North Central Research Station

Fire Weather Patterns

It has long been recognized that atmospheric conditions play a critical role in affecting the behavior and severity of wildland fires as well as the probability of their occurrence.  Fire managers rely heavily on current observations and forecasts of local, regional, and synoptic atmospheric conditions to prepare for fire suppression and prescribed fire activities.  Understanding the relationships of large-scale upper and middle atmospheric processes to regional-scale fire-weather systems is critical for establishing reliable forecasts of fire-weather and for assessing how potential large-scale changes resulting from a globally changed climate might influence fire occurrence and severity in the US.

One of the most comprehensive observational studies of atmospheric synoptic-scale linkages with high fire danger was performed by Schroeder et al (1964).  Through subjective analyses, they were able to identify specific surface and 500 mb circulation patterns conducive for high fire load index values for 14 different regions of the US.  As an extension of this study, a statistically based classification of middle atmospheric circulation patterns and lower atmospheric temperature and moisture patterns prevalent at the onset of severe wildland fires in different regions of the continental US has been performed.  Although many factors in addition to atmospheric conditions prior to and during a wildland fire determine the probability of a severe fire occurrence, this study addresses only the synoptic-scale atmospheric circulation, temperature, and moisture fields as forcing mechanisms for fire occurrence.

Archived US wildland fire data for the period 1971-1991 were obtained for fires that burned more than 1000 acres.  Fire occurrence data were organized according to the geographic locations of the fires.  For those days when wildland fires occurred in each defined region, the 1200 GMT 500 mb geopotential height fields, the 0000 GMT 850 mb temperature fields, and the 0000 GMT lower atmospheric relative humidity fields were extracted from the National Center for Environmental Prediction's (NCEP) Limited Fine Mesh (LFM) Model initialization fields data base.  Gridded geopotential height anomaly fields were calculated by subtracting the appropriate 1971-1991 gridded monthly averages of 500 mb geopotential heights from the extracted LFM initialization fields.  These anomaly fields provided the basis for determining the prevalent circulation patterns at the onset of severe fires in each of the six defined regions.

The 500 mb geopotential height anomaly fields were subjected to empirical-orthogonal-function (EOF) analyses to determine the principal components of the anomaly fields.  Those EOFs accounting for the most variance in the observed geopotential height anomaly fields were used as guidance in establishing a set of synoptic 500 mb circulation pattern classifications that characterize the atmosphere at the onset of most severe fires in each of the six defined regions.  Average gridded 850 mb temperature and lower atmospheric relative humidity anomaly fields associated with each of the synoptic 500 mb circulation patterns were also determined from the LFM data base.

Results from the EOF analyses indicate that there are typically two or three specific middle atmospheric (500 mb) synoptic circulation patterns associated with severe wildland fire occurrence in each of the six regions.  These circulation patterns lead to specific lower atmospheric (850 mb) synoptic temperature patterns and lower atmospheric (surface to 700 mb) synoptic relative humidity patterns.

Region 1 (NW) Region 2 (NC) Region 3 (NE)     Region 4 (SW) Region 5 (SC) Region 6 (SE)

The 500 mb circulation patterns and the associated lower atmospheric temperature and moisture patterns over the US can influence the Lower Atmospheric Severity Index (LASI), also known as the Haines Index.  This atmospheric index represents the potential for severe wildfire occurrence based solely on lower atmospheric temperature and moisture conditions.  Current and forecasted LASI values can be obtained from the University of Wisconsin Nonhydrostatic Modeling System

Publications can be obtained from your local library, the appropriate journal, or the authors, if supplies remain.

• Synoptic circulation and temperature patterns during severe wildland fires.  Heilman W. 1995. In: Proceedings of the Ninth Conference on Applied Climatology, Jan 15-20 1995, Dallas TX. American Meteorological Society, Boston MA. p 346-351.
• Synoptic weather types associated with critical fire weather.  Schroeder M, Glovinsky M, Hendricks V, Hood F, Hull M, Jacobson H, Kirkpatrick R, Krueger D, Mallory L, Oertel A, Reese R, Sergius L, Syverson C. 1964. Pacific Southwest Forest and Range Experiment Station, Berkeley CA. 492 pp.

For more information:

• Warren E Heilman  Project Leader, Research Meteorologist