It seems as if our recent warm weather should be moving us towards an earlier thaw. But by how much? My gut feeling was that three very warm days should advance the ice-out date by about three days, but I’ve been holding to my estimate of May 15-18 until I could get better analysis. That arrived today, from Sean C.
His newest graph shows what rapid progress we’ve made, and that the latest weather forecast suggests a faster melt will continue. If your browser has shrunk the graph to fit your screen, you can click on the graph to see it at full size.
Here’s a summary for anyone who doesn’t remember, or didn’t see the old graph: the steep red line represents a really fast thaw (2007), while the green line shows a sluggish spring (2004).
The general idea is we need enough warm weather to melt the winter’s ice. For a cold winter like we just had, we need a thawing index of about 200, marked by the horizontal dotted red line.
The blue dots represent our actual progress towards that target, based on our recorded daily mean temperatures. You can see that we’ve been piling on the warm days, almost matching the red line.
The dotted yellow line is Sean’s old forecast from the previous version of this graph. It’s based on the long-term weather forecast we had in mid-April. He used the forecast high and low to estimate the daily mean temperature as halfway in between, and plotted a prediction based on that.
The solid yellow line is Sean’s new prediction for daily mean temperatures based on the latest forecast. You can even see how today’s cool, cloudy weather set us back a little: look how the yellow line drifts away from the red after the last blue dot.
Sean looked back at the actual highs and lows recorded each day and compared them to the mean temperatures calculated by Environment Canada after the fact, and he’s learned the mean is likely to be a degree or two warmer than the midpoint. He’s revised his new yellow line accordingly.
As we get into May, we’re not using such long-term forecasts as we were two weeks ago; Sean can now work mostly from the fourteen-day outlook. The newest forecasts are milder, so overall, the outlook is quite a bit better.
Sean’s new track shows us thawing by May 11. That sounds about right to me.* Remember, we’re talking about 100% ice-free on Lake of the Woods. Your favourite island may be reachable before that date.
*Sean’s graph is temperature oriented, but we could still have variations in humidity, wind and sunshine that could speed things up or slow them down. I like to give a range, so I’m hedging my bets and saying May 10-15.