Saturday, May 04, 2013

Los Angeles Wildfire FAQ

When the LA Times is showing pictures like this
and the MODIS Image of the Day shows a smoke plume like this, I field a few questions.
I thought that others might have the same questions, but don't know who to ask.

Take a look at the burn area map.
Q: What do the colored lines mean?
A: The red line contains the burned area.  The blue lines represent the evacuation areas.  Some places further stratify with mandatory and voluntary evacuation zones.  But, some people outside those areas will want to evacuate anyway for health or mobility reasons.

Q:  What does 20% contained mean?
A: Fires are fought along the perimeter so the number refers to the percentage of the fire perimeter (red line) that is under control--where the firefighters are reasonably sure that the fire won't spread beyond that line.

Q: I live further away from the fire than my neighbor who is closer in distance to the fire.  Why do I have to evacuate and he doesn't?
A: Recall what I wrote in Fire is a river that runs uphill (and you can see the video evidence in Fire is a river that runs uphill redux):
On flat land, in the absence of winds, a fire will burn itself out after it has exhausted the fuels in the fire perimeter. To fight the fire, you need only create a firebreak which denies the fire of the fuel it needs to maintain itself.

Flames and hot embers rise. On a hill (still in the absence of winds), the fire will keep moving uphill, picking up fresh fuel, until it can climb no further.
Strong winds can blow the fire downhill and over ridge tops, as has happened in this fire.  Evacuation zones are set by risk; uphill and downwind areas are at higher immediate risk.

Q: Why are the firefighters pulling back and not doing anything?  The fire is still raging!
A: The problem is further complicated by shifting winds.  Wildfires in our region mostly start when we have an offshore wind pattern--the winds come from inland and blow towards the ocean.  But, we typically experience an afternoon sea breeze--the winds come from the ocean and blow inland.

There is a huge variability and uncertainty in the onset time of the afternoon sea breeze and firefighters have to pull back at the earliest possible time of onset.  Below is the weather prediction for the fire area on Friday morning.  Notice the wind shift was predicted to take place between 8 AM and 2 PM?
Here's the wind at a nearby weather station. It's as close to what actually happened as we know.
Notice that the winds rotated around, coming briefly from the *west*? That wasn't expected.

To efficiently and safely allocate firefighting resources, we need good wind predictions.  We are at a disadvantage because we live right next to the "big blue data void" aka the Pacific Ocean.  But, NOAA does run a (sparse) string of marine buoys and our government runs weather satellites and numerical weather prediction centers.  But, don't get too complacent.  Many scientists are sidelined due to austerity budgets (myself included).

3 comments:

  1. Stay safe-and may this ill-advised austerity end soon

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  2. Fire - a much bigger threat than earthquakes.

    You didn't mention how a fire can generate its own winds. That's part of what fueled the Oakland fire a couple of decades ago.

    I hope you are far from the fire and that the smoke isn't causing breathing issues.

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  3. @Neefer

    I didn't want to get into fire meteorology and entrainment. But, it is clearly visible in the LAT photo.

    I thought it might be too confusing for novices. But, you are right. Depending on how close you are to the fire, the microscale winds created by the fire can be stronger than the mesoscale (land-sea breezes) and global scale (jet stream) wind patterns.

    Oy, vey. It's only early May. It's going to be a long fire season...

    I'm fine, but RA, my roomie at PR Weekend, lives close to the fire.

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