WMOD00285 ‎(2)‎

• Title: Feasibility Study of Wintertime Cloud Seeding to Augment Arizona Water Supplies
• Date: 1/1/1987
• State: AZ
• Weather Modification - Doc Type: Report

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4. Applicability of Cloud Seeding to Arizona Mountains

4.1 Winter storm patterns

Certain weather patterns favor the occurrence of wintertime precipitation over
the Arizona mountains. The degree of presence of some features, such as
moisture in the atmosphere at low and mid-levels, may determine whether a
storm system deposits none to heavy precipitation at locations of interest.
Of importance to this study are the synoptic-scale (at least a few hundreds of
kilometers) patterns that frequently lead to precipitation occurrences over
the Arizona mountains during winter.

A review of attendant synpotic conditions for higher frequency storm events,
depositing precipitation during winter (November through April) led to the
i dent ifi cati on of several promi nent weather' patterns. Gi ven in fi gures 3-7
are several maps indicating in progression, height contour patterns at the
500 mb (millibar) pressure level (about 5.6 km or 18,000 ft) that are asso-
ciated with a producer of moderate to heavy amounts of winter precipitation
over the Arizona mountains. This system, indicated as A-I, is characterized
by the initial presence of a cold air mass over the British Columbia region
whi ch is advected southward by a strong stl"engtheni ng 1 ow-p res sure wave.
Depending on such features as the temperature of the plunging air mass and
timing, the system moves into southern CalHornia and develops a closed cir-
cul ati on whi ch frequently slows its progressi ve movement. The strong and per-
sistent circulation leads to afTl>le moistur1e advection from the Pacific Ocean
into Arizona.

On the surface, cold air moves southward into the Great Basin and a low
pressure wave on the cold front strengthens over the Nevada-Utah area. As the
front approaches, precipitation commences and continues intermittently until
the air dries in northwesterly flow aloft sometime after the passage of the
upper-air wave axis. Intermittent precipitation may occur for one to three
days. Very cold, slow moving systems may produce substantial amounts of pre-
cipitation. Such storms may be followed by a similar pattern in a few days.
If the planetary-scale (at least several thousand kilometers) pattern is
quasi-stationary, there may occur a series of such systems of varying strength

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