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Stable conditions lead to drought in Western United States Ridge of high pressure just won't go away. The result? Some of the biggest forest fires in a decade. August 16, 2000 More than 80 large fires continue to scorch almost 370,000 hectares across 13 western US states and western Canada, as stable high pressure dominates over these regions. To date, this year, 1.92 million hectares have burned in the United States, twice the average for the last decade. Twelve people have died as a result of the fires since March. To help fight the outbreaks, 500 experienced fire fighters from Australia, Canada, New Zealand, and Mexico will arrive on the scene this week They will assist more than 20,000 US firefighters. This satellite image from August 15 shows intensive fires in Idaho (you can see the heat represented as orange) spreading thick smoke to the north east into Montana. The big dry in America's west has been caused by a ridge of high pressure that just won't go away. NOAA's climate prediction center reckons the situation is unlikely to change for some time. Image courtesy of NOAA. What's causing the big blaze? Drought is the big factor, along with hot temperatures and strong winds. But behind these factors lies a persistent ridge of high pressure dominating the western States all summer. The north eastern States, meanwhile, are enjoying a cool and moist summer, with green grass and mushrooms - the product of a warm band of cloud, associated with a trough, that - like the western ridge of high pressure - just won't go away. This isobar and temperature forecast map for August 15, 00 gmt, shows a situation that has remained static all summer in North America. A ridge of high pressure extends over the south-western States from the sub-tropical Pacific and under that balloon of high pressure are clear skies, a gentle southerly air flow and hot temperatures. After months of this kind of weather, just mention the word forest fire and something will ignite. Note the cooler arm of lower pressure over the eastern seaboard. Climate models caught off guard According to Vernon Kousky of NOAA's Climate Prediction Center in Maryland, the ridge of high pressure has taken up residence farther west than normal. "A stronger than normal ridge was anticipated to occur, " he writes, "But predicting the location proved to be difficult. "Our official seasonal climate outlooks reflected a stronger than normal ridge just to the east of the Rockies, which would have led to warmer than normal conditions throughout the eastern half of the nation. "However, with the stronger than normal ridging occurring over the Intermountain West, the abnormal warmth has occurred over the West and the Great Plains, while many sections of the East have been cooler than average. "This pattern, accompanied by drier than normal conditions in the West and wetter than normal conditions in the Northeast, has been highly persistent during July and August, with no signs of any changes in the immediate future." Kousky notes that La Nina is probably a factor - due to its global influence. But he points out that sea surface temperatures in the equatorial Pacific are returning to normal. "It is, however, difficult to assess the relative importance of a variety of factors, including La Nina, natural variability, blocking, soil moisture, etc," Kousky writes. "In the next 10 years we may see incremental advances in our ability
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