Arctic Vortex, Tilts, Troughs, and Ridges — Wilderness North

Celebrating 30 Years of Wilderness North  –          

Arctic Vortex, Tilts, Troughs, and Ridges

It is spring and you can tell because the sun at mid-day is way above the horizon. The temperatures may introduce a little doubt about what season we are in, however they are nudging upward, albeit slowly. Many seasonal events and processes are some weeks behind schedule, including ice out on various lakes, gardening and so on. Cooler and wetter than normal describes all the weeks of spring in Thunder Bay and the Northwest region. April of 2014 was the 5th consecutive month with cooler than average monthly temperatures.

The exceptional cold in the recent winter was blamed on the “polar vortex”. This term became a media favourite although it was in use by meteorologists as far back as the 1960s. Ironies abound in meteorology, and one of them includes that many of the polar regions of the planet during the past winter were warmer than normal.

When weather systems stall, bad things, i.e. severe weather, often result. In late April a massive low pressure system was moving very slowly northeast from Nebraska. A variety of weather warnings were posted from Manitoba and Northwestern Ontario down to the Gulf of Mexico. A common warning was for strong winds. Thunder Bay had a heavy snow warning, west in Manitoba and south in Minnesota the concern was for flooding with more rain or snow, and in the American south the fear was for tornadoes, torrential rains and flash flooding.

Many or most of these warnings were followed by the predicted severe conditions. Thunder Bay experienced rain and mainly snow while some locations near Winnipeg had flooding and evacuations. Multiple tornadoes in the southern United States claimed 36 lives. Remarkable rainfall during two days with 600 to 660 mm (an average year of precipitation in Thunder Bay) next to the Gulf of Mexico caused loss of life and considerable property damage.

Okay, the snow is all gone now, but this photo illustrates an April challenge we faced near my place. Yes, that is 37 cm/14.5 inches of new snow on the ground!

Troughs and Ridges
We refer to troughs and ridges occasionally in weather forecasts. Typically, troughs are synonymous with cool environments, and ridges flag warmer situations. One of the most important aspects of forecasting troughs is how they are shaped. Their orientation can dictate whether a storm will strengthen, weaken, or remain unchanged. There are three types of tilts, a positive, negative and neutral.  A trough with a positive slope is oriented from the lower left of the point of origin to the upper right (southwest to northeast). A neutral trough is oriented from south to north and a negatively tilted trough is positioned southeast to northwest. A negatively tilted trough generally has the most importance and potential for severe weather because the positioning can allow a storm system to grow and strengthen. Colder air is on top of the warmer with more humid air feeding from, at, or near the surface. Instability is created, allowing for convection and strengthening of a storm system. The opposite is true for a positively tilted trough. Colder air is not directly on top of the warmer air so there is very little, if any instability. However, a positively tilted trough can change and become negative with enough momentum from winds in the upper levels of the atmosphere.

I was a speaker at a conference in Fort Frances (west of Thunder Bay) the day before the April snow in the above photograph. I mentioned the term “Arctic vortex” and mused about “negative tilt” – perhaps this will become a media term sometime in the future. My unofficial forecast for people driving back to Thunder Bay suggested that the trough would assume a negative tilt and deliver a prolonged precipitation event to Thunder Bay and inland areas. Much of this did come down as snow, especially at higher elevations inland from the lake.

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