In my last article, I said we’d continue our discussion on cold-weather precipitation, but before diving back into that icy topic there have been a few unique weather events across our region that deserve a bit of a closer look.
The first event was the high winds that worked their way across the three Prairie provinces last Tuesday and Wednesday. A very strong area of low pressure that developed in Alberta late last Monday quickly moved into northern Manitoba by Wednesday, dragging a cold front with it. This low combined with a rebuilding ridge of high pressure and helped to generate warning-level winds across a large portion of all three Prairie provinces. The numbers below shows the top reported wind speeds across Manitoba for that period.
- Dauphin – 102 km/h
- McCreary – 98 km/h
- Roblin – 94 km/h
- Portage la Prairie – 93 km/h
Then, a couple of days later, late-season tropical heat moved in, pushing daytime highs into the low to mid-20s across much of southern and central Manitoba. This push of mild air actually did originate in the tropics, as a large area of high pressure over eastern North America helped to create a large flow of tropical air that worked all the way into Central Canada. Winnipeg was the only station I saw that officially broke a record last Friday, when the temperature topped out at 25.0 C, shattering the old record of 23.9 C set back in 1958. What was interesting about this mini-heat wave was the warm overnight temperatures that occurred overnight on Friday over central and eastern parts of southern Manitoba. If you walked outside late Friday or early Saturday morning, it felt more like summer, as overnight lows only fell into the upper teens.
The last unusual weather event was the weak band of thunderstorms that developed over south-central Manitoba late in the afternoon on Saturday. While the storms were not very strong, they did generate enough lightning and thunder to be noticeable. Thunderstorms can happen at any time of the year, but between October and March they are pretty unusual.
OK, now on to cold-weather precipitation. In the last issue we discussed the general process that produces most of the precipitation in our region, and that was the Bergeron process. This is the process that allows for the rapid development of ice crystals which, in turn, leads to the development of snowflakes. While snow is the most common form of winter precipitation it is not the only frozen form of precipitation we see. Before we go into the details on these forms of precipitation, let’s remove one type of frozen precipitation that just doesn’t belong in the winter: hail.
Hail, while being ice, does not form during cold weather. For hail to form you need strong up- and downdrafts in the atmosphere that can take an ice crystal repeatedly between the liquid and frozen areas of a cloud. This allows the buildup of ice into layers to create a hailstone. In order for this to happen you need a warm section of cloud that contains large amounts of liquid water, along with strong updrafts, which are only found in thunderstorms. While hailstones are not a winter or cold-weather type of precipitation, there is a close relative that is similar to and often confused with hail: ice pellets.
Ice pellets, also known as sleet, look somewhat like hailstones as they are made of solid ice, but they are not large, tending to be only a few millimetres across, and they do not form in the same way. Ice pellets are formed when falling snow encounters a shallow layer of warm air that either partially or entirely melts the snowflake. This melted flake than re-enters a layer of cold air near the ground where it freezes again, forming the hard ice pellets. Ice pellets are hard enough that they will bounce when they hit the ground and are difficult to break, making them seem like hailstones.
The next form of frozen precipitation is the snow pellet. These form when a snowflake partially melts and then refreezes, creating a small layer of ice over a small pellet of snow. These snow pellets can be hard enough to bounce when they hit the ground, but are soft enough to break easily. As well, where ice pellets will typically be clear, snow pellets are white due to the inner layer of snow.
Next week we’ll take a look back at October’s overall weather numbers, then look ahead to see what the most recent weather outlooks call for this winter.