WMOD00400

• Contract/Permit #: 14-06-D-6801
• Title: Structure and Seedability of San Juan Storms
• Date: 11/1/1976
• Weather Modification - Doc Type: Report

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and thereby further destabilize the storm. During this neutral stage a con-

vergence zone begins to form over Pagosa and parallel to the continental

divide. The convergence at the surface usually begins on the mountain side

first or simultaneously with the beginnings of convergence on the upwind

or valley side. The seeding material typically does not reach Wolf Creek

Pass during this stage. If it rises into the clouds rather than being

advected toward the west and northwest, then the seeding effect most likely

occurs over the San Juans west of Wolf Creek Pass.

During the stable and neutral stages the thermodynamic stability and

wind shear above mountain level are often such that there is dynamic in-

stability present. Consequently clouds associated with gravity waves some-

times develop upwind of the mountain. The "aves are almost always present

over and do"mlind of the mountain during these stages. Breaking Kelvin-

Helmholtz waves were observed on two separate occasions while flying downwind

of Wolf Creek Pass.
The upwind edge of the cloud during the stable and neutral stages is at

least several miles up"ind of the San Juans. In examining satellite photo-

graphs it was common to observe a continuous band of clouds extending from

California and Arizona to the San Juans.

c. Unstable stage

During this stage convectively unstable clouds are present. The upwind

edge of the clouds is typically near Navajo Reservoir. The cloud bases occur

at 2.5 to 3.0 km and the subcloud winds converge rapidly toward the conver-

gence zone over Pagosa. It is common in the unstable stage to observe

significant convergence into the convergence zone over Pagosa at all levels

at and below 3.6 km.