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Winter weather
- December 5, 2007 12:43 PM |
- By Your Voice
Environment Canada recently announced Canada may be in for the coldest winter in 15 years. For many Canadians the chill is already being felt, as much of the country was blasted with early winter storms last week.
What can we expect in the months ahead and how should you prepare for the cold weather?
Richard Moffet is a meteorologist with Environment Canada and chief of the Analysis and Prognosis section at the Canadian Meteorological Centre. He took your questions on winter weather, read his responses below.
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Comments (10)
How does the Canadian Meteorological Centre predict temperatures for an upcoming season?
Richard Moffet: To forecast the weather, Environment Canada uses sophisticated numerical models that predict the evolution of the atmosphere. For longer range forecasts such as seasonal outlook, we use an ensemble approach. We run 4 models per day that differ slightly in the way some physical processes are performed (parameterized.) On December first, for example, we use the integrations of the last ten days of model runs and from this ensemble of 40 members, we establish for each site in Canada the temperature and precipitation forecast.
From these forecasts, we determine in which class the forecast will fall: normal, below normal or above normal. The skill for temperature forecast varies from one region to another but it usually is about 60 per cent. You may find the map of this winter’s forecast here.
Why is the Probability of Precipitation never reported as 50% in the forecasts?
Richard Moffet: A 50 per cent chance of an event could be misinterpreted. 50 per cent probability for an event which at observation time will be observed or not , hit or miss, could be interpreted by the public as not really making a forecast but as guessing. Probability of Precipitation in itself is understood by people in many different ways.
Is the cold weather due to global warming?
Richard Moffet: In the last 10 years, climatologists have noticed that temperatures have gradually increased, globally. Regionally, this has not necessarily been the case every year where local or specific influences have played a role.
This year our forecast for below normal temperature for much of Canada stands out from the forecasts of the previous years. One effect that might lead the models to forecast this year’s colder winter in Canada is the presence of La Nina off the coast of Peru. Over a large area off Peru, the sea surface temperatures are presently below normal temperatures by one or two degrees. The La Nina phenomenon usually lasts through the winter and into the spring and it influences the atmospheric flow locally — but eventually over North America.
Statistically speaking, La Nina years have been linked to colder than normal winter temperatures for the Central U.S. and the Canadian Prairies. To come back to the question, the cold weather forecast this year does not signal a halt in the global warming scenario or is not due to it.
Winter precipitation sometimes falls as freezing rain and other times as snow. What determines the difference and how can meteorologists predict which it is likely to be?
Richard Moffet: The type of precipitation that we experience at the surface highly depends on the temperature profile (distribution) with height. If temperatures are negative from the surface and up, then we will get snow. If the temperature in the lowest 500 or 1000 meters is above zero, most likely it will be rain.
Freezing rain will occur when an above freezing layer of air lies aloft, deep enough to have rain form. If at the surface rests a layer of colder air, in the below freezing mark, then we will experience freezing rain, meaning that rain will freeze as it hits the surface. If the extent of the sub-zero temperature surface layer is deep enough rain can become ice pellets.
To predict the precipitation type, we use numerical prediction models that show the evolution of the atmosphere and thus forecast the temperature at all levels. We also take into consideration known local effects that our model may not forecast well.
Briefly explain why "La Nina" means colder weather in western Canada while "El Nino" usually results in warmer weather.
Richard Moffet: Not all La Nina years have meant colder winter for much of Canada but statistically most have meant so. Many inputs influence the atmospheric flow but ocean temperatures are surely one of them. The relationship between La Nina and temperature patterns over much of Canada is not direct.
La Nina being a prolonged negative sea surface temperature anomaly off the coast of Peru —over quite a large area —it does influence the atmospheric flow over the north Pacific in a way that the colder Arctic air masses can come down central North America more often than usual. El Nino is the opposite of La Nina, it’s a positive sea surface anomaly off the coast of Peru.
Here in Alberta, does our snow come down from the North or across the Rockies from the West?
How does topography affects snowfall patterns?
I would like to find out about my region in west-central Alberta (about 100 k East of the Rockies). Two hours drive to the NE, Edmonton and northern Alberta get significantly more snow than we do, and they get it more often. Even Saskatchewan gets more snow.
For some reason, the weather systems depositing snow take a sharp left turn before they arrive here. Is it our proximity to the mountains, or could there be another reason? The mountain parks (Banff, Jasper) get their share of snow too, so I don't see how the mountains pose an obstacle.
Perhaps I shouldn't complain. Just an hour south of us, they get even less snow. Rocky Mountain House can be snow-covered, while Sundre and Olds are bare.
Mystified in RMH
Richard Moffet: Clouds and precipitations form when air is forced to rise, leading to a drop in temperature of the rising air and in the capacity for the air to hold as much humidity. One way to force air to rise is to have a wind go across a mountain range.
This is the case with the Coastal range and the Rockies in Western Canada for example, with the main winds being westerly. In the windward side of the mountains, we have significantly more precipitation than in the leeside. In the lee of the Rockies, where westerly winds are descending and drying we have what is called a shadow effect on precipitation, meaning less precipitation. Not only is the air warming and drying as it descends but it has also shed a lot of humidity by precipitating on the windward side.
In Alberta, with an easterly wind in the lower levels though you will experience some orographic lifting leading to some precipitation. On the very local scale, the mountain and lakes can make a difference.
Weather forecasters predict this to be the coldest winter in 15 years. I think I read that it will be .75 to 1 degree colder on average.
What would the typical person notice this winter compared to others with such a smal variance in the average temperature? Or, is the media making a big deal about a variance that wouldn't normally be noticeable by the typical person?
Richard Moffet: In fact, the threshold at which we depart from normal to below normal temperature varies from region to region, it is available here:
For many regions, it is in the order of one degree with some places in the west having a threshold of two degrees or so. This is not much, and I agree with you that it would not be noticeable by a typical person, but since the yester winters have been above normal, it means a difference of a few degrees from previous years.
In some regions, like Ontario, it may mean also that partitioning of precipitation between snow and rain will be different this year. The overall precipitation amounts may be the same but it will be more in the form of snow this year.
Do climate models suggest that frigid arctic air can be pushed further south due to the more chaotic weather activity?
Richard Moffet: In seasonal forecasting, we look at the cumulative effects of weather systems in the three month period and not at the individual weather systems at any time during these three months. This being said, if below normal temperature are forecast it is probably due to an increase of cold air outbreaks, coming from the Arctic. But this situation cannot be called chaotic.
Colder winters, and La Nina are part of the climate, past and future. In fact, some older folks might argue that if the forecast holds, we’ll have a winter like we used to.
The Weather Network does not agree with your prediction of a cold winter because "it uses a different model." What are these models, and how are they different?
Thank you.
Richard Moffet: I do not have a copy of the Weather Network forecast and cannot comment on it. Weather Centres around the world all use two types of tools. The main tool is to run numerical dynamical models describing the evolution of the atmosphere, like we do at Environment Canada.
The amount of sophistication can vary a lot from one model to the other and these models are global. Furthermore, more than one model and/or one integration of these models are used to come up with the forecast. The second tool is to link statistically the forecasts to observed or recent atmospheric or oceanic forcings.
The description of the Canadian models used will be available on December 12th at Environment Canada’s " rel="nofollow">Weatheroffice website.
How accurate are probability of precipitation (P.O.P.) predictions? Are Monday's P.O.P. predictions more accurate for Tuesday than they are for Thursday or Friday? Can the P.O.P. number be relied upon as a rough proxy for the likely intensity of the precipitation as well as its likelihood? E.g. if the P.O.P. is at 10%, is it safe to assume that if it does rain it will only rain a little bit?
Richard Moffet: Forecast accuracy is always very high in the short term. For day one, depending on the season and location it might be accurate more than 90 per cent of the time. The accuracy is not dependent on the day of the week though but on the lead time of the forecast. For seasonal forecasting the accuracy drops quite fast and the skill is often very low.
This is reflected in the very prudent wording of the seasonal precipitation forecast. Precipitation varies spatially quite significantly and is thus harder to forecast than temperature. Probability of precipitation pertains to the likelihood of the event and has no significance as to the amounts or the intensity of the precipitation. You might have a 20 per cent chance of a heavy downpour for example.