Hurricane Ida vaulted to category 1 hurricane strength at 1:15 pm. EDT Friday, and is expected to continue to intensify before it crosses western Cuba and enters the Gulf of Mexico Friday night. At 2 p.m. EDT Friday, Ida made landfall on Cuba’s Isle of Youth with sustained 75 mph winds and a central pressure of 987 mb. Ida’s winds increased by 40 mph in the 24 hours ending at 2 p.m. EDT Friday, exceeding the National Hurricane Center (NHC) minimum qualification for rapid intensification, which is a 35-mph increase in 24 hours.
NHC is predicting that Ida will rapidly intensify Saturday and Sunday and be a category 3 hurricane when it makes landfall in Louisiana on Sunday night. With little lead time to prepare and Ida expected to rapidly intensify right up until landfall, the situation for Louisiana appears particularly serious. Update (5 p.m. EDT Friday): NHC is now predicting that Ida will approach the coast on Sunday as a category 4 hurricane with sustained winds of 140 mph.
A mandatory evacuation has been issued for portions of New Orleans outside of their levee system, and a voluntary evacuation for inside the levee system. A mandatory evacuation has also been issued in parts of Plaquemines Parish, Louisiana; the Air Force Hurricane Hunters have also begun evacuating their fleet of 10 aircraft from Keesler Air Force Base in Mississippi to Texas.
On Friday afternoon, satellite images and radar from Grand Cayman showed Ida had a vigorous area of heavy thunderstorms steadily growing more organized and more intense. Upper-level winds out of the southwest were creating about 10 knots of wind shear, keeping heavy thunderstorm activity suppressed on the southwest side of Ida’s center. However, a major burst of heavy thunderstorms had blossomed over Ida’s center, forming an expanding area of high cirrus clouds with very cold cloud tops, called a central dense overcast (CDO). These characteristics are hallmarks of an intensifying storm, and radar imagery and reports from the Hurricane Hunters early Friday afternoon indicated that Ida had built a ragged but complete eyewall and closed off an eye.
Track forecast for Ida
There is not much mystery about where Ida is headed. A ridge of high pressure centered near the Southeast U.S. coast will keep Ida moving to the northwest at between 10 and 15 mph for the next three days, and there is little doubt that Louisiana will receive a direct strike. Landfall is likely to occur between 6 p.m. Sunday night and 4 a.m. Monday morning.
Ida expected to dump dangerous flooding rains
On Friday afternoon, Ida was bringing heavy rains to Jamaica and the Cayman Islands, and to much of Cuba. Kingston, Jamaica, recorded 7.32 inches of rain in the 48 hours ending at 8 a.m. EDT Friday. Cayman Brac in the Cayman Islands recorded 6.46 inches of rain in the 24 hours ending at 8 a.m. EDT Friday. Top winds at the station Friday were sustained at 31 mph, gusting to 45 mph, at 10 a.m. EDT.
Ida is embedded in a moist airmass, and its modest forward speed of 10-15 mph over the next three days will make it a prolific rainmaker. The NHC is predicting total rainfall amounts of 6-10 inches for Jamaica, and 8-12 inches for the Cayman Islands and western Cuba.
Steering currents are predicted to weaken on Monday after Ida makes landfall on the U.S. Gulf Coast. The storm is likely to slow to a forward speed of just 5-10 mph Monday through Wednesday, resulting in an extended period of downpours in the southeastern U.S. NHC is predicting rainfall amounts in excess of 10 inches along about a 150 mile-wide swath of the U.S. Gulf Coast, with one isolated area of 15 or more inches (Figure 3). This precipitation will be capable of causing major flooding, because soil moisture is in the 80th percentile or higher over most of this region.
Heavy rains from an upper-level low pressure system were affecting much of coastal Louisiana on Friday afternoon, and 1-2” of rain were expected to fall by Saturday morning, before Ida’s rains arrive. Unfortunately, a plume of deep moisture associated with Ida was over this region, and a Flash Flood Watch was issued late Friday morning for portions of coastal Louisiana and Mississippi, warning of localized rainfall amounts of up to 6” over just a 1-2 hour period, causing flash flooding.
Intensity forecast for Ida
Ida will have conditions supporting intensification up until landfall Sunday night/Monday morning on the U.S. Gulf Coast. However, passage over the western tip of Cuba, with its narrow strip of mountains rising over 2,000 feet, may disrupt Ida for six hours or so.
In the Gulf of Mexico, wind shear will be light to moderate (5-15 knots), the atmosphere will be moist, and sea surface temperatures will be very warm, 29-31 degrees Celsius (84-88°F). Very deep, these warm waters give the system plenty of ocean heat to power it. Ida is predicted to pass over a warm eddy in the Gulf of Mexico with unusually high heat content on Sunday, and the heat from this eddy can fuel its rapid intensification (see Tweet by NHC hurricane specialist Eric Blake below).
One potential impediment to intensification could be wind shear caused by upper-level outflow from the Eastern Pacific’s Tropical Storm Nora. Nora is predicted to be near hurricane strength Saturday through Monday as it moves along the coast of southwestern Mexico toward the Baja Peninsula: If the storm is strong enough, its upper-level outflow could bring higher wind shear over the Gulf of Mexico. (Nora is forecast to remain a hurricane well north into the Gulf of California and could bring heavy rain to the southwest U.S. later next week.) However, with Ida now a hurricane, it will be able to establish its own upper-level outflow that should be able to keep Nora’s shearing winds at bay.
The 0Z and 6Z Friday morning runs of three of the top intensity models – the HWRF, HMON, and COAMPS – predicted that Ida would rapidly intensify into a category 4 hurricane by Sunday as it approached landfall on the U.S. Gulf Coast, and the 12Z Friday SHIPS model gave a 41% chance that Ida would be a Cat 4 before landfall. NHC’s 11 a.m. EDT Friday forecast predicted a category 3 hurricane with 120 mph winds at landfall. A reasonable uncertainty range for Ida’s landfall intensity would be category 2 to category 4, between 100- and 150-mph winds, and it’s worth keeping in mind the history of category 5 U.S. landfalls, as mentioned in the Tweet by Brian McNoldy below.
Update (5 p.m. EDT Friday): The 18Z Friday SHIPS model gave a 73% chance of Ida becoming a Cat 4 before landfall and a 31% chance its sustained winds would reach 135 knots (155 mph), just below the 157-mph Cat 5 threshold. The official NHC forecast now predicts that Ida will approach the coast Sunday afternoon as a category 4 storm with 140 mph winds.
Storm surge forecast for Ida
Ida will grow larger as it approaches landfall in the U.S., and NHC is predicting that tropical storm-force winds will span an area 250 miles across at landfall. This large wind field will be capable of driving a highly destructive storm surge of 7-11 feet to the east, or right, of where the center makes landfall. If Ida intensifies to a category 4 storm and tracks to the right side of the current NHC cone of uncertainty, a storm surge of 15 feet could threaten New Orleans’ levees. A surge of that magnitude is close to the system’s design limits, and will be higher than any observed since the levee system was rebuilt after 2005’s catastrophic strike by Hurricane Katrina. Update (5 p.m. Friday): With NHC now calling for Ida to reach category 4 strength before landfall, potential inundation levels have been raised to 10-15 feet from Morgan City, Louisiana, to the mouth of the Mississippi River.
New Orleans levee system to get a major test
In the wake of the unthinkable devastation wrought by Hurricane Katrina in New Orleans in 2005, Congress approved a $14.5 billion upgrade to the city’s flood defenses—the Hurricane and Storm Damage Risk Reduction System (HSDRRS). It consists of a 139-mile system of levees, and walls and gates designed to protect against a 1-in-100-year storm surge, equivalent to what a category 3 hurricane would bring.
The new flood defense system in 2012 underwent a stern test with Hurricane Isaac. Isaac was a large, slow-moving Category 1 storm with 80 mph winds that brought to New Orleans a storm surge characteristic of a Category 2 storm. A surge as high as 12-14 feet assaulted portions of the new levee system. The new flood defenses performed admirably, giving confidence that they can withstand the 15-foot storm surge that a 1-in-100-year category 3 hurricane might bring.
The Louisiana coastline has experienced more than 60 hurricane strikes since 1851, but only one that brought a storm surge to New Orleans capable of overwhelming the new levee system. That storm was the deadliest hurricane in Louisiana history, the 1893 Chenier Caminanda hurricane, which hit the coast south of New Orleans as a category 4 storm. More than 2,000 people died in that storm, mostly from storm surge.
One interesting aspect of the new levee system is the “armoring” that has occurred: Fabric mats that grass can grow through have been installed along the inside of key stretches of the levee system to greatly reduce or eliminate the chances of overtopping causing a levee breach. The Army Corps says the armoring will mean New Orleans will flood to a depth of no more than five feet in an overtopping situation, greatly reducing damage. That’s much lower than the 15-20 feet of inundation in the lowest parts of New Orleans from the multiple levee breaches and floodwall failures during Hurricane Katrina in 2005.
Disturbance 98L in the eastern Atlantic likely to develop
In the eastern Atlantic, midway between the coast of Africa and the Lesser Antilles, lies a tropical wave designated 98L. Satellite imagery showed the disturbance had developed a well-defined surface circulation, but had only a small area of heavy thunderstorms. Over the next day, 98L will be moving west-northwest at 10-15 mph, and it is then expected to turn north on Saturday, well before reaching the Lesser Antilles. 98L may pass close to the Azores Islands on Wednesday.
The system is over warm waters with sea surface temperatures (SSTs) of 27 degrees Celsius (81°F), and some modest development of 98L may occur until wind shear grows prohibitive on Tuesday. NHC, in a 2 p.m. EDT Tropical Weather Outlook, gave 2-day and 5-day odds of development of 80%. The next name on the Atlantic list of storms is Julian.
Disturbance 97L in the central Atlantic may also develop
About 600 miles east of Bermuda in the central Atlantic lies a tropical disturbance designated 97L, which has grown more organized over the past day. Conditions are marginally favorable for 97L to develop over the next two days, with sea surface temperatures of 28-29 degrees Celsius (82-84°F) and moderate wind shear of 10-20 knots.
The atmosphere surrounding 97L is dry, with a mid-level relative humidity of 50%, which is slowing development. The system has some model support for development by this weekend, with a predicted track that will take 97L to the northeast, potentially bringing it near the northwest Azores Islands on Monday. NHC, in a 2 p.m. EDT Friday Tropical Weather Outlook, gave 97L 2-day and 5-day odds of development of 40% and 60%, respectively. The next name on the Atlantic list of storms after Julian is Kate.
Bob Henson contributed to this post.
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