December 2021 Pattern Eases West U.S. Drought

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December 2021 upper air 500 MB height anomalies identify the prevailing weather pattern. A West Coast upper trough propels a stormy regime. In the East both the polar and subtropical jet streams arc into a ridge position bringing historic warmth. Note the upper air pattern runs parallel to the regional SSTA regimes in the northeast Pacific and western North Atlantic.

Fig. 1: Upper air 500 MB height anomalies for December 2021.

Discussion: December 2021 upper air 500 MB height anomalies identify the prevailing weather pattern (Fig. 1). A West Coast upper trough propels a stormy regime. In the East both the polar and subtropical jet streams arc into a ridge position bringing historic warmth. Note the upper air pattern runs parallel to the regional SSTA regimes in the northeast Pacific and western North Atlantic.

The upper trough in the West has enabled heavy snow with high water content to initiate. The percent of normal snow water equivalent is running >200% of normal in the Sierra Nevada to the Grand Canyon region (Fig. 2). The percent of normal precipitation is running 120% to 180% of normal for much of the West since October (Fig. 3)

Fig. 2-3: Percent of snow/water equivalent across the West for Dec. 30, 2021 and percent of normal precipitation for the water year beginning October 1, 2021.