Fantala Tied for Strongest on Record for Indian Ocean; Massive Flash Flood in Houston

Fierce Tropical Cyclone Fantala stormed to Category 5 strength north of Madagascar over the weekend with an impressive burst of strengthening, making the cyclone as strong as any on record anywhere in the Indian Ocean. Fantala’s estimated peak sustained winds of 150 knots (173 mph), averaged over 1 minute by the Joint Typhoon Warning Center, are tied with those of Tropical Cyclone Agnielle (November 1995 peak 1-minute winds of 150 knots) as the strongest in the Southwest Indian Ocean. Fantala and Agnielle both top the record holder for the North Indian Ocean (Super Cyclonic Storm Gonu, June 2007, peak 1-minute winds of 145 knots). Reliable satellite-based records for the Indian Ocean only go back to 1990, but Fantala’s power is still remarkable, and quite evident in satellite imagery. [Update: An earlier version of this post had stated that Fantala was unrivaled as the strongest in the Indian Ocean. Thanks to Phil Klotzbach, CSU, for bringing Cyclone Agnielle to our attention. Klotzbach also includes Tropical Cyclone Monica as an Indian Basin storm, based on its peak 1-minute winds of 155 knots occurring west of longitude 135°E. Definitions vary on the boundary of the Indian Ocean in this area. Monika reached peak strength north of Australia in the Arafura Sea, which is considered by several sources, including the CIA World Factbook, to be part of the western Pacific Ocean.]


Figure 1. A visible image of Tropical Cyclone Fantala collected at 1025Z (6:25 am EDT) on Monday, April 18, 2016, by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on board the Aqua satellite. The north tip of Madagascar can be seen at bottom. Image credit: NASA Goddard MODIS Rapid Response Team.


Figure 2. An enhanced infrared image of Tropical Cyclone Fantala collected at 1020Z (6:20 am EDT) on Monday, April 18, 2016, by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on board the Aqua satellite. Image credit: RAMMB/Colorado State University.

How Fantala got so strong, and what lies ahead
Conditions were highly favorable for Fantala to intensify. Wind shear has been fairly low over the storm for the last couple of days, around 10 knots, and sea surface temperatures along Fantala’s track have been around 29 - 30°C, plenty warm for tropical development and roughly 1 to 2°C above average (see Figure 3 below). On the other hand, the heat content of the upper ocean has not been particularly large over the last several days (see Figure 4), which implies that a storm as strong as Fantala could easily churn up cooler water. This makes it even more impressive that Fantala has managed to hang onto Category 4/5 strength for more than 48 hours, especially given its relatively slow motion.

After moving northwest for the last couple of days, Fantala is now crawling westward at about 4 knots as it embarks on a tight cyclonic loop that will turn its course around nearly 180 degrees to a southeast bearing. By late this week, Fantala may veer toward the southwest and could eventually approach Madagascar, though the JTWC projects it to be only a Category 1 or 2 cyclone by week’s end.


Figure 3. Departures from the seasonal norm (anomalies) in sea surface temperature across the globe, averaged for the period from March 13 to April 9, 2016. Image credit: NOAA/ESRL/PSD.


Figure 4. Oceanic heat content (OHC) in the upper part of the southwest Indian Ocean as of 06Z (2:00 am EDT) Monday, April 18, 2016. The forecast track of Fantala over the next 120 hours is outlined by hurricane symbols. Over the last several days, Fantala moved northwestward over an area of OHC of less than 35 kilojoules per square centimeter (the blue-green color on the map). Tropical cyclones are more likely to undergo rapid intensification when OHC is greater than 50 kilojoules per square cm. Fantala's initial strengthening from tropical storm to Cat 3 strength (Apr. 12 - 14] occurred around longitudes 65°E - 70°E, while Fantala was passing over heat content between 50 and 100 kilojoules per square cm. Image credit: RAMMB/Colorado State University.


A string of basin records for tropical cyclone strength
Many parts of the tropics have seen record-warm sea surface temperatures in 2015 and 2016, triggered by a strong El Niño on top of longer-term warming caused by human-produced greenhouse gases. These unusual readings have added fuel to the fire of tropical cyclone production. Along with Fantala’s record-setting performance in the Indian Ocean, two other ocean basins have seen their strongest cyclones on record in the past six months, as measured by 1-minute wind speeds confirmed in post-storm analyses. (Many agencies around the world calculate averages based on longer intervals, such as 10 minutes.)

Northeast Pacific: Hurricane Patricia, October 2015, 215 mph
Southwest Pacific: Tropical Cyclone Winston, February 2016, 180 mph



Figure 5. Residents of an apartment complex in the Greenspoint area of north Houston use an air mattress to evacuate their flooded homes on Monday, April 18, 2016. Image credit: AP Photo/David J. Phillip.

Houston walloped by massive flash flooding
At least two fatalities and more than 1200 high-water rescues were reported as the Houston area was socked on Monday morning by a huge mesoscale convective system (MCS) that drifted southeast across the area, dumping eye-popping amounts of rain: 6” - 8” over central Houston, with 12” - 18” common over the far western suburbs (see Figure 7). While individual thunderstorms often weaken after dark, the large mass of thunderstorms that makes up an MCS will often persist overnight and into the next morning, as the MCS cloud tops radiate heat to space and instability is enhanced. Countless roads and homes were flooded throughout the Houston area on Monday morning, and at one point power was out to more than 100,000 customers. City offices and mass transit lines were forced to close.


Figure 6. VIIRS infrared satellite imagery of the intense thunderstorms moving into the Houston area at 0835Z (3:35 am CDT) Monday, April 18, 2016. The Sabine River, separating Texas and Louisiana, can be seen in white against the east edge of the bright red, which denotes very cold cloud tops. Image credit: RAMMB/Colorado State University and Suomi NPP.


Figure 7. Rainfall amounts across Harris County, including the greater Houston area, for the 24 hours ending at 3:37 pm CDT on Monday, April 18, 2016. Image credit: Harris County Flood Control District.


The morning deluge drew some comparisons to Tropical Storm Allison, which devastated Houston as it lingered over Texas for several days in June 2001. Allison’s total rains were far heavier than today’s, topping 30” in places. However, today's flood was "flashier" than Allison, with extremely heavy rains over a short period. As noted by Capital Weather Gang, the 9.92" of rain at Houston Intercontinental Airport as of 3 pm CDT is the second heaviest calendar-day amount since airport record-keeping began in 1969, behind only the 10.34" recorded on June 26, 1989, during the city's "first" Tropical Storm Allison. It is also the second highest calendar-day total for any official Houston location going back to 1889, although it pales next to the nation's astounding 24-hour rainfall record of 43", recorded in Alvin, just south of Houston, on July 25, 1979.

Because the Houston area is so flat, water easily drains off paved areas and collects on the surface during high rainfall rates, so it doesn’t take an Allison-level event to produce widespread flooding. Houston’s west and northwest suburbs have experienced major growth over the last 10-20 years, which may be exacerbating the effect of a given rainstorm. As of midday Monday, the Buffalo Bayou in west Houston (Piney Point Village) was projected to crest at 61 feet, only about 3 inches short of its record 61.2-foot crest from March 4, 1992. However, as shown in Figure 7, it looks unlikely to hit that projection. Downtown, the Buffalo Bayou’s expected crest of 33.1 feet is far below the 42 feet observed in Allison and the 1935 record of 49 feet.

The complex of storms across Houston developed in weak upper-level flow near the edge of a sprawling upper-level low that brought 40” to 50” of snow across the foothills west of Denver, Colorado. At lower elevations, a foot or more of wet snow was recorded across large parts of the Front Range urban corridor. As expected, a rich stream of moist air from Texas to Nebraska led to streaks of very heavy rain throughout the Southern and Central Plains. Amounts over the weekend topped 7” west of Fort Worth, TX, and moderate to major river flooding is occurring over parts of southwest Oklahoma and northwest Texas. Amounts of 4” - 6” were common across southwest Nebraska.

We’ll be back on Tuesday with a new post.

Bob Henson


Figure 8. In west Houston (Piney Point Village), the Buffalo Bayou surged more than 28 feet in less than 18 hours. The floodwaters from Monday morning’s rain will fall short of reaching the 61-foot crest projected earlier Monday morning, although more rain is possible Monday night and later in the week. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service


Figure 9. Kaicee Crowley walks through floodwaters to get belongings out of her stranded car at the North Main Street exit off I-45 in Houston on Monday, April 18, 2016, as White Oak Bayou comes over its banks and floods the freeway. Image credit: Karen Warren/Houston Chronicle via AP.