I didn’t do a Signs of Spring post yesterday, but if you’re interested, snow is retreating wherever the sun shines, but lingering in shadowy places. I’ve seen a pair of Mallards, my first ducks of the year. Also a pigeon and a pair of Whisky Jacks. I heard a second-hand report (hearsay!) of a robin. Cyclists are emerging from their winter dens.
And now onto the main topic.
Contributor Sean C. is finally confident that we’ve reached the inflection point, the date when our daily mean temperature rises above freezing on a lasting basis. He figures this took place on Saturday, April 13th.
That means that from now on, the ice will be melting steadily.
Here’s the first of his graphs, depicting the severity of the winter.
Each of the downward spikes represents a winter, ending on the date of the inflection point. The depth of the spike represents the severity of the winter, and the width of the spike represents the duration of the ice-making period. In a nutshell, the winter that just ended was not the worst, but it was a very close match for the previous winter, which was pretty bad.
You can click on these graphs to see them full-screen and a bit bigger.
Here’s Sean’s graph comparing the way some recent winters unfolded.
In this figure, each winter gets a line in a different colour, and tracks across the calendar until it ends on the inflection date. A short shallow line, like 2012’s medium blue one at the top, represents a mild winter that started to thaw early. The awful winter of 2014 (which was also the deepest downward spike on the first graph) is the grey line that slopes down and down, making ice until mid April. Above that are two lines that track together at the end. The dark blue one is from a year ago, and the dark red one is this year. These years look as closely matched on this graph as they do on the first.
Okay. We’ve established that it was a crummy winter that dragged on into the middle of April. Now what?
Well, now Sean uses some math to figure out how much heat we’ll need to melt the ice formed over such a winter. He works out a thaw index based on the severity of the winter. The index lets him make predictions based on the long-term forecast, and it works like this: If a day has a mean temperature of 5ºC, we add five points to the total. If a day has a mean temperature of 7º, we add seven, and so on*. Then he works out how many of those points we’re likely to need to thaw this winter’s ice.
Sean’s first version of this graph used the same thawing index he forecast last year: 200. However, when I asked Sean if it might be better to use the actual index of 242 that came to pass, he reconsidered, and issued a new version of the graph using an index of 240.
The amended graph now replaces the earlier version on this post. The higher index adds another day to the estimated time until we’re ice-free.
On this graph, the red line represents a really warm spring (2007) that hit 200 points in early May and the green line depicts a cold spring (2004) that took much longer. Please note: this graph isn’t about the thaw in those years, it’s just about how rapidly we accumulated enough warm weather. The very short blue line hiding in the lower left corner is 2019, with data points marked by blue dots to show the actual daily mean temperature achieved. The yellow line is what the weather forecast says we’re likely to get. If the forecast is accurate, the blue line will grow along the yellow path, and Sean will extend the yellow line as new forecasts come into effect. (There are longer forecasts, but plotting them day by day at this point would be wishful thinking)
The horizontal dotted red line indicates the target thaw index of 240 estimated for this year, while the vertical dotted red line marks the date we might reach it, based on the trend in the longer-term forecasts.
*A sort of fun thing we learned about daily mean temperatures: you might suppose that if the daytime high was 10ºC, and the overnight low was 0ºC, then the mean daily temperature would be 5ºC. That turns out to be not quite right. Actually, such a day is likely to produce a mean temperature closer to 6ºC. We seem to spend more hours near the high than the low. Perhaps this is due to spring’s long days and short nights.
If you’ve been following along, you’ll have noticed that the predicted date for reaching the target of 240 points is May 87th. May 18th is bad. It’s even worse than last year, mainly because the temperatures forecast for the next weeks are not high.
Still, there are other factors to consider.
The index itself is not precise; it’s an educated guess based on turning a limited set of past data into a mathematical formula.
A more positive influence might be ice quality. All that snow this winter made for some poor ice. If it’s weaker and softer, it should melt faster.
Then there are the usual wild cards: rain, wind and sunshine.
In summary, this year looks a lot like last year. Rosy forecasts haven’t panned out, and a normal ice-free date in early May looks unlikely.
Plan on the middle of May and cross your fingers.