Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 230450
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1150 PM CDT FRI AUG 22 2014

...UPDATED AVIATION DISCUSSION...

.SHORT TERM /TONIGHT/...
ISSUED AT 327 PM CDT FRI AUG 22 2014

THROUGH EARLY EVENING...STORMS HAVE FIRED IN TWO AREAS ACROSS
CENTRAL IOWA.  THE NORTHERN MOST LINE OF STORMS IS ASSOCIATED WITH A
FRONTAL BOUNDARY THAT IS EVER SO SLOWLY MEANDERING SOUTH.  FLOW IS
WEAK AND THERE IS LITTLE CONVERGENCE AND NO SHEAR TO SPEAK OF SO THE
STORMS WILL BECOME OUTFLOW DOMINANT AND SHOULD FALL APART BY 00-01Z.
FURTHER SOUTH STORMS HAVE FIRED ALONG A MOISTURE AXIS.  THIS AXIS
WAS EVIDENT ON RADAR FOR MUCH OF THE DAY AND STORMS FIRED AFTER
SUFFICIENT HEATING DESTABILIZED THE ATMOSPHERE.  THESE STORMS WILL
BE A BIT MORE ROBUST AND PRODUCE LOCALLY HEAVY RAINFALL...SOME
STRONGER WIND GUSTS AND POSSIBLY SOME SMALL HAIL.  AGAIN...THESE
STORMS SHOULD MOVE EAST SOUTHEAST AND DIMINISH SHORTLY AFTER 00Z.

THE REST OF EARLY TO MID EVENING SHOULD BE RELATIVELY QUIET THOUGH
THINGS WILL FESTER THROUGH THE EVENING SO ISOLATED STORMS WILL STILL
BE POSSIBLE.

FOR THE REST OF TONIGHT...UPPER RIDGE OVER IOWA WILL GET PUSHED TO
THE EAST WITH THE AXIS BY 12Z SAT INTO WESTERN IL BY A DEEPENING
TROUGH OVER THE ROCKIES.  SEVERAL WEAKER SHORTWAVES WILL EJECT OUT
OF THE BASE OF THE TROUGH PUSHING ACROSS NORTHERN/WESTERN IOWA.  IN
ADDITION...THE FRONTAL BOUNDARY FROM A SURFACE LOW IN WESTERN KS
WILL LIFT NORTHWARD AS THE SURFACE LOW SHIFT TO THE NORTHEAST.  TO
THE WEST OF IOWA THERE IS PRETTY IMPRESSIVE SHEAR AND STRONG THETA-E
ADVECTION THAT WILL WORK INTO THE WEST AND NORTH AND GRADUALLY SHIFT
MORE NORTH AND EAST.  ONE AGAIN THE 4KM WRF HAD A DECENT HANDLE ON
ALL OF THE CONVECTION OF THIS AFTERNOON AND TONIGHT SO I DID PUT
SOME WEIGHT ON ITS SOLUTION FOR TONIGHT.  STORMS SHOULD FIRE OUT
WEST OF IOWA THIS AFTERNOON AND EVENING AND WILL PROBABLY MERGE INTO
A CLUSTER OF STORMS THAT WILL THEN SHIFT INTO THE SOUTHWEST THIRD OF
SO OF IOWA DURING THE OVERNIGHT.  THERE MAY BE ISOLATED STORMS
THROUGHOUT THE EVENING BUT THE MAIN PRECIP EVENT WILL OCCUR OVER THE
SOUTHWEST BETWEEN 08Z-12Z BEFORE LIFTING NORTH.  SOME STORMS WILL
PRODUCE HEAVY RAINFALL BUT THERE MAY ALSO BE A COUPLE SEVERE STORMS
AS WELL...MAINLY OVER THE FAR WESTERN PORTION OF THE CWA.  IT WILL
BE A MUGGY NIGHT WITH LOWS IN THE UPPER 60S OVER FAR NORTHERN IOWA
AND 70S ELSEWHERE.  DEWPOINTS WILL STAY IN THE UPPER 60S AND LOW TO
MID 70S SO FOG WILL ONCE AGAIN DEVELOP...ESPECIALLY ACROSS THE
NORTH.

.LONG TERM /SATURDAY THROUGH FRIDAY/...
ISSUED AT 327 PM CDT FRI AUG 22 2014

ACTIVE WX PATTERN EXPECTED AT LEAST THROUGH THE MIDDLE OF NEXT
WEEK...WITH HOT AND HUMID SUMMERTIME WEATHER GIVING WAY TO FALL-LIKE
WEATHER.

MID/UPPER-LEVEL WIND SPEED MAX PLUNGING SOUTHWARD ALONG THE PAC
COAST...ON THE WEST SIDE OF THE WESTERN U.S. TROF...WILL HELP
FACILIATE ITS INTENSIFICATION BEFORE IT LIFTS OUT. IN RESPONSE...THE
EASTERN U.S. RIDGE WILL AMPLIFY INTO THIS WEEKEND. A VORT MAX OVER
THE SW U.S. WILL CONTINUE LIFTING NORTHEASTWARD. BY THE BEGINNING OF
THE LONG TERM FCST PERIOD...LINGERING CONVECTION FROM OVERNIGHT WILL
CONTINUE LIFTING NORTHWARD AHEAD OF A NORTHWARD-ADVANCING FRONTAL
BOUNDARY. THE BOUNDARY IS EXPECTED TO BE IN THE VICINITY OF THE IA-
MN BORDER...LIKELY JUST NORTH OF IT...BY LATE AFTERNOON SATURDAY.
SOUTH OF THE BOUNDARY...TEMPERATURES WILL WARM MORE THAN WHAT WAS
SEEN TODAY. PREVIOUS FCST DISCUSSION ACKNOWLEDGED MIXING ISSUES OVER
THE PAST FEW DAYS AND ON SATURDAY...AND AGREE WITH THAT THINKING
HOWEVER EXPECT IT TO BE MORE THAN WAS SEEN TODAY AS EVIDENCED BY
STRONGER SURFACE WINDS THAN WAS SEEN TODAY. THUS HAVE NUDGED TEMPS
UP A DEGREE OR TWO FROM THE GOING FORECAST FOR SATURDAY AFTERNOON.

ADDITIONAL CONVECTION IS POSSIBLE SATURDAY NIGHT AS ANOTHER
MID/UPPER-LEVEL DISTURBANCE MOVES THROUGH THE CENTRAL/NORTHERN
PLAINS. BEST FORCING WILL REMAIN WEST AND NORTH OF THE CWA...AND
HAVE PAINTED POPS AS SUCH.

SUNDAY IS EXPECTED TO BE THE WARMEST DAY OF THE FCST PD WITH IA
SOLIDLY WITHIN THE WARM SECTOR. NAM HAS CONTINUED TREND FROM
PREVIOUS FCST OF BEING THE WARMEST WITH FCST HIGHS WELL INTO THE
90S. NAM IS SUGGESTING MORE MIXING ON SUNDAY THAN ON SATURDAY AS
EVIDENCED BY LOWER DEWPOINTS AND STRONGER SURFACE WINDS. FOR NOW
HAVE NOT CHANGED SUNDAY/S MAX TEMPS MUCH FROM PREVIOUS FCST. HEAT
INDEX VALUES WILL BE AS HIGH AS AROUND 100F.

HIGHER CHANCES OF SHRA/TSRA OCCUR SUNDAY NIGHT AS UPPER-LEVEL TROF
TO OUR WEST EJECTS NORTHEASTWARD AND PUSHES A BOUNDARY INTO IA.
MODELS SUGGEST THE BOUNDARY WILL HANG UP IN THE STATE...LIKELY
MAKING IT AS FAR SOUTHEAST AS CENTRAL IA. INSTABILITY AND FORCING
WILL BE PLENTIFUL ENOUGH TO RESULT IN AT LEAST SCATTERED SHRA/TSRA
DURING THE NIGHT. AS THE UPPER LEVEL TROF EJECTS...ADDITIONAL
UPSTREAM ENERGY WILL HELP ANOTHER TROF TO DEVELOP IN ITS PLACE OVER
THE NORTHERN U.S. ROCKIES.

WITH BOUNDARY REMAINING IN THE REGION...HAVE KEPT SLIGHT CHANCE POPS
GOING INTO MONDAY. AFTER THIS POINT...THE MODEL SOLUTIONS DIVERGE
LEADING TO INCREASING FORECAST UNCERTAINTY. GFS BRINGS THE SECOND
TROF THROUGH FASTER THAN THE EURO. EXPECT THE BOUNDARY TO MOVE
THROUGH THE CWA BY WEDNESDAY. HAVE INCREASED POPS BEGINNING TUESDAY
AND KEPT THEM IN PLACE THROUGH LATE IN THE DAY ON WEDNESDAY.

AFTER THE FRONTAL PASSAGE...A FEW DAYS OF FALL-LIKE WEATHER WILL
PREVAIL WITH HIGH TEMPS ACROSS THE NORTHERN CWA POSSIBLY NOT LEAVING
THE LOWER 70S ON WEDNESDAY.

&&

.AVIATION...23/06Z
ISSUED AT 1150 PM CDT FRI AUG 22 2014

DELAYED CONVECTION UNTIL PAST 12Z SATURDAY FOR ALL TAF SITES AS
THE 23/00Z HOPWRF AND HRRR ARE IN GOOD AGREEMENT WITH CONVECTION
SPREADING ACROSS SOUTH-CENTRAL IOWA WITH THE HRRR MORE WIDESPREAD
FURTHER NORTH. HAVE MENTION OF VCTS AT FOD/DSM/OTM WITH A LITTLE
HIGHER CONFIDENCE SITES WILL SEE STORMS. HAVE MVFR TO IFR VIS WRT
TO FOG PAST 08Z TONIGHT. MIXING DEVELOPS LATE SATURDAY MORNING AND
KEPT MENTION OF GUSTS THROUGH THE AFTERNOON HOURS.

&&

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

&&

$$

SHORT TERM...FAB
LONG TERM...ZOGG
AVIATION...PODRAZIK


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