WEATHER REPORT
Sunday, January 26, 2025
It's cloudy on Flat Lake this morning. The low was 34°. The snow has settled to a snow pack depth of 4” on land.
Ice roads are wet with puddles on the lakes. I measured 24" of ice in my plowed parking lot a couple weeks ago.
We expect rain showers today, with a high temperature in the upper 30s. Rain and snow showers are likely tonight, with a low temperature in the low 30s. We'll have cloudy skies tomorrow, with a high temperature in the mid 30s. Total rainfall over the weekend is expected to amount to 1½ to 2 inches. Flooding in low lying areas is possible. A flash flood watch has been issued for the Little Su, due to an avalanche blocking the stream in Hatcher Pass that could release suddenly.
How does the lake freeze?
When the temperature of the water in the lake near the surface reaches 39°, we know that freeze-up will begin soon. This is because 39° is the temperature at which water reaches it's maximum density.
As the weather gets colder in the fall, the lake begins to cool. The surface water, being in contact with the cold air, drops in temperature until it reaches 39°. At 39° water reaches maximum density, so it's heavier that the water below it, and it sinks. This displaces the warmer, deeper water, which rises to the surface. At the surface, it's exposed to the cold air, and it cools until it reaches 39°. It then sinks, and displaces warmer water below, which is forced to the surface. This process continues until all the water from top to bottom in that area has reached 39°, so it's all the same density. This is called “lake turnover”. This happens more quickly in shallow areas, which is why the shoreline and shallow areas typically freeze first.
At that point, the top layer of water won't sink. It will remain in contact with the cold air, and will continue to cool off until it reaches 32°, and can begin to freeze. In cold temperatures, this can happen in a matter of hours. If the air is calm, ice will begin to form, and freeze-up begins.
