Category Archives: Meteorology & Physical Oceanography

“Perfect Storm” Set to Occur on 21st Anniversary of Original Historic Event

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Hurricane Sandy this afternoon, currently churning over the Bahamas and gradually moving northward. Image courtesy the NOAA National Environmental Satellite Data and Information Service (NESDIS).

The odds of a potentially historic meteorological event occurring in the vicinity of the northeastern United States next week are increasing. The players on the field are as follows: Hurricane Sandy traveling northward along the east coast of the U.S., a warmer-than-average Gulf Stream, a very deep upper-level trough over the central U.S. currently bringing snow to Colorado, and unusually strong high-latitude blocking (a very negative North Atlantic Oscillation / NAO). While Hurricane Sandy is currently bringing some rain, wind, and rip currents to South Florida, Sandy has the potential to bring even bigger problems to the Northeast. An increasing number of model forecasts are now “phasing” Sandy with the mid-latitude trough, and given the amount of upper-level jet energy available in this setup, this could become a particularly powerful phasing event.

Something similar happened in late October through early November 1991. It was known as “the Perfect Storm”, resulting in 13 fatalities and caused > $200 million in damages to the northeastern U.S. and fishing and shipping interests. In the Perfect Storm, northward-moving Hurricane Grace phased with a mid-latitude trough, similar to the one over the Central U.S. today. Normally a hurricane weakens as it moves northward, as it encounters an increasingly unfavorable environment. This means greater wind shear, drier air, and lower sea surface temperatures. However, with phasing events, the tropical system merges with the mid-latitude system in such a way that baroclinic instability (arising from sharp air temperature/density gradients) and extremely divergent air at the upper-levels more than compensates for a decreasingly favorable environment for tropical systems. The Perfect Storm deepened to 972 mb, and was at its strongest while out over the open ocean (but still whipping the coast with strong winds and heavy surf):

The co-location of an anomalously deep upper-level trough (left) directly over a strong surface cyclone (right) off the coast of the Northeastern United States during the Perfect Storm: Oct 30, 1991. Image courtesy of Pennsylvania State University meteorology department.

While there is still inherent uncertainty in the forecast, especially considering we are at least 5 days away from the phase, the majority of the numerical guidance has now come into agreement that a phasing event will occur precisely on the 21st anniversary of the Perfect Storm somewhere between the mid-Atlantic states through Maine or potentially the Nova Scotia region. Most of the models now indicate even stronger jet dynamics will occur next week than occurred during for the Perfect Storm, and that today’s storm could potentially deepen to well below 960 mb or even below 950 mb. The fact that the Gulf Stream is anomalously warm for this time of year means that Sandy will weaken less as a tropical system than it otherwise would have prior to the phase. Also, a very strong blocking scenario (very negative NAO) has developed over the north Atlantic means that the cyclone will be very slow moving, and is likely to retrograde westward into the northeastern U.S. rather than continue out to sea like most recurving extratropical cyclones do. While it is too early to pin-down exact impacts from the system at this time, it is likely that portions of the coastal Northeast will experience a damaging storm surge, significant beach erosion, and a prolonged severe wind and heavy rain event. Meanwhile, interior regions of western Pennsylvania into Ohio may simultaneously be experiencing heavy snowfall. Stay tuned!

The Global Forecast System (GFS) forecast for next Tue, Oct 30, 2012. Note that the cyclone is stronger and closer to the coast than during the Perfect Storm. Image courtesy of Pennsylvania State University meteorology department.

Will Komaromi
Ph.D. Student
Meteorology & Physical Oceanography
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Tropical Storm Watch for Southeast Florida as Sandy Heads North

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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?

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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

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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

The Peak of Hurricane Season Has Arrived

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Today, September 10th, marks the climatological peak of hurricane season. The waters are nice and warm with lots of tropical waves traveling off the coast of Africa. It is this time of year that people tend to think of when they hear the word “hurricane,” with the African Easterly Waves developing into classic Cape Verde-type hurricanes. It is not surprising that during the peak of hurricane season we have 2 named storms (Tropical Storm Leslie and Hurricane Michael) with a third system likely to form soon. In case you were wondering, it would be called Nadine if it reaches tropical storm strength.

With that in mind, let’s look at the season so far. We have had 13 named storms, 7 hurricanes, and 1 major hurricane. We had an early start to the season with 2 named storms before the official June 1st start. Tropical Storm Beryl, who made landfall in Jacksonville Beach, Florida made history as the strongest May storm on record with maximum sustained wind speeds of 70 mph (just below the 74 mph hurricane cut off) before landfall. A few weeks later there was Tropical Storm Debby who dumped large amounts of rain across Florida, especially on the west coast. Miami hasn’t been directly effected yet, but did get rainbands and weak tropical storm force winds as then Tropical Storm Isaac passed just below the Keys. Hurricane Isaac later went on to strengthen in the Gulf of Mexico and sat on top of Louisana, unleashing rain and winds for what seemed like forever. The major impact with Isaac was the flooding, as natural levees were topped and the Mississippi River overflowed into the surrounding areas. Ironically, this occurred around the 7th anniversary of Hurricane Katrina, and although New Orleans was fine this time, others were not so lucky. The first major hurricane was Hurricane Michael in the Atlantic which was a Category 3 for 6 hours.

Rain and flooding have been the main story so far this season, but there is still the second half to go. Let’s see what the rest of the season has to bring, and keep our fingers crossed. Hurricane season officially ends on November 30th.

-Angela Colbert
Meteorology & Physical Oceanography
Graduate Student
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How Do Hurricanes Get Their Names?

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Isaac, Kirk, and Lesile, oh my! As hurricane season is ramping up to its peak, all those storms swirling around in the Atlantic can get confusing. To help stop this confusion, we name tropical storms and hurricanes to more easily and clearly communicate information about them (we use numbers for tropical depressions). However, this was not always the case.

Back in the day, hurricanes were referred to by their position (latitude-longitude) or in some cultures, named after saints. This was not only hard to communicate, but confusing to the public about warnings. Thus after World War II, the navy began flying into the storms and referred to them by the international phonetic alphabet (Able, Baker, Charlie, Dog…and my personal favorite, Love) from 1950-1952. In 1953, it was decided that the storms would be named by women’s names. This was a common military practice when speaking about ships and planes, so it was carried on. The storm naming responsibility landed in the hands of the National Hurricane Center until 1977, when they relinquished naming rights to a regional naming committee with the World Meteorological Organization.

It wasn’t until 1979 that the current naming procedure was put into place. The committee decided to have 6 revolving lists of names that would repeat. These names include both male and female names that alternate and that are common in English, Spanish, or French speaking cultures. This means that the current list of storm names for the 2012 season will be repeated in 2018. The names are in alphabetic order with the “A” name used for the first storm of that year, even if it forms before the official start of hurricane season on June 1st or if the previous season did not use all the names on the list. The storm alphabet excludes the letters Q, U, X, Y, Z (how many names can you think of with those letters?). In the uniquely hyperactive seasons (let’s say 2005) when all the names in the storm alphabet are used, names are given following the Greek alphabet (Alpha, Beta, Gamma…).

The only exception for these names is when a storm has a significant enough impact that it would be inappropriate to continue using the name. That name is then retired. Some notable retired names are Andrew (1992) and Katrina (2005). When this happens, the committee meets to discuss and replace the name on the list.

My guess is that there will be no Isaac in 2018, so we will have a new “I” name to enter the list rotation. Any guesses on what the new name will be? Leave your guess in the comments.

-Angela Colbert
Meteorology & Physical Oceanography
Graduate Student
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