Here is the hydrology portion of this morning’s Area Forecast Discussion from the NWS in Des Moines

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.