"Cat 7" Hurricane Patricia Threatens Western Mexico

"Category 7" Hurricane Patricia Threatens Western Mexico. The hurricane is generating astonishing sustained winds of 200 MPH (325 KPH), according to the latest (1500UTC/ 1000CDT) advisory from the U.S. National Hurricane Center.

The Saffir-Simpson Hurricane Wind scale rates hurricanes on a scale of 1-5, depending on the maximum sustained winds. The strongest hurricanes, categorized as category 5 storms, produce maximum sustained winds exceeding 155 mph, and inflict catastrophic wind damage. However, one limitation to this bounded scale is that hurricanes with winds of 160 mph and 200 mph are both classified as category 5 storms.

 Infrared satellite imagery from the CIMSS Tropical Cyclone Group at the University of Wisconsin depicts category-5 Hurricane Patricia approaching Western Mexico on Friday, October 23, 2015.

If the Saffir-Simpson Hurricane Wind scale were a continuous scale, with no upper bound, Patricia might be classified as a category-7 hurricane. Considering that category-3 hurricanes produce winds of 115-135 mph, and cat 4 hurricanes produce winds of 135-155 mph, if we continued using 20 mph wind categories, cat 5 hurricanes would be categorized 155-175 mph, cat 6 hurricanes 175-195 mph, and rare cyclones like Patricia and Super Typhoon Haiyan, which impacted the Philippines in 2013, could be classified as cat-7 tropical cyclones, with winds exceeding 195 mph.

I'm not too serious about re-writing the hurricane classification system (at least not today), but it is good to question the status quo and make sure we accurately depict the amazing intensity of the most severe hazards.

Visible satellite imagery from the CIMSS Tropical Cyclone Group at the University of Wisconsin depicts category-5 Hurricane Patricia approaching Western Mexico on Friday, October 23, 2015. This 1-km resolution image shows Patricia's tightly-wrapped eye, which is common in the most intense hurricanes.

Some may argue that once winds reach 155 mph, there is no difference in wind destruction. From a wind engineering perspective this may be true, however, other hurricane hazards, like storm surge, are generally related to pre-landfall maximum sustained winds (although other factors, like hurricane size, are also important.)

I co-authored a paper with Dr. Barry Keim in 2014 that found a significant relationship between storm surge heights and pre-landfall winds, with hurricane winds 18 hours before landfall producing the best relationship with surge levels. We also found a non-linear relationship between storm surge heights and pre-landfall winds. According to this study, we found that doubling pre-landfall hurricane winds increases surge potential by a factor of 4.59. This means that the strongest hurricanes move much more sea water than "average" hurricanes.

Storm surge heights relate better to pre-landfall winds than wind speeds at landfall. This graphic shows that wind speeds 18 hours before landfall correlate best with surge heights (see Needham and Keim (2014)). This is a concern as Hurricane Patricia approaches Mexico's West Coast with maximum sustained winds of 200 mph (325 kph).

These principles should concern us when we consider Patricia's pre-landfall winds of 200 mph (325 kph). From a storm surge perspective, the only approximate analog to Patricia in the literature is Hurricane Kenna, which produced pre-landfall winds of approximately 165-170 mph (270 kph), and generated a 16.5 (5 m) storm surge at San Blas, Mexico (Franklin 2002). Kenna also generated 10 ft (3.05 m) waves at Puerto Vallarta (Franklin 2002).

Needham et al. (2015) provides an overview of storm surge data sources, observations and impacts for Western Mexico. Kenna's storm surge is the highest observed coastal flood event for this region, according to the SURGEDAT global storm surge database (see  http://surge.srcc.lsu.edu/).

Patricia is substantially stronger than Kenna and will make landfall farther south. The area to the west of Manzanillo will likely be most vulnerable to storm surge. Coastal interests in this region should brace for an exceptional wind event, with catastrophic storm surge flooding. I expect that Patricia's surge heights will likely exceed 16.5 ft (5 m), and will be accompanied by large, destructive waves. This would be the largest storm surge in the modern history of Western Mexico.

 Storm surge heights to the west of Manzanillo may exceed 16.5 ft (5 m), as Hurricane Patricia approaches Mexico's west Coast. This photo shows a sunset over Manzanillo.

Image: Wikipedia Commons

Moisture from Patricia will stream into the southern U.S. and exacerbate the heavy rainfall event that has already started in Texas. I am in Galveston, Texas, today, where the coastal population is bracing for substantial flooding from the combination of high "tides" (surge) and heavy rainfall over the next several days. I will write another post soon about the flood potential in Texas, including some on-the-ground pics I took this morning.

REFERENCES

Franklin, J.L., 2002: Tropical Cyclone Report, Hurricane Kenna, 22-26 October, 2002, National Hurricane Center, Miami, Florida.

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Needham, H.F., B.D. Keim, and D. Sathiaraj, 2015: A Review of Tropical Cyclone-Generated Storm Surges: Global Data Sources, Observations and Impacts. Reviews of Geophysics, Ahead-of-Print version posted online June 30, 2015 and available at: http://onlinelibrary.wiley.com/doi/10.1002/2014RG000477/full.

Needham, H.F., and B.D. Keim, 2014: Correlating Storm Surge Heights with Tropical Cyclone Winds at and before Landfall. Earth Interactions, 18, 8, 1-26.