Understand Hurricane Forecast Information

Understand Forecast Information. According to the NOAA for Hurricane Preparedness, rely on forecasts from the National Hurricane Center and your local NWS office. Know your alerts & the difference between a Watch and Warning. Focus on potential impacts, regardless of storm size or category. Know that deadly hazards occur well outside of the Forecast Cone. (Image credit: NOAA National Weather Service)

Hurricane Forecast Information—Rely On Good Information to Make Good Decisions

Hurricane Preparedness Resources

• Hurricane Hazards and Risk Factors
• Make a Hurricane Evacuation Plan
• Hurricane Preparedness Kits and Supplies
• Hurricane Insurance Checkup and Updates
• Prepare Your Home for Hurricanes
• Help Neighbors with Hurricane Preparedness
• Complete Your Hurricane Preparedness Plan
• Emergency Preparedness Tips
• How to Prepare for a Power Outage
• 10 Tips to Survive a Hurricane Disaster
• FEMA Recommends a Generator
• Hurricane Disaster Preparedness

Tropical Cyclones

Hurricane Milton October 8, 2024. NOAA GOES East Satellite Image

Understanding tropical cyclones and their potential for damage is a key preparedness step. A cyclone is an air mass that rotates around a center of low atmospheric pressure with inward spiraling winds. They have a counterclockwise rotation in the northern hemisphere, and clockwise in the southern hemisphere. The Coriolis effect, a deflective force due to the differences in the Earth’s rotation between the equator and the poles, governs the direction of rotation.

Tropical cyclones get their energy from warm, tropical ocean water. They are the largest, most powerful storms on the planet.

Understand the difference in tropical cyclone types and corresponding forecasts, and take appropriate action.

Potential Tropical Cyclone

A developing storm that forecasters are confident will significantly impact land with tropical storm force winds and possibly storm surge as it develops. The National Hurricane Center uses the same naming convention as with tropical depressions, starting with one, two, three, etc. For example, Potential Tropical Cyclone One.

Designating a developing storm as a potential tropical cyclone allows forecasters to issue watches and warnings the same as a tropical storm or hurricane, and those within the watch or warning area should put their emergency plans into action.

Tropical Depression

A tropical depression is a low-pressure system that has organized into a cyclone. Its energy comes from warm ocean water, and it rotates about a central low.

Less powerful than tropical storms or hurricanes, they can still cause significant damage with heavy rain and wind. Tropical depressions have winds less than 39 MPH. Whereas images of hurricanes usually have a well-defined rotation and center, rotation in tropical depressions is less apparent.

Tropical Storm

When a tropical depression gains enough energy and organization, it becomes a tropical storm and the NHC assigns it a name from the list of names for the current year. Tropical storms can cause significant damage and carry tremendous amounts of water. Wind speeds range from 39 to 73 MPH.

Lower wind speed does not mean less danger. Tropical Storm Allison flooded over 70,000 homes and left 30,000 people homeless in Houston Texas after dumping up to 40 inches of rain. 23 people died in Texas, and 41 altogether along a path from Louisiana to southeast Pennsylvania. TS Allison was the costliest tropical cyclone that never attained major hurricane status. It was the first ever tropical storm to have its name retired.

Hurricane

Hurricane Helene early forecast and projected path with initial watches and warnings, 2-3 days before landfall. National Hurricane Center Graphic Outlook

Hurricanes are powerful storms with sustained winds over 73 MPH, and as high as 190 MPH with even higher gusts. Most have easily recognizable rotation in satellite photographs and well-organized convection. They can span over a thousand miles in width.

Infamous hurricanes include Andrew, which devastated portions of Florida and as a result, changed building codes and attitudes about a hurricane’s potential for damage. Hurricane Katrina flooded New Orleans with storm surge up to 41 feet high, overwhelming levees and floodwalls and submerging portions of the city under 20 feet of water for weeks.

Forecasters rank hurricanes based on wind speed from category 1 to category 5.

• Category 1: 74-95 MPH
• Category 2: 96-110 MPH

Hurricanes of category 3 strength of higher receive the major hurricane classification.

Major Hurricane

Hurricane Andrew was at category 5 when it hit the Bahamas and a very strong category 4 with 155 MPH sustained winds when it hit the southern tip of Florida. It remained at category 4 across the Gulf of Mexico until making a second landfall at Morgan City, Louisiana at category 3.

• Category 3: 111-130 MPH
• Category 4: 131-155 MPH
• Category 5: ≥156 MPH

Subtropical Cyclone

Subtropical cyclones share characteristics between tropical cyclones and extratropical cyclones. Since 2002, they receive names from the same lists as tropical storms and hurricanes. They tend to be larger than tropical storms and gain energy from the atmosphere instead of warm ocean water. They initially form out of extratropical cyclones over colder water and higher latitudes, often between early December and late may, outside the hurricane season.

The maximum wind field extends farther from the center than a tropical cyclone. Less intense storms follow the naming convention of tropical cyclones. Subtropical depression or subtropical storm. Once it attains hurricane force winds, the NHC considers it a fully tropical hurricane, even if it still has subtropical characteristics.

Extratropical Cyclone

A hurricane may undergo a transition cycle in which it loses the distinct characteristics that make it tropical, while remaining a dangerous storm.

• Tropical cyclones have a warm core, they are compact without associated weather fronts, and the strongest winds are near the surface. Their energy comes from warm ocean water.
• Extratropical cyclones have a cold core and are driven by contrasts between colliding warm and cold air masses. They tend to be elongated and much larger than a hurricane, have associated weather fronts, and their strongest winds are high in the atmosphere.

Hurricane Sandy was transitioning or had transitioned to an extratropical cyclone about 2 1/2 hours before landfall near Brigantine, New Jersey.

Post-Tropical Cyclone or Remnant

A post-tropical cyclone or remnant low has lost its strong convection, but as an extratropical cyclone, it can still produce hurricane force winds and heavy rainfall.

As a remnant low, the cyclone is losing strength. Winds have fallen below tropical storm force (Less than 39 MPH). It can still regenerate if conditions are favorable.

After a hurricane or tropical storm makes landfall, it no longer has an energy source from the warm ocean. A hurricane degrades to a tropical storm, then a depression, and then a post-tropical depression. Eventually, another weather system will absorb the remnant low.

Tropical Storm Cristobal made landfall on Louisiana, degraded to a tropical depression, and traveled all the way to Wisconsin where it finally became post-tropical, crossed lake superios, and continued on all the way to Hudson Bay.

Tropical Extratropical and Subtropical Cyclone Hazards

Water

Water is the number one hazard associated with tropical cyclones. Storm Surge alone causes more death and destruction than any other of the other hazards combined, including wind.

Storm Surge

Hurricane Harvey’s Storm Surge Over a Texas Road

The winds of a hurricane or tropical storm circle inward and push water into a mound at the center of the storm. At the same time, higher pressure outside the storm pushes down on the water which rises at the low pressure center. The result is a mound of water that moves with ahead of the storm. If storm surge occurs near high tide, the water is even higher. Heavy surf can acerbate the effects of storm surge and cause more damage.Storm surge from Hurricane Katrina along the Mississippi Gulf Coast was up to 28 feet with inundation 6-12 miles inland, overwhelming levees and floodwalls.

The National Hurricane Center issues Storm Surge Watches and Warnings.

Torrential Rain Leads to Inland Flooding, Flash Floods, and Mudslides

Hurricane Helene unleashed extensive landslide activity across the southeast United States. This USGS Photo was taken in Vilas, North Carolina.

Although storm surge can contribute to inland flooding, torrential rain is the primary cause. Inland flooding occurs because the waterways that remove water from the land can’t keep up with the rainfall. Rivers and streams reach capacity and their banks overflow.Heavy, localized rain can cause a sudden rise in water levels. Water follows the path of least resistance and travels downhill. Heavy rain in one area can lead to a flash flood miles downstream. The flood often pushes a deadly mix of rocks, trees, and other debris of ahead of it, moving it like a thick liquid that can take out bridges and wash away homes.

Torrential rain can also cause mudslides, another deadly force. Hurricane Helene produced more than 2000 mudslides across southeastern states.

Wind

Hurricane Patricia had a record wind speed of 215 MPH in 2015. Hurricane Melissa reached wind speeds of 185 MPH. Category 5 storms are the strongest, but also the least common.

The extreme winds of a hurricane can turn almost any object into a projectile. This is why your safe space should be an interior room with reinforced walls.

Keeping wind out of a home is key to keeping the roof. Wind that reaches inside the house creates a high interior pressure compared to the low pressure outside. The resulting lift can tear a roof from the framing.

Hurricanes are cyclones and very often spawn tornadoes, another type of cyclone. The EF1 and EF2 tornadoes associated with hurricanes usually don’t last very long, but they can cause significant damage. Yet another reason to have a secure and reinforced safe room.

Power Outages

This home with lights in a dark neighborhood during a power outage has power from their standby generator in the yard. A standby generator can run for days or even weeks during the extended outage following a hurricane.

When Major Hurricane Ida hit New Orleans in 2021, it blew a transmission tower carrying eight feeder lines into the Mississippi River. Over a million people lost power. The restart of a small power plant in the city restored power to some critical services like hospitals, but many residents went weeks without power.

Generator service dealers reported a surge in service calls for routine maintenance required after 4-8 days for most standby generators.

Widespread power outages are common following a hurricane, often in hot, humid conditions. The humid heat wave that followed Ida was directly responsible for at least 10 fatalities. Most residents who stayed lost the means to keep food fresh. Within hours after the power goes out, refrigerators become warm enough to make food unsafe.

Rip Currents and Heavy Surf

Days before a hurricane reaches land or passes by, while it is hundreds of miles away, it can produce spectacular waves that attract crowds. Many venture onto piers and for the thrill of seeing these monster waves up close.Less visible are the Rip Currents that can sweep an unsuspecting swimmer or someone who was knocked from a pier by a wave up to half a mile from shore. Swimming against a rip current is futile, and the swimmer will tire without making any progress.

Escape a Rip Current by swimming parallel to shore, not against the current, to swim out of the current. No one can swim against the current. National Weather Service Graphic—Rip Currents Know Your Options

Rip currents are sometimes called undertows because they can pull someone off their feet, even in relatively shallow water.

Stay away from the big waves, obey warning signs, and know what a rip current looks like before you go into the water. Swim near lifeguards.

If you are caught in a rip current, swim parallel to the shore until you are out of the current. The current will take you further out, but you’ll still be close to shore and can eventually escape and make the swim back to shore.

Watches Warnings Advisories Outlooks

The National Hurricane Center issues tropical cyclone outlooks, advisories, watches, and warnings.

Tropical Weather Outlook (TWO)

A tropical weather outlook discusses current disturbances within the Atlantic Basin and any potential for development over the next 7 days. A probability forecast for the next 48 hours and 7 days are given a Low (10-30%), Medium (40-60%), or High (70-100%) probability. On the graphical outlooks, these are shown as yellow, brown, and red hatched areas.

Tropical Weather Outlooks are available every six hours from May 15 to November 30 for the Atlantic Basin.

When reconnaissance aircraft report significant changes in a disturbance, the NHC issues Special Tropical Weather Outlook.

Advisories

When any tropical cyclone (depression, tropical storm, hurricane) is present, the NHC issues a Tropical Cyclone Public Advisory (TCP) every six hours on a regular schedule at 5 am, 11 am, 5 pm, and 11 pm during Daylight Saving Time (4 am, 10 am, 4 pm, and 10 pm Standard Time).

Whenever watches or warnings are in effect, intermediate advisories provide updated information every 3 hours. Watches and warnings may came during a regular TCP, or intermediate TCP, or even a Special Advisory Package to for an unexpected and significant change in the cyclone.

Public Advisories contain five pieces of key information.

1.      The summary table provides the current position, distance from a well-known reference point, maximum sustained winds, current direction, and how fast it is moving.

2.      A summary of all the watches and warnings with recent changes noted.

3.      A discussion of the cyclone’s current characteristics including location, motion, intensity, pressure, and a description of the general track over the next 72 hours. It may also include a 5-day forecast track and intensity if necessary

4.      Hazards to land, usually within the expected time frame of watches and warnings, including storm surge, wind, rainfall, tornadoes, and rip currents. This section may contain weblinks for local, graphical hazard information.

5.      The last section states the time of the next planned advisory.

Forecast Cone

Note the wind field in this advisory graphic by the NHC covers most of Florida with Tropical Storm Force Winds

The cone is not the size of the storm, nor is it the probable damage area of the storm. It is the storm’s uncertain path. The cone is narrow at the beginning because the exact center of the cyclone is known. 5 days later, it might fall hundreds of miles to either side of the cone’s middle.

The forecast cone is one of the most misunderstood forecast products.

In the accompanying graphic for Hurricane Helene, the brown circle at the bottom tip of the cone represents the wind field of the hurricane. As is clearly shown in the this graphic, the hurricane is much wider than the width of the cone over most of its path, and about the same size as the end of the path.

The predicted path in the days ahead is uncertain, and the cone widens over time, often resulting in a bulbous appearance as in this graphic.

Tropical Cyclone Advisories Every 6 Hours When Cyclone is Present

Watch

The NHC issues watches for Tropical Storms, Hurricanes, and Storm Surge 48 hours before they expect conditions to occur.

It is possible that a storm that forms close to land, such as in the Gulf or the Caribbean, there might be less than 48 hours before the storm arrives.

If you live on the coast, check the storm surge watches and warnings if you live close to the coast when the NHC issues the first tropical storm or hurricane watch.

Warning

The NHC issues Warnings for Tropical Storms, Hurricanes, and Storm Surge 36 hours before the expected conditions occur.

The forecast path and time of arrival may change dramatically over time. Hurricane Helene’s path didn’t change much between the first watch and landfall, but the time of landfall was almost 8 hours earlier than initially expected.

Be ready to leave if you are under a storm surge warning. Have your emergency kit ready to go, the car full of gas. If authorities give evacuation orders, leave immediately to avoid being stuck in traffic.