Irma’s Projected Path Shifts West; Storm Expected to Restrengthen to Category 5

As of the 5 PM advisory from the National Hurricane Center (NHC), dangerous Hurricane Irma was packing 155 mph maximum sustained winds and tracking just north of due west off the Cuba coast.

The new advisory provides a couple of surprises. One, Irma’s path has shifted more to the west. As a result, the West Coast of Florida and western South Florida is under more of a threat from Irma. That said, the NHC has not backed off its storm surge forecast of 5-10 feet for places like Miami. So, so far, that vulnerable city is not out of the woods — particularly for southern sections of the city.

(Official track shifts west for Irma as the Hurricane Center now predicts the storm will restrengthen to category 5 intensity over the Florida Straits after raking the coast of Cuba. Image source: The National Hurricane Center.)

This is likely due to the fact that Irma has a very large circulation with tropical storm force winds extending outward up to 160 miles from its center and hurricane force winds extending up to 60 miles from the storm’s center. So a west coast landfall in South Florida has the potential to still bring hurricane conditions to places like Miami. That said, if the track continues to shift west, Miami may dodge a bullet as our concerns shift to places like Fort Myers and possibly Tampa.

The NHC’s full statement on present storm surge potential is as follows:

SW Florida from Captiva to Cape Sable…8 to 12 ft
Cape Sable to Boca Raton including the Florida Key…5 to 10 ft
Venice to Captiva…5 to 8 ft
Anclote River to Venice including Tampa Bay…3 to 5 ft
Boca Raton to Flagler/Volusia County line…3 to 6 ft

So basically all of South Florida from Cape Coral to Boca Raton is looking at a 5-12 foot storm surge according to the present NHC forecast. That includes Miami, Ft Lauderdale, the Keys, and the Fort Myers area.

(The NHC’s 5 PM storm surge inundation map shows the potential for significant flooding from South Miami to the Cape Coral area and on out to the Florida Keys. For reference, blue regions are expected to see more than one foot of water above ground, yellow more than three feet, orange more than six feet, and red more than nine feet.)

The second surprise in the recent official forecast is that the NHC now briefly expects Irma to regain category 5 status as it crosses the Florida Straits. Projected 36 hour intensity from NHC is for a storm packing 160 mph winds at that time. This increase in strength now jibes with a number of model forecasts that show Irma tapping much warmer than normal Gulf Stream waters just prior to striking Florida.

It’s worth noting that intensity forecasts are sometimes tough to nail down and the NHC is quick to caution that fluctuations in storm strength are likely. In any case, this is a very dangerous storm that bears watching.




Powerful Irma Threatens to Put South Florida Underwater, Spill Lake Okeechobee

Near category five strength Irma represents a major flood threat from storm surge and rainfall to South Florida. Due to its large size, strong winds, its movement toward shore atop rising seas, and ability to push a tall and wide-ranging surge of water over far-flung coastlines, Irma has the potential to put major cities like Miami under water. In addition, expected 10-15 inch rainfall over Lake Okeechobee threatens the integrity of an aging dike which, if overtopped, could result in severe flooding of inland communities.


As of the 5 PM advisory from the National Hurricane Center, Irma was a top-strength category 4 hurricane packing 155 mile per hour maximum sustained winds and a minimum central pressure of 925 mb. The storm is presently tracking just north of Cuba along a westerly or west-northwest path. It is expected to turn north by Saturday, ultimately making landfall somewhere in South Florida.

Like Harvey, Irma is very moisture rich. Like Harvey, Irma is set to interact with a deep trough dipping down over the Eastern U.S. Like Harvey, Irma is tapping warmer than normal surface waters off Florida which is helping the storm to maintain a high intensity. And like Harvey we can confidently say that the record-breaking and long-lasting high intensity of Irma has been fueled by human-forced climate change — with some weather models indicating a risk that Irma could restrengthen on approach to Florida as it crosses over the 3.5 degree F warmer than normal waters of the Gulf Stream.

Unlike Harvey, Irma is expected to continue moving after making landfall. And this movement will prevent the kind of prolonged event that occurred during Harvey — with a tropical system raining out over the same region for days and days on end. That said, Irma’s extremely strong winds presently at 155 mph and what is likely to be a very powerful storm surge pose a threat to most locations along the Florida Peninsula — especially South Florida. As with other recent hurricanes like Sandy and Matthew, Irma presents an even greater threat from storm surge flooding due to higher overall ocean levels as a result of melting glaciers in places like Antarctica and Greenland. So Irma’s massive predicted surge is running in on a higher ramp than that of decades past.

(The NAM 3 kilometer model shows a very intense 896 mb storm off South Florida by 10 PM Saturday. This model forecast shows Irma strengthening to a very extreme Category 5 storm over the much warmer than normal waters of the Gulf Stream. Official forecasts from the National Hurricane Center still call for a weaker, but still strong and dangerous, Category 4 or 5 system threatening South Florida at about this time. Image source: Tropical Tidbits.)

According to the National Hurricane Center, preliminary expected storm surges range from 8-12 feet for SW Florida from Captiva to Cape Sable, 5-10 ft from Cape Sable to Boca Rato including the Florida Keys, 5-8 ft from Venice to Captiva, 3 to 6 ft from Boca Raton to the Velusia County line, and 3 to 5 ft from Anclote River to Tampa (note that both Florida coasts expect moderate to severe storm surges and that these totals are increased and expanded from the 2 PM NHC advisory).

To put these numbers in perspective, pretty much all of South Florida, including most of the city of Miami is below 10 feet above sea level. A 10 foot storm surge with breaking, wind-driven waves on top, would therefore have catastrophic impacts for this region (see graphic below). As Irma approaches, these already significant storm surge projections may rise further even as impacts from storm surge are likely to expand up the coast.

(A ten foot rise in base sea levels as could occur during Irma’s storm surge would put most of South Florida under water. Storm surge projections for this region are presently 8-12 feet and 5-10 feet. Note that storm surge impact can vary widely based on location and that changes in Irma’s projected path is likely to alter its storm surge related impacts. Image source: Climate Central.)

Though Irma has been compared with Andrew, we must note that Irma is a significantly larger storm — dwarfing the tiny but intense Andrew. As a result, Irma has the ability to deliver a lot more in the way of a powerful surge of water to both Florida coasts. And where Andrew’s damages were primarily due to extremely high winds, Irma’s damages are likely to come from both wind and water — with the potential for very severe storm surge and flood-based destruction.

In addition to the problem of Irma’s likely large and wide-ranging surge, a second issue is the fact that there’s some concern that an aging dike holding water back from communities near Lake Okeechobee might not withstand projected rainfall totals from Irma of 10-15 inches. Though not Harvey-level rainfall amounts, these rains would come in very intense bands over the course of perhaps one day. Such heavy rainfall could cause the lake to over-top the dike — resulting in severe flooding for downstream communities.


(Irma’s heaviest rains are expected to fall over Lake Okeechobee — adding to an already significant flood risk to South Florida. Image source: NOAA.)

The seventy year old dike is presently vulnerable not just due to its age, but also due to the fact that a construction project aimed as shoring up the dike is underway. This rebuild in progress makes the dike even more vulnerable to heavy rains and to large waves that would be stirred up on the lake by hurricane force winds. The Army Corps of Engineers has reassured the public that a dike breach is unlikely — as its most vulnerable section in the southeast has already been strengthened. Concern remains, however, that flooding from the dike could combine with a backing up of canals due to storm surge to swamp communities far inland from the coast.



A Visibly Extreme Jet Stream in Advance of Irma

On Tuesday, I wrote this blog about how Jet Stream behavior and related severe weather during summer of 2017 jibed with the findings of recent climate science. About how human-forced polar warming appears to be impacting extreme summer weather patterns by altering the upper level winds — with a particular focus on impacts to North America.

Yesterday, I looked at the upper level wind patterns running over North America in advance of Irma’s approach and saw this:

(Classic ridge-trough pattern like that identified by Dr Jennifer Francis and Dr Michael Mann. One that, according to their related research, increases the likelihood of certain kinds of extreme weather patterns and events. One that these scientists associate with polar warming set off by human-caused climate change. Image capture from 1500 UTC on September 6. Image source: Earth Nullschool.)

It’s a classic high amplitude wave form in the Jet Stream. One that shows an extremely deep trough digging all the way down to the Gulf Coast in the east and arching back up into a pointed ridge north of Alaska and into the Arctic Ocean in the west. This kind of high amplitude wave pattern is not typical. Or if such a pattern did appear in the past, it tended not to stick around for so long. But during this summer, such intense high amplitude ridges have been forming again and again over the west and such deep troughs have been forming again and again in the east.

New Precipitation and Temperature Extremes

The most apparent visible effect of this ridge-west — trough-east pattern has been to produce record heat, drought, and wildfires in the west and record rainfall in conjunction with an extremely stormy weather pattern in the south and east. You can plainly see this dipolar relationship in the precipitation and temperature anomaly maps provided by NOAA below:

These maps cover precipitation and temperature observations for the last 30 days compared to climatological averages. In the west we find that precipitation for large regions has been less than 10 percent of normal (less than 1/10th normal). Meanwhile temperatures in the west have ranged between 1 and 4 C above average. In the south and east, large regions have seen between 200 and 800 percent of typical precipitation amounts (2 to 8 times the norm). Temperatures, meanwhile have ranged between 1 and 3 C below average.

This is the very definition of heightened extremes. Looking at the prevalent upper level air pattern over the U.S. for the summer of 2017, it’s clear that south to north upper level winds pulling air up from the Equatorial zone toward the pole are facilitating one side of the extreme and that a countervailing upper level wind originating near the pole and running south toward the tropics is driving the opposite extreme.

Slowing Upper Level Winds in a North-South Orientation Weakens the Steering Currents

Unfortunately, prevalent and long lasting heat or heavy rainfall isn’t the only apparent impact of this new pattern. Another aspect of this extreme dipole is a weakening of the west to east steering currents that typically begin to pick up in a region between 25 and 30 degrees North Latitude and to intensify further beyond the 30 N line. This effect is due to the fact that upper level wind patterns are oriented more in a north-south (west) or south-north (east) direction and due to the fact that under such large Jet Stream meanders the upper level steering winds tend to slow down.

(It’s not just Harvey and Irma. Weak upper level steering currents are contributing to a long range potential that Jose might loop back to strike South Florida.)

For Hurricanes like Harvey and Irma, stronger west to east steering winds have had two protective effects for the United States. First, they have helped storms to keep moving — working to generally prevent the kind of long duration stall we saw that helped to produce such catastrophic flooding during Harvey. Second, they have tended to deflect storms away from the U.S. East Coast. And for Irma, what this means is that this storm is more likely to strike the U.S. East Coast if the upper level steering winds that would typically turn it to the east are weak.

This is a dynamic upstream aspect of human-forced polar warming. One that produces added extreme weather risks on top of those already generated by warming ocean waters — which increase peak potential storm intensity — and rising atmospheric water vapor — which helps to add latent heat, lift and related convective available potential energy that increases top limits for storm intensity and heavy rainfall.

And as we sit here hoping and praying that Irma will re-curve away from the U.S. east coast, we should consider how polar warming may be helping to make such a terrible strike more likely — increasing risks to so many people and to so much that we all hold dear.



Dr Jennifer Francis

GFS Model Runs illustrated by Earth Nullschool

Extreme Weather Events Linked to Climate Change’s Impact on Jet Stream

This is the Pattern Climate Scientists Warned us About


Hat tip to Scott

Hat tip to Wharf Rat

Models Show Irma Tracking Toward 88 Degree (F) Waters Before Setting Sights on Florida, Georgia and South Carolina

As of yesterday and today, Irma was the strongest storm ever to form in the Central Atlantic. Fueled by record atmospheric and ocean heat and related high atmospheric moisture content, the storm plowed into the Leeward Islands of Barbuda, St. Martin and Anguilla as a top-strength Category 5 monster hurricane.

(Alex Woolfall takes shelter in a concrete stairwell in St. Martin to avoid Irma’s catastrophic winds. It’s worth noting that hurricanes are heat engines. Tapping 87 F sea surface temperatures and producing 100 percent humidity would result in the very hot conditions Alex was experiencing 7 hours ago. We’re all pulling for Alex and those like him who were trapped in the belly of this massive beast. His last report was at 5:45 AM.)

As Irma’s eyewall began to pass over Barbuda, a reporting station recorded a wind gust of 155 mph before it was knocked out. That island of 1,800 people is now completely cut off from the outside world. Having just experienced winds in excess of those hosted by Andrew and Camille, it is likely that catastrophic damage was inflicted.

On St. Martin, which also passed through Irma’s eye and most intense wind bands, initial reports are also showing very considerable damage. Four of the strongest buildings on the island have been destroyed. And it is expected that most structures across this French/Dutch shared island which is home to 75,000 have seen moderate to catastrophic damage.

(Footage this morning, apparently taken from a camera near the airport at Simpson Bay in St. Martin shows debris, flooding, and very strong winds.)

Anguilla, which is north of St. Martin and is home to another 15,000 souls, passed through the northern eye wall. This is typically the most intense part of a hurricane. So far, reports from Anguila are spotty. But the damage there is likewise expected to be catastrophic.

As of the 5 PM advisory, according to the National Hurricane Center, Irma is still a devastatingly powerful Category 5 monster hurricane hosting maximum sustained winds of 185 mph. The storm had seen some weakening due to apparent eyewall replacement and mild wind sheer — which pushed pressures back up to 920 mb from a low of 914 mb last night earlier today. However, this weakening was not significant enough to impact Irma’s amazing wind intensity. Since that time, Irma’s central dense overcast has thickened while pressures have dropped back down to 914 mb as of the 5 PM advisory.

(Irma tracking just slightly north of the officially projected path from the NHC as of early afternoon on Wednesday. Image source: The National Hurricane Center.)

As the storm passes toward the Virgin Islands, roars by Puerto Rico, and howls into the Turks, Caicos and Bahamas, it is likely that some weakening will occur. Despite this fact, the storm is expected to maintain Category 5 intensity through at least the next 48 hours. After 72 hours, the official forecast calls for Irma to drop to strong Category 4 intensity and eventually a strong Category 3 by Monday. However, some models like the GFS show Irma again strengthening as it taps very warm waters off Florida.

(Very hot sea surface temperatures off Florida could provide fuel that allows Irma to strengthen a second time as predicted in forecast models like the GFS. Image source: Earth Nullschool.)

Most models are now starting to settle on a consensus that brings Irma toward Florida and along a course that may threaten Georgia and South Carolina. The GFS model shows Irma tapping extremely hot sea surface temperatures in the range of 88 degrees Fahrenheit (about 3.5 F hotter than average) and pumping up again into very strong Category 5 intensity with an 895 mb minimum central pressure off Florida by Sunday. This would be a stronger intensity than the 914 mb reached last night by measure of pressure alone.

(The GFS, ECMWF and other major models are starting to agree on a consensus track which has Irma raking the Florida coast before threatening Georgia and the Carolinas. Image source: The National Hurricane Center.)

GFS shows the storm raking most of the Florida coast as it bounces from one landfall or near landfall to another across the eastern seaboard before making a final landfall as a 924 mb monster along the Georgia-South Carolina border. Meanwhile, another major model — the Euro (ECMWF) — has the storm following approximately the same path at a lower intensity.

Though the GFS modeled intensity does not jibe with the official forecast — which calls for weakening of Irma to strong Cat 4 and then strong Cat 3 status — we should not completely rule out the GFS prediction due to those very warm ocean surfaces mentioned above. If predicted wind shear does not emerge, then it would allow Irma to more effectively tap those very warm waters off Florida and hit a second peak intensity. And if such a forecast were realized, it would produce a seriously catastrophic disaster for the U.S. East Coast.

(Models are starting to come into consensus on Irma’s track — which is zeroing in on it raking the Florida coast and then slamming into Georgia or South Carolina — but forecast intensity varies widely. GFS shows Irma off Florida at an intensity stronger than her present extreme strength by Sunday. Image source: Tropical Tidbits.)

Of course, the official forecast track and intensity — in which a strong Category 4 storm rakes coastal Florida and then tracks up into Georgia or South Carolina to make final landfall as a strong Cat 3 is bad enough. So in this case, we are looking a present forecast scenarios in which models are starting to come into consensus on track that range from bad (official Cat 4 and then Cat 3 intensity storm impacting Florida, Georgia and the Carolinas) to worse (GFS potential for a very strong cat 5 storm threatening the U.S. Southeast Coast).




The National Hurricane Center

Earth Nullschool

Tropical Tidbits

878 mb Storm Off North Florida — The Model Forecast for Irma that no one Wants to See Happen

As the United States struggles to recover from severe damage inflicted by one hurricane made far worse by climate change, another powerful storm is brewing over the hotter than normal waters of the tropical North Atlantic.

As of the 5 PM Atlantic Standard Time statement from the National Hurricane Center, Irma was positioned about 1,100 miles east of the Leeward Islands in the central tropical Atlantic. The storm hosted a small circulation, packing 110 mph winds and a minimum central pressure of 973 mb. Over the next few days, according to the Hurricane Center, Irma is presently expected to reach major hurricane status with 130 mph maximum sustained winds.

(Category 2 Irma in the Central Atlantic seems relatively innocuous. But NHC guidance indicates the potential for Irma to develop into a major hurricane over the next five days. Some of the longer range models, however, are producing some rather worrying forecasts. Image source: National Hurricane Center.)

The Hurricane Center is clear to note that it uncertain at this time if Irma will ultimately threaten the Bahamas or the mainland U.S. But the Center cautions that all interests remain watchful and prepared as the storm could pose a risk over the coming days:

It is much too early to determine what direct impacts Irma will have on the Bahamas and the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season.

Looking beyond the official forecast, some of the our best long range model runs are putting together some seriously scary predictions for Irma. By next week, the Global Forecast System (GFS) model shows Irma as a 878 mb monster hurricane looming about 300 miles off Florida. 878 mb would represent the lowest pressures ever recorded in a hurricane in the Atlantic (The present strongest Atlantic storm was Wilma at 882 mb. The devastating Labor Day Hurricane hit 892 mb.). And it would almost certainly represent the strongest storm in our records ever to venture so far North. 878 mb roughly corresponds with maximum sustained winds in excess of 170 mph and possibly as high as 200 mph or more. And we’ve never seen something like that threatening the Central Atlantic U.S. East Coast in all of the modern era.

(A storm stronger than Wilma and approaching Tip’s record 870 mb intensity off North Florida and not in the Caribbean? GFS says it’s possible. Let’s hope for the sake of much that is precious and dear to us that this model forecast does not emerge. Image source: Tropical Tidbits.)

The model then slams the storm into Cape Hatteras just after midnight on Monday, September 11 as only a slightly weaker Category 5 range storm at 910 mb. The storm proceeds north into the Hampton Roads area early Monday morning retaining approximate Cat 5 status at 919 mb. After roaring over this highly populated low-lying region, the storm enters the Chesapeake Bay at 934 mb by noon on Monday — in the Category 4 range and still stronger than Hurricane Sandy — before crossing up the Bay and over the D.C. region by evening the same day at 958 mb (approx Cat 3).

To say this would be an absolute worst case disaster scenario for the Mid-Atlantic is an understatement. A storm of this intensity would produce 10-20 foot or higher storm surges, devastating winds, and catastrophic rainfall throughout the Outer Banks, Hampton Roads and on up the Chesapeake Bay. But unlike Harvey, it would be a fast-moving event. More like a freight train than a persistently worsening deluge.

This long range model scenario is not, however, an official forecast. It’s just what the GFS atmospheric computer models are presently spitting out. And such long range predictions from a single model, no matter how reliable, should be taken with at least a pinch of salt. That said, we should certainly, as the NHC recommends, keep our eyes on Irma and keep our response plans ready.

(Sea surface temperatures in the North Atlantic off the Southeast Coast are between 1 and 1.8 C above average. Atmospheric moisture levels are quite high as is instability. So as with Harvey, we have quite a lot more fuel than normal available for a hurricane to feed on. Image source: Earth Nullschool.)

We should also note the context in which this present extreme potential emerges. Ocean surfaces in the North Atlantic off Florida are very warm with temperatures around 30.5 degrees Celsius (87 F) near the Bahamas. This is about 1.8 degrees Celsius above the already warmer than normal climatological average. Atmospheric moisture and instability in this region of the North Atlantic are also quite high. These two conditions provide fuel for hurricanes that do enter this region. They are conditions that are linked, at least in part, to human-caused climate change. And they are similar to the conditions that amplified Harvey’s intensity just prior to landfall.

So though the GFS forecast described above is far from certain, we should absolutely listen to the NHC’s urging for us to pay attention to what could be another dangerous developing storm. One that appears to at least be physically capable of defying previous weather and climate expectations. Let’s just hope it doesn’t.


The National Hurricane Center

Earth Nullschool

Tropical Tidbits

List of Most Intense Tropical Cyclones

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