Special Updraft Alert: Flash flood event possible Thursday

Thursday has all the earmarks of a potentially damaging and life-threatening flash flood event for parts of southern Minnesota.

After a beautiful day today, the remnants of Hurricane Blanca are streaming northeast through the Rockies toward Minnesota. All indications support a potential flash flood event across southern Minnesota on Thursday.

The only question seems to be precisely where the heaviest rainfall bands will set up. Not going for the dramatic here, but multiple model runs have cranked out anywhere from 3- to 6-inch rainfall totals for southern Minnesota.

If we see that much rain, flash flood watches and warnings are likely somewhere in southern Minnesota and northern Iowa Thursday.

All residents of southern Minnesota should take time to review flash flood safety for Thursday.

Event synopsis

Remnants of Tropical Storm Blanca are streaming northeast today across the Rockies toward Minnesota. The tropically generated moisture plume is clearly visible over Colorado and Utah on the GOES infrared satellite loop.

NOAA

On June 8, Blanca became the earliest tropical cyclone on record to make landfall on Mexico's Baja Peninsula. The system produced rare, early June rain in Arizona and near-record dew point levels.

Moisture from Blanca will be sucked into developing low pressure tracking northeast toward the Upper Midwest Thursday. The likely result will be heavy rainfall totals across southern Minnesota and northern Iowa.

NOAA

The core of the heaviest rains could generate 1- to 3-inch rainfalls per hour, according to Wednesday morning's Twin Cities National Weather Service forecast discussion.

The core of uncertainty at this point centers around the eventual location of the heaviest rainfall band. Will it set up over the Twin Cities, or further south in southern Minnesota and northern Iowa?

The Twin Cities NWS elaborates in this morning's forecast discussion.

THE NMM HAS THIS CONVECTION ACROSS SOUTHERN MN...WHICH WOULD GIVEN THE TROPICAL AIRMASS WOULD LIKELY RESULT 2-3 IN/HR RAINFALL RATES ON ALREADY SATURATED SOILS AND LEAD TO FLOODING. HOWEVER...THE ARW KEEPS THE MAJORITY OF THE CONVECTION ACROSS IOWA. THE HOPWRF TS PRIMARILY FOCUSES THE CONVECTION ALONG THE EAST/WEST ORIENTED WARM FRONT. WHAT THAT MEANS IS THERE UNCERTAINTY IN THE PLACEMENT OF THE HEAVIEST PRECIP...AND FOR THAT REASON NO FLOOD WATCHES WERE ISSUED. THE LAST 2 RUNS OF THE HOPWRF-TS HAVE 6-8IN TOTALS. GIVEN THE ENVIRONMENT...IT SEEMS VERY REASONABLE THAT A FEW LOCATIONS WILL MEASURE OVER A HALF A FOOT OF RAIN. THE QUESTION IS WHERE. THE RUN INITIATED OFF THE GFS 09.18 WAS DIRECTLY ACROSS THE METRO...WHILE THE 10.00 RUN HAS SHIFTED THE PRECIP AXIS SOUTH FROM SIOUX FALLS THROUGH MANKATO...TO EAU CLAIRE. THIS MOST RECENT RUN SEEMS MOST REASONABLE GIVEN THE BALANCE BETWEEN FORCING AND INSTABILITY. ALSO...THE CIPS ANALOGS SHOW THE HEAVIEST PRECIP ALONG/SOUTH OF I- 90...WHICH MEANS CLIMATOLOGY ALSO SUPPORTS THE SOUTHERN SOLUTION.

Rainfall potential

Here's a look at various solutions to the rainfall set up Thursday.

The Twin Cities NWS grids favor 1 inch to 3 inches from north to south across the metro, with heaviest totals of 3 inches or more in southern Minnesota.

The National Oceanic and Atmospheric Administration's latest rainfall maps agree with the southern solution for the heaviest rains, with some totals of 4 inches or more in northern Iowa and topographically flood-prone southeast Minnesota.

NOAA

Specific model solutions for the Twin Cities generate some incredible rainfall totals of 3 inches to 4 inches or more for the Twin Cities. If the southern solution wins, these numbers may fall dramatically later today in subsequent model runs.

The models giveth. The models taketh away.

We dry out Friday and Saturday. The longer-range forecast favors rainfall spaced every two to three days. In other words, typical June.

NOAA

Have a plan for potentially heavy rains Thursday, especially in southern Minnesota, northern Iowa and Wisconsin communities, including La Crosse.