Archive for Saturday, 5 September 2015

Storm Surge–Plain and Simple (Part 1)

Source: Storm Surge–Plain and Simple (Part 1)

Storm Surge–Plain and Simple (Part 1)

POSTED ON JUNE 17, 2014

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You may have heard that NHC is unveiling an experimental storm surge graphic this hurricane season.  We mentioned in our first blog post on May 29 that we would be discussing the background and interpretation of this graphic.  There’s a lot to cover, so instead of throwing it all at you in one shot, we are going to do a three-part series on the new graphic and communication on storm surge in general.  Here’s what we plan on covering:

Part 1:  Why do we need a storm surge graphic?

Part 2:  How is the storm surge graphic created?

Part 3:  How should you interpret the storm surge graphic?

High Water Memo

So, let’s get on with Part 1.  First, let’s look back at a little history.  Way back in 1955, the U.S. Weather Bureau issued a memo (figure to the right) to weather offices along the coast, directing them to refer to any water rise produced by a hurricane or tropical storm in terms of “above normal tide levels,” and those rises were to be specified in ranges to account for uncertainty.  Believe it or not, that policy went unchanged for over 50 years!  In 2008, the NHC Public Advisories for Hurricane Ike referred to storm surge like this:

“COASTAL STORM SURGE FLOODING OF UP TO 20 FEET…WITH A FEW SPOTS TO NEAR 25 FEET…ABOVE NORMAL TIDES ALONG WITH LARGE AND DANGEROUS BATTERING WAVES…CAN BE EXPECTED NEAR AND TO THE EAST OF WHERE THE CENTER OF IKE MAKES LANDFALL. THE SURGE EXTENDS A GREATER THAN USUAL DISTANCE FROM THE CENTER DUE TO THE LARGE SIZE OF THE CYCLONE. WATER LEVELS HAVE ALREADY RISEN BY MORE THAN 5 FEET ALONG MUCH OF THE NORTHWESTERN GULF COAST.”

For many years, we didn’t have the technology, nor sufficient accuracy in our track forecasts, to be any more specific in our Public Advisories.  The best we could do was give an estimate of the highest storm surge expected with a general description of where that surge could occur relative to the center of the storm.  Unfortunately, many times these statements were too vague for emergency managers and other decision makers to make sound decisions before a storm.  One question a statement like this could not answer:  “How far inland could the storm surge go?”

Another issue had to do with what are called vertical datums.  We’ll leave the more technical discussion of vertical datums for another blog post, but what you need to know for this discussion is that a vertical datum is simply a reference point.  The water level height caused by the combination of storm surge and the tide must be attached to some point of reference.  The operative question is “the height of the water level is 6 feet above what?”  The problem was that many people either weren’t specifying what the datum was, or they were confusing one datum with another.

Here’s an example, again using Ike, where confusion set in.  The figure below shows output from the National Weather Service SLOSH model indicating simulated water level heights from Hurricane Ike along the Texas and Louisiana coasts.  What’s the first thing that jumps out at you?  The first question many people have is why do the values increase (go from 15 feet to over 21 feet) as you move inland from the coast into Chambers and Jefferson Counties in Texas?  Shouldn’t the deepest water have occurred at the immediate coast?  The subtlety here is that the water level in this picture is depicted relative to a datum called NAVD88.  So, the water levels in Chambers and Jefferson Counties were more than 21 feet above NAVD88, not 21 feet above the actual ground at those locations.

Ike Surge NAVD

Luckily, there’s a way to display how much water was sitting on normally dry ground, which is what most people typically envision when given storm surge heights.  Since we know what the elevation of the land is at each location, relative to the same vertical datum used for the surge data itself, we can subtract the land elevation from the surge heights to get a good idea of how high the water was above the ground at each location.  The next figure is the same simulation for Ike but instead shows this subtraction at play.  Notice any differences from the previous image?

Ike Surge Ground

Now it should all make sense.  The highest values (about 15 feet above ground level) are located along the immediate coast and decrease as you move inland.

Recent hurricanes like Katrina, Rita, and Ike showed that we needed to make some changes in the ways that we communicate storm surge information.  And thankfully, we now have the technologies and capabilities to go beyond simplified text statements in the Public Advisory.  In Part 2 of this series, we’ll talk about the Probabilistic Storm Surge product, how it accounts for uncertainties in the storm surge forecast, and how it is being used to create the Experimental Potential Storm Surge Flooding Map for this hurricane season.

— Robbie Berg and Jamie Rhome

Posted Saturday, 5 September 2015 by Culebra Snorkeling and Dive Center in Culebra Posts & Reviews

The Ups and Downs of Predicting Tropical Cyclone Formation: The Role of Atmospheric Waves

 The Role of Atmospheric Waves

POSTED ON JUNE 25, 2015

A previous blog entry described the new NHC five-day tropical cyclone formation (or genesis) products.  In this blog entry, we discuss the factors that go into these predictions.

The primary tool used at NHC for five-day tropical cyclone genesis forecasts is global numerical modeling.  Global models can predict many of the environmental factors that influence tropical cyclone formation, and the skill of these models has been improving with time.  More tropical cyclone formations are being forecast with longer lead times, and weather prediction models show fewer “false alarms” than in the past.  Recent studies suggest, and forecaster experience seems to confirm, that a consensus of the available model guidance usually outperforms any single model.   This “two heads are better than one” approach works as long as the models (or heads) are somewhat independent of one another.  In addition, NHC is currently evaluating a few statistical techniques that use the global model output to produce objective guidance designed to assist hurricane specialists in developing the probabilities of formation issued in the Tropical Weather Outlook.

Kelvin Waves and the Madden-Julian Oscillation

Global model guidance is not the only tool available to NHC forecasters, however.  Researchers have learned that a majority of lower latitude tropical cyclone formations are associated with waves in the atmosphere moving through the global Tropics from west to east.    Two particularly important wave types are the Convectively Coupled Kelvin Wave (CCKW), which circumnavigates the equator in about 15 to 20 days, and the Madden-Julian Oscillation (MJO), which transits the globe in 30 to 60 days.  These waves are normally initiated by large areas of thunderstorm activity over tropical regions, especially near India and southeastern Asia.  These waves are different in both frequency and direction of motion from the more well-known tropical waves that originate over Africa and often spawn tropical cyclones as they move westward across the Atlantic and eastern North Pacific basins.

Tropical cyclone formation often accompanies the passage of the “active phase” of either the faster-moving CCKWs or the slower-moving MJO.   Figure 1 shows tropical cyclone tracks over a 37-year period in active and inactive phases of the MJO as the wave moves around the globe, along with increased or decreased rainfall anomalies associated with the two phases of the MJO (Zhang 2013).   In the figure, the active phase of the MJO for the Atlantic occurs in panel (a), while for the eastern Pacific the active phase occurs in panel (d).  The less active phases for these two basins fall in panels (c) and (b), respectively.

Figure 1. Tropical cyclone tracks in active and inactive phases of the MJO and increased (green) and decreased (purple) rainfall anomalies associated with the two phases of the MJO (from Zhang 2013). Panel (a) shows the active phase of the MJO for the Atlantic, and (d) shows the active phase for the eastern Pacific. Panels (b) and (c) show the less active phases for both basins.
Figure 1. Tropical cyclone tracks in active and inactive phases of the MJO and increased (green) and decreased (purple) rainfall anomalies associated with the two phases of the MJO (from Zhang 2013). Panel (a) shows the active phase of the MJO for the Atlantic, and (d) shows the active phase for the eastern Pacific. Panels (b) and (c) show the less active phases for both basins.

This concentration of tropical cyclone activity occurs because each type of wave temporarily makes large-scale environmental conditions, such as vertical wind shear or atmospheric moisture, more conducive for tropical cyclone formation.  Although not every wave causes a tropical cyclone to form, pre-existing disturbances have a greater likelihood of developing into tropical cyclones after the passage of a CCKW or the MJO.  High-activity periods can last as long as a week or more with the MJO, but are generally followed by days to possibly weeks of little to no activity during the inactive phases of these waves, when large-scale conditions become unfavorable for tropical cyclone formation.  Forecasters use real-time atmospheric data and other tools to diagnose the location and motion of these important catalysts for tropical cyclone formation.

Here is an example from the 2014 hurricane season of how forecasters used these atmospheric signals.  The graphic below, called a Hovmöeller diagram, shows where large areas of rising air (cool colors) and sinking air (warm colors) exist near the equator as a function of time.  The dashed black contours depict the active phase of successive CCKWs, and the solid red contours show the inactive phases.    In this particular case, forecasters noted that there was a strong CCKW moving through the eastern Pacific in the middle part of October.  Extrapolating the wave forward in time, along with numerical models forecasts of the wave’s location and strength, suggested that a tropical cyclone could form within a few days over the far eastern Pacific from a disturbance that was already in the area.  The green dot indicates where Tropical Storm Trudy formed, a day or two after the CCKW passed the disturbance.  Although CCKW tracking is only a secondary factor in determining a Tropical Weather Outlook forecast, a basic knowledge of this atmospheric phenomenon is an important part of the process.

active inactive phase
Figure 2. Hovmoeller diagram showing large areas of rising air (cool colors) and sinking air (warm colors) near the equator as a function of time

Forecasters consider many factors when preparing the five-day genesis probabilities for the Tropical Weather Outlook, including explicit forecasts from the global models and knowledge of any ongoing CCKWs or the MJO.   In addition, the final NHC forecast also reflects the current trends of the disturbance, which are weighted much more heavily in the two-day outlook, but also can affect the five-day forecast as well.  There are several ongoing research projects that will hopefully yield objective probabilities and other tools designed to help better predict tropical cyclone formation.  These tools, in combination with the dynamical guidance from numerical models, should improve the quality of genesis forecasts and perhaps in the next five years extend reliable tropical cyclone formation forecasts from five days to one week.

— Eric Blake and Todd Kimberlain

Source: The Ups and Downs of Predicting Tropical Cyclone Formation: The Role of Atmospheric Waves

Posted Saturday, 5 September 2015 by Culebra Snorkeling and Dive Center in Culebra Posts & Reviews

Cyclones and Warnings and Names, Oh My!

Source: Cyclones and Warnings and Names, Oh My!

Posted Saturday, 5 September 2015 by Culebra Snorkeling and Dive Center in Culebra Posts & Reviews

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