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17 A typical storm evolves through four separate stages. The stages are labeled stable, neutral, unstable and dissipation in Fig. 3.1. Most storms evolve through all four stages (c.L, 28-29 December, 27-28 January, 29-30 January, 30-31 January and 16-17 March). The length of time of the various stages lasts up to 6 to 9 hours. Parts of the conditions such as blocked flmoJ', 500 mb temperatures dropping belmoJ' -23C or convective instability are sometimes missing on particular storms but the basic sequence is invariant. Fig. 3.2 depicts a typical mesoscale surface streamline analysis for each of the four stages. a. Stable stage During this stage the clouds are forming and become deep and stably stratified with blocked flow often present upwind of the mountains. The 500 mb temperatures are almost always much warmer than -23C. Near the time of onset of precipitation the generators are ignited if the randomized decision ~vas to seed. The surface winds are from the south across New }lexico because of induced surface convergence resulting from strong diver- gence aloft at the exit zone of the approaching jet core. As the stably stratified surface winds from the south reach the San Juan mountains they are deviated toward the west or northwest. Occasional downslope flow occurs during this stage from off the San Juans south of Wolf Creek Pass. b. Neutral stage During the neutral sta'ge the atmosphere may contain some weak embedded convective instability. If blocking flow is present most of the destabiliza- tion results from cold air advection aloft. During this stage the blocking flO\\', if present, is removed. 1.Je speculate that one of the functions whi'ch the embedded convection serves is to remove the stably stratified air mass. This allO\oJ's Harm, moist air to advect into the lower levels from the southwest I I I I I I I I I I I I I I I I I I I