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Climate of 2001
animated fire pictureWestern U.S. Wildfires
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National Climatic Data Center, 17 August 2001

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August Wildfires
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Contents of This Report:

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Top of Page Overview

The western fire season is in full swing as a combination of above normal temperatures and drought have contributed to numerous large wildfires. The national level of preparedness as defined by the National Interagency Fire Center increased to the highest point on the August 16, as more than a half a million acres were burning in 42 large fires across the United States. Many of the fires were lightning induced, aided by low relative humidity and strong winds. Fire activity as of mid-August was near to or slightly above the 10-year average.

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Top of Page Long-term Drought Index

Much below normal precipitation last winter across the West has resulted in very dry antecedent conditions, with timber and grasslands quite susceptible to ignition from dry thunderstorm produced lightning strikes. The Palmer Drought Index map shows long-term (cumulative) drought and wet spell conditions. By the end of July 2001, long term drought continued across a large part of the inter-mountain West. On these maps, the red shading denotes drought conditions while the green shading indicates wet conditions. Severe to extreme drought covered much of Nevada, Washington, Oregon, Idaho, Montana and Wyoming. U.S. Animated PDI
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The August 14 edition of the Drought Monitor combines the effects of short term and long term drought and shows that many of the major wildfires are coincident with areas experiencing the most severe drought conditions. More precipitation has fallen along coastal areas of Washington and Oregon curtailing the fire threat there.

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Top of Page Precipitation Rankings & Time Series

In July 2001, most western states received above normal precipitation. However, this rainfall was not enough to ease the long-term dryness that has plagued the region. The maps below show the precipitation rankings by state for 3 months, 6 months and 12 months ending July 2001, with a ranking of 1 reflecting record dryness.

3 month Precipitation rankings
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6 month Precipitation rankings
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12 month Precipitation rankings
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The table below contains precipitation ranks for selected states for the period August 2000-July 2001 and the corresponding 12 month precipitation departure (based on the 1961-1990 normals). Clicking on the rank will display the corresponding time series. The ranks are based on the period 1895-2001 with a rank of 1 reflecting record dryness and 106 reflecting record wetness.

State Rank Departure
Washington 3 10.99
Oregon 3 9.32
Idaho 6 4.71
California 17 6.25
Nevada 37 0.94

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Precipitation Analysis

Maps of percent-of-average precipitation (below) show the persistence of below normal precipitation amounts throughout much of the western U.S. Average precipitation during the past 12 months has been within 50-90% of normal. But when averaged within the most recent 3 month period, precipitation amounts have been more variable, generally ranging between 30 - 70% of normal across fire affected areas. (These maps were provided by the Western Regional Climate Center (WRCC) using data from the Climate Prediction Center and the National Climatic Data Center.)

18-Month % of Average 12-Month % of Average 6-Month % of Average 3-Month % of Average


Maps of the Standardized Precipitation Index (SPI) (also provided by the WRCC) for the same periods also show dry conditions across many of the same areas. The SPI provides a single numeric value to precipitation which facilitates comparisons across regions with different climates. Above normal precipitation in July has done little to offset long-term precipitation deficits in the 12 month period.

18-Month SPI 12-Month SPI 6-Month SPI 3-Month SPI

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Climate and Wildfires - Answers to Frequently Asked Questions

  • How does weather and climate affect the incidence of wildfires?

    The climate related variables that affect the severity of the wildfire season are those that affect soil moisture content, vegetation density, and the moisture content of live vegetation. Extended periods of above-normal temperatures and below-normal rainfall are key factors that contribute to an active wildfire season. Available moisture is rapidly lost due to high evapotranspiration rates under hot and dry conditions. If the losses due to evapotranspiration are not replaced through precipitation, below-normal soil and vegetation moisture levels increase the potential for wildfire development.

  • Are a lack of precipitation and hot temperatures the only factors that affect the severity of the wildfire season?

    No. Although extended periods of hot and dry weather contribute to the incidence of wildfires, several other factors play important roles. Vegetation density, wind speeds, humidity, and the incidence of lightning producing thunderstorms are critical factors that help determine the extent to which wildfires develop.

  • Has it ever been hotter than it is in the western U.S. today?

    Yes. Although three month temperature averages for the West region were at record levels, average monthly temperatures in July alone were not as noteworthy. Nevada set a record for warmest May-July in the 1895 to present record.

  • Has it ever been drier than it is in the western U.S. today?

    Yes. Although severe and extreme drought conditions are widespread throughout the western US, many areas have experienced drier periods than in the recent past. On a regional basis, the last 12 months were third driest across the Northwest region and twentieth driest for the West region.

For additional information on the 2001 wildfire season please see the National Interagency Fire Center web site or the U.S. Forest Service Fire and Aviation web site. Satellite images of the wildfires in the West can be obtained from NOAA's Operational Significant Events Imagery
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Top of Page For all climate questions other than questions concerning this report, please contact the National Climatic Data Center's Climate Services Division:

Climate Services Division
NOAA/National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801-5001
fax: 828-271-4876
phone: 828-271-4800
email: questions@ncdc.noaa.gov

For further information on the historical climate perspective presented in this report, contact:

Scott Stephens
NOAA/National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801-5001
fax: 828-271-4328
email: Scott.Stephens@noaa.gov

or

Dimitri Chappas
NOAA/National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801-5001
fax: 828-271-4328
email: Dimitri.H.Chappas@noaa.gov


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