Sandy’s Historic Encounter with the Northeast U.S. Looking Very Likely

Hurricane Sandy formed just four days ago north of Panama. In its short lifetime so far, it has claimed 21 lives in Jamaica, Haiti, and Cuba and unexpectedly intensified to a strong Category 2 storm immediately after exiting mountainous eastern Cuba. It passed over the central Bahamas on Thursday with 105mph sustained winds, and brought tropical storm conditions to Miami, West Palm Beach, and the southeast Florida peninsula. Today, conditions over southern Florida are improving as Sandy crawls north at 6mph. As of 11am this morning, Sandy’s maximum sustained winds are 80mph (a Category 1 hurricane) and the wind field is expanding. Tropical storm force winds (35mph+) now reach 275 miles from the storm’s center, and weaker though still noteworthy winds extend approximately 800 miles from the center. It is located 190 miles due east of West Palm Beach, FL.

The official track from the National Hurricane Center shows Sandy moving slowly to the north through Saturday, then northeast for a couple of days before getting pulled back westward toward the coast. Tropical storm watches and warnings are in effect for the U.S. east coast from the southern tip of Florida to the Outer Banks of North Carolina.

The longer range forecast is becoming more certain as nearly every model now agrees on a similar track and evolution. Unfortunately, the solution being converged upon is a devastating and historic “perfect storm” scenario for the entire northeast U.S. from the coast to hundreds of miles inland.

There’s a fine line between over-hyping a situation, and giving plenty of advanced warning prior to a potentially catastrophic situation. When it comes to a forecast for something like this, think of a spectrum of possibilities: a chance that it won’t be so bad and all the hype is overkill, a chance that it will live up to or even exceed the hype, and then the middle ground of a bad storm, but nothing to panic about. Of course, we don’t know with 100% certainty which of these possibilities will be realized, but since the high-end impact scenario is presently a very real one, it would be prudent to over-prepare and be safe than under-prepare and regret it.

In an effort to aid forecasters as much as possible, weather balloons are being released four times per day rather than the typical two times per day across the entire nation. These enhanced upper-air observations upstream of the storm are fed into models, and starting with more accurate knowledge of the true state of the atmosphere would ideally lead to more accurate forecasts. Secondly, there are two aircraft flying around in the storm this morning: one from NOAA’s Hurricane Research Division and one from the Air Force’s Hurricane Hunter fleet. This will help guide storm-scale data assimilation, while the additional weather balloons will help with large-scale environmental data assimilation. These resources are being utilized due to the looming “perfect storm” scenario unfolding over the next few days.

From Sunday through the middle of next week (and beyond?), this system is forecast to intensify while expanding at the same time. This would bring extraordinary storm surge and rainfall to the entire mid-Atlantic and northeast U.S. regions. If you lived in those areas during Irene last year, consider that a practice run. This setup is truly rare and is not your typical hurricane or Nor’easter riding up along the coast.

Several days of heavy rain combined with the strong wind can easily result in widespread flooding, tree damage, and power outages. The Monday-Tuesday period should be core of the worst weather in the northeast, but it won’t just end abruptly either. Immediately along the coast, very high storm surges combined with beach erosion could lead to significant seawater flooding. The full moon on Monday will make the normal high tides even higher, and raising the base sea level.

We will have another update tomorrow when Sandy is located east of Georgia and South Carolina and about one day prior to impacts being felt in the majority of the mid-Atlantic region.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

Tropical Storm Watch for Southeast Florida as Sandy Heads North

On Monday morning, the season’s 18th Tropical Depression formed in the central Caribbean Sea, between Jamaica and Panama. It quickly intensified to a tropical storm, and was named Sandy. Since then, it has been drifting very slowly and is only now reaching Jamaica and has just reached hurricane intensity. Its rainbands extend far from the center however, and are affecting Jamaica, Dominican Republic, Haiti, Cuba, the Bahamas, and soon, south Florida. As of 11am today, the maximum sustained winds are 80mph and tropical storm force winds extend 140 miles from the center. This is the 10th hurricane of the 2012 Atlantic season.

Sandy is forecast to pass over Jamaica today, then eastern Cuba on Thursday morning, the Bahamas on Thursday night into Friday morning, then head northeast along the southeast U.S. coastline. Depending on what affect Jamaica and Cuba have in its intensity and structure, there is a possibility that it could become a hurricane again over or near the Bahamas when it’s at its closest approach to Florida.

Here in the Miami area, we are under a tropical storm watch as of Wednesday morning, and can expect tropical storm conditions to begin later today and last through Saturday with the worst conditions on Thursday night into Friday. This includes very heavy rain and gusty winds in rainbands, strong rip currents, coastal flooding, and beach erosion. Our own Hurricane Portal has several resources of local interest posted.

By Monday and Tuesday, many models are showing the beginning stages of an extremely dangerous and significant storm for the entire mid-Atlantic and northeast regions of the U.S. Sandy may lose some or all of its tropical traits, but even as a subtropical or extratropical cyclone, it could be a huge player for a lot of people next week. More and more models (and ensemble members) are showing Sandy interacting with a strong mid-latitude trough somewhere off the coast of North Carolina and Virginia and becoming ominously intense.

Tracks from various models (colors) and ensemble members (gray). (SFWMD)

By Saturday, when it has crossed out of the Caribbean and into the Atlantic, and has passed over Jamaica and Cuba, models will hopefully be a bit more certain about whether Sandy will head out to sea or head into the northeast U.S.

Finally, today is the seven year anniversary of Hurricane Wilma’s landfall on Florida. Although it made landfall on the southwest coast of the peninsula, it zipped across and will still a destructive storm on the east side of the peninsula. That was the last time the U.S. was hit by a major (Category 3+) hurricane.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

What Happens Underwater During a Hurricane?

We think we’re pretty familiar with hurricanes – strong winds, storm surge, flooding rains, ominous satellite images from space, and radar loops when they get near land. But what goes on at and below the ocean’s surface when a hurricane passes overhead? Quite a lot, actually!

Effects on the ocean properties

The upper levels of the ocean are typically strongly stratified by temperature and by salinity. That is, colder, saltier water lies below the warmer, fresher water near the surface. When a hurricane comes by, it mixes everything up, resulting in a muddled and more homogeneous upper ocean. That means the surface water is cooler and saltier than it was previously was, and deeper water is warmer and less salty than it previously was. However, in very shallow coastal areas, the copious amount of fresh cold rain water from the hurricane can actually reduce the temperature and salinity of the near-surface water.

Time series of the vertical profile of temperature and salinity from the ocean’s surface down to 200m, and spanning one day prior to the hurricane’s passage through 2.5 days after the passage. The dramatic mixing down to approximately 150m is evident. Time in days relative to the passage is listed along the horizontal axis. This particular case is from Hurricane Frances (2004) on 1 September. (Sanford et al., 2007)

The colder surface water upwelled by the hurricane can actually be a fairly significant player in controlling the hurricane’s intensity. A strong slow-moving hurricane will upwell cold water much more effectively than a weaker and/or fast-moving hurricane. And since hurricanes require warm ocean water to fuel their “engine”, that upwelling can end up weakening the storm. The trail of upwelled cooler water left behind a storm is called a “cold wake”, and shows up clearly on maps of sea surface temperature.

Map of sea surface temperature before (left) and after (right) Hurricane Isabel in 2003. Isabel’s track from the eastern Atlantic all the way into the mid-Atlantic coast is evident by the cold wake left behind. (NASA/GSFC)

Intense hurricanes can generate 60’+ waves, and at the ocean surface, the boundary between the water and the air becomes nebulous. Amidst the formidable waves, sea spray and foam streak horizontally across the surface at high speed, blurring the view of the ocean’s surface in this photo from an aircraft flying through a hurricane.

Photo of the sea state under Category 4 Hurricane Isabel taken from 400 feet above the surface. Note that the aircraft was not in or near the eyewall at this time or altitude. (Will Drennan, RSMAS)

But below the ocean’s surface, the currents and turbulence beneath those waves can also be quite destructive. Unlike places above the surface, the ocean doesn’t “forget” about the storm very quickly… strong currents and turbulence have been known to exist up to a week after the storm passes overhead. Damaging currents can extend down to at least 300 feet below the surface, capable of dismantling coral reefs, relocating ship wrecks, breaking oil pipelines, and displacing huge volumes of sand on the seabed.

Simplified schematic showing the parts of an ocean wave. At the surface, there are crests and troughs. Crests are separated by a wavelength. The depth to which a wave’s effects can be felt depends on the wavelength and wave height.

Effects on marine life

Some studies conducted in the Caribbean Sea have shown that in the year following a hurricane, coral cover is reduced by 15-20% (more or less, depending on the intensity of the hurricane) in the affected areas. There are several factors that go into the negative effect on coral: 1) the turbulent water breaks it, 2) the days of muddied water reduces the amount of sunlight reaching the algae in coral tissue, 3) the fine suspended particles clog the pores, and 4) the tremendous amount of rain reduces the salinity of the shallow ocean in the immediate area which can stress coral.

Large self-propelled marine animals such as sharks seem to be minimally affected, since they can detect tiny changes in pressure as larger waves at the surface approach, as well as the reduced surface pressure associated with the storm itself, and go deeper or leave the area. However, hurricanes have been known to result in tremendous numbers of dead fish, crabs, sea turtles, oysters, etc due to reduced amounts of dissolved oxygen in the water, rapid salinity changes, and violent surf.

Just like us up here on the surface, marine life suffers for months to several years from the death and destruction following a hurricane.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

October Ushers in Highest Likelihood of Hurricanes in South Florida

In the 2012 Atlantic hurricane season, we have had 14 tropical storms so far, 8 of which became hurricanes, and just 1 of those became a major (Category 3+) hurricane. An extremely unusual aspect of the season is that essentially all of the hurricane activity occurred north of 25N! Only Ernesto briefly reached minimal hurricane intensity in the tropics just prior to hitting the Yucatan peninsula in early August. All other hurricanes were in the subtropics or mid-latitudes. The season’s only major hurricane so far (Michael) formed from an upper-level cold low pressure system, not from an easterly wave or anything connected to the tropics.

As we head into October, the fifth month of the official Atlantic hurricane season, it’s very important for us in south Florida to realize that this is the greatest hurricane risk month. More hurricanes directly hit or affect southern Florida in October than in any other month. In the graphic shown below, the gray circle is 300 miles across and centered on far western Broward County – designed to include all of southern Florida and immediate surrounding ocean. Any storm of hurricane intensity (sustained winds of 75mph+) whose center passed within that circle is shown in the colored lines, and the legend in the lower right corner associates the color with a category on the Saffir-Simpson scale (yellow is Category 1, orange is Category 2, etc). Finally, the coastal counties are shaded by historic landfall frequency, with darker reds corresponding to more frequent, and pale reds corresponding to less frequent. The monthly tally of tracks passing through the circle is indicated in parentheses below the month. Keep in mind that all of these storms were hurricanes – tropical storms and depressions are not included; and most importantly, never focus on exactly where the center of the track is. Destructive winds, tornadoes, flooding rains, and inundating storm surges can and do occur for hundreds of miles away from the center; so even tracks on the fringe of the circle likely brought severe weather conditions to the mainland.

Another interesting aspect of these maps is that in August and September, southern Florida is most likely to get struck by a storm coming from the southeast. But in October, the dominant direction is from the southwest… due to storms coming from the Gulf of Mexico and western Caribbean, the more favored areas for hurricane formation later in the season.

On the topic of landfalls, do you remember the last time a major hurricane made landfall on the U.S.? It was Hurricane Wilma, on the morning of October 24, 2005, and it hit southern Florida at Category 3 intensity. That was 2,535 days ago, an utterly unprecedented span between major U.S. hurricane landfalls. Streaks that even approach this long are very rare, and only two other spans of over 2,000 days have occurred since 1900. Why has it been so long? Pure luck. There have been several major hurricane landfalls since 2005 in other countries (Nicaragua, Mexico, Cuba, Virgin Islands, Bahamas, and close encounters in Jamaica and Belize), so it definitely isn’t correct to say that seasons since 2005 have been quiet … far from it. No one knows exactly when our lucky streak will end, but I can say with 100% certainty that it will end eventually!

Finally, we are still tracking Hurricane Nadine, which has been around since September 11 (the wave that would become Nadine actually left the African coast on September 7). It’s currently about 550 miles west of the Azores, and is a Category 1 hurricane with 75mph sustained winds. To accurately compare its longevity to historical storms, we need to discount the extra-tropical portions of its lifetime; doing so yields a total of 19.00 days as of this morning at 8am, which is certainly quite long, but not long enough to set any records just yet. This table puts Nadine’s lifetime in perspective among the record holders:

1st 28.00 days Hurricane #3* (1899)
2nd 27.25 days Hurricane Ginger (1971)
3rd 24.75 days Hurricane Inga (1969)
4th 22.00 days Hurricane Kyle (2002)
5th 21.00 days Hurricane #4* (1926)
… (four other storms) …
10th 19.00 days Hurricane Nadine (2012)

* These storms were prior to the satellite era when it was very easy to miss some of a storm’s existence, so these totals are likely underestimates.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

Hurricane Isaac Makes Landfall – Tropical Storm Kirk is Born

Isaac was upgraded to a hurricane just before landfall, and actually continued to strengthen as it got closer and closer to the coast. Not only that, it also stalled, and is sitting in basically the same place for 12 hours and counting. The full radar loop of Isaac’s approach from the New Orleans radar is very illustrative for both the intensification and the stalling. It first clipped the Mississippi delta on Tuesday evening, then the center moved offshore just a little, and came ashore again about 70 miles west several hours later. However, hurricanes are large, and damaging effects are always felt very far from the exact center.

As expected, the storm has caused massive power outages (half a million and increasing) and substantial storm surge. The surge was almost perfectly forecast by the National Hurricane Center, and peaked at about 11′ in Shell Beach LA, 8′ in Bay St. Louis MS, 4′ in Mobile Bay AL, 3′ in Pensacola FL, and reports of significant surge in Destin FL. The storm surge will again be a major contributor to the damage, even for a low-end Category 1 hurricane. Storm surge is intentionally no longer a part of the Saffir-Simpson intensity scale.

The rainfall is another big factor, as was very well forecast by HPC. Below is the 24-hour rainfall estimate ending at 6am CDT this morning (so obviously the final values will be higher).

In what seems to be a Circle of Life in the tropics, just as Hurricane Isaac makes landfall and is destined to dissipate into nothing, Tropical Storm Kirk is born far to the east with no threat to land. For more on Kirk’s current position and development, visit Tropical Atlantic Update.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy

Quick Look: Hurricane Isaac vs Hurricane Katrina

Much to the relief of everyone in southern Florida, Isaac never did get too organized or intense after passing by Haiti and Cuba. It maintained a steady tropical storm intensity as it skimmed by Key West, as it made the journey across the Gulf of Mexico. It has just been upgraded to a hurricane off the Louisiana coast, but it could have been much, much worse.

During the early morning hours on August 29, 2005, Hurricane Katrina made landfall in eastern Louisiana as an enormous Category 3 storm. It intensified from a tropical storm to a minimal hurricane as it passed over Miami and the southern Florida peninsula on August 25th, then took full advantage of ideal conditions in the eastern Gulf of Mexico and dramatically intensified to a monster Category 5 storm on August 28th. Something similar was certainly possible with Isaac, and haunting similarities were everywhere – the dates, the tracks, the size, the landfall location, but one key difference remained: the intensity.

Hurricane Katrina (2005 – top) vs. Hurricane Isaac (2012 – bottom) at exactly 10:15am on August 28th.

I also made a comparison image of the two storms as they appeared on satellite at exactly the same date and time, just seven years apart (10:15am on August 28th). At this time, Katrina had 165mph sustained winds, while Isaac had 70mph sustained winds.

As far as Isaac goes, it is now a hurricane with 75mph winds as of 11:20am this morning. This is the first time that it has reached hurricane intensity during its entire 12-day journey across the Atlantic. It is just hours from landfall, and just hours from the exact landfall time of Hurricane Katrina seven years ago. You can monitor the storm with a long radar loop from New Orleans. Rainfall totals along the northern Gulf coast are expected to be in the 12-18” range, and the storm surge could be significant between the center of the storm and places for hundreds of miles east of the center as its circulation pushes the ocean out ahead of it and onto the coastline.

Luckily, we aren’t looking at a repeat of one of our country’s largest natural disasters, but it acts to keep us vigilant and prepared.

Brian McNoldy
Senior Research Associate
& Author of Tropical Atlantic Update
Follow Brian on Twitter: @BMcNoldy