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Breaking: Flood forecasts lowered again!

Posted at 7:39 PM on March 24, 2011 by Paul Huttner (4 Comments)
Filed under: Crow River, Flooding, Minnesota River, Mississippi River, St. Croix River, Winter/spring 2011

Here's some potentially good news in a sea of bad weather this week.

Most river level forecasts have been revised lower in the next week.

The latest updates from the hydrologists at the NCRFC in Chanhassen have lowered river level forecasts for some river locations as much as 3.5 feet from forecasts earlier this week.

The changes are largely due to the latest round of cold weather this week. The cold "locks up" moisture by freezing it in place on the landscape, instead of creating the runoff that would happen with above freezing temps.

You can see all the latest river data here, but here are a few select hydrographs for some key river points in southern Minnesota.

Keep in mind that specific forecast river crest "numbers" and timing are best estimates within a wider overall forecast range. Forecasts will likey be modified in the coming days as new data is fed into hydrologic models.

Mississippi @ St. Paul: Forecast rise to 18.9' next week.
(Down 3.5 feet from earlier forecast!)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5th.MSSSTP@.png

Crow River @ Delano: Forecast crest at 18.8' Saturday night.
(Down 1.7 feet from earlier forecast) Forecast to fall below 17 feet again next week!

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5t crowsky.png

Minnesota River @ Mankato: Forecast crest at 25' Saturday.
(Down 2 feet from forecast earlier this week.)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5th.MNMKTTO.png

Minnesota @ Henderson: Forecast crest at 737.4' Sunday.
(Down 1.1 feet from forecast earlier this week)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5th.MN HENDU.png

Minnesota @ Jordan: Forecast crest at 32' Monday & Tuesday.
(Down 1 foot from earlier forecast)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5t MN JORRDY.png

Minnesota @ Shakopee: Forecast crest 715.8' Tuesday & Wednesday.
(Down 1.2 feet from earlier forecasts)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5t savage.png

Minnesota @ Savage: Forecast crest at 713.4 Thursday.
(Down from 0.6' from earlier forecasts)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5t MNreally savage.png

St. Criox @ Stillwater: Forecast rise to 87.1' Thursday.
(Down 0.7' from earlier forecasts)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5t MN STILLY.png

Let's hope these latest forecasts reflect an overall trend that keeps floodwaters a little more in check the next few days.

Next snow Tuesday?

I know, I know. I hate to even mention it. But after what looks like a few much needed dry days through the weekend, it looks like our next chance of snow could be on the horizon.

Remember models can and often do change dramatically 5 days out.

The GFS insists on brining another potentially potent low pressure system into the Upper Midwest by next Monday night and Tuesday. Take a deep breath and look at the meteogram below.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 14 1 earl 1 5t gfssky.PNG
(Click to enlarge...if you dare)

Yep. That's 8+ inches of snow on the chart for the 12z morning run, but less from the 18z afternoon run. Remember, we don't issue snowfall forecasts 5 days in advance and there is some indication that system could steer south. Also the GFS has grossly overforecast snow totals for the last few events. And the European model seems to suggest the southward track which could leave Minnesota high and (thankfully) dry next week.

But be aware that there is at least the possibility of significant snow next Tuesday.

Hang in there; it still looks warmer by next weekend!

PH

Comments (4)

Hi Paul,

Could you explain the numbers on the y-axis on the graphs? Why are the Minnesota River stages in the 700s (except for at Jordan and Mankato) and much lower for the Mississippi and St. Croix?

Thanks!

Posted by Nikki | March 25, 2011 9:33 AM

Paul,

I have a problem. I have been listening to a lot of MPR lately and the anchors and Newscasters for both MPR and NPR are using the term 100-year storm or 100-year flood all wrong. They have even asked people why they are happening so frequently. From my Hydrology classes in college I learned that the terms 100-year storm simply means that in any given year you have a 1 in a 100 (or 1%) chance of having a storm of that size. They same would hold true for 50 & 500-year events (2% and 0.2% respectively). If I am correct, would please correct the Morning edition people. It's been bugging me all season.

Thanks.

Posted by Chris | March 25, 2011 12:11 PM

Paul -

What analysis do you have for those of us in the Red River Valley? Our forecast was updated and doesn't look quite as lovely. We'd all love to hear your take on things.

Posted by Melissa | March 25, 2011 12:31 PM

"Could you explain the numbers on the y-axis on the graphs?"

As you probably suspected, it's the height of the surface of the river above a particular benchmark height. What's confusing is that different river gaging stations use different benchmark (datum) levels. You will see a reference to the datum for each location at the bottom of each graph.

Gage Datum
A horizontal surface used as a zero point for measurement of stage or gage height. This surface usually is located slightly below the lowest point of the stream bottom such that the gage height is usually slightly greater than the maximum depth of water. Because the gage datum is not an actual physical object, the datum is usually defined by specifying the elevations of permanent reference marks such as bridge abutments and survey monuments, and the gage is set to agree with the reference marks. Gage datum is a local datum that is maintained indepen­dently of any national geodetic datum. However, if the elevation of the gage datum relative to the national datum (North American Vertical Datum of 1988 or National Geodetic Vertical Datum of 1929) has been determined, then the gage readings can be converted to elevations above the national datum by adding the elevation of the gage datum to the gage reading.
Source: www.weather.gov/glossary/index.php?letter=g

Posted by Paul S | March 25, 2011 4:51 PM

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