April 29, 2022: Sean’s Graphs

I know people have been waiting for Sean Cockrem’s graphs. Now that we’ve finally seen temperatures shift to mostly above freezing, he was able to set yesterday as Inflection Date, and start work on predicting how long the thaw might take.

Sean writes:

At long last Iast, I’m ready to declare [April 27th] as the 2022 inflection date. Usually I wait a few days after to make sure that the measured daily means are above zero, but based on how far into April we are and with the 14 day having consistent warm temperatures, I’m going to make the executive decision for this declaration.

This was the third coldest winter so far this millenium. Only the 2013/14 and 2008/09 winters were colder. This was also the latest inflection date we have had in the last 20 years of data collection.

For the forecast, I have set our target thawing index at 190 which is based on an average 1:10 thawing index:freezing index. I’ve entered the forecast data for the next month or so. Keep in mind everything after a week is a guess, and everything after two weeks is based on seasonal temperatures. This year has been anything but seasonal. Having said all that, there is a glimmer of hope that we will be ‘Ice Out’ right around May 20th, which is the Friday of the long weekend. If the temperatures stay below seasonal, it’s going to be a long shot for complete ice out, which is not to say that a good portion of the lake will be open by then.

As far as the season goes, this one will be down to the wire for the next 3 weeks closing in on the long weekend. A bit more interesting on our end than other winters where the ice is off in late April. Fingers crossed we get some sunny warm days.


I recommend clicking on these graphs to see a larger version. Then you should click on that to see the graph enlarged to it’s full resolution.

Let’s start with the updated shark-fin graph.

The Shark-fin graph plots the severity of each winter and marks the beginning and end of each thaw.

The key points to keep in mind:

The downward spikes plot the temperature trend of the winter. A long winter makes a wider spike, and a severe winter makes a deeper spike.

The thaw commences when the Mean Daily Temperature rises above freezing (on a lasting basis) and that date is marked with a blue X. This is the Inflection Date.

The thaw ends when the lake is entirely ice free. That date–when we reach it–is marked with a red dot.

What can we tell about the winter we just had? It was the second coldest on the graph, and as Sean said, this is the latest Inflection Date we’ve seen since we started keeping records. (2003 was the year I started taking pictures, so Sean hunted down the weather data back to that year. Before that, we don’t know when the ice was gone.)

Summary: It was a long cold winter, so we assume thick ice. It was a late start to the thaw, so that puts us behind.

On to the next graph.

The Cumulative Freezing Index compares the temperature path of each winter.

This graph takes some of the same data, but focuses strictly on the winter. Each winter’s Cumulative Freezing Index is shown as a separate line that ends on the Inflection Date, and this past winter is represented by the heavy dashed line.

Key points:

The years are synchronized to the calendar on this graph, so some years have already become colder than others at the starting line of December 1st.

We had a mild fall, so we got off to a good start.

This years line drops lower than most, because we had a lot of cold weather.

It extends further to the right than any other line because it took so long to get to Inflection.

Now comes the speculative graph: the Forecast.

This one takes some explaining. In the first two graphs, we gave the winter a “score”.

The Cumulative Freezing Index is a single number that sums up how much cold weather we had over the course of the winter. This winter’s number was -1887.* That’s nasty.

*Refresher: Sean derives this number by using a simple technique. Once the Mean Daily Temperature drops below freezing in the autumn, he takes note of it every day. If the MDT is -3°C one day, the line drops three points. If the MDT is -5°C the next day, the line drops a further five points.

Sean tracks the rising temperatures after Inflection Day, too. Then, using my dates for ice-out, he looked for a relationship between the freezing index and the thawing index. This got him a rule of thumb: for every ten points on the freezing index, you need one point on the thawing index to get rid of the ice.* That’s a starting point.

There are other factors that can hasten or delay the thaw. Sunshine is always helpful. Rain and wind can speed the thaw along, especially if the timing is right.  Strong currents are a plus. On the negative side, snow is the worst, but clear nights and cloudy days don’t help either. This forecast doesn’t try to predict any of those things. They’ll be wild cards that may mess with this simple prediction, or they might balance out.

*does that seem strangely unbalanced to you? For the purposes of Ice Patrol, there are only two seasons: the one where it’s below freezing, and the one where it’s above. Generally speaking, we have more summer than winter here: the summer spikes on the shark-fin graph are bigger than the winter ones. But the thaw is just a few weeks out of the year. If the thawing index had to equal the freezing index to melt the ice, it would take nearly all summer to thaw the lake. Also, the lake is not frozen to the bottom, because water is weird stuff, but that’s a whole other story.

In any case, this forecast is an estimate. Sean figures that we’ll need to accumulate roughly 190 points to get rid of the amount of ice this winter should have created.

So he goes to the weather forecast to see how long that might take, and you know how those are. As Sean suggested above, the seven day forecast is broadly reliable. (When it’s off, it’s usually in the timing.) After that, the fourteen day forecast is more of an educated guess, and the really long-term stuff basically just goes, “I don’t know… normal, I guess?”

The forecast graph tracks how long it might take for the weather to melt the ice.

So here’s how it looks for now. The horizontal yellow line is set at 190 points. That’s our target. The vertical grey line is the start of the May Long Weekend. That’s not a deadline, it’s more of a  wish. The smooth blue line is the weather forecast. It hits the target on Friday of the May Long.

There are two other lines. The red one shows how the Thawing Index racked up in the best year, and the green one represents the worst spring we know of.

The red line is the easier one to understand. 2007 was a lovely spring. If we could have those sorts of temperatures this year, the thaw would go a little faster. But as far as our records go, that’s a best-case scenario, and a long shot.

The green line is confusing. 2004 was a dreadful spring, and in fact the whole summer was exceptionally cool. The temperature trend that spring was feeble. So how did the thaw go? Not bad. It had been a middling kind of winter (~1500 points), so the target that year was lower. Despite the chilly spring, the lake was clear of ice by mid-May, significantly better than the ten-to-one rule of thumb would have predicted. In terms of modelling, 2004’s data made it an outlier.

In conclusion: Lake of the Woods is on track to be clear of ice right around the long weekend. If the weather forecast is right, and if the rule of thumb holds true this year.

Thanks, Sean!

Keep in mind that we should see steady progress between now and then. Marinas will open. Many islands will be accessible a week or more before the lake is entirely ice-free. That may also be true of Falcon Lake and West Hawk.

Shoal Lake plays by it’s own rules, and often lags a few days behind Lake of the Woods.

Further north, the long weekend and the opening of fishing season may face problems.

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