Cone of Uncertainty: What It Actually Means

Tropical storms and hurricanes that move slowly near the coast or inland are among the most feared by forecasters.

Sometimes these tropical cyclones may not have powerful wind speeds, but the threats posed can be amplified due to their sluggish pace.

Here's a look at what a slow speed means for rainfall potential and some other typical threats from a storm that's in no hurry to exit a region.

A slow-moving storm brings relentless rainfall

• In focus: The slower a storm moves, the more incredible the rainfall amounts can become. A storm chugging along at just 5 mph has a rainfall potential upwards of 30 inches, while one moving briskly at 20 mph typically produces much smaller rain totals in any given location. When this occurs near mountainous terrain, rainfall totals can be further enhanced.
• Reinforced: A landfalling major hurricane that is moving at an average or faster-than-average pace can drop less rainfall than a slow-moving tropical depression or storm.

• Why you should pay attention: Freshwater flooding fatalities now greatly outweigh deaths from other hurricane hazards, including wind speeds – and this proportion of deaths to other hazards continues to grow.
• Poster child for flooding: Hurricane Harvey is the most extreme example of major flooding caused by a storm that stalls out or moves slowly. Catastrophic damage occurred in Houston and other parts of southeastern Texas after Harvey spent days soaking the state. Much of this rain fell well after the storm was no longer a hurricane.
• Recent example: Hurricane Sally (2020) punished the Alabama and Florida coasts with two days of tropical storm conditions, which led to up to 30 inches of rain. Swift water rescues were needed in southern Alabama and the western Florida panhandle as roads and bridges washed out.
• Tropical storms can be deadly, too: Tropical Storm Allison and its remnants loitered for days in the Houston area in June 2001, unleashing more than 40 inches of rain that resulted in massive flooding and 23 deaths in Texas alone.

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Sluggish storms keep coastal communities inundated longer

• In focus: Storms that stall or move at a snail's pace near the coastline pile up water and prolong the coastal flooding potential. In addition, large battering waves over a longer period can worsen damage to the coastline, including severe beach erosion. This is because the winds blow onshore for a longer period.
• Why you should pay attention: The persistence of winds can keep water pinned along the coast and in inlets, bays and rivers, causing damage to homes, businesses, boats and other structures for hours to days.
• Flooding example: Hurricane Sally (2020) daddled near the northern Gulf Coast producing 5 to 7 feet of storm surge in some communities from Alabama to Florida, including in Pensacola as shown below. The storm's slow motion toward the coast meant that the storm's storm surge lasted more than three days.
• The reverse can be true, too: Hurricane Sally also emptied Mobile Bay on the backside of the storm’s circulation. Water levels dropped in the Bay by up to 7 feet.

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Winds also last longer

• In focus: Tropical storms and hurricanes can down trees no matter what speed they move, but when the winds continue for hours and hours, impacts can be enhanced.
• Why you should pay attention: Prolonged wind events increase the stress on trees and other structures. When combined with hours of heavy rainfall, trees are more likely to fall in soggy soil conditions even at lower wind speeds. Power outages and damage to homes and vehicles become increasingly likely as a storm drags on.

MORE ON TROPICAL STORMS AND HURRICANES:

• What The Cone Of Uncertainty Means, And What It Doesn’t
• Changes Ahead For The 2025 Hurricane Season
• How A Hurricane Hundreds Of Miles Away Can Change Your Plans

Jonathan Belles has been a digital meteorologist for weather.com for 9 years and also assists in the production of videos for The Weather Channel en español. His favorite weather is tropical weather, but also enjoys covering high-impact weather and news stories and winter storms. He's a two-time graduate of Florida State University and a proud graduate of St. Petersburg College.