Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 261721
AFDDMX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1221 PM CDT FRI SEP 26 2014

.SHORT TERM /TODAY/...
ISSUED AT 347 AM CDT FRI SEP 26 2014

PESKY UPPER LOW THAT ROTATED OVER THE UPPER RIDGE OVER THE PAST FEW
DAYS CONTINUES TO BE OVER THE AREAS WITH ITS ELONGATED CENTER
EXTENDING FROM WESTERN IL INTO ERN KS.  PROGS ARE CONSISTENT IN WITH
FORECAST OF THIS FEATURE TO LIFT BACK NORTHWEST OVER IOWA TODAY.
THE DYNAMICS ARE VERY DIFFUSE..BUT ENOUGH SUFFICIENT MOISTURE IS IN
PLACE AND WILL BE DRAWN INTO IA TODAY...ESPECIALLY THE SOUTHEAST
PART OF THE CWA  WILL INTRODUCE ISOLATED LIGHT SHOWERS...BUT DON`T
EXPECT THESE TO AMOUNT TO MUCH.  TEMPS TODAY WILL BE A LITTLE ABOVE
THOSE OF YESTERDAY WITH THE STATE PRETTY UNIFORM IN THE LOW 80S.

.LONG TERM /TONIGHT THROUGH THURSDAY/...
ISSUED AT 347 AM CDT FRI SEP 26 2014

LITTLE CHANGE WAS MADE TO THE LONG TERM PERIOD AS THE FORECAST TRENDS
ARE STILL BEING REFLECTED IN THE MEDIUM RANGE MODELS. ALL MODELS
RETROGRADE A WEAK UPPER LOW TO THE NORTHWEST THEN NORTH OVER
IOWA THROUGH THE WEEKEND. THIS IS THE SAME LOW THAT HAS ROTATED
AROUND THE REGION FOR THE PAST COUPLE OF DAYS. THERE IS SOME BROAD
SCALE WEAK FORCING WITH THE LOW AS WELL AS SOME WEAK INSTABILITY.
YESTERDAY SHOWED THIS LOW TO BE CAPABLE OF PRODUCING SHOWERS
ACROSS CENTRAL IOWA AS IT MIGRATES AROUND. MUCH LIGHT THE PAST
COUPLE DAYS THERE WILL BE ISOLATED SHOWERS AGAIN ON SATURDAY
DURING THE PEAK HEATING HOURS OF THE AFTERNOON. THE LOW FINALLY
PUSHES OFF FOR SUNDAY AND MONDAY. THE FORECAST IS DRY FOR THIS
PERIOD AND TEMPERATURES SHOULD BE QUITE WARM FOR END OF
SEPTEMBER/BEGINNING OF OCTOBER.

FOR LATE MONDAY THROUGH WEDNESDAY A LARGE AND PROGRESSIVE TROUGH
WILL CARVE ITSELF OUT OVER THE ROCKIES AND PACIFIC NORTHWEST.
THERE REMAINS NO GOOD TAP OF MOISTURE BUT THERE IS SOME
INSTABILITY. THERE WILL LIKELY BE VORT MAXES EJECTING OUT OF THE
BOTTOM OF THIS TROUGH BUT THE MODELS SEEM TO WANT TO LIFT THOSE TO
OUR WEST AND NORTH SO TUESDAY MAY BE DRY AS WELL. BY TUESDAY NIGHT
THE UPPER TROUGH PUSHES THE BLOCKING HIGH INTO THE GREAT LAKES AND
AT LEAST WESTERN AND CENTRAL IOWA WILL SEE CHANCES FOR SHOWERS AND
THUNDERSTORMS. THE SYSTEM EVOLVES INTO A CLOSED LOW...OR NEARLY
CLOSED LOW OVER NORTH DAKOTA BEFORE SHIFTING INTO CANADA BUT THE
MAIN TROUGH WILL STILL RESIDE OVER THE ROCKIES. WITH IOWA
REMAINING IN SOUTHWEST FLOW AND THE DEEP TROUGH TO THE WEST SHORT
WAVES COULD EASILY EJECT OUT OF THE BASE OF THE TROUGH AND WOULD
COME RIGHT ACROSS IOWA. THERE IS NOT MUCH INDICATION FROM MODELS
OF THE SYSTEM DOING THIS. SO WHILE THERE WILL BE A PROLONGED
PERIOD OF SHOWER AND THUNDERSTORM CHANCES THE THREAT OF SEVERE
STORMS AT THIS TIME IS UNCERTAIN.

IF WE COULD TAP SOME BETTER MOISTURE OR IF A STRONGER WAVE
EJECTING OUT OF THE BASE OF THE TROUGH WOULD PUSH ACROSS IOWA YOU
COULD NOT RULE OUT A STRONG/SEVERE STORM TUESDAY NIGHT/WEDNESDAY.
THE TROUGH WILL FINALLY MOVE INTO THE GREAT LAKES BY FRIDAY BUT
THE MODELS ARE NOW QUITE DIVERGENT FROM FRIDAY ON. THE EURO PUSHES
THE TROUGH INTO THE GREAT LAKES RATHER QUICKLY BEFORE UPPER LEVEL
HIGH PRESSURE REBUILDS OVER THE WESTERN U.S. WHILE THE GFS REALLY
AMPLIFIES THIS RIDGE THUS DEEPENING THE TROUGH OVER THE MIDWEST.
THE GFS SOLUTION WOULD NOT ONLY BE WETTER BUT CONSIDERABLY COLDER
AS WELL. WE WILL NEED TO CONTINUE TO MONITOR TO SEE HOW THE MODELS
RESOLVE THIS DIFFERENCE.

&&

.AVIATION...26/18Z
ISSUED AT 1219 PM CDT FRI SEP 26 2014

UPPER LEVEL LOW WILL CONTINUE TO INFLUENCE SITES. CONDITIONS WILL
REMAIN PRIMARILY VFR...THOUGH VISIBILITIES MAY DROP TO MVFR OR
LOWER NEAR 12Z WITH BR DEVELOPMENT AT SITES. IN
ADDITION...SCATTERED SHRA/TSRA MAY AFFECT SITES THROUGHOUT THE
PERIOD. HOWEVER...HAVE KEPT MENTION OUT DUE TO UNCERTAINTIES IN
TIMING AND LOCATION.

&&

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

&&

$$

SHORT TERM...MS SEP 14
LONG TERM...FAB
AVIATION...AWB


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