![]() Beware of advancing tropical storms as well. Look for strong winds that accompany the flow. Often critical weather patterns follow the frontal passage that brings extremely dry air due to a strong westerly or northwesterly flow. In the southeastern US, drought is frequently associated with the La Niña state of the southern oscillation pattern or a blocking ridge aloft near the Atlantic coast. Cold fronts produce relatively short lived periods of high winds and instability that can produce extreme fire behavior. In the northern plains, Great Lakes, and the northeastern US, pre-frontal high pressure from the Pacific, Northwestern Canada, and Hudson Bay all can produce very dry conditions. The area between the upper ridge and upper trough has the most critical upper air pattern because of unstable air and strong winds aloft that descend to ground level.Įast of the Rocky Mountains, most critical fire weather patterns are associated with the periphery of high-pressure areas, particularly in the pre-frontal and post-frontal areas. Foehn winds occur on the lee side of mountain ranges and are typically very strong, often occurring suddenly with drastic warming and drying. Cold front passages are important to firefighters because of strong, shifting winds, and unstable air that can enhance the smoke column or produce thunderstorms. The surface pressure patterns of most concern are those associated with cold fronts and terrain-induced foehn winds. Most periods of critical fire weather occur in transition zones between high- and low-pressure systems, both at the surface and in the upper air. In brush and timber fuels, drought becomes an important precursor by increasing fuel availability. In both cases, an unusually dry airmass for the region and season must also occur. The critical fire weather patterns that support these conditions can be separated into two primary categories: those that produce strong surface winds, and those that produce atmospheric instability. The four critical weather elements that produce extreme fire behavior are low relative humidity, strong surface wind, unstable air, and drought. Critical Fire Weather Patterns Program for Climate, Ecosystem, and Fire Application (CEFA).Synthesis of Knowledge of Extreme Fire Behavior: Volume 1 for Fire Managers, Chapter 3.Because HDW is affected by heat, moisture, and wind, seasonal and regional variability can be found when comparing HDW values from different locations and times. HDW was designed to be very simple – a multiplication of the maximum wind speed and maximum vapor pressure deficit (VPD) in the lowest 50 or so millibars in the atmosphere. It combines weather data from the surface and low levels of the atmosphere into a first-look product. The Hot-Dry-Windy Index (HDW) was designed to help users determine which days are more likely to have adverse atmospheric conditions that make it more difficult to manage a wildland fire. Thunderstorm Dynamics, Outflows, and Downbursts. ![]() ![]() Breakdown of the Upper Ridge and Cold Frontal Passage. ![]()
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