Everything but the kitchen sink
This past week has featured a very wide array of weather conditions, and the system which arrived on Thursday and continued into Friday presented a complicated forecast scenario with many unknowns. We began with warming sunshine nearing 50 degrees for the first time in months but it was short lived, and by sunset the snow was falling steadily and once again the ground was covered. What made this happen was once again a collision of warm and cold air masses which were loosely connected to a low pressure system in Canada. As the warm air moved in it was forced up into the sky, causing it to cool down and the moisture fell as snow. Simple enough, and we have seen many snow events this year just like this scenario.
What happened next was different, however, and things became more interesting. As the air masses bumped into each other (did you hear the thunder?) several different layers were created, each having different temperatures – and on different sides of the freezing point. As a result those of us on the ground experienced what is called “mixed precipitation”.
We wrote about the size of snow flakes previously and described how the size is determined by the amount of moisture in the air. A snowflake which starts out at a high altitude where the air is very much below freezing but then drops through a layer of warmer air will melt, lose its snowflake shape, and become a small bit of ice called sleet.
If the upward motion of the air is strong enough these ice particles may be shot back up into the colder layers before hitting the ground and accumulate more ice on their surface. Several trips on this elevator ride can produce large chunks which we know as hail.
Freezing rain was yet another outcome of recent system. This happened when the precipitation was liquid on its journey through the warm layers of the sky, but landed on subfreezing pavement or structures causing a layer of ice to form rather than individual particles. With our substantial snow cover we had many areas that experienced freezing rain as well as snow, sleet, and rain.
The unusual sequence of stuff falling from the sky had an end result of no shovels this time and a bit of relief as some of the snow pack melted. The next time the forecast is for mixed precipitation you can appreciate how it is formed.