Happy Meteorological Spring: Monday snow then 40F next week? Why “Concrete Frost” boosts flood risk

Happy Meteorological Spring

It's over!

Meteorological winter (and many Minnesotans least favorite month of February) ended at midnight.

Hellooooo March.

While it doesn't look like we'll rush headlong into spring this year, it is likely that a month from today there's a decent chance we'll see scenes like this popping up in southern Minnesota.

Image: Paul Huttner -MPR News

The Minnesota Twins hope so...it's the earliest home opener since moving outdoors to Target Field...just 1 month away on April 1st this year.

As the final data trickles in Friday for the winter that was, it appears we ended up about +1F vs. average in the temperature department overall this winter. That's a good -6F vs. last winter in most of Minnesota... but in reality much closer to what passes for "average" these days.

We're still counting up the inches on snowfall, and I have little doubt we'll add more inches (maybe as soon as Monday) to the 35.5" in the metro we've shoveled and played in so far.

In this edition of Updraft we look ahead to spring and we introduce a shiny new weather term..."concrete frost."

How does it increase or risk for spring flooding in parts of Minnesota? Flood and drought at the same time? Only in Minnesota.

Plus, signs of a spring like thaw are already in sight. How warm will it get next week? How much snow will be left by next Friday?

Things to ponder as we launch into "Meteorological Spring 2013."

February snow boosts flood risk:

It seems counterintuitive, but Minnesota is likely to have both flood...and drought this spring.

Say what?

Here's the deal.

Our February snow blitz has laid down 2" to 5" of water content in the snowpack over Minnesota. The highest concentration is in western and north-central Minnesota watersheds that feed into the Red, Minnesota and Mississippi Rivers.

Image: Twin Cities NWS/Hydrology

As that snow melts in the coming weeks that water will runoff into the rivers. Add any heavy spring rainfall...or a rapid warm up and you increase the chances for spring flooding.

Now we can throw in an extra "incentive" for flooding in southern Minnesota this spring.

"Concrete Frost."

Remember our wet rainy systems in December followed by a rapid "flash freeze?" That water was basically frozen in place from the metro south to the Iowa border. The hard...impervious layer of ice on top of the ground under the snow cover acts like "concrete kryptonite" to spring rains.

Image: Twin Cities NWS/Hydrology

Any rainfall that hits the "concrete frost" will quickly runoff into rivers. That increases the risk for flooding...and an inch or two of spring rain may cause flooding that parched...thawed soils would normally gulp down.

Until the concrete frost thaws...the dice are loaded in favor of flooding from heavy spring rainfall...and drought depleted soils are unable to soak up the much needed rainfall.

Flood and drought.

Twin Cities NWS Hydrologist Dianne Cooper has put together an excellent video describing the concept of "concrete frost."

The frost and snowpack...and any additional snow and spring rain has boosted flood forecasts to "normal" in much of Minnesota and "high" along the Red River near Fargo and Wahpeton.

Some details from the Twin Cities NWS & North Central River Forecast Center in Chanhassen.

Here's a closer look at where things stand on the Minnesota River at Montevideo.

Forecast: Cool & quiet weekend & big thaw next week?

March comes in like a "chilly lamb" this year.

My MPR colleague and UM Climate expert Dr. Mark Seeley has an interesting look at the numbers behind the old saying "In like a lion, out like a lamb" a preview of this week's "Weather Talk."

Topic: The Lion and the Lamb Climatology of March for MSP

March months which have come in "like a lion and out like a lamb" or in "like a lamb and out like a lion" are remembered for both their storminess and temperature deviation.

Standard deviations in daily maximum and daily minimum temperature are generally in the 10 to 11 degree F range during March for the Twin Cities. Occasionally daily temperature deviations exceed one standard deviation during the first and last weeks of the month.

Using temperature records for the first and last week of March from the Twin Cities (1900-2012) and looking for opposite temperature patterns based on approximately one standard deviation statistically (plus or minus 11 degrees F from normal), the following characteristic years fell out......

"In like a lamb/out like a lion" "In like a lion/out like a lamb"

(March starts mild, finishes cold) (March starts cold, finishes mild)

1921 1920

1923 1925

1924 1943

1934 1945

1937 1955

1964 1978

1966 1989

......... 1995

......... 2005

Temperature records confirm these years, 16 in all, fell into one category or the other. But 16 out of 113 years is only 14 percent of the time when this old saying has been true, at least based on MSP temperature standard deviations for March. Looks like the March will begin with near normal temperatures.

Mark also highlights how a wet February shaved a few points off drought percentages in Minnesota.

Precipitation was generally abundant during the month of February. It was the wettest February statewide since 2007. Many observers reported over 2 inches of precipitation, most of which came as snowfall.

For some the moisture was record-setting for the month, including 2.45 inches at Breckenridge, 2.49 inches at Benson, and 2.57 inches at Rothsay. For most areas of the state February brought the most snowfall for the winter as well. Itasca State Park, Breckenridge, Bemidji, Pelican Rapids, Wheaton, and Hermantown reported over 20 inches for the month, while Ottertail, Rothsay, and Battle Lake reported over 25 inches. Some of these values were record-setting for the month.

For the most part the moisture was welcome. As we concluded the month the US Drought Monitor reduced the total area of the state designated to be in severe to extreme drought from 84 percent to less than 70 percent of the state landscape.

March Lamb... with Monday snow??

As we search for warmer days I see (bright) a ray of hope next week. I also see some potential snow between now and then.

The models have been hinting at a potential snow system for Monday,The latest model trends suggest this system may clip the metro and southern MN with snowfall Monday.

Image: NOAA via Iowa State University

Milder Pacific breezes kick in by next Wednesday & Thursday. By then most of our remaining 5" snow cover in and near the metro should be fading fast. That should allow us to warm up into the 40s if everything goes right next Thursday and Friday.

Image: NOAA GFS via Iowa State University

Weather fingers and toes crossed!

PH