Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 030437
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1137 PM CDT TUE SEP 2 2014

.SHORT TERM /TONIGHT/...
ISSUED AT 322 PM CDT TUE SEP 2 2014

SURFACE HIGH PRESSURE...WHICH BROUGHT CLEAR AND COOLER WEATHER TO
THE REGION TODAY...WILL SHIFT EASTWARD ACROSS THE MISSISSIPPI RIVER
TONIGHT. LOW TO MID LEVEL MOISTURE WILL QUICKLY RETURN TO THE STATE
AFTER MIDNIGHT AS SOUTHERLY FLOW SETS IN BEHIND THE HIGH. THIS IS
WELL REPRESENTED IN THE 925 MB MIXING RATIO FIELD...WHICH JUMPS FROM
9.5 TO 15.5 G/KG OVER SOUTHWEST IOWA BETWEEN 06 AND 12Z WED. A
SUBTLE SHORTWAVE AT 500 MB AND A SMALL ASSOCIATED VORT MAX
APPROACHING FROM NEBRASKA...COUPLED WITH THE STRONG 925-850 MB THETA-
E ADVECTION AT THE NOSE OF THIS RETURNING AIRMASS...WILL LIKELY
FORCE A FEW ELEVATED SHOWERS AND THUNDERSTORMS AFTER 08-09Z IN
WESTERN IOWA. THESE CELLS WILL TRACK NORTH AND EAST OVER THE LAST
SEVERAL HOURS OF THE PERIOD. MODELS HAVE BEEN RATHER INCONSISTENT IN
THE PLACEMENT OF ANY PRECIP...WITH THE NAM THE MOST AGGRESSIVE BY
FAR AND DEVELOPS A LARGE CLUSTER OF STORMS ON THE INSTABILITY
GRADIENT/LLJ NOSE. OTHER MODELS ARE SUBSTANTIALLY DRIER. TRENDED
TOWARDS THE OVERALL CONSENSUS SOLUTION AND KEPT MOSTLY SLIGHT TO
LOW CHANCE POPS IN THE FORECAST FOR WESTERN IOWA AT 09Z AND
SPREADING ACROSS THE CWA BY 12Z.

.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
ISSUED AT 322 PM CDT TUE SEP 2 2014

ON WEDNESDAY A WEAK MID LEVEL IMPULSE MOVING OVERHEAD SHOULD
PRODUCE A FEW THUNDERSTORMS...THOUGH THE LACK OF A FORCING
MECHANISM IN OUR AREA MAY RESULT IN THE STORMS BEING CONFINED
FURTHER NORTH ACROSS MINNESOTA WHERE A SYNOPTIC WARM FRONT WILL BE
LOCATED. NEVERTHELESS MAINTAINED THE INHERITED 20 POPS ACROSS OUR
FORECAST AREA...DIMINISHING IN THE AFTERNOON. MEANWHILE...WITH THE
AFOREMENTIONED FRONT TO OUR NORTH AND BRISK SOUTHERLY FLOW EXPECT
A RETURN OF WARMER AND MORE HUMID CONDITIONS BY WEDNESDAY
AFTERNOON...WHICH WILL PERSIST THROUGH WEDNESDAY NIGHT AND INTO
THURSDAY WITH WINDS VEERING MORE TOWARD THE SOUTHWEST. EXPECT
ADDITIONAL THUNDERSTORM DEVELOPMENT WEDNESDAY NIGHT AS THE LOW
LEVEL JET KICKS IN...WITH INITIATION LIKELY IN MINNESOTA ALONG THE
SYNOPTIC BOUNDARY AND SOME QUESTION AS TO WHETHER THE RESULTING
ACTIVITY WILL MOVE CLOSER TO DUE EAST AND STAY NORTH OF OUR AREA
OR WHETHER IT WILL DIVE SOUTHEAST AND CLIP OUR NORTHEASTERN
COUNTIES. AGAIN...HAVE MAINTAINED 20 POPS IN THE NORTHEAST TO
ACCOUNT FOR THIS.

ON THURSDAY WITH WINDS VEERING TO SOUTHWEST EXPECT EVEN BETTER
DAYTIME HEATING ACROSS MOST OF THE AREA WITH LOWER 90S POSSIBLE.
MEANWHILE...A VIGOROUS IF COMPACT MID/UPPER LEVEL DISTURBANCE WILL
MOVE QUICKLY ACROSS NORTH DAKOTA AND NORTHERN MINNESOTA ON
THURSDAY...WITH THE ATTENDANT SURFACE LOW PRESSURE SYSTEM PUSHING
A COLD FRONT INTO NORTHWEST IOWA DURING THE AFTERNOON. DURING MOST
OF THE DAY WARMER TEMPERATURES ALOFT MAY INHIBIT CONVECTIVE
DEVELOPMENT ALONG THE ADVANCING FRONT...BUT LATE IN THE DAY AND
ESPECIALLY THURSDAY NIGHT AS THE FRONT PROGRESSES ACROSS THE REST
OF IOWA EXPECT FAIRLY WIDESPREAD COVERAGE OF THUNDERSTORMS
ESPECIALLY ACROSS THE SOUTHEASTERN TWO THIRDS OR SO OF THE STATE.
GIVEN THE INSTABILITY IN PLACE AHEAD OF THE FRONT THERE IS SOME
POTENTIAL FOR SEVERE WEATHER...ESPECIALLY FROM LATE AFTERNOON INTO
THE EARLY NIGHTTIME HOURS...AS WELL AS LOCALLY HEAVY RAINFALL WITH
STORMS POSSIBLY DEVELOPING PARALLEL TO THE FRONT AND TRAINING OVER
THE SAME AREAS. THIS THREAT WILL BE ASSESSED IN MORE DETAIL
TOMORROW AND THURSDAY.

AFTER THE COLD FRONT PASSES THROUGH THURSDAY NIGHT SOME SHOWERS
AND STORMS WILL LINGER ACROSS SOUTHERN IOWA ON FRIDAY BEFORE
EXITING...BUT OTHERWISE THE LARGE AREA OF COOL HIGH PRESSURE
MOVING IN BEHIND THE FRONT WILL DOMINATE OUR WEATHER FOR SEVERAL
DAYS. THE RESULT WILL BE THE FIRST SHOT OF REAL AUTUMNAL AIR WITH
SUNNY AND DRY CONDITIONS...HIGHS IN THE LOWER 70S...AND LOWS IN
THE MID 40S TO MID 50S THIS WEEKEND. BY MONDAY MORNING A LONGWAVE
500 MB TROUGH WILL BE COMING ASHORE OVER THE PACIFIC U.S.
COAST...RESULTING IN A TURN TO SOUTHWESTERLY FLOW ALOFT OVER THE
PLAINS AND IOWA AND USHERING IN SOMEWHAT MILDER AND MUCH WETTER
WEATHER FOR ABOUT THE FIRST HALF OF THE WORK WEEK...AT THIS TIME
MOST LIKELY BETWEEN MONDAY NIGHT AND WEDNESDAY...WITH MULTIPLE
ROUNDS OF THUNDERSTORMS AND HEAVY RAINFALL POSSIBLE. THIS IS
PRETTY FAR OUT AT 7 TO 8 DAYS AWAY IN THE FORECAST SO THERE IS
STILL A LOT OF UNCERTAINTY REGARDING EXACTLY HOW THIS SCENARIO
WILL EVOLVE.

&&

.AVIATION...03/06Z
ISSUED AT 1137 PM CDT TUE SEP 2 2014

LITTLE CHANGES MADE TO THE AVIATION FORECAST WITH UNCERTAINTY
STILL HIGH IF THUNDERSTORM ACTIVITY WILL IMPACT ANY TAF SITE WITH
MAINLY ISOLATED CHANCES THROUGH THE FORECAST PERIOD. WINDS HAVE
SHIFTED AROUND TO THE SOUTH ACROSS MUCH OF THE AREA...WITH THE
SOUTHERLY TO SOUTHEASTERLY WINDS EXPECTED TO INCREASE BY MIDDAY
WEDNESDAY. LOW VFR TO HIGH MVFR CIGS STILL EXPECTED BY WEDNESDAY
MORNING AND THROUGH THE REMAINDER OF THE DAY.

&&

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

&&

$$

SHORT TERM...SKOW
LONG TERM...LEE
AVIATION...BEERENDS


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