The Weather Guys

Our storm on Wednesday night and Thursday of last week was the first strong storm of the autumn/winter season. As you found yourself caught in the strong winds, you may well have wondered how do storms like this one come to be.

That has been the central motivating question in meteorological science for most of the past 100 years.

During that time meteorologists have learned a great deal about how such storms are formed. In most instances, two or more days before the storm is noticed at the surface of the Earth, processes are at work in the upper troposphere.

Specifically, at the height of the jet stream (about 6 miles above the surface), weak downward vertical motions begin to drag the layer of the atmosphere known as the tropopause downward into the middle troposphere, the lowest layer of the Earth’s atmosphere where all of its weather occurs.

This process eventually results in the creation of a mid-level vortex, a region of counterclockwise rotating winds, at about 3 miles above the ground. Once generated, this vortex is then moved around by the atmospheric winds in its vicinity.

At the forward side of this moving vortex, the air is forced to rise. Such upward vertical motion evacuates air from the lower troposphere, lowering the pressure at the surface. Simultaneously, the upward vertical motion produces clouds and precipitation.

So long as the mid-level vortex continues to intensify and move, so too does the surface cyclone. In many cases, the mid-level vortex eventually becomes quasi-stationary and positioned directly above the surface cyclone. This usually marks the end of the intensification for the storm though it can still deliver high impact weather at this stage.

Over the past weekend, southern Wisconsin experienced its first cold snap of the season with widespread morning lows in the upper 20s on Friday and Saturday mornings.

Often, cold snaps in autumn are short-lived, as this recent example was, affecting usually one or two nights at most.

There are a variety of reasons for this brevity. First, cold snaps at this time of year require a substantial southerly excursion of cold air from high latitude Canada to get us cold. Though the Arctic night creeps ever farther south each day after the autumnal equinox, its southerly progress is slow.

Consequently, cold air production is limited to the very highest latitudes well into November, which means that any cold air that makes it as far south as Madison is not well connected to a broader reservoir of cold air that would allow the cold to remain.

Second, there is little snow on the ground even in central Canada at this time of year and so the cold air that does migrate southward is modified (warmed) to a greater degree on that migration at this time of year than in the late fall and winter when snowcover is widespread.

Third, the lack of snow on the ground in Madison itself limits the longevity of cold snaps. This is even true in the wintertime. Overnight cooling over a snowfield can reinvigorate the chill of a cold snap and render it longer lasting as a result. So, if you were complaining about the cold this weekend, be glad we are still in October and much more likely to experience a quick rebound.