Wednesday, 19 November 2003: 10:30 AM
Characteristic Composite Charts Associated With Peak Fire Season In Vermont
An eleven year database of known fires reported in Vermont (1991-2001), provided by the Vermont Department of Forests, Parks, and Recreation, shows the months of April and May as the state's peak fire season. This time period is typically referred to as the pre-greenup time of year when leaves have not yet become fully developed on trees. Dead fuels, any non-living organic matter that will burn, are maximized during this period and have the greatest potential to burn. The database is utilized to examine days in which fires were reported in Vermont during April and May. These events are divided into two categories: days with surface relative humidity (hereafter SRH) less than 30 percent and days with SRH greater than 30 percent. The 30 percent SRH value is chosen because it is a critical element with respect to local red flag criteria. Red flag criteria consists of meteorological parameters (sustained winds of 13-23 knots and SRH less than 30 percent), long term dryness, and the vegetation status that all combined result in increased fire danger. NCEP reanalysis data and a local computer program are utilized to create composite charts of characteristic mean sea-level pressure and 500 hPa heights valid at 1200 UTC on the days in April and May in which at least one fire was reported. Two composite patterns are presented which show the large scale meteorological conditions associated with fire occurrence during Vermont's typical peak fire season. One common pattern, with surface high pressure to the west of Vermont and a trough of low pressure to the east, is conducive to producing SRH less than 30 percent. Another pattern, with high pressure to the east of Vermont and low pressure to the west, results in winds speeds of 13-23 knots and gusts as high as 35 knots. These patterns exhibit the critical meteorological parameters that factor into local red flag conditions. Recognition of such patterns, and understanding their impact on the fire environment, will be instrumental for meteorologists at the National Weather Service in Burlington, Vermont when providing necessary products (fire weather watches and/or red flag warnings) and services to the local fire weather user community. This information can then assist users with any land management decisions related to pre-suppression and planning.