The 500-hPa circulation pattern during January featured above-average heights at high latitudes and below-average heights in the subtropics and lower mid-latitudes (Fig. E9). The main surface temperature departures during the month included above-average temperatures over much of the United States , the high latitudes of the North Atlantic , northern Europe , and western Russia (Fig. E1). The main precipitation anomalies reflected well above-average totals over the southwestern and midwestern United States (Figs. E3, E5, E6).

a. United States

In the West and Midwest , extremely large precipitation totals (300%-400% above normal) began in mid-December and continued through mid-January. During this period, near-record warmth covered the South and East. These signals were the primary contributor to the large monthly mean temperature anomalies (Fig. E1) and precipitation departures (Figs. E3, E5, E6) recorded during January. These conditions were associated with a persistent large-scale anomalous circulation that spanned the eastern North Pacific and North America (Fig. A2.1, top right). Regional aspects of this pattern included a blocking ridge in the Gulf of Alaska , an amplified trough over the southwestern U.S. , both of which were centered 30°-40° longitude west of the climatological ridge and trough positions. The Hudson Bay trough was notably weaker than average, as an upper-level ridge dominated the eastern U.S.

In the West, precipitation totals for the 30-day period 18 December - 17 January 2004 -05 reached 200+ mm across California and 75-150 mm across much of the Southwest and Inter-Mountain region. Most of California , northern Arizona , Utah , Nevada , southern Colorado , and southern Wyoming recorded more than 400% of average precipitation during the period. January marks the fifth straight month of above-average precipitation in the Intermountain region, and the fourth straight month of above-average precipitation in Southern California (Fig. E5). During January, this above-average precipitation resulted from the amplified trough in the West, along with a pronounced southward shift of the mean jet stream and storm track (Figs. E10, T21). This exceptionally focused storm track led to the same areas being hit by several major, moisture-laden winter storms.

The Madden-Julian Oscillation (MJO) also contributed to the enhanced precipitation in the West, through its impacts on the East Asian jet stream and downstream circulation anomalies. This impact was seen in mid-to-late December when the East Asian jet core and jet exit region were retracted westward in response to enhanced MJO-related convection over the equatorial Indian Ocean and suppressed convection near the date line (Figs. T11, T12). These rainfall features led to a pronounced westward shift of the mean upper-level ridge and trough positions across the eastern North Pacific and North America, meaning that the Southwest was situated downstream of a mean upper-level trough instead of beneath the climatological mean ridge (Fig. A2.1 top right). As the enhanced convection associated with the MJO moved eastward to the date line in early January, the East Asian jet stream under-cut the blocking ridge and eventually extended all the way to California . This evolution contributed to the intensity and extended duration of precipitation in the Southwest in early January.

In the Midwest , precipitation totals during 18 December - 17 January 2004 -05 reached 150+ mm (200%-400% of average) from Arkansas and Missouri northeastward to Pennsylvania . Nearly all of this precipitation resulted from major storms that occurred on 23 December, and 2-6 and 11-13 January. During the first storm all of the precipitation fell as snow, with accumulations of 10-20 inches recorded throughout the Ohio Valley region. The second and third storms occurred during a major warm-up in early January. The excessive rainfall from these storms, combined with rapid snowmelt, produced significant flooding from northeastern Oklahoma to Pennsylvania .

These winter storms were embedded within a persistent jet stream that extended northeastward from the central Baja Peninsula to New England . Each storm was associated with strong frontal boundaries and deep northward flow of moisture from the Gulf of Mexico , which contributed substantially to the extreme nature of the precipitation events.

b. North Atlantic and Europe

Above-average 500-hPa heights were again observed across the high latitudes of the North Atlantic during January (Fig. E9). This pattern reflected a strong positive phase (+1.6) of the North Atlantic Oscillation (NAO) (Table E1, Fig. E7), and has prevailed in four of the last five months. These conditions were associated with an enhanced flow of marine air into northern Europe and Scandinavia , which contributed to ongoing above-average temperatures in these areas (Fig. E10). The North Atlantic also saw a continuation of exceptionally warm SSTs at both high latitudes and in the subtropics. This warmth reflects the ongoing warm phase of the Atlantic multi-decadal mode that began approximately in 1995 (Goldenberg et al. Science, 2001).

2. Southern Hemisphere