Flood Science Storms Weather

Hurricane Eta Slams Nicaragua with catastrophic wind, rain and storm surge. Eta, or its remnants, could move into the Gulf next week.

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November 3, 2020

Hurricane Eta slammed into the coast of Nicaragua this morning, generating category-4 hurricane winds of 145 mph, pushing a massive saltwater flood, and opening up the first chapter of a widespread flood event that will feature extensive inland flooding and mudslides.

Infrared satellite image of Hurricane Eta just before landfall along the Nicaragua coast on Tuesday, November 3, 2020.
Source: www.weathernerds.org.

Eta’s slow trek through Central America could result in some areas observing more than 16 inches of rain. Steep, mountainous slopes not only enhance such rainfall, through a process known as orographic lift, but then enhance rainfall runoff and mudslides, creating widespread catastrophic damage. The GFS accumulated rainfall map shows the location in Nicaragua, north of the landfall location, where more than 16 inches of rain could fall between Monday evening and midday Wednesday (see below).

Accumulated rainfall map from the GFS global weather model. Effective from Monday evening at 6PM through Wednesday at 12PM, Nicaragua time.
Source: www.tropicaltidbits.com

Powerful hurricanes that struck Nicaragua in 1906 and 2007 provide guidance for storm surge potential along the coast. Hurricane Felix struck northeastern Nicaragua as a category-5 hurricane with 160 mph winds in 2007, generating a storm surge of at least 18 feet, which remains the highest storm surge on record in Central America, according to the global storm surge database, U-Surge (www.u-surge.net). The storm in 1906 made landfall as a category-3 hurricane and generated a storm surge of 15 feet.

Tracking map for Hurricane Felix, which struck Nicaragua as a category-5 hurricane in 2007. Source: https://www.nhc.noaa.gov/data/tcr/AL062007_Felix.pd

The intensity of Hurricane Eta falls between these two historical events, enabling them to provide some rough guidance of storm surge potential in this part of the world. A data-driven perspective on storm surge potential takes into account past storms and then bumps the surge potential up or down from there, depending on differences in storm characteristics.

One important difference between hurricanes Felix and Eta is the track direction. Hurricane Felix struck the coastline with a perpendicular track, whereas, Eta’s track to the west-southwest is more of a shore-parallel track, producing offshore winds that suddenly reverse to onshore after the eye passage. These type of tracks suppress storm surge generation a bit because the biggest surge does not push in until a location is on the back side of the eye.

Another important difference is storm size. Eta’s area of hurricane-force winds are quite small, only extending 25 miles from the center of circulation, according to the latest NHC update. By contrast, Hurricane Felix generated hurricane force winds out to 40 nautical miles from the center of circulation. This comparison lowers Eta’s surge potential in relation to Felix.

However, Eta’s slow forward motion should serve to slightly increase storm surge potential. As of the 0700AM EST National Hurricane Center advisory this morning, Eta’s forward motion had slowed to 4 mph to the west-southwest.

Balancing out the differences in storm track, wind speed and storm size, between Hurricane Eta and past high-profile hurricanes in Nicaragua, a reasonable storm surge estimate from Eta would be in the 14-17 foot range. This water level would be sufficient to produce catastrophic damage, as large waves pushing in on top of the storm surge would destroy natural and built objects on low-lying ground.

Will Eta or its remnants track into the Gulf next week?

The official National Hurricane Center (NHC) track takes Eta slowly across Nicaragua and Honduras over the next several days, before the storm curves to the northeast. The NHC forecast weakens Eta into a depression, but redevelops it into a tropical storm in the Caribbean Sea over the weekend. Such transitions often border a fine line where a tropical cyclone may actually lose its closed circulation center, and its name, but the remnants redevelop.

Forecast track for Hurricane Eta, from the National Hurricane Center on Tue Nov 3 at 700AM.

Either way, we should keep an eye on this system going into next weekend and beyond. At this point, both the Euro and American (GFS) global circulation models redevelop this system and bring it into the southern Gulf. The Euro model is faster, bringing it into the southern Gulf by the end of next week, whereas the American model is a few days behind that. Both maps are depicted below.

The GFS (American) global circulation model map for Mon Nov 16 at 1200PM CST. Source: www.tropicaltidbits.com.

We must remember that the purpose of consulting weather models so far into the future is to get a “general idea” of what COULD happen. Beyond the 5-6 day range, forecasts get more speculative and often change dramatically. The big picture is that a hurricane could potentially track into the southern Gulf of Mexico in the 8-12 day range. Given the predominant circulation pattern for tropical systems to track towards the east or northeast in the Gulf during November, the state of Florida should pay especially close attention to this long-range forecast.

The European model for Thursday November 12, 2020, at 600PM CST. Source: www.tropicaltidbits.com.