FXUS63 KDMX 111144 AFDDMX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 544 AM CST THU MAR 11 2010 .UPDATE AVIATION DISCUSSION 11/12Z... .SHORT TERM /TODAY/... LOW OVER EASTERN KS/WESTERN MO IS PULLING NORTH WITH TIME THIS MORNING AND OCCLUDING AS IT DOES. MAIN AREA OF STRONG THETAE ADVECTION IS LIFTING NORTH ALONG THE UPPER LEVEL BOUNDARY WITH PRONOUNCED DEFORMATION AXIS FORMING OVER THE WESTERN SECTIONS OF THE FORECAST AREA NOW. CHALLENGES REMAIN WITH REGARDS TO TEMPS TODAY. WARM SECTOR IS MAKING IT INTO THE EASTERN AND NORTHERN SECTIONS TODAY WITH OCCLUSION MORE THAN LIKELY KEEPING WEST AND NORTHERN SECTIONS COOLER. HAVE RAISED TEMPS EAST INTO THE LOWER 50S FOR THE AFTERNOON HOURS. DRY SLOT ALSO APPROACHING SOUTH AND PRECIP HAS BEGUN TO CUT OFF THERE. WILL EXPECT TO SEE DEF AXIS GRADUALLY LIFT NORTHWEST WITH TIME AS THE LOW BACKS NW AS WELL. MOST OF THE AREA WILL DRY OUT. NARROW RIBBON OF INSTABILITY IS PROGGED TO DRIFT OVER EASTERN AND NORTHERN SECTIONS LATE THIS MORNING AND EARLY AFTERNOON. LEFT CHANCE FOR ISO THUNDER IN THIS AREA...THOUGH PROGS SHOW THAT CAPE IS WEAKENING WITH TIME SO AM NOT EXPECTING ACTIVITY SOUTH OF US NOW TO REMAIN SUSTAINED ACROSS THE AREA TODAY. QPF AMOUNTS OF ONE HALF TO TWO THIRDS OF AN INCH WEST/NORTHWEST WILL ADD TO RIVER FLOWS OVER THE NEXT SEVERAL DAYS. CURRENT HYDRO WATCH REMAINS ACTIVE AND WILL MONITOR FOR POTENTIAL ICE JAMS. FOR THE MOMENT...SKUNK RIVER NEAR AMES HAS SETTLED DOWN. WILL MAINTAIN FLASH FLOOD WATCH THROUGH THIS EVENING...STILL POSSIBLE THAT ANY INCREASE IN ICE ACTION COULD CAUSE ADDITIONAL THREAT THROUGH MIDNIGHT TNT. BY LATE AFTERNOON H850 TEMPS WILL DROP TO BELOW ZERO WEST AND CENTRAL. THOUGH SOME -SN MAY MIX IN WITH PRECIP ALOFT...FEELING IS THAT PRECIP WILL BE LIGHT ENOUGH AND THE LOWER LAYERS WILL BE TOO WARM TO SUPPORT ANY EXTENDED PERIOD OF LIGHT SNOW MIXTURE. .LONG TERM /TONIGHT THROUGH WEDNESDAY/... UPPER LOW WILL SPIN ALONG THE MISSOURI RIVER VALLEY ALONG THE NEBRASKA/IOWA BORDER TONIGHT WITH MUCH OF THE CWA IN SUBSIDENCE AND DRY SLOT. HAVE CUT BACK ON POPS OVERNIGHT WITH ONLY SCHC IN THE SOUTHEAST WITH HIGH CHC POPS IN THE FAR WEST NEAR THE THE DEEPER SATURATION ON BACKSIDE OF DRY SLOT WHERE DEFORMATION AREA WILL BE. SOME CONCERN AS TO PCPN TYPE AS WELL WITH COOLER AIR WRAPPING AROUND THE WESTERN AND SOUTHERN SIDES OF THE UPPER LOW. HAVE GENERALLY LEFT JUST RAIN GIVEN RELATIVELY LIGHT PCPN AND TEMPERATURES HOLDING AT OR SLIGHTLY ABOVE FREEZING OVERNIGHT INTO FRIDAY MORNING. THE UPPER LOW WILL SLIDE SOUTHWARD ON FRIDAY WITH MAIN DEFORMATION AREA STILL NORTH AND WEST OF THE CWA AND GRADUALLY WEAKENING AS WELL. HAVE ALSO LOWERED POPS DURING THIS PERIOD WITH FORCING REMAINING LIMITED AND DEEPER SATURATION NEARLY NON-EXISTENT. HOWEVER...BY FRIDAY NIGHT INTO SATURDAY...A SHORTWAVE WILL ROTATE FROM THE SOUTHWEST SIDE OF THE CUTOFF VORTEX ALL AWAY AROUND TO THE NORTHERN SIDE BY SATURDAY. THIS WILL BRING A SLUG OF MOISTURE AROUND THE UPPER LOW WITH AN INCREASING THREAT OF RAIN LATE FRIDAY NIGHT INTO SATURDAY COMING IN FROM THE EAST. TEMPERATURES PROFILES WILL ACTUALLY WARM DURING THIS TIME AS ANY REMAINING COLD AIR GETS ASSIMILATED INTO THE SYSTEM. THE SYSTEM WILL BEGIN TO GRADUALLY PULL TO THE EAST ON SUNDAY INTO EARLY NEXT WEEK WITH THE THREAT OF PCPN COMING TO AN END. HOWEVER...WE MAY SEE CLOUDINESS LINGERING AS LOW LEVELS REMAIN FAIRLY MOIST IN THE CYCLONIC CURVATURE ON BACKSIDE OF THE TROF. LITTLE CHANGE TO FORECAST INTO EARLY NEXT WEEK AS UPPER RIDGING BUILDS INTO THE REGION WITH SOME SLOW MODERATION IN TEMPERATURES. && .AVIATION... 11/06Z...SURFACE LOW NOW DRIFTING INTO SOUTHERN IOWA. DRY SLOT HAS MOVED INTO SOUTH CENTRAL THOUGH CIGS ARE NOW VLIFR THERE. STILL SOME INSTABILITY ACROSS EAST AND NORTH EARLY TODAY THROUGH THE LATE MORNING HOURS. VSBYS AND CIGS TO REMAIN IN THE IFR/VLIFR SOUTH/WEST WITH SOME MVFR CIGS NORTHEAST. AS THE LOW DRIFTS BACK TO THE NORTHWEST...SOME IMPROVEMENT IN CIGS EXPECTED AFT 17Z AS WARMER TEMPS AND BOUNDARY LAYER INCREASES IN HEIGHT THIS AFTERNOON./REV && .HYDROLOGY... WIDESPREAD FLOOD EVENT...DRIVEN BY COMBINATION OF RAPID SNOW MELT AS WELL AS OBSERVED AND FORECAST RAINFALL...IS ALREADY UNDERWAY OR WILL START WITHIN THE NEXT FEW DAYS...IN MOST LOCATIONS. WE/RE NOT LOOKING AT ANOTHER 2008 EVENT HOWEVER. THE FLOODING IS STARTING MAINLY FROM SOUTH TO NORTH. BASINS OF MOST CONCERN ARE FIRST THE DES MOINES/RACCOON...FOLLOWED BY THE CEDAR/IOWA THEN SKUNK. MODERATE TO MAJOR FLOODING EXPECTED AT MANY LOCATIONS IN THE DES MOINES/RACCOON RIVER BASINS. AS MANY PEOPLE HAVE NOTICED...WE ISSUED OUR RIVER FLOOD WARNINGS MUCH FARTHER IN ADVANCE OF THE FLOODING THAN IS TYPICAL. THIS WAS DUE TO OUR HIGH CONFIDENCE IN THE FLOODING TO OCCUR. /NWS POLICY FOR RIVER FLOOD WARNINGS REQUIRES AT LEAST AN 80 PERCENT CONFIDENCE LEVEL IN RIVER FLOODING TO OCCUR. OUR CONFIDENCE IN THE FLOODING TO OCCUR WAS IN EXCESS OF 90 PERCENT...AND NEAR 100 PERCENT IN MOST LOCATIONS./ SEVERAL DAYS AGO OUR RIVER MODELS WERE SHOWING SIGNIFICANT RISES ON MANY RIVERS OWING SOLELY TO SNOW MELT. EARLY THIS WEEK OUR CONFIDENCE BECAME HIGH IN MODERATE TO HEAVY RAINFALL AS WELL...SO WE ELECTED TO GET THE WORD OUT AS EARLY AS POSSIBLE. ICE JAMS HAVE BEEN PROBLEMATIC SINCE LAST NIGHT...FIRST STARTING ON THE LOWER RACCOON RIVER AND HAS SINCE EXPANDED TO SEVERAL CREEKS AND SMALL STREAMS MAINLY FROM US HWY 30 SOUTHWARD. AREA OF PRIMARY CONCERN NOW IS SQUAW CREEK IN THE AMES AREA. THE CITY OF AMES ADVISES THAT THEIR CREWS HAVE BEEN WORKING...WITH LIMITED SUCCESS...TO MITIGATE THE ICE JAM THERE. THEY ARE CONCERNED ABOUT FLASH FLOODING POTENTIAL TONIGHT INTO TOMORROW...THUS WE ISSUED A FLASH FLOOD WATCH FOR THAT AREA. ICE JAM CONCERNS WILL SHIFT NORTHWARD WITH TIME...OVER THE NEXT FEW DAYS. AFTER THE AMES AREA...ANOTHER PLACE OF CONCERN WILL BE ON THE DES MOINES RIVER IN THE FORT DODGE AREA. ICE JAMS THERE HISTORICALLY HAVE BEEN PROBLEMATIC...AND TEND TO OCCUR WITHIN A FEW DAYS OF ICE JAMS OCCURRING ON THE LOWER RACCOON RIVER IN THE DES MOINES AREA. RIVER FORECASTS DO INCLUDE FCST PRECIP THRU SATURDAY MORNING. PER METEOROLOGICAL DISCUSSION ABOVE...WE MAY SEE ADDITIONAL SEASONALLY MODERATE RAINFALL THIS WEEKEND. OUR PRESENT THINKING IS THAT THIS ADDITIONAL RAINFALL WOULD EITHER RESULT IN CRESTS BEING REACHED MORE QUICKLY...OR THE RIVERS FALLING MORE SLOWLY ONCE THEY DO CREST. WE DO NOT EXPECT THE RAIN TO ADD SIGNIFICANTLY TO THE CRESTS LIKE WHAT HAPPENED IN 2008 IN SOME LOCATIONS. WE WILL HAVE TO CONTINUE MONITORING THIS SITUATION. && .DMX WATCHES/WARNINGS/ADVISORIES... FLASH FLOOD WATCH THROUGH THIS EVENING FOR STORY COUNTY. && $$ SHORT TERM...REV LONG TERM...COGIL AVIATION...REV HYDROLOGY...ZOGG