Number-crunching measures accuracy of weather warnings

by Richard Gleeson
Northern News Services

NNSL (Sep 29/97) - The accuracy of daily weather forecasts may be open to debate, but when it comes to severe Northern weather, the warnings are right more often than not.

The Arctic Weather Centre in Edmonton, where all forecasts for the NWT are made, keeps tabs on how accurate it's been forecasting weather that will be "potentially dangerous to either health and life or property."

Federal budget cutbacks brought an end to tracking the success of general weather forecasts in the early '90s, said the centre's Brian Wiens.

"One of the ongoing projects is to generate statistics for general forecasts," said Wiens. "It is certainly seen as important, but just hasn't made it to the top of the list for a few years now."

The centre uses a number of methods to judge its accuracy as it explained in a response to enquiries made by News/North a month ago.

Numbers the centre has compiled indicate that, from January 1996 to April this year, 70 per cent of occurrences of dangerous weather in the North were detected.

So far this year, to April, that number has risen to 75 per cent.

Another measure of accuracy indicates how many weather warnings were false alarms.

Since 1995, forecasted dangerous weather failed to materialize 37 per cent of the time. About 38 per cent of warnings from January to April this year proved false.

The Arctic Weather Centre has a target of 12 hours advance notice for all warnings. In general, warnings are not issued more than 24 hours in advance.

Severe thunderstorms, less frequent in the North than they are in the South, are more difficult to predict. At most, three hours notice is given.

"A thunderstorm will typically take only 10 to 30 minutes to develop from initiation to fully developed condition," said Wiens.

"In southern Canada radar covers a good portion of the populated areas. With a radar you can see something starting to develop, because you're getting a picture every 10 minutes."

No weather radar is used in the NWT.

Warning formulas
Dark clouds stretch to the horizon, the wind is picking up and the radio cackles out a blizzard warning.

Snow is coming, but how much, and exactly how heavy will it fall? The Arctic Weather Centre defines dangerous weather with numbers.

The following is a list of what you can expect to see, assuming the warning is accurate, soon after a warning is issued.

heavy snowfall: 15 centimetres or more in 24 hours or less
heavy rainfall: 50 millimetres or more in 24 hours or less
freezing rain: one hour or more of freezing rain
wind: steady winds of 60 kilometres per hour or gusts of 90 km/h
blizzard: visibility of less than 1 km and wind speed of 40 km/h and temperatures of 0 Celsius or less
severe thunderstorm: wind speed of 90 km/h or more or rainfall of at least 25 mm per hour or hail at least 12 mm in diameter or tornado funnel cloud or water spout
tornado: confirmed sighting of a tornado
weather (generic): used when more than one of the above criteria apply or when conditions change between criteria