September 8, 2017

Update #5: Hurricane Irma strengthening, watch some cool visuals

Posted by larryohanlon

As Hurricane Irma barrels through the Caribbean and towards the mainland U.S.A., AGU wants to keep you updated with what scientists are talking about now. Check back for updates on science happening now from Eos staff writer JoAnna Wendel and science writer Larry O’Hanlon.

Update #5, 3 pm EDT, 8 Sept: Irma strengthens again

Well, like some meteorologists suspected: the eye wall replacement process is complete, and Hurricane Irma seems to have strengthened. In the National Hurricane Center’s 2 pm EDT update, they write that Irma’s winds are now sustained at 250 km/hr (155 mph). Pressure has dropped to 925 millibars (it was 927 millibars a few hours ago).



Here are some cool visuals I’ve come across today:

A GOES-16 satellite view of Irma’s lightning:



An animation showing the difference in size between Hurricane Andrew, a category 5 storm that buffeted Florida in 1992, and the current Hurricane Irma, which is much larger.



More GOES-16 imagery, showing a close-up view of Irma’s turbulent eye.



Animations showing the three hurricanes, Irma, Jose (now a category 4) and Katia (a category 2):




Update #4, 12:20 pm EDT, 8 Sept: Is Irma poised to strengthen?

As Hurricane Irma bears down on south Florida, it may strengthen once again. 

Meteorologists have been noting that the “eye wall replacement” of the hurricane’s inner eye is nearly complete. This is a process by which the walls of the inner eye break down and reform. Scientists aren’t exactly sure why this process occurs, but it could result from the turbulence from the strong hurricane winds.

Irma is also heading for relatively warm water–the exact fuel it would need to strengthen. Hurricanes gain their strength by sucking up warm air from warm sea surfaces.



Watch this 84-hour evolution of Irma:



The last update from the National Hurricane Center listed Irma’s sustained winds at 240 km/h (150 mph), with hurricane warnings issued for much of the tip of the Florida peninsula. 


Update #3 10:30am EDT, 8 Sept. 2017: Don’t get fooled by a downgraded Hurricane Irma

Looks like south Florida is still in Hurricane Irma’s cross-hairs. This morning at 8am EDT, the National Hurricane Center (NHC) released its latest update for Irma, which has now sustained winds at 240 kilometers per hour (150 miles per hour).


Storm surge warnings are in place for the Florida peninsula—this means that life threatening flooding could occur. From the Florida Keys up the peninsula to Bonita Beach, Venice, and the Jupiter Inlet, storm surges could reach 1.5-5 meters above ground (that’s about 5-10 feet). The NHC also issued hurricane warnings for the same areas.

From the Washington Post: “It’s not a question of if Florida’s going to be impacted, it’s a question of how bad Florida’s going to be impacted and where the storm ends up,” William “Brock” Long, administrator of the Federal Emergency Management Agency, said at a briefing on Friday.

Since Irma tore through the small Leeward Islands on Wednesday, it’s been downgraded to a category 4. However, meteorologists urge people to avoid thinking that the storm will be less destructive. After all, Hurricane Katrina was only a category 3 when it made landfall in Louisiana and devastated New Orleans.

For one thing, Hurricane Irma is still extremely wide, flinging hurricane-force winds 110 kilometers out from the eye and tropical storm force winds 295 kilometers outward, according to the NHC. Secondly, as I learned yesterday (see Update #2), the rainfall could be dangerous even far outside the hurricane’s main track.


Finally, meteorologists have pointed out that the slight weakening could be due to a process within the hurricane called an “eyewall replacement cycle,” in which the hurricane’s central eye breaks down and rebuilds. This is common with very strong hurricanes, according to the Weather Prediction Center.


This is one of the reasons meteorologists watch the pressure of a storm (see Update #1 below)—a lower pressure tells them that wind is rushing upwards, intensifying the vigorous cycling of winds required to power a hurricane. So even though Irma’s pressure is currently at 927 millibars, this could change.


Update #2, 3:30 pm EDT, 7 Sept. 2017 – Danger even outside Irma’s main track 

Two tweets caught my eye in the last hour, and the both emphasized the dangers of Hurricane Irma’s impending mainland U.S.A landfall this weekend: Even outside the main track of the hurricane, the potential for dangerous rainfall is high. 



“Obviously, along the path of [Hurricane Irma] there’s going to be significant rainfall,” said Ed Vallee, a meteorologist and author of the above tweet. However, once hurricanes like this one make landfall, they expand, which means “the rainfall is much more expansive” than the main track of the hurricane. So far, Irma is expected to barrel through Florida, Georgia, and the Carolinas. Vallee is in Fayetteville, North Carolina, which is about 160 kilometers (100 miles) inland, and he’s preparing for 3-6 inches of rain. 



Of course, where this rain falls depends on where the hurricane does end up hitting Florida. And that depends on where it begins its northward journey, according to Irma is expected to make landfall in southern Florida on Sunday, but it may turn north before that and grind along the coast. If it hits the peninsula directly, that band of dangerous rain outside the cone would be symmetrical, covering an area double the main track of the hurricane. If Irma does end up turning north on Saturday, that band of rain will shift. Vallee says it’s important to keep checking the National Hurricane Center and the Weather Prediction Center for updates.


Update #1, 2pm EDT, 7 Sept. 2017 – What does a hurricane’s pressure mean?

In all the online discussion about Hurricane Irma, I keep seeing one measurement stick out: Irma’s pressure. Yesterday, a National Hurricane Center (NHC) scientist Eric Blake tweeted that Irma’s pressure had dropped to 914 millibars. What does that even mean? Who cares about the pressure?

Well, the pressure in a hurricane matters because it tells scientists something about the storm’s relative strength. A hurricane forms when warm air rushes upwards, condenses, and forms clouds, over and over and over again. Areas of low pressure form as that air rushes upwards, and cooler air sinks. This cycling is what creates a hurricane, and it’s this cycling continuing across a huge, warm Atlantic ocean that gave Hurricane Irma its strength.

So when the NHC announces Irma’s pressure, and when that pressure is lower than the last measurement, that means more air is rushing upwards, strengthening the storm. Typical pressure at sea level is about 1013 millibars, said Jill Trepanier, a hurricane expert at Louisiana State University. Pressures around 980 millibars give you a thunderstorm, and anything below that can lead to a tropical storm or hurricane.

Yesterday, Irma’s pressure clocked in at 914 millibars, meaning that it’s continuing to strengthen, Trepanier said. In contrast, when Hurricane Andrew made landfall in Florida in 1992, its pressure measured 922 millibars.

In the NHC’s latest update, from 11am EDT today, the pressure had risen slightly to 921 millibars. Does that mean the hurricane is weakening? Not necessarily, Trepanier said. As hurricanes travel, the walls of the inner swirling eye can break down, causing a slight weakening of the storm. Usually, however, a new eyewall forms, and the pressure drops yet again. Trepanier suspects that within 12 hours, this 921 millibar reading will decrease once again.

2005’s Hurricane Wilma holds the record for lowest pressure (and thus, strongest storm) in the Atlantic, clocking in at 882 millibars. But the title for lowest pressure measured anywhere on Earth goes to 1979’s Typhoon Tip, at 870 millibars. The typhoon carried with it maximum wind speeds of 306 kilometers per hour (190 miles per hour). Although the storm’s strength greatly diminished by the time it slammed into Japan’s largest island of Honshu, its rain still caused mudslides, destroyed bridges, flooded homes, and killed 99 people.


12 pm EDT, 7 Sept. 2017

Meteorologists have labeled Hurricane Irma as one of the strongest hurricanes ever recorded in the Atlantic. Wind speeds of about 298 kilometers per hour (185 miles per hour) have been sustained for more than 24 hours—that shatters Hurricane Allen’s (1992) record of 18 hours. In the last 2 days, Irma has demolished buildings in the Caribbean Islands, knocked out power for over 1 million people, and now threatens south Florida.



What made this storm so strong? A perfect combination of temperature and location, writes Andrew Freedman at Mashable.

“For the weather obsessed among us, monitoring Irma has been like watching an elite sprinter as they break a record time that no one thought was beatable,” he wrote.

The storm formed off Africa’s Cape Verde, a location which has historically produced monster hurricanes, NOAA research scientist Neal Dorst told Live Science. Forming here allows the developing storm ample time to gain strength as it billows across the Atlantic Ocean.



And that strength comes from warm sea surface temperatures across the equatorial Atlantic, Freedman writes, allowing Irma to grow and grow.

Hurricanes need warm air for fuel. Water vapor from the warm ocean air rises and forms clouds, which releases heat. This pattern repeats and repeats, creating a larger and larger column of clouds. As that water vapor condenses and releases heat, the heat needs somewhere to go—so it moves away from the storm, quickly dropping the pressure. More warm air from below rushes towards the low-pressure area, creating more clouds and more thunderstorms. As the cycle continues, wind whips the column into a tropical storm. Once these winds exceed 119 kilometers per hour (74 miles per hour), the tropical storm becomes a hurricane.

In the Caribbean, from the Lesser Antilles to the Florida Keys, warm water and air temperatures made conditions ripe for hurricane strength, Freedman writes.

The next factor a hurricane needs is a calm wind environment, which is called low wind shear, Freedman continues. This means that the wind is blowing in relatively the same direction at relatively the same speed, allowing a symmetrical cyclone to form. Fortunately for Irma, winds across the Atlantic remained calm as the storm moved.

Finally, Hurricane Irma is huge—stretching some 640 kilometers across (400 miles), and its structure allows for a double ring of hyper intense winds to whip around its eye…