Flood watch silver lining: Slow thaw easing snow melt runoff, so far

Flood risk still moderate to high

Temperature for the Twin Cities
Observed temperature for the Twin Cities in March 2023
Twin Cities National Weather Service office

Yes, it’s been a cold and snowy March. Yes, it seems like winter is dragging on forever this year. But our reluctant spring has a silver lining.

The frequent freeze-thaw cycles mean the 2 to 7 inches of water that were trapped in the snow cover are releasing more slowly into area rivers, and that may be slightly easing our significant flood risk this spring.

Freeze-thaw cycles this month

This March in Minnesota has featured nearly continuous freeze-thaw cycles. You can see on the graphic at the top of this post how almost every day has recorded daily temperatures cycling above and below freezing in the Twin Cities.

We’ve avoided rapid temperature rises into the 50s, 60s, or even 70s that often occur in March. That’s a good thing when it comes to melting the snow cover nice and slowly. A slower melt means a slower runoff profile into our rivers, which can help mitigate flood risk.

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You can clearly see the effect on the daily water levels on Minnehaha Creek in the Twin Cities west metro over the past week. See how water levels rise and then fall each day as the temperature changes?

Water levels in Minnehaha Creek
Water levels in Minnehaha Creek
Minnehaha Creek Watershed District

The higher rise on the chart is driven by rainfall on March 21 and 22.

You can also see the water temperature reflects the daily diurnal temperature change — the temperature difference between the minimum at night (low) and the maximum during the day (high) — over the past week. See how they rise during the day with the air temperature, then fall to around 32 degrees at night as the air temperature falls below freezing?

Water temperatures in Minnehaha Creek
Water temperatures in Minnehaha Creek
Minnehaha Creek Watershed District

So our constant freeze-thaw cycles slow down the runoff rate as the snow melts more gradually into our river systems. If we had a big warmup and more rainfall, you would see huge and prolonged runoff spikes as snowmelt rapidly rushes into the rivers.

Flood risk still high

The National Oceanic and Atmospheric Administration’s latest overall flood risk is still much higher than average in our region. The latest flood report updated Monday reflects current and forecast weather conditions.

NOAA hydrologists are also seeing the potential benefits of our slower runoff profile in area rivers. But the risk of major flooding still lurks, especially if we get a quick warmup with additional rainfall.

There is some good news! The 7-10 day temperature and precipitation pattern is favorable for a slow melting period through the end of March (details later in the briefing). Though it almost may be too slow, which makes us more vulnerable to a fast April warm-up or heavy rain event.

As always, the threat of seeing major flooding will still depend on what kind of rainfall/temperature patterns we get as we move into April.

The threat of moderate to major flooding remains high on some area rivers.

There’s still a good 2 to 7 inches of snow water equivalent in the snow cover across Minnesota and Wisconsin.

Snow water equivalent
Snow water equivalent

Snow cover in our region is still in the 90th percentile historically for this date.

Snow water rank
Snow water equivalent rank

Here’s the latest outlook for the Mississippi River at St. Paul.

Mississippi River at St. Paul (STPM5)

A slight decrease in the numbers from two weeks ago, but still quite high:

Minor: 83% vs. 26% normal

Moderate: 71% vs. 20% normal

Major: 47% vs. 13% normal

And here’s the outlook for the St. Croix River at Stillwater.

St. Croix River at Stillwater (STLM5)

The probability of seeing Major Flood Stage now tops 80%:

Minor: >95% vs. 25% normal

Moderate: 92% vs. 19% normal

Major: 81% vs. 11% normal

Cool wet pattern continues

The overall weather pattern still favors cooler and wetter than average weather over the next two weeks.

NOAA 8 to 14-day temperature outlook
8 to 14-day temperature outlook
NOAA 8 to 14-day precipitation outlook
8 to 14-day precipitation outlook

So while we’re not out of the woods yet on flood risk, our reluctantly cool spring has been a benefit.

So far.

Stay tuned.