The Regenesis of Low Pressure Systems
in the Lee of the Canadian Rockies
and the Difficulties Forecasters Face:
A forecasting challenge for the cities of
Edmonton & Calgary, Alberta.


The regenesis of Low Pressure systems on the lee side of the Canadian Rockies is a forecasting problem that, depending on the season and the atmospheric setup, can present a daily challenge to the region's forecasters.

On the windward side of the Canadian Rockies is a series of mountain ranges that eventually lead right back to the Pacific Ocean. In that wide expanse between two of Mother Nature's largest geographic features there are relatively few stations that are in place to accurately monitor and evaluate the atmosphere and the development of Mid-Latitude Cyclones. As well, radar coverage is virtually non-existent in that region. The Province of British Columbia makes up the entire windward side of the Canadian Rockies. With a land area of nearly 950,000 square kilometers, or roughly four times the size of Great Britain, ( the area is significantly under-represented by only four widely dispersed radar sites. While physical geography dictates most of the limitations in setting up more stations, and, the effectiveness or usefulness of their data, the end result is none-the-less the same: Poor coverage of existing weather. This presents the first variable in problem.

The second problem is understanding what happens once a disturbance crosses the Rockies in the upper levels. The disturbance can trigger either development or redevelopment in the foothills. The problem is made even worse when the baroclinic zone is stronger on the lee side. This was a frustrating issue in the first several months of 2008 for forecasters in the City of Edmonton. When systems crossed the mountains the fronts were becoming sharper, and more pronounced. It helped to generate instability and resulted in many instances of narrow bands of heavy precipitation that would cut across the forecast region. Many meteorologists spent much time trying to explain either why there wasn't snow when they had forecasted it only hours earlier, or, alternatively where the record breaking "freak spring snow storm" came from. On April fourth, 2008 the consensus among forecasters and the national weather service of Environment Canada was an initial forecast for three to five centimeters of snow in Edmonton, Alberta with slightly heavier snow to fall over areas north of the city from a Low pressure that was expected to pop over the Rockies and track generally right across the Province's capital city. But, on the morning of April 5, 2008 residents of Edmonton, Alberta woke up to fifteen centimeters of snow on the ground, and a Snowfall warning stating to expect that even more was on the way. In reality, when the low re-developed, it was further south than anticipated, which brought heavier snow to Edmonton (instead of to the north) causing a blown forecast. Further to the northeast, in the town of Vegreville, Alberta, the heaviest snow that the system had to offer was measured at twenty-four centimeters. This large amount of snow fell in only a few short hours, unless you were living only fifty kilometers to the south of Edmonton. In that short distance away the depth of the fresh snowfall dropped off dramatically, measuring in at only four centimeters that day. Similar scenarios played out for the following two days throughout the region.

Looking at the complexity of the physical process, we'll start on the windward side, where a column of air quickly stretches, then begins to shrink vertically. It is being compressed and forced to expand laterally as it moves up the windward side of the Rockies implying a negative vorticity tendency. This diverging column will therefore acquire anticyclonic vorticity, starting it on a path of heading towards the equator. This movement equator ward increases the air parcels relative vorticity and decreases the effect of the coriolis force. All of this coupled together causes the column to reappear on the lee side further to the south of the initial latitude it started at on the windward side. During descent on the lee-side, the flow of air is warmed and the column of air starts to stretch back out. This action initiates the spin up of a low in foothills, and the result is upper level divergence. The entire movement creates a net gain in relative cyclonic vorticity, which sends the air parcel pole ward. Mid-Latitude Weather Systems, T.N. Carlson (Braun-Brumfield, Inc), p. 203- 207

Without being able to fully track the initial disturbance on the windward side, calculating the location and strength of regenesis on the lee side can be difficult. Another kink in the calculation of forecasting regenesis is the physical formation of the mountain range. It is not simply a giant, uniform wall of rock. The peaks and valleys and topography can easily and quickly have an influence on weather patterns.

From the physical world to the digital domain, computer models have a hard time pinpointing exactly where that secondary low will form as well. But, when a lee trough does appear, east winds are created. In the winter, the east winds maintain cold temperatures and can contribute to high wind chills and snow. In the summer months, the east winds can act as a forcing mechanism through upslope flow, triggering instability with the low-level moisture. This can result in severe thunderstorms. When you have shear in the low levels created by combining a lee trough, upper level winds and cooling, you are facing the ingredients that can lead to the potential development of damaging hailstorms and, worst case scenario, possibly deadly tornadoes.

In the transition season of spring in the lee of the Rockies it is thankfully too warm to create blizzard conditions, and just too dry to develop severe thunderstorms. But, the instability can still be there to give localized heavy snow, as was noticed on April 5, 2008.

To complicate matters even more, a parent low on the windward side of the Rockies isn't always required for the formation of a weak elongated trough in the lee of the mountains. They can form with the help of relatively strong 700mb cross-mountain winds. "Accordingly, a positive vorticity tendency occurs south of the developing low; in the absence of synoptic-scale vorticity or thermal advections, the track of the low is toward the south." Mid-Latitude Weather Systems, T.N. Carlson (Braun-Brumfield, Inc), p. 210

It would be advantageous to forecasters (and to the public in general who live on the lee side of Canada's Rocky Mountains) to have better atmospheric monitoring capabilities, in the form of higher numbers of reporting stations installed in the Province of British Columbia by the Federally owned and operated weather service Environment Canada. This sparsely inhabited area of the country is the location of the windward side of the mountain range, and the gateway to the development of weather patterns that can have a significant impact on the rest of the country. More accurate forecasting tools could help in the prediction of moisture for agricultural producers in the prairie producing provinces, or provide travel advisories in the event of winter storm development, or help give more timely warning in the case of severe summer weather that rolls of the foothills.

The two major centers in Alberta are Edmonton and Calgary. They are tied together with one of the busiest traveling corridors in western Canada, be it by air or land. Many people depend on the accuracy of the forecast to help plan their movement. All of this while forecasters are, in every practical sense, having to start from square one, instead of being able to study the approaching weather from any great time or distance in advance.

Better understanding of the regenesis of low-pressure systems in the lee of the Canadian Rockies has the potential to help empower the population to make wiser and more effective personal, business, or safety decisions.