In May a few media outlets were hawking an early and severe wildfire season here in the Northwest, due to low snowpack and low river levels., neither of which have a large impact on wildfires in our region. Well, we are now at the start of the traditional wildfire season and the reality is that there have been LESS fires and LESS acreage burned than normal in our region. As I write this, there is one moderate-size fire burning in Washington State (600 acre fire in the Olympics), and the fires in the western U.S. have burned less area and were fewer in number than normal (for proof of this check out this website by the National Interagency Fire Center).
Why less fires than usual? First, precipitation was only a bit below normal during the past winter/spring. There is no precipitation drought, but a snow drought, with precipitation falling as rain rather than snow in the mountains. Thus, eastern Washington started the warm season with near normal soil moisture and normally moist vegetation.
Second, May was unusually wet in parts of eastern Washington and Oregon, keeping the surfaces moist, even though temperatures were above normal.
Third, there has been a dearth of lightning the past month. This is VERY important because most fires are caused by lightning around here.
But the situation is changing rapidly now and becoming more threatening...the details of which I will describe below
Before we look ahead, it is useful to consider the fire climatology in our region (see below). Lightning fires are dominant over human caused. Lightning can produce the big surges of fires that can overwhelm fire-fighting resources. You see the one large spike in human-caused fires? That is July 4th! Peak wildfire season is in August.
So lightning is a critical element in controlling the frequency of wildfires. Thus, you can have hot, dry years that produce few major fires if there isn't much lightning.
To get a major wildfire season you need the surface to be dry, and particularly for vegetation (dead or alive) to be dry and ready to burn. You dry the surface with a combination of lack of precipitation and warm temperatures/sunshine. Why warm temperatures and sun? Because they cause water to evaporate from the surface.
Around here summers are generally dry, except for occasional thunderstorms, which are generally found over and east of the Cascade crest. So temperature is the key element in drying things out.
We thus come to the first problem: temperatures have been MUCH warmer than normal during the past month. To show this, here is the departure from normal of maximum temperatures during the last 30 days. Wow. Six to ten degrees above normal in much of eastern Oregon an Washington, as well as western Oregon.
This warmth, plus precipitation at or slightly below normal (which is NOT much even during normal years) has caused the surface to dry. As a result, various soil moisture indices have indicated rapid declines of soil moisture. For example, the popular Palmer Drought Severity Index (PDSI) shows moderate to severe drought conditions in the upper soil layers over eastern Washington and California.
So the surface conditions are drier than normal, with the Forest Service folks estimated we are about 2-3 weeks ahead of normal (early to mid July conditions). Unusual amounts of grass over grew in eastern Washington in May due to the wet conditions that month. That grass has now dried out and has become a fire threat. That is why there were a series of fast burning grass fires in eastern Washington during their warm spell last week. They were put out quickly and did little damage...but they are a warning.
So lets look into the future using the state-of-the-art prediction tools at our disposal. First, there is a threatening situation during the next week. Over the weekend, temperatures are going to warm rapidly as a huge ridge of high pressure builds over the region. Here is the latest forecasts by weather.com for Wenatchee. For Friday through Wednesday, the highs will be above 105. Not good. The surface will be toasty dry.
What about lightning? The latest UW WRF MODEL forecast suggests that starting this weekend the potential for thunderstorms will increase dramatically. To show this, here are forecasts of something call CAPE (Convective Available Potential Energy)....a measure of the amount of instability in the atmosphere.. for several times starting Friday afternoon. Instability is needed for thunderstorms. Substantial numbers of thunderstorms are possible from Friday through Tuesday.
Friday afternoon, lighting could strike the Oregon Cascades and northern CA.
More on Saturday afternoon, with the risk spreading into eastern WA
The bottom line is that with very dry conditions in place, multiple lightning-caused fires are quite possible. Fire folks need to get ready.
But what about the rest of the summer? Our seasonal forecast models (like the NOAA CFSv2) can give us some insights. The temperature forecast for July though September is for much warmer than normal conditions from northern CA into British Columbia (see below) This implies drying conditions. Not good.
Precipitation? Wetter than normal over the Rockies, with moisture extending into eastern Oregon (see below). No precipitation signal over eastern WA. This precipitation will be mainly for thunderstorms, which means lots of lightning over the Rockies, Sierra, and southern Cascades.
Where it has been very wet (the Rockies), the fire risk might be reduced by the moisture. But the places on the western periphery (like the southern Oregon and northern CA Cascades) might be hit by a lot of lightning--and it will be dry there. And these thunderstorms might drift into eastern Washington. Substantial threat.
It is important to note that lightning caused fires are a natural part of the ecology of the region, NECESSARY for the life cycle of several of our local plant species. And the extreme warmth this year is probably the result of natural variability--not human-caused global warming. The real problem is that our civilization has injected population into a natural fire-prone environment and thus some folks are at risk, forcing us to intervene to stop fires in many locations.
Washington State now has a statewide burn ban in effect---a very good idea. Considering the risk, perhaps they should go further, such as a total fireworks ban, including the sales of all fireworks. The burn ban does not extend to Federal lands--is that possible? This would be a good idea, as would clearing brush/grass around structures in fire-endangered areas.
We are dealing with the conditions of 2070 this summer and it will be necessary to use more extensive methods than normal to reduce human-caused fires. Control of lightning is in other hands.