Cyclone Pam Weakens to Category 4; Remembering the Hesston Tornado Outbreak

By: Jeff Masters and Bob Henson , 11:13PM,GMT on March 13,2015

Tropical Cyclone Pam has weakened slightly to a top-end Category 4 storm with top sustained winds of 155 mph, according to the 2 pm EDT Friday advisory from the Joint Typhoon Warning Center (JTWC). The official tropical cyclone warning center for the area, the Fiji Meteorological Service, estimated that Pam's central pressure remained at 899 mb at 2 pm EDT Friday, the same value as at 8 am. Satellite loops show that Pam is still a huge and intimidating storm, with a prominent 14-mile diameter eye and a very large area of intense eyewall thunderstorms with extremely cold cloud tops. The clouds have warmed in recent hours, as the strong updrafts in the eyewall have weakened and are no longer pushing the cloud tops to the base of the stratosphere. Steady weakening should continue over the weekend, but Pam will still be capable of bringing tropical storm-force winds and destructive waves to the northern portion of New Zealand on Sunday.


Figure 1. Tropical Cyclone Pam as seen by the VIIRS instrument on the Suomi satellite at 10:42 am EDT March 13, 2015. At the time, Pam was a Category 5 storm with 165 mph winds, and was just southeast of Efate Island, where the capital of Vanuatu, Port Vila, lies. Image credit: @NOAASatellites.

Pam's eyewall clobbered three major islands of the South Pacific island nation of Vanuatu on Friday. Even though Pam had weakened slightly to 155 mph winds, the southern islands of Erromango (population 2,000) and Tanna (pop 29,000) likely took the worst punishment from the storm, due to the fact these islands were hit by the stronger right-front quadrant (southeast) side of the storm. Pam was at its peak strength, 165 mph Category 5 winds, when its weaker left (southwest quadrant) eyewall passed over the eastern side of Efate Island, Vanuatu's most populous island (population 66,000.) However, since the weather station in the capital city of Port Vila survived the storm and reported winds below tropical storm force, this most populous (southwest) portion of the island may have escaped severe damage. Despite this fact, there is a good chance that Pam will be the most expensive natural disaster in Vanuatu's history.

25 Years Ago Today: An Early Outbreak for Tornado Alley
The U.S. is in the midst of an extremely quiet March for severe weather thus far, with no tornadoes and only three reports of large hail. Things looked far different 25 years ago today, as the central Great Plains endured its worst late-winter tornado outbreak on record. From March 11 to 13, 1990, more than 60 tornadoes plowed across Tornado Alley, from north Texas to Iowa. The meteorological setup for this outbreak was classic, but the seasonal timing wasn’t. Never before or since have such strong tornadoes been observed so far northwest so early in the year.


Figure 2. Tracks and intensities of the tornadoes observed on March 13-14, 1990, across the central Plains. Image credit: National Weather Service.

The standouts of this outbreak were two long-lived F5 tornadoes that raked central Kansas on the late afternoon of March 13. The twisters were the first in a family of five generated by a single supercell thunderstorm. The first one developed near the town of Pretty Prairie, west of Wichita, and went on to strike the town of Hesston near the end of its 48-mile path, destroying more than 200 homes. As this F5 tornado narrowed and dissipated, a new one formed about a mile to its north within the same larger-scale circulation. An analysis by eminent tornado scientist Theodore “Ted” Fujita found that the two twisters likely rotated around a common axis as the first one dissipated and the second one expanded, delivering a glancing blow to the town of Goessel along its 22-mile path. There have been only four other days since 1950--and none so early in the year--with at least two tornadoes producing F5 or EF5 damage (May 25, 1955; May 15, 1968; April 3, 1974, and April 27, 2011).

Despite the strength and longevity of the Hesston and Goessel tornadoes, they caused only two fatalities. The low death toll becomes even more impressive when you consider the limits of communication and warning technology circa 1990. At that time, the National Weather Service office in Wichita still relied on a 1957-vintage, pre-Doppler weather radar that tracked precipitation but provided no wind data. ”We would draw what we saw on a reflection plotter and then trace contours on a paper map overlay to locate a storm geographically,” recalled Randy Steadham, who was filling in as meteorologist in charge of the Wichita NWS office. The emergence of a classic hook echo on radar, plus a number of eyewitness reports, helped the office to track the storm closely and issue updates roughly every 9 minutes, which made it to the public via radio, TV, and NOAA Weather Radio.


Video 1. A compilation of videos of the F5 tornado that ended up striking Hesston, KS, on March 13, 1990.

The long-lived Hesston tornado was one of the first to be widely captured on videotape, a feat made possible by the new availability of affordable video cameras. According to storm-chase documentarian Blake Naftel, the only previous F5 to be videotaped from multiple perspectives was the one that struck near Wheatland, PA, on May 31, 1985. After the March 1990 outbreak, a team led by Jonathan Davies drew on a number of photos and videos and a post-storm ground survey to carry out detailed analysis of the Hesston tornado’s evolution and structure and its “handoff” to the the Goessel twister. Their results appeared in a 1994 paper in the Bulletin of the American Meteorological Society (PDF available at top of linked page). The group’s work touched on several longstanding challenges, including the task of distinguishing between tornadoes and sub-vortexes. ”In a sense, the more information we have, the more confusing and problematic the events become,” they wrote. And though the notion of high-resolution digital video cameras tucked within cellphones lay far in the future, the authors did note: “Given the proliferation of inexpensive, simple video cameras, it is increasing(ly) likely that at least some video images will be obtained for many, if not most, significant tornado events.”

The Wichita NWS office maintains a page on the Hesston/Goessel tornadoes with a number of photos, videos, and writeups. A slideshow from the Wichita Eagle captures the tornadoes’ devastation. Another long-lived supercell to the north produced an F4 tornado or tornado family that tracked across more than 120 miles in Nebraska. The NWS office in Hastings, NE, covers the outbreak’s northern extent in more detail.

This week’s WunderPoster: Frost flowers
As winter winds up in the Northern Hemisphere and flowers begin to dot the landscape, this week’s WunderPoster (Figure 3, right) harks back to the cold-season beauty of frost flowers. These intricately branched features can blossom where very cold, dry, and calm air settles over a thin layer of lake ice or sea ice. All WunderPosters can be downloaded in formats suitable for posters or postcards.

Now it’s your turn: help us create a WunderPoster!
To celebrate our 20th anniversary, we’re looking to you to provide inspiration for a new WunderPoster. Our “picture yourself here” website has all the details.