10/10/26 – After a life measured in days, Hurricane Isaias went from an area of low pressure in the Bay of Campeche to a killer, Cat 3 hurricane to a post-tropical storm now causing flash flooding in the Appalachians? So, what are the forces that cause hurricanes to form, strengthen and weaken?
I dedicate this post to curious readers and budding scientists still in school. Please share it with your students, children and grandchildren.
What Causes Hurricane Formation?
The forces listed below support formation of an organized tropical cyclone.
Key force or condition
What happens physically
Effect on the developing storm
An existing weather disturbance
Thunderstorms gather around a low-pressure area, such as a tropical wave.
Provides the starting circulation around which a storm can organize.
Warm ocean water
The ocean supplies heat and water vapor to air above it. Water temperatures around 80°F (27°C) generally favor development.
Supplies energy for sustained thunderstorms.
Moist air
Humidity limits evaporation and cooling within developing thunderstorms.
Helps thunderstorms persist and organize.
An atmosphere that supports rising air
Warm, moist air rises, cools, and condenses, releasing heat.
Builds deep thunderstorms that help strengthen the circulation.
Low vertical wind shear
Winds change relatively little with height, allowing the lower and upper circulation to remain aligned.
Helps thunderstorms organize around the surface center.
Sufficient distance from the equator
Earth’s rotation deflects moving air through the Coriolis effect.
Helps organize a rotating circulation; tropical cyclones rarely form within about 5 degrees of the equator.
Warm, moist air flows inward over the ocean and rises in thunderstorms. An organized hurricane develops a central eye, and outflow high above the storm.Source: NOAA
Surface air picks up heat and water vapor, then carries that energy into the eyewall.
Fuels stronger thunderstorms and circulation.
Warm water extending deep below the surface
Wind-driven mixing brings up relatively warm water instead of quickly exposing cold water.
Sustains the energy supply as the hurricane churns the ocean.
Concentrated heating in the eyewall
Rising water vapor condenses and releases heat close to the center.
Strengthens the warm core and helps central pressure fall, increasing winds.
Low or no vertical wind shear
The circulation remains more vertically aligned.
Allows eyewall thunderstorms to organize and heat the core efficiently.
Moist surrounding air
Less dry air enters and disrupts eyewall updrafts.
Helps maintain vigorous thunderstorms around the center.
Effective outflow aloft
Air rising through the eyewall spreads outward near the top of the storm.
Supports the continuing flow of air inward below and upward through the eyewall.
An increasingly organized inner core
Thunderstorms form a more continuous eyewall; the eyewall may contract.
Concentrates the strongest circulation closer to the center and can accelerate strengthening.
Low wind shear lets circulation remain vertically aligned, helping thunderstorms concentrate heat around the center and favoring strengthening. Contrast this image with the one below. Source: NOAA
These conditions cause hurricanes to weaken as they move.
Key force or condition
What happens physically
Effect on the hurricane
Strong vertical wind shear
Winds at different heights tilt the circulation and displace thunderstorms from the surface center.
Disrupts the organized eyewall and reduces heating of the core.
Dry air entering the inner core
Evaporation cools air within thunderstorms, encouraging downdrafts and interrupting moist updrafts.
Erodes eyewall thunderstorms and can open gaps in the eyewall.
Cooler ocean water
Less heat and water vapor transfer from the ocean to the atmosphere.
Reduces the energy available to sustain the hurricane.
Cold water mixed upward by the storm
Strong winds churn the ocean and can bring colder water to the surface.
Allows the hurricane to weaken its own energy supply, especially over a shallow warm layer.
Landfall
Access to ocean heat and moisture diminishes; surface friction increases, and terrain can disrupt circulation.
Slows winds and breaks down the eyewall, although heavy rain can persist.
An eyewall replacement cycle
An outer eyewall develops and intercepts inflow feeding the original eyewall, which weakens and disappears.
Often temporarily lowers maximum winds while expanding the wind field; strengthening may resume.
Strong wind shear tilts the circulation and disrupts the organized flow of heat and moisture, inhibiting development and potentially weakening the storm. Landfall, dry air, and cooler water also cause weakening. Source: NOAA.
Isaias encountered wind sheer shortly before reaching land. That lowered its intensity at landfall from Cat 3 to Cat 2.
Note that “strengthening” and “weakening” usually describe changes in maximum sustained winds. A weakening hurricane can still produce destructive storm surge and prolonged, intense rainfall. Hurricane Harvey is an excellent example.
Harvey struck near Rockport as a Category 4 hurricane with 130 mph sustained winds.
Weakening, August 26
It weakened to a tropical storm within approximately 12 hours, then maintained winds around 40 mph for the next couple of days.
Prolonged flooding
Weak steering currents caused Harvey to stall near the coast. Part of its circulation remained over the Gulf, sustaining its moisture supply and repeated torrential rainfall.
The key point: Harvey weakened as a windstorm, but remained a catastrophic rainstorm.
A lower hurricane category does not necessarily mean a lower flood threat.
In fact, Harvey produced the highest recorded rainfall total ever from a single tropical cyclone in the continental United States: 60.58 inches near Nederland, Texas, according to the National Weather Service.
Posted by Bob Rehak on 10/10/2026
3329 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2026/10/Shear-Weakens.jpg?fit=1100%2C324&ssl=13241100adminadmin2026-10-10 17:49:042026-10-10 17:49:05What Causes Hurricanes to Form, Strengthen and Weaken?
10/9/26 @ 10AM – This morning, Hurricane Isaias intensified into a major Category 3 Hurricane with sustained winds of 120 mph. Isaias should make landfall near Pensacola late tonight or early tomorrow morning, according to the National Hurricane Center.
Increasing forward motion should bring strong winds far inland over the southeast U.S. into the Tennessee and Ohio River Valleys, causing widespread power outages.
The National Hurricane Center (NHC) expects tropical-storm-force winds to arrive on the Florida panhandle by 2PM today.
Arrival Time
Tropical-storm-force winds now extend outward 185 miles from the center and hurricane force winds 30 miles.
Damaging wind gusts are possible over much of central and northern Alabama, extreme eastern Mississippi, and portions of western and northern Georgia, especially in areas of higher terrain.
Rainfall and Flooding Potential
Isaias will bring areas of heavy rainfall to the central U.S. Gulf coast and much of the Southeast through the weekend.
Considerable flash and urban flooding, along with significant river flooding, will be possible.
Storm Surge Up to 9 Feet
NHC predicts damaging storm surge up to 9 feet in the western Florida Panhandle, with lesser amounts elsewhere.
Maximum values will occur near and to the east of where the center crosses the coast.
Infrared satellite images show an eye surrounded by intense thunderstorms.
Infrared satellite image as of 9:16 CDT this morning
Tropical Tidbits shows this animated IR Satellite Loop. However, an eye is not apparent on the Geo-Color view.
Near Peak, but Increasing Forward Speed and Wind-Field Width
The hurricane is passing over a warm deep water eddy and that is likely helping the intensification process in the face of increasing vertical shear. Isaias has turned toward the NNE and is increasing in forward speed.
Isaias is likely near peak. However, it is possible that the hurricane could intensify a bit more this morning before strong shear begins to impact the system.
The wind field will greatly expand later today into tonight as Isaias makes landfall and moves inland while interacting with an upper level trough to the west.
Origin of Name
Isaias is pronounced “ees-ah-EE-ahs” – four syllables – with the emphasis on the third. That is NHC’s official pronunciation.
According to ChatGPT, Isaías is the Spanish and Portuguese form of Isaiah, the biblical prophet’s name. It comes from Hebrew Yesha‘yahu, meaning “Yahweh is salvation,” or “salvation of the Lord.”
Why use a name that English speakers may find difficult? NHC follows name lists maintained by the World Meteorological Organization’s (WMO) Hurricane Committee. Atlantic lists deliberately balance English, French and Spanish names because storms affect countries speaking all three languages. The lists rotate every six years. Isaias replaced Ike after the destructive 2008 hurricane caused Ike’s name to be retired. NOAA
WMO itself says storm names should be instantly recognizable for safety. The multilingual approach explains the choice, although it clearly creates a pronunciation hurdle for many English speakers.
Posted by Bob Rehak on 10/9/26 based on information from NHC and Harris County Meteorologist Jeff Lindner
3328 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2026/10/Isaias-10.9.26.jpg?fit=1000%2C1000&ssl=110001000adminadmin2026-10-09 09:59:552026-10-09 09:59:56Isaias Now a Major Hurricane
9/8/26 – At 4 am CDT today, the National Hurricane Center (NHC) issued an advisory stating that Tropical Storm Isaias had intensified into a hurricane overnight. NHC expects winds to increase to near major hurricane status in the next 24 hours and says the storm will produce life-threatening surge up to 7-feet high across the northern Gulf Coast from the mouth of the Mississippi to the Florida panhandle.
Isaias is now moving toward the east-northeast near 9 mph and should turn toward the northeast later today followed by a turn northward on Friday. NHC expects landfall along the U.S. northern Gulf Coast late Friday or early Saturday.
A dropsonde launched from an Air Force Reserve hurricane hunter found average winds of 76 kt – the basis for upgrading Isaias to a hurricane at 0330 UTC.
Environmental Factors Affecting Forecast
Since that time, Isaias has continued to intensify, but with some northward tilt with height, likely due to moderate westerly vertical wind shear that continues to hinder circulation. However, circulation has increased since yesterday.
Aircraft fixes indicate that Isaias continues to maintain an east-northeastward heading. The most critical steering features:
A subtropical ridge centered east of the Bahamas
A mid-level trough along the northern Gulf coast
Low pressure digging in from the Rocky Mountains.
The net effect:
Isaias should make a left turn to the northeast and the north over the next 24-48 hours.
NHC
The forecast has not changed appreciably though models continue to disagree over the exact track (east or west) after the turn to the north. The storm should become post-tropical over the Tennessee Valley.
The intensity forecast remains difficult. Isaias is fighting around 20 kt of westerly vertical wind shear, and an eyewall keeps coming and going.
Infrared image of circulation at 5 AM CDT
Other favorable environmental factors for further intensification include 30 C sea-surface temperatures and abundant moisture. Shear should not increase appreciably for the next 24 hours. In fact, the hurricane has intensified despite shear. So NHC expects significant intensification in the short-term. making Isaias just shy of major hurricane intensity in 24 hours.
However, in the final 12-24 hours before Isaias makes landfall on the north Gulf coast, vertical wind shear should ramp up to 40-50 kt. This will likely impart some weakening. At the same time, the hurricane’s wind field may become broader.
Regardless, NHC expects Isaias to remain a dangerous hurricane through landfall with gusty winds extended well inland.
Key Messages from NHC
Key Messages:
1. Isaias should strengthen and become a strong hurricane when it approaches the northern Gulf Coast on Friday. A life-threatening storm surge is expected along a portion of the northern Gulf Coast,and a Storm Surge Warning remains in effect from the Mouth of the Mississippi River to Steinhatchee, Florida. Residents in the warning area should follow advice given by local officials and evacuate if told to do so.
2. Damaging hurricane force-winds will extend across the Florida Panhandle, southern Alabama, and southern Mississippi. A Hurricane Warning remains in effect from Ocean Springs, Mississippi to the Bay/Gulf County Line, Florida. Residents there should prepare for extended power outages.
3. Isaias will bring areas of heavy rainfall to the central U.S. Gulf coast and much of the Southeast Friday into the weekend. Flash and urban flooding, along with isolated river flooding, will be possible.
NHC Graphics Illustrate Risks
The graphics below show current predicted risks.
Isaias track forecastRainfall forecastIsaias flash-flood risk Isaias wind field
Isaias is the latest “first hurricane of the Atlantic season” on record.
Posted by Bob Rehak on 9/8/2026 based on NHC information
3327 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2026/10/Isaias-9.8.26-5am.jpg?fit=1000%2C1000&ssl=110001000adminadmin2026-10-08 05:57:092026-10-08 05:57:10Isaias Now a Hurricane
What Causes Hurricanes to Form, Strengthen and Weaken?
10/10/26 – After a life measured in days, Hurricane Isaias went from an area of low pressure in the Bay of Campeche to a killer, Cat 3 hurricane to a post-tropical storm now causing flash flooding in the Appalachians? So, what are the forces that cause hurricanes to form, strengthen and weaken?
I dedicate this post to curious readers and budding scientists still in school. Please share it with your students, children and grandchildren.
What Causes Hurricane Formation?
The forces listed below support formation of an organized tropical cyclone.
These conditions favor formation, but their presence does not guarantee that a hurricane will develop. Sources: NOAA’s hurricane overview and formation explanation. www.noaa.gov.
What Causes Hurricanes to Strengthen?
Once a tropical storm develops, alignment of the conditions below can intensify it into a hurricane (sustained winds 74 mph or greater).
Sources: NOAA’s ocean conditions research and hurricane modeling explanation.
What Causes Hurricanes to Weaken?
These conditions cause hurricanes to weaken as they move.
Isaias encountered wind sheer shortly before reaching land. That lowered its intensity at landfall from Cat 3 to Cat 2.
Sources: NOAA’s satellite interpretation guide, hurricane FAQ, and eyewall research.
Effect on Flooding
Note that “strengthening” and “weakening” usually describe changes in maximum sustained winds. A weakening hurricane can still produce destructive storm surge and prolonged, intense rainfall. Hurricane Harvey is an excellent example.
In 2017, Harvey’s winds weakened dramatically before much of Houston’s catastrophic flooding occurred. The National Weather Service documents the sequence below.
The results were extraordinary:
The key point: Harvey weakened as a windstorm, but remained a catastrophic rainstorm.
In fact, Harvey produced the highest recorded rainfall total ever from a single tropical cyclone in the continental United States: 60.58 inches near Nederland, Texas, according to the National Weather Service.
Posted by Bob Rehak on 10/10/2026
3329 Days since Hurricane Harvey
Isaias Now a Major Hurricane
10/9/26 @ 10AM – This morning, Hurricane Isaias intensified into a major Category 3 Hurricane with sustained winds of 120 mph. Isaias should make landfall near Pensacola late tonight or early tomorrow morning, according to the National Hurricane Center.
Increasing forward motion should bring strong winds far inland over the southeast U.S. into the Tennessee and Ohio River Valleys, causing widespread power outages.
The National Hurricane Center (NHC) expects tropical-storm-force winds to arrive on the Florida panhandle by 2PM today.
Arrival Time
Tropical-storm-force winds now extend outward 185 miles from the center and hurricane force winds 30 miles.
Damaging wind gusts are possible over much of central and northern Alabama, extreme eastern Mississippi, and portions of western and northern Georgia, especially in areas of higher terrain.
Rainfall and Flooding Potential
Isaias will bring areas of heavy rainfall to the central U.S. Gulf coast and much of the Southeast through the weekend.
Considerable flash and urban flooding, along with significant river flooding, will be possible.
Storm Surge Up to 9 Feet
NHC predicts damaging storm surge up to 9 feet in the western Florida Panhandle, with lesser amounts elsewhere.
Maximum values will occur near and to the east of where the center crosses the coast.
Infrared satellite images show an eye surrounded by intense thunderstorms.
Tropical Tidbits shows this animated IR Satellite Loop. However, an eye is not apparent on the Geo-Color view.
Near Peak, but Increasing Forward Speed and Wind-Field Width
The hurricane is passing over a warm deep water eddy and that is likely helping the intensification process in the face of increasing vertical shear. Isaias has turned toward the NNE and is increasing in forward speed.
Isaias is likely near peak. However, it is possible that the hurricane could intensify a bit more this morning before strong shear begins to impact the system.
The wind field will greatly expand later today into tonight as Isaias makes landfall and moves inland while interacting with an upper level trough to the west.
Origin of Name
Isaias is pronounced “ees-ah-EE-ahs” – four syllables – with the emphasis on the third. That is NHC’s official pronunciation.
According to ChatGPT, Isaías is the Spanish and Portuguese form of Isaiah, the biblical prophet’s name. It comes from Hebrew Yesha‘yahu, meaning “Yahweh is salvation,” or “salvation of the Lord.”
Why use a name that English speakers may find difficult? NHC follows name lists maintained by the World Meteorological Organization’s (WMO) Hurricane Committee. Atlantic lists deliberately balance English, French and Spanish names because storms affect countries speaking all three languages. The lists rotate every six years. Isaias replaced Ike after the destructive 2008 hurricane caused Ike’s name to be retired. NOAA
WMO itself says storm names should be instantly recognizable for safety. The multilingual approach explains the choice, although it clearly creates a pronunciation hurdle for many English speakers.
Posted by Bob Rehak on 10/9/26 based on information from NHC and Harris County Meteorologist Jeff Lindner
3328 Days since Hurricane Harvey
Isaias Now a Hurricane
9/8/26 – At 4 am CDT today, the National Hurricane Center (NHC) issued an advisory stating that Tropical Storm Isaias had intensified into a hurricane overnight. NHC expects winds to increase to near major hurricane status in the next 24 hours and says the storm will produce life-threatening surge up to 7-feet high across the northern Gulf Coast from the mouth of the Mississippi to the Florida panhandle.
Isaias is now moving toward the east-northeast near 9 mph and should turn toward the northeast later today followed by a turn northward on Friday. NHC expects landfall along the U.S. northern Gulf Coast late Friday or early Saturday.
A dropsonde launched from an Air Force Reserve hurricane hunter found average winds of 76 kt – the basis for upgrading Isaias to a hurricane at 0330 UTC.
Environmental Factors Affecting Forecast
Since that time, Isaias has continued to intensify, but with some northward tilt with height, likely due to moderate westerly vertical wind shear that continues to hinder circulation. However, circulation has increased since yesterday.
Aircraft fixes indicate that Isaias continues to maintain an east-northeastward heading. The most critical steering features:
The net effect:
The forecast has not changed appreciably though models continue to disagree over the exact track (east or west) after the turn to the north. The storm should become post-tropical over the Tennessee Valley.
The intensity forecast remains difficult. Isaias is fighting around 20 kt of westerly vertical wind shear, and an eyewall keeps coming and going.
Other favorable environmental factors for further intensification include 30 C sea-surface temperatures and abundant moisture. Shear should not increase appreciably for the next 24 hours. In fact, the hurricane has intensified despite shear. So NHC expects significant intensification in the short-term. making Isaias just shy of major hurricane intensity in 24 hours.
However, in the final 12-24 hours before Isaias makes landfall on the north Gulf coast, vertical wind shear should ramp up to 40-50 kt. This will likely impart some weakening. At the same time, the hurricane’s wind field may become broader.
Regardless, NHC expects Isaias to remain a dangerous hurricane through landfall with gusty winds extended well inland.
Key Messages from NHC
Key Messages:
1. Isaias should strengthen and become a strong hurricane when it approaches the northern Gulf Coast on Friday. A life-threatening storm surge is expected along a portion of the northern Gulf Coast,and a Storm Surge Warning remains in effect from the Mouth of the Mississippi River to Steinhatchee, Florida. Residents in the warning area should follow advice given by local officials and evacuate if told to do so.
2. Damaging hurricane force-winds will extend across the Florida Panhandle, southern Alabama, and southern Mississippi. A Hurricane Warning remains in effect from Ocean Springs, Mississippi to the Bay/Gulf County Line, Florida. Residents there should prepare for extended power outages.
3. Isaias will bring areas of heavy rainfall to the central U.S. Gulf coast and much of the Southeast Friday into the weekend. Flash and urban flooding, along with isolated river flooding, will be possible.
NHC Graphics Illustrate Risks
The graphics below show current predicted risks.
Isaias is the latest “first hurricane of the Atlantic season” on record.
Posted by Bob Rehak on 9/8/2026 based on NHC information
3327 Days since Hurricane Harvey