Issued by NWS Des Moines, IA
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000 FXUS63 KDMX 100232 AFDDMX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 930 PM CDT SAT MAY 9 2009 .SHORT TERM.../TONIGHT/ WNWLY FLOW ACROSS THE NRN PLAINS AND UPPER MIDWEST SOUTH OF AN UPPER LEVEL LOW OVER THE RED RVR VLY OF THE NORTH WILL STEER WEAK IMPULSES OVER THE REGION TONIGHT. A SFC LOW OVER NE SD AT 2 PM SEEMS TO BE MOVING SLOWER SOUTHEAST THAN 12Z MODELS WANT TO MOVE IT. STEEP LOW AND MID LEVEL LAPSE RATES ARE A BIT OF A CONCERN LATE TODAY INTO THIS EVENING. HOWEVER...AVAILABLE MOISTURE IS THE LIMITING FACTOR. CURRENT UPSTREAM CONDITIONS DO SHOW MODERATELY ENHANCED CUMULUS AND REGIONAL RADARS SHOW A FEW SHOWERS OVER ERN SD. NAM IS THE MOST BULLISH WITH PRECIP POTENTIAL OVER CENTRAL IOWA THROUGH THIS EVENING...AND THE LATEST RUC IS SHOWING MORE THAN BEFORE. WILL POPS IN THE 15 TO 30 PERCENT RANGE DEPENDING ON COLLAB WITH SURROUNDING OFFICES FOR ISOLATED TO SCATTERED SHOWERS/TSTMS. IN ANY EVENT...RAINFALL...IF IT OCCURS WILL BE LIGHT...AND SHOULD BE OVER BY LATE EVENING. .LONG TERM.../SUNDAY THROUGH SATURDAY/ FOR SUNDAY AN UPPER RIDGE BUILDS OUT WEST PRODUCING A BIT OF TROUGHING OVER THE CENTRAL U.S./UPPER MIDWEST. WATER VAPOR SHOWS A SEVERAL WEAK VORT MAXES RIDING IN THE UPPER FLOW...THE AFOREMENTIONED ONE FOR TONIGHT AND ONE AGAIN FOR SUNDAY RIDING IN A QUASI NORTHWESTERLY FLOW. THE EURO IS A BIT MORE BULLISH ON A RESULTANT SHORT WAVE ACROSS SW/SRN IA THAN THE GFS BUT NEITHER MODEL HITS THIS HARD OR PRINTS OUT MUCH IN THE WAY OF QPF. WITH WEAK CAPE AND VIRTUALLY NO LOW LEVEL SHEAR...I KEPT A MENTION OF ISOLATED STORMS IN THE FAR SOUTH THROUGH PEAK HEATING BUT NOTHING MORE THAN THAT. SOUTHERLY/SW FLOW BEGINS OVER THE FORECAST AREA MONDAY AS A LARGE SURFACE HIGH PUSHES EAST PUTTING CENTRAL IOWA IN RETURN FLOW. SOME THETA-E ADVECTION OCCURS LATE MONDAY/MONDAY NIGHT BUT MOISTURE ISN`T REALLY DEEP YET AND THE MODELS DIFFER SLIGHTLY ON WHEN/WHETHER ANOTHER SHORTWAVE STREAMS BY THE FORECAST AREA. FOR THIS REASON I LEFT MONDAY AND MONDAY NIGHT DRY THOUGH MONDAY NIGHT MAY NEED AN INTRODUCTION OF POPS IF TIMING ISSUES BECOME RESOLVED. THE MAIN SYSTEM FOR TUESDAY NIGHT THROUGH WEDNESDAY NIGHT STILL ON TRACK AND LITTLE CHANGES MADE TO THE GOING FORECAST FOR THIS PERIOD. BROAD UPPER LEVEL TROUGH STILL FORECAST TO MOVE INTO THE UPPER MIDWEST TUESDAY/WEDNESDAY TIMEFRAME WITH A COLD FRONT PLODDING ACROSS IOWA TUESDAY NIGHT THROUGH WEDNESDAY. THE BEST CAPE WITH THIS FEATURE PASSES ACROSS CENTRAL IOWA LATE TUESDAY/TUESDAY NIGHT BUT THE BETTER SHEAR COMES WITH THE FROPA WEDNESDAY. THINGS STILL NOT COMING TOGETHER ENOUGH TO RAISE CONFIDENCE ON SEVERE POTENTIAL BUT CONFIDENCE IS HIGH THAT A PRECIP EVENT WILL OCCUR. AS A RESULT AND IN ACCORDANCE WITH NEIGHBORS...POPS WERE BUMPED UP TO HIGH CHANCE/LIKELY CATEGORY. THE SYSTEM IS QUITE PROGRESSIVE SO POPS WERE PULLED FROM WED NIGHT AND THURSDAY. MODEL DIFFERENCES ON A SYSTEM FOR NEXT WEEKEND BUT GENERAL AREA OF CONCERN IS NORTHERN IOWA SO SLIGHT CHANCE POPS WERE INTRODUCED FOR FRIDAY/SATURDAY TIMEFRAME WITH THE BETTER BET BEING SATURDAY. TIMING ISSUES WILL BE BETTER RESOLVED LATER. && .AVIATION...10/06Z LINGERING CLOUDS AND -SHRA WILL BE OUT OF THE PIX TO OUR NORTHEAST TNGG WITH LGT WINDS AND VFR CONDS OVR NGT. WK UPR LVL IMPL WILL MOVE SEWD OUT OF THE RCKYS AND BRUSH THE SWRN PART OF THE STATE ON SUN. THERE IS AN ISOLD CHC FOR A TSRA IN THE KDSM AND KOTM AREAS DURG THE LATE AFT HRS UNTIL SNST...CERTAINLY CIRCUMNAVIGABLE THOUGH. NRN TAF SITES WILL BE PCPN FREE WITH ONLY HI CLDS. EXPECT WNDS TO BE NWLY 10 TO 15 KTS...NOT GUSTY LIKE ON SAT. VFR CONDS WITH LGT NWLY WNDS GOING INTO SUN NGT. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM...AVIATION...MS MAY 09 LONG TERM...FAB
