Thunderstorm events that caused major increases in wildfire activity across Western Washington were analyzed in an effort to identify common upper level flow patterns. The composite charts (sea level pressure and 500mb heights) shown on this page were constructed from three separate thunderstorm events that showed similar flow patterns at 500 mb. The northward advection of subtropical moisture around an upper level ridge centered over the western states is a critical fire weather pattern for Washington. Thunderstorm activity associated with this synoptic pattern can result in serious fire control problems for wildland firefighers due to the high number of new fire starts from lightning activity.

The pattern is characterized by high amplitude, meridional flow across the Pacific with a deep trough of low pressure over the eastern Pacific and a strong, ridge of high pressure aloft over the Western States. Southerly flow aloft around the back side of the upper level ridge will oftentimes advect mid- and high-level moisture north into the Pacific Northwest from central and northern California - especially if the pattern persists for several days as is usually the case.

The mean sea level pressure pattern shows a broad thermal trough of low pressure along the west coast from California north into the interior lowlands of Western Washington. Surface high pressure is situated to the north over northern British Columbia and to the east over the northern and central Plains. The presence of the thermal trough over Western Washington, along with large-scale subsidence associated with the upper level ridge, results in a very warm, dry and stable air mass over the Pacific Northwest. Marine is is typically quite shallow and generally confined to the immediate coast. At 00Z, the lower levels of the air mass are characterized by a well mixed, dry adiabatic boundary layer.

Thunderstorm activity with this synoptic pattern initially develops along the Sierra Nevada Mountains in California. It then shifts north - usually along the Oregon Cascades - into Washington over the period of several days. Diurnal, air mass type thunderstorms usually develop during the late afternoon and early evening hours, oftentimes over the higher terrain. The activity then shifts north before dissipating around midnight. Each day the activity shifts further north.

The first surge of moisture into the Pacific Northwest may bring an outbreak of dry lightning on either side of the Cascades. Because weather condition are generally hot and dry prior to the initial surge of moisture, lightning-caused wildfire starts are common due to high ignition probabilities caused by critically dry fuel conditions. In time, the lower levels of the air mass will become moist enough to support "wetting" rainshowers with most of the thunderstorms.

Watch for mid-level triggers ( vorticity centers or weak shortwaves) rotating around the perimeter of the upper level high. Lift associated with these features can result in widespread, nocturnal thunderstorms which persist into the morning hours of the following day.




	LIGHTNING PATTERN #1 - Close Upper Level Lows



	LIGHTNING PATTERN #2 - Ejecting shortwaves



	LIGHTNING PATTERN #3 - Subtropical Moisture



	LIGHTNING PATTERN #4 - Shortwave in NW flow pattern

Please refer any questions or comments about this web site to: john.werth@noaa.gov