Spring Floods: Why the dice are loaded

It's no surprise that the NWS came out with a potentially dire preliminary flood forecast today. All the ingredients are already in place for major spring flooding on virtually all rivers in the Upper Midwest.

The NWS is right to say if you live anywhere near a river or stream and are at risk of being flooded, buy flood insurance today! It takes 30 days for flood insurance policies to kick in.

Here are the updated preliminary spring flood outlooks from NWS.

-Twin Cities (Mississippi, Minnesota & Crow Rivers)

-Red River Valley

Here's the headline form Grand Forks NWS:

"For the Southern Red River Basin: Current spring flood outlooks indicate that much of the southern basin, including the Fargo-Moorhead area, already has a higher risk of spring flood levels than were seen in advance of the 2006 or 2010 major flood events. There is a 20 to 25 percent chance that areas near Fargo-Moorhead could see flood levels approach the record levels set back in the early spring of 2009. Continued much above normal snowfall through the remainder of the winter will likely continue to drive that risk slightly upward in later outlooks."

Here are three main reasons why major flooding is highly likely along Minnesota's rivers this spring.

1) A wet fall:

You may recall the unprecedented record floods in September that sent some rivers to all time record highs. Late summer and autumn rainfall was several inches above average in much of the region. That means rivers, streams, lakes and soils were already at capacity going into the freeze.

In fact 2010 was the 2nd wettest on record in Minnesota history, according to the Minnesota Climatology Working Group.

Top Ten Annual Mean Precipiation Records for Minnesota

1895-2010

Rank Value Year

---------------

1 33.92 1977

2 33.64 2010 *

3 33.27 1965

4 33.22 1968

5 32.54 1991

6 32.32 2005

7 32.31 1905

8 31.68 1986

9 31.64 1993

10 31.57 1903

Snow melt this spring will send already brimming rivers quickly higher.

2) Heavy winter snowfall:

One look outside the window tells you all you need to know about Minnesota's snow pack. Our series of massive winter storms have blanketed the Upper Midwest with heavy snow cover.

Snow depth exceeds 16" in much of the eastern Dakotas and western Minnesota.

The water content (snow water equivalent-SWE) in that snow pack ranges from 3" to 8" in much of the region.

Water content in snow pack (snow water equivalent) shows 4" to 8" in Minnesota & Red River Valley watersheds.

That's like 6" rain storm waiting to be unloaded into area rivers once melting begins in the spring. The damage is already done; so to speak...that snow isn't going away before spring.

3) La Nina:

The deck is already stacked in favor of major spring floods. La Nina may be the "wild card."

Our strong (but fading?) La Nina episode has delivered on the statistically favored colder than average winter in the Upper Midwest. There is also a bias toward cool wet springs in la Nina years. A wet spring could add more fuel to the fire in the spring flood scenario.

If heavy spring rains occur, the rapid snowmelt will send rivers into shock flood mode.

What to watch for:

The weather in the next 8 weeks is critical, and will determine how severe spring flooding will be.

Worst case scenario:

-Heavy late winter snows with high water content. The "Panhandle Hookers" that wind up in the Texas-Oklahoma Panhandle region bring wet heavy snows. We've been lucky so far that while many of our storms have produced heavy snowfall totals, the snow:water ratio has been relatively dry...often around 15:1. A few more wet storms will add a lot of water to already charged snow pack.

-Late thaw...rapid spring warm up. If the snow continues to accumulate, then we get a rapid period of warm weather in March that will send torrents of water into area rivers in a shot period of time.

-Heavy spring rains. A big slow moving spring storm with heavy rains would also send a "shock wave" of high water into area rivers. This is the nightmare scenario that will keep flood forecasters and river residents up at night over the next two months.

Best case scenario:

There are several factors that could mitigate flooding this spring.

-Below average snowfall through March

Our 55.4" of snowfall this winter has put the Twin Cities on pace for a top 10 snowfall season. If we get average snowfall for the rest of the winter season (about 22") that would put the metro at about 77" for the season...and vault us into top 10 territory and put us near the 5 snowiest winter threshold of 81.3".

If we are somehow able to continue the current "snow drought" into the spring and end up below that 22" average...that would help...some.

-Gradual thaw?

If we see a nice slow warm up this spring, that would help discharge snow melt into area rivers a little at a time. Ideal scenario? Days above freezing and night below freezing to generate a nice slow "controlled" discharge of snow melt.

-Little spring rain

Another factor that could mitigate spring flooding would be a dry spring. Unfortunately there is a bias toward wetter than average springs in La Nina years. Anything can happen though. A nice dry March and early April would help...a lot.

Stay tuned!

PH