEST MORE
CONVECTION REGARDLESS OF THE SOLUTION. HAVE GONE DRY BY WED NIGHT
AND THU HOWEVER AS EVEN THE SLOWER ECMWF SUGGESTS NO PRECIP.

PERSISTENCE OF HEAT WILL BE A PROBLEM OVER THE WEEKEND...BUT BETTER
MIXING POTENTIAL MAY JUST LOWER DEWPOINTS ENOUGH TO KEEP HEAT
INDICES OUT OF ADVISORY RANGE.  IN FACT...HEAT INDICES SAT AND SUN
MAY BE A TOUCH LOWER THAN TODAY.

&&

.AVIATION...22/18Z
ISSUED AT 1242 PM CDT FRI AUG 22 2014

PATCHY STRATUS WILL PLAGUE NORTHERN TAF LOCATIONS FOR A COUPLE MORE
HOURS BUT THE FOG HAS FINALLY LIFTED/DISSIPATED.  IN GENERAL THERE
WILL BE VFR CIGS BUT BRIEF PERIODS WHERE STRATUS MAY DRIFT OVER TAF
LOCATIONS WILL YIELD IFR CIGS.  SOME OF THE HI-RES MODELS SUGGEST
STORMS TO DEVELOP ALONG/SOUTH OF I-80 AROUND 21Z.  SAT ALREADY
SHOWING CU BUBBLING AND WITH THE BOUNDARY APPROACHING I COULD NOT
LEAVE KDSM AND KOTM DRY 21Z-00Z.  STORMS WILL BE ISOLD AT BEST
THOUGH SO JUST A MENTION OF VCTS IN THOSE TAFS.  LATE TONIGHT A
BETTER CHANCE FOR STORMS SW OF A LINE FROM KFOD TO KOTM WITH FOG
AND MVFR/LOCAL IFR COND ONCE AGAIN NORTH OF THAT LINE.

&&

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

&&

$$

SHORT TERM...LEE
LONG TERM...SMALL
AVIATION...FAB


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