Despite the heavy toll, the storm system that spawned the tornado wasn't unusual for this time of year, say meteorologists. The high death toll resulted from the twister's path through a commercial area including a hospital, a nursing home, a row of crowded restaurants and several large stores. The winds, while as high as 198 miles per hour, weren't unusual for powerful springtime tornadoes in parts of the U.S.

Highlighting the unpredictability of such lethal storms, the weeks before the twister marked an unusual lull in the number of tornadoes that normally occur this time of year, experts said. More than half the season's severe tornadoes usually strike in May and June. During the first three weeks of May, however, the number of powerful twisters had dipped to historic lows, federal meteorologists said.

"We were so far below normal in the first three weeks of May that we may not catch up to normal for the month," said meteorologist Harold Brooks at the National Severe Storms Laboratory in Norman, Okla. [ wsj ]

April was especially busy, contributing to one of the most severe spring tornado seasons on record. NOAA officials estimate there have been almost 1,000 tornadoes this year, double the total of a typical year, with storms raging from Missouri to North Carolina. This year's tally of 50 fatal tornadoes compares with about 20 in an average year.

Forecasters expect severe thunderstorms, which can spawn tornadoes, to persist for the next several days, heightening the potential for more twisters before the storm system is blown out to sea and dissipates.

Strong tornadoes are "likely" Tuesday over Oklahoma, Kansas, and other areas, with the storm system moving eastward Wednesday to Southeast Missouri, Central Illinois, and surrounding states, the National Weather Service said.

The twister in Joplin comes on the heels last month of the largest one-day outbreak of tornadoes to date, when 226 twisters were reported during a single 24-hour period on April 27, largely in Alabama and Mississippi. All told, the cluster of tornadoes, which continued into the following day, killed more than 340 people.

This spring's outbreak surpasses any since 1953, when 519 people were killed at a time when forecasters lacked the technology and ability to warn people well before storms hit. The 1953 toll included 116 people killed when a tornado struck Flint, Mich., 114 from one in Waco, Texas, and 90 from a tornado in Worcester, Mass. The toll from that season was so heavy that it prompted the federal government to help set up ground-based radar systems that allowed local weather forecasters to track storms across regions, rather than relying solely on observational reports from weather bureaus outside their areas.

Final numbers on tornadoes and deaths this year won't be available for months because early tallies include duplicate reports—and some unreported storms. NOAA officials warn that for now the tornado numbers will appear high until local weather forecasters can confirm the final count.