california_assessment_2005

california_assessment_2005 - 18 December 2004 17 January...

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1 18 December 2004- 17 January 2005: U.S. Storms and Flooding in the West and Midwest Exceptional Warmth in the Midwest and East Gerald D. Bell and Wayne Higgins Climate Prediction Center/NOAA/NWS/NCEP Contents: 1. Introduction pp. 1-2 2. Precipitation pp. 2-3 3. Snowfall in the West p. 3 4. Flooding in the West and Midwest p. 3 5. Exceptionally warm temperatures in the Midwest and East pp. 3-4 6. Atmospheric Circulation pp. 4-8 7. Links to the Madden-Julian Oscillation (MJO) pp. 8-9 8. Summary pp. 9-10 1. Introduction The period 18 December – 17 January 2004-05 featured a series of major winter storms that produced 300%-400+% of normal precipita- tion in the western and midwestern United States (Fig. 1). Periods of ex- ceptional warmth were also observed across the central and eastern United States. These conditions were related to a large-scale upper-level circula- tion pattern characterized by a block- ing ridge in the Gulf of Alaska, an am- plified trough over the southwestern U.S., and a broad ridge in the East. Associated with this circulation, the jet stream and storm track in the West were shifted well south of normal and entered the United States over south- ern California, as compared to their climatological mean position over the Pacific Northwest. For both the West 18 December - 17 January 2004-05 Total Precipitation (mm) a Departure from Normal (mm) b Percent of Normal (%) c Normal (mm) d 25 50 75 100 125 150 175 200 25 50 75 100 125 150 175 200 -150 -100 -50 -25 25 50 75 100 125 150 175 200 25 50 75 125 150 200 250 300 350 400 Fig. 1. Precipitation during the 30-day period 18 December – 17 January 2004-05: (a) total (mm), (b) departure from normal (mm), (c) percent of normal, and (d) normal (mm).
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2 and Midwest, the heavy precipitation events were also associated with deep moisture supplied from the tropical North Pacific and Gulf of Mexico, re- spectively. In the West, the enhanced precipitation was partly related to a strong Madden-Julian Oscillation (MJO), a tropical disturbance that influences tropi- cal rainfall patterns on time scales of 30-60 days. At times the MJO produces ENSO-like features that cause large-scale circulation anomalies over the Pa- cific Ocean and western U.S. such as those observed during the period of interest, and can trigger extreme precipitation events in the western United States. 2. Precipitation a. Southwest For the 30-day period 18 December - 17 January 2004-05 precipitation totals were 200+ mm (8+ inches) across California (Fig. 1a). In southern California and the southern Sierra Mountains these amounts were generally 150-200 mm (6 to 8 inches) above average (Fig. 1b), and more than four times the normal for the period (Figs. 1c, d). Large precipitation totals of 75-150 mm (3 to 6 inches) were also observed across much of the Southwest and Inter-Mountain region of the West during the period, with northern Arizona, Utah, Nevada, southern Colorado and southern Wyoming all recording more than 400% of normal precipitation. This excessive precipitation resulted from
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california_assessment_2005 - 18 December 2004 17 January...

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