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Disclaimer |
Critical Fire Weather Patterns
Close Upper Level Lows
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| Upper Level Low cases are characterized by a closed, upper- level low off the Oregon or northern California coast with a negative-tilt, upper level ridge axis along the Rocky Mountains from the desert southwest to central British Columbia. Southerly flow aloft around the leading edge of the low will oftentimes bring mid- and high-level moisture north into the Pacific Northwest from central and northern California - especially if the low persists for several days.
At the same time, the air mass over the Pacific Northwest begins to destabilize as cooler air aloft begins to push inland over a very warm air mass near the earth's surface. The pattern is usually accompanied by strong, low-level cooling in Western Washington though as low-level, onshore increases when the surface thermal trough shifts into Eastern Washington. (See the composite mean sea level pressure pattern depicted below left). As this occurs, the air mass over Western Washington becomes increasingly more stable while it becomes more unstable east of the Cascades. The area most favorable for thunderstorm development is usually centered over eastern Washington, where vertical motions are enhanced due to the strong couplet of low level convergence associated with the surface thermal trough and an area of strong upper level divergence in the northeast quadrant of the 500mb low. Flow around the upper level low can advect thunderstorm activity - which usually develops during the late afternoon and early evening hours along the east slopes of the Cascades - into the North Cascades National Park and the northwest interior of Western Washington. Thunderstorm activity usually dissipates by midnight, however, if a mid-level trigger is present, thunderstorm activity can persist throughout the night.Composite maps for this type of flow pattern were created from the 5 thunderstorm events listed below.
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 Please refer any questions or comments about this web site to: john.werth@noaa.gov
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