National Fire Danger Rating System

Weak southerly flow aloft between the upper level trough and the upper level ridge will in time advect mid- and/or high-level moisture north along the Sierra Nevada Mountains of California into eastern Oregon and eastern Washington. Diurnal thunderstorm activity is common with this pattern - especially along the east slopes of the Cascades in Washington and Oregon. Thunderstorms usually develop during the late afternoon and early evening hours as convective temperatures in the area are reached. The activity continues into the early evening hours, usually dissipating by midnight in most areas. Initial thunderstorm activity tends to be high-based due to an extremely dry, sub-cloud base air mass. Dry lightning can be frequent and widespread with these patterns and can cause numerous fire starts if antecedent weather prior to the initial thunderstorm activity had been hot and dry.

As the upper level trough ejects inland, thunderstorm activity will increase and become more widespread as synoptic-scale lift increases vertical motions in the mid and upper-level troposphere. Oftentimes, the thunderstorm and lightning activity persists throughout the night.

Note: Thunderstorm activity with this type of pattern is usually concentrated in Eastern Washington where the presence of the thermal trough and the advection of cooler air aloft results in increasing environmental lapse rates. The air mass west of the Cascades becomes more stable as increasing onshore flow brings marine air back into the interior lowlands. However, if the layer or marine air is too shallow to make it deep into the Cascades, thunderstorm activity can develop along the west slopes of the Cascades during the late aternoon and early evening hours.