Red Flag Warnings and Flash Flood Watches

Friday, August 9, 2013
1:35 P.M.

Flash Flood in Gobi, Mongolia in 2004 – Uploaded by Wikimedia User QFL247

It would seem as though the current watches and warnings posted around Washington contradict each other. How can you be at risk of forest fires and flash floods at the same time? Rain mitigates the damage done by fires, so wouldn’t floods extinguish them altogether?

Not necessarily. Recall from my previous post that many forest fires are initiated by bolts of lightning from thunderstorms. Some of these thunderstorms are wet, and others are relatively dry. Nevertheless, if one tree is caught on fire in an arid area and the trees and ground are dry enough and the winds strong enough, the fire will overcome the obstacles of rainfall and begin to spread. It’s after these fires have died down that the danger for flash flooding begins.

Rainfall reacts extremely differently in forests and in impervious surfaces like the concrete we see in the city. In forests, rainfall is absorbed by the relatively permeable ground, and it is only when the rainfall is extremely heavy or the ground is previously saturated that a flash flood could occur. Rainfall is not absorbed by concrete or asphalt and simply puddles on the surface, leaving a place more susceptible for flash flooding. On December 14, 2006 between the hours of 4 to 5 P.M., Seattle was deluged with a rainstorm that dropped an inch of rain in one hour across the heart of the city. Had this occurred in a forest, flash flooding would have not been a problem. Due to the inability of much of the urban surfaces to absorb water, however, Seattle did see flash flooding, and it took the life of one woman in Madison Valley as she drowned in her basement due to rising waters surrounding her house. Madison Valley is very close to my house, and the only reason our home wasn’t inundated was because we are not in an urban watershed that is particularly susceptible to it. Madison Valley is shaped like a bowl and funnels all the water to the bottom of the valley, where this woman lived.

When forests are burnt, the ground beneath them becomes much less permeable. As such, they present a much higher flash flood risk to areas in which their runoff flows to. After a patch of earth has been scorched, thunderstorms are still often nearby while the fire has either moved or been extinguished. These thunderstorms can still drop prodigious amounts of rain in very short periods of time, and since the newly-burnt ground can’t absorb as much water as the forest that was once there, the flash flood is much higher.

Here are the National Weather Service’s definitions of a Red Flag Warning and a Flash Flood Watch.

A RED FLAG WARNING MEANS THAT CRITICAL FIRE WEATHER CONDITIONS ARE EITHER OCCURRING NOW…OR WILL SHORTLY. SCATTERED LIGHTNING IN DRY FUELS CAN CONTRIBUTE TO MULTIPLE FIRE STARTS…ALONG WITH ERRATIC AND GUSTY WINDS. FIREFIGHTING AGENCIES WILL WANT TO PLAN ACCORDINGLY.

A FLASH FLOOD WATCH MEANS THAT CONDITIONS MAY DEVELOP THAT LEAD TO FLASH FLOODING. FLASH FLOODING IS A VERY DANGEROUS SITUATION.

So yes, they can, and often do, coexist.

And it’s not just Washington. The entire Pacific Northwest is experiencing critical fire conditions. Firefighting agencies will have their work cut out for them over the next few days. Both of these pictures were obtained from the National Weather Service.

The meteorological synopsis has largely remained the same. An upper low will drift northward from California and weaken, but as it does so, it will provide instability and moisture to support convective thunderstorm activity over the Cascades each afternoon. Shortwave troughs – disturbances in the mid to upper regions of the atmosphere that cause upward motion ahead of them – associated with the upper low will roll through the area tonight and Saturday evening and will provide additional lift and convection and therefore an increased chance of thunderstorms.

I’m 99% sure that Washington will see at least one wildfire over the weekend. The only question is where.

I’m experimenting with different styles of writing, so if this seems a bit dry, don’t worry. I’m still the same ole’ Charlie. 🙂

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2 thoughts on “Red Flag Warnings and Flash Flood Watches

  1. It takes too long for a parcel of forest land to completely burn, in order to have torrential rains from the SAME thunderstorm event cause worse flash flooding BECAUSE of the fire. Now if you have some forest fires and then a new thunderstorm moves in a few days later, it's a different story.

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