Quiet tornado season wakes up; severe thunderstorm in India leaves 1 million homeless

After a record quiet start to the 2010 severe weather season, the atmosphere finally unleashed one of its classic violent spring weather days yesterday. NOAA's Storm Prediction Center tallied 32 tornado reports in Colorado, Kansas, and Texas. Fortunately, most of the storms occurred over uninhabited areas, and no injuries or major damage were reported. Our severe weather expert Dr. Rob Carver, has the details on the action in his blog today. The action was focused in a region the Storm Prediction Center had put in their "Moderate Risk" area for severe weather in yesterday's outlook. This was the first "Moderate Risk" region declared so far this year, which is a record for the latest day in the year this has occurred. According to Rich Thompson, a forecaster at NOAA's Storm Prediction Center, the previous record latest date for a "Moderate Risk" severe weather day was March 21, 2005. So, we beat the previous record by an entire month, which is a remarkable feat. There has been only one tornado death so far this year, the lowest death toll this far into the season on record. Typically, about half of the 80 or so tornado deaths we average per year have occurred by this point in the season. Yesterday's preliminary tornado count of 32 came close matching the preliminary U.S. tornado count for the entire month of March--36. March was the fourth-quietest March for tornado activity since record keeping began in 1950. The three-year average for March tornadoes is 138. One reason for the quiet tornado season has been the "upside down" winter and early spring we've experienced over North America over the past 3 1/2 months. Temperatures in Canada have been the warmest on record during this period, but have been unusually cold over the southern U.S. and Gulf of Mexico. Since the instability that severe thunderstorms need to occur comes from warm, moist air from the Gulf of Mexico encountering cold, dry air from Canada, this year's unusual "upside down" configuration has led to a much more stable than usual atmosphere over tornado alley in the U.S.


Figure 1. Doppler radar storm-relative velocity of the mile-wide tornado that affected western Kansas at 4:40 MDT April 22, 2010. No damage or injuries were reported from the tornado. A second tornado's Doppler radar signature is also visible at the bottom of the image.

Another severe weather outbreak expected today and Saturday
The Storm Prediction Center has outlined another "Moderate Risk" region of concern for severe weather today, this time centered over Arkansas, Mississippi, and northern Louisiana (Figure 2.) Tomorrow, the action moves eastwards and will be centered over Mississippi, Alabama, and Tennessee. This 3-day severe weather event will probably end up being one of the most significant of the year. Our severe weather expert Dr. Rob Carver will blog about today's action after it is over. As usual, you can follow today's severe weather outbreak with our interactive tornado page and our severe weather page..


Figure 2. Severe weather risk areas for Friday April 23, 2010.

Tornado and associated severe weather leave 1 million homeless in India
The U.S. has the world's most violent and numerous tornadoes, but second place goes to Bangladesh and eastern India. There, warm moist air from the Bay of Bengal often encounters cold, dry air from the Himalayas, setting up the instability needed to support severe thunderstorms. On Tuesday, April 13, and very unstable airmass (CAPE values > 3000) with strong westerly wind shear set up over eastern India, providing the classic set-up for supercell thunderstorms. Radar loops from the Kolkatta radar that day show a severe thunderstorm formed over extreme northeast India, near the Bangladesh border, and moved southeast into Bangladesh. The thunderstorm appeared to form a "bow echo", a configuration that often generates strong winds in excess of hurricane force near the bowed-out portion of the radar echo. Winds of 75 mph affected a large area of densely populated land, killing 137, severely damaging or destroying 200,000 homes, and leaving 1 million homeless. A weak tornado may have accompanied the storm. This may be the greatest number of people ever left homeless by a severe thunderstorm in world history.


Figure 3. Radar image from the Kolkatta Regional Meteorological Centre of the Indian Meteorological Department, showing the severe thunderstorm that killed 137 people and left 1 million homeless. Thanks go to Steve Nesbitt of UIUC for saving this image.

Jeff Masters