EF-4 tornado kills 6 in Texas; Mahasen makes landfall in Bangladesh, killing 10

After going twelve months with a record-low tornado death toll of just seven people, last night we received a jolting reminder that tornadoes typically kill a lot more people than that in the U.S. A deadly tornado swept through Granbury, Texas near 8 pm CDT, killing six and injuring up to 100. The weather system that spawned the Granby tornado also unleashed a mile-wide twister that hit Cleburne, about 25 miles southeast of Granbury. Damage was heavy in Cleburne and a state of emergency declared, but only seven minor injuries were reported. A third tornado hit the small town of Millsap, about 40 miles west of Fort Worth, causing roof damage a destroying a barn, but caused no injuries. Preliminary figures indicate that a total ten tornadoes touched down in Texas last night, and NWS damage survey teams are out today to determine the exact total and how strong they were. The National Weather Service out of Fort Worth has issued a preliminary rating of EF-4 to the Granbury tornado, making it the first tornado stronger than EF-0 reported in May 2013. The storms also dumped softball-sized hail up to 4" in diameter in Mineral Wells, TX.


Video 1. The Granbury, Texas tornado of May 15, 2013.

Thursday's tornado was the deadliest U.S. tornado in over a year. The last time six people died in a U.S. tornado was on April 14, 2012, during an EF-3 tornado that hit Woodward, Oklahoma. The last Texas tornado that was deadlier occurred on April 24, 2007 in Maverick County, when an EF-3 tornado hit Eagle Pass, Texas, killing seven. Texas has had one other tornado death in 2013, from a twister that hit on February 21, 2013, in Sabine County. The region of Texas hit by last night's tornadoes has few basements, which may have contributed to the death toll. According to underground member Seattleite, "In this part of Texas basements are very uncommon. The reason is due to the soil, it is basically clay. It contracts and expands with temperature and moisture levels on the order of a foot or more in a typical year. The pressure from this can cause basement walls to cave. They can be built, but it costs at least an extra $20,000+, as they surround the basement with a sand-like barrier to handle the changes in the ground."


Figure 1. Softball, anyone? One of the 4" hailstones that fell near Mineral Wells, Texas on May 15, 2013. Image from Patrick Vondra via Twitter.

Is the 2012 - 2013 tornado drought over?
Thanks to the cold spring in the Midwest during 2013, and the 2012 Midwest drought, the 197 EF-1 and stronger tornadoes that occurred during May 2012 - April 2012 was an all-time minimum for any twelve-month period since at least 1954, wrote tornado researcher Harold Brooks at the U.S. Severe Weather Blog (previous minimum: 247 tornadoes from June 1991-May 1992.) The death toll of just seven was also a record low for any twelve-month period since 1950. Amazingly, this tornado drought occurred less than two years after the record maximum: 1050 EF-1 and stronger tornadoes from June 2010 - May 2011. The extraordinary contrast underscores the crazy fluctuations we've seen in Northern Hemisphere jet stream patterns during the past three years. Call it "Weather Whiplash" of the tornado variety. A blog post by meteorologist Patrick Marsh of NOAA's Storm Prediction Center argues that the record 12-month tornado maximum of 1050 EF-1 and stronger tornadoes from June 2010 - May 2011 was a 1-in-62,500 year event. The record 12-month minimum of 197 EF-1 and stronger tornadoes that occurred from May 2012 - April 2013 was a 1-in-3000 to 1-in-4000 year event. In Marsh's words: "Anyway you look at it, the recent tornado "surplus" and the current tornado "drought" is extremely rare. The fact that we had both of them in the span of a few years is even more so!"

Our tornado drought may be at its end, as the latest forecasts from NOAA's Storm Prediction Center call for an active severe weather pattern Saturday - Monday. The current forecast calls for just a "Slight Risk" on Saturday over the Northern Plains, but the threat will grow on Sunday and Monday as a powerful spring weather system gathers strength over the center of the country.

Saturday's main threat areas: SD to NE, and northern KS
Sunday : IA, parts of MO/KS, to central/eastern OK
Monday : IL/MO to OK/TX border


Figure 2. MODIS image of Tropical Cyclone Mahasen taken at 06:50 UTC Thursday May 16, 2013. Mahasen made landfall about two hour prior to this image as a tropical storm with 50 mph winds. Image credit: NASA.

Tropical Storm Mahasen hits Bangladesh
Tropical Storm Mahasen hit the Bangladesh coast near 08 UTC Thursday, May 16 near a place called Feni north of Chittagong. Mahasen was a tropical storm with top winds of 50 mph at landfall. Satellite observations suggest that the storm was becoming much more organized just before landfall, and it is fortunate that the storm ran out of time to intensify when it did. Mahasen likely brought a storm surge of up to a meter (3.3 feet) to the coast of Bangladesh, but it is the storm's rains that are causing the main problems. Satellite rainfall forecasts made at landfall show that Mahasen could dump up to 20 inches of rain along a swath through Bangladesh and into Northeastern India. These rains will be capable of causing destructive flooding, and ten deaths have already been reported in Bangladesh from the storm. At least eight people have been killed in Sri Lanka due to landslides triggered by Mahasen's heavy rains, and a boat carrying refugees capsized on Monday, killing eight and leaving 50 missing.

First tropical storm of the year, Alvin, forms in the Eastern Pacific
The official start of hurricane season in the Eastern Pacific is Wednesday, May 15, and Mother Nature emphatically agreed, bringing us the first named storm of the year, Tropical Storm Alvin. With wind shear a moderate 10 - 20 knots and the storm currently struggling to hold itself together, it currently appears unlikely that we will see a Hurricane Alvin. The storm is moving west-northwest into the Central Pacific, and is not a threat to any land areas.

I'll have a new post this afternoon, on an important climate change study released today.

Jeff Masters