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Does a rain storm reduce or spread an existing fire?

Believe it or not, some types of rain are not ideal for fighting fires

Chris Murphy

Wednesday, July 23, 2014, 8:07 AM - When I began working at The Weather Network (15 years ago) I remember asking then Director of the Forecast Centre, Bob Anderson, what is good weather and bad weather in terms of forest fire behavior?

His words stuck with me and I pass them along here.

First off, as you no doubt surmised on your own, rain is good and wind is bad. But rain comes in many forms, intensities, and what about thunderstorms?

That said; does a storm reduce or spread an existing fire?

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Let's begin with the ideal situation. If I had a magic wand that I could simply wave and say, "Give me this kind of weather to put out this fire ..." this is what I would prescribe:

A slow-moving, low pressure system would be the first order of business. It helps to both lower the temperatures, while increase the humidity.

That's Step 1.

An initial period of drizzle or light rain lasting for at least eight hours is helpful. This will not put out the fire, especially a large one, but would serve to soften the ground. A hard ground does not absorb water and run-off consequently occurs.

The ideal scenario would then call for a period of approximately 24 hours of continuous - or near continuous - light to moderate rain, giving approximately 25-40mm of accumulation.

This would be especially effective if this storm lacked lightning and no wind of consequence.

Follow this up with a week's worth of near or below seasonal temperatures, plenty of clouds and a few showers and the previous vulnerable landscape and campfire bans are a thing of the past.

All this is assuming that the region is not in extreme drought conditions. If you've ever heard the expression, "What this place needs is a hurricane," that is an example of extreme drought. But we are not talking about that here.

So what about the worst case scenario? Thunderstorms without rain.

You may have heard the expression "dry Lighting" persistent hot and dry conditions and can produce its own weather, sometimes in the form of a thermal Low.

This happens in the British Columbia interior after a prolonged period of 30-plus C temps and little to no humidity. The hot sun bakes the ground, forcing air to rise, but without the resident moisture you get the lightning minus the rain.

Throw in some erratic wind and the stick of dynamite has been lit.

Lying in between these two scenarios ranges another scenario playing off the expression bigger is better (not always true).

A torrential rainstorm lasting an hour may drop 50 mm, but if this is the first rain in a while, most of it is lost to run-off. Furthermore, it will create an incredible amount of smoke, without truly dousing the hotspots.

Also, this could possibly cause a flood and still not put out the fire! A forecast of 15-20mm of rain over a day or two is a MUCH better scenario, despite the reduced rainfall.

This week, the fire zones in B.C. and Alberta are going to experience a scenario closer to the ideal than the worst case.

A word of caution, however: Thunderstorms are in the forecast meaning some rainfall totals will exceed forecast model projections.

And with thunderstorms, comes the unpredictable wind component.

Hats off to our firefighters for their incredible efforts each and every year. Looks like you'll get some help this week.

Forest fires started by humans - and most often by accident - make up 60% of the approximately 9,100 fires that occur in Canada each year. Lightning started fires, though fewer, are by far the largest - often burning an area 10 times that of those started by people.

If you have any more questions on this topic, or another, use the comment section below and I will respond to you next time out!

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