WMOD00548

• Contract/Permit #: 6-07-DR-20020
• Project Name: Sierra Cooperative Pilot Project
• Title: Satelite Studies During the 1976-77, 1977-78 Sierra Cooperative Pilot Project
• Date: 2/1/1979
• Weather Modification - Doc Type: Report

36

and high clouds disappeared at middle levels, with warm, moist air

having intruded, giving continuous clouds in the vertical as detected

a
.

by rawinsonde. Stability increased in the 85 to 70 kPa layer but the

air below 85 kPa was conditionally unstable yet drier than within the

.
;

stable layer.

The main surface low pressure center stayed offshore from British

Columbia throughout the duration of the storm. A cold front extended

far south of the low into northwest California and beyond as precipi-

tation began. Some distance behind the cold front, clearing out of

the overlying cirrus shield occurred along a ragged cloud edge seen in

the satellite images.

Precipitation increased as the front approached the target and was

fairly steady with time. As the front progressed eastward, a secondary

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low formed on it along the northern California border, moving into north-

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west Nevada by 0900 GMT with the cold front. A strong southerly low-

level jet with winds of 25 ms-l at 600 m preceded the front on the

Sheridan soundings prior to 0900 GMT. CTT was lowest during this time

and except on the 0000 GMT sounding, a 1 to 2 km gap between the upper

and lower cloud decks was present. Most of the precipitation fell

prior to frontal passage during the time of the low-level jet. A lack

of heavily precipitating convective cells is indicated by the precipi-

tation records in Figure 16. The orographic precipitation was enhanced

after 0700 GMT at Blue Canyon and Donner Pass near the time of frontal

passage. The precipitation rate at Truckee, on the downwind side of

the crest, was unaffected by the passing of the cold front.

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Southern reaches of the cold front extending offshore into the

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Pacific from south-central California didn't progress eastward as did