Impacts on North America

Severe large-scale weather and climate events, such as major floods, droughts, heat waves, and cold waves, affect all parts of the United States, often with large social and economic impacts. Over the past year, severe drought has plagued portions of the southern and western U.S., with estimated agricultural losses in Texas alone reported at greater than $1B. During the same period, much of the East has suffered from recurrent periods of heavy precipitation, at times bringing severe flooding. In just one month, February, a major cold-air outbreak covering more than half of the country and flooding over parts of the Pacific Northwest and eastern U.S. resulted in significant loss of life and property damages in excess of several hundred million dollars. Questions of basic interest to the public, are "Why?" - that is, can we identify the causes for these extreme events, and "Where and When?" - can such events be predicted and, if so, at what time range? Research in NOAA is actively addressing these questions.

Identifying Causes of Extreme Events

Extreme weather phenomena range from localized flash floods that can occur in minutes to droughts which persist for decades or longer and affect large parts of continents. This range reflects the equally wide array of processes that contribute to, and link together, weather and climate variability. One of the fundamental tasks of diagnostic meteorology is to identify the primary processes responsible for particular forms of this variability.

Over the past year, two of the most prominent and economically significant U.S. weather stories have been the drought over the Southwest and the excessive precipitation over parts of the East. As indicated by Dr. Ropelewski of the CPC, both the drought in the Southwest and the excessive rainfall in the East are related to certain unusual features of the jet stream. Extending back as far as last fall, important features for U.S weather have included an unusually strong and persistent high pressure ridge over the Southwest and a somewhat weaker (and more variable) low pressure trough in the East (Figure 1). The high pressure ridge had several effects on weather over the Southwest, including a tendency to divert Pacific storm systems well to the north of the region, to oppose inflow of moisture from the Gulf of Mexico, and to produce descending air motions near and to the east of the high pressure center.

All of these factors suppress precipitation over the region, and it is therefore reasonable to consider this persistent ridge as the primary direct, or immediate, cause of the Southwest drought. In contrast, although a trough in the East favors storm development and moisture transport into the region, the links to the observed excessive precipitation are more complex. Several distinct phenomena have contributed to the recurrent heavy precipitation, including large-scale storm systems, often with heavy snowfall (predominantly winter), a nearly stationary frontal boundary (predominantly spring) and, most recently, a hurricane.