Month: July 2014
Teenagers today seem to enjoy taking words and employing them as a new part of speech, especially if it results in the use of fewer syllables. Thus we have the verb fail used as a noun in place of failure, the verb invite used as a replacement noun for invitation, and so on. This has given us such linguistic classics as what an epic fail or where’s my invite. My teenage daughter has managed the inverse transformation, telling me that she has “no time to piano today”. Texting and Twitter can be blamed for much of this, of course, but the hurricane community’s gift to the Lexicon of Lazy Locutions originated nearly two decades ago. The noun that represents our particular role in the decline and fall of the English language is the subject of today’s blog post.
If you lurk around the dark meteorological corners of the Internet, or even if you just watch weather broadcasts during hurricane season, you’ve probably come across expressions like Invest AL94. With the accent on the first syllable (IN-vest rather than in-VEST), this is not an insider’s instruction to sell your AAPL stock at $100, but rather it’s a reference to a specific “investigative area” – a weather system for which a tropical cyclone forecast center is interested in collecting specialized data sets or running model guidance.
Accounting for Invests
NHC has responsibility for identifying these invests, or disturbances of interest, in the Atlantic basin. NHC and the Joint Typhoon Warning Center (JTWC) have shared responsibility for designating invests in the eastern Pacific, while the Central Pacific Hurricane Center (CPHC) has this responsibility in the central Pacific. The rest of the globe (for this purpose at least) belongs to JTWC.
NHC, CPHC, and JTWC prepare their forecasts and advisories on a computer platform known as the Automated Tropical Cyclone Forecast system (or ATCF). Tropical cyclones are followed on the ATCF using identifiers such as AL032014; this particular identifier would decode as the Atlantic basin’s third tropical cyclone of the 2014 season. Invests are given identifiers using the numbers 90 through 99 in place of the cyclone number, so the first Atlantic invest of 2014 was AL902014, or AL90 for short. After AL992014 is used, we would cycle around and reuse AL902014, so unlike the ATCF identifiers for true tropical cyclones, invest identifiers are not unique.
Once NHC or one of the other forecast centers “opens an invest”, data collection and processing is initiated on several government and academic websites, including those at the Naval Research Laboratory (see example to the right) and the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin. Information on these sites, along with the standard suite of models run on the invest, then helps NHC prepare the genesis forecasts that appear in the Tropical Weather Outlook.
It’s important to recognize that the designation of a disturbance as an invest does not correspond to any particular likelihood of development of that system into a tropical cyclone. Indeed, we will open an invest in part to help us determine what that likelihood is. Also, and particularly near the beginning of the season, it’s not uncommon for NHC to create one or more invests solely to test data flow or model processing scripts. The Tropical Weather Outlook should always be consulted to determine the significance or potential threat of an invest disturbance.
No Insider Trading
ATCF databases have traditionally been posted to NHC’s public FTP server to facilitate the exchange of information with modelers and other quasi-operational groups, and to make model guidance available to the private sector. Unfortunately, posting of the ATCF data to the FTP site allowed some pre-decisional information to leak out to those who knew exactly where to look. For example, one could find the command that renumbers the invest system AL902013 to the tropical cyclone AL012013. Renumbering an invest is a process the NHC Hurricane Specialist needs to invoke in order to prepare the first advisory on a tropical cyclone, even though the final decision to release that advisory might not be made for another hour or two. In the early years of the FTP site, these leaks seemed to fly under the community’s radar, but over time had become increasingly known. Anticipation of new cyclones began to cause problems for us and for our partners in emergency management and the media, some of whom would infer or even prematurely announce that we were going to start advisories. (And indeed, occasionally we’ve had to change our minds and not initiate advisories on a renumbered invest).
In 2014, NHC has made some changes to how data from the ATCF get publicly posted. The most significant of these is the establishment of a blackout period, during which changes made to the ATCF storm ID and some other parameters will not flow to the FTP server. The blackout period begins 90 minutes prior to the nominal advisory release time (e.g., 9:30 a.m.) and ends at the nominal advisory release time (e.g., 11:00 a.m.). Quasi-operational websites that make use of ATCF data will now draw their data from the FTP server rather than NHC’s internal databases. In this way, everyone will be able to learn about an NHC advisory, and know for sure that it’s coming, all at the same time when that advisory is released. We want to emphasize that while the blackout period will restrict the release of pre-decisional information, it will not restrict the distribution of model guidance used by private-sector forecasters.
The Closing Bell
One final thought on invests. NHC knows that lots of folks, including non-meteorologists, look at the tropical cyclone models. While we make model data available on our FTP site for use by the meteorological community, and the public can find these data displayed all over the Internet, we deliberately avoid enhancing their visibility or prominence by posting model plots on our own website. This is particularly important for invests, where the model guidance is notoriously poor and erratic, partly because many of these models were never designed to be run on disturbances. NHC’s Hurricane Specialists work hard, based on their knowledge and experience, to interpret all the available models and other data in the formulation of their official forecasts and warnings, and in so doing help NHC continue to be America’s calm, clear, and trusted voice in the eye of the storm. And hopefully stay out of Weird Al Yankovic’s sequel to “Word Crimes.”
— James Franklin
In our last storm surge post, we talked about the need for a storm surge graphic and why we use “above ground level” to communicate storm surge forecasts. Now we’ll discuss how we create the new storm surge graphic.
But first, we need to touch on how forecast uncertainty relates to storm surge forecasting.
Putting All Your Eggs in One Basket
The exact amount of storm surge that any one particular location will get from a storm is dependent on a number of factors, including storm track, storm intensity, storm size, forward speed, shape of the coastline, and depth of the ocean bottom just offshore. Needless to say, it’s a complex phenomenon. Although we’re getting better on some aspects of hurricane forecasting, we still aren’t able to nail down the exact landfall of the storm or exactly how strong and big the storm will be when it reaches the coast. This means that there is a lot of uncertainty involved in storm surge forecasting. Here’s an illustration showing why all of this is important.
Here’s the forecast track for a Category 4 hurricane located southeast of Louisiana and only about 12 hours away from reaching the northern Gulf Coast:
Here’s the question: how much storm surge could this hurricane produce in Mobile, Alabama, and Pensacola, Florida (marked on the map)? If we take this forecast and run it through SLOSH (the National Weather Service’s operational storm surge model), here’s what you get:
The forecast has this hurricane making landfall near Dauphin Island, with the center moving northward just west of Mobile Bay along the black line. You can see from this map that water levels will rise to at least 14 ft. above NGVD29 (the particular reference level we are using in this scenario) in the upper reaches of Mobile Bay while they will rise to about 2 ft. above NGVD29 in the Pensacola area. What’s the problem with this storm surge forecast? It assumes that the track, intensity, and size forecasts of the hurricane will all be perfect. This is rarely, if ever, the case.
Here’s what actually happened with this hurricane. The storm turned ever so slightly toward the east and made landfall about 30 miles east of where the earlier forecast had shown it moving inland. Despite the shift, this was a good track forecast–30 miles is more or less typical for a 12-hour error. So, what kind of storm surge resulted from the actual track of this hurricane? If we take the actual track of the storm and run it through SLOSH, here’s what we get:
Since the center of the hurricane actually moved east of Mobile Bay, winds were pushing water out of the bay, and the water was only able to rise about 4-5 ft. above NGVD29 near Mobile. On the other hand, significantly more water was pushed toward the Pensacola area, with values as high as 12 ft. above NGVD29 in the upper reaches of Pensacola Bay.
This scenario was an actual storm–Hurricane Ivan in 2004. If emergency managers in Pensacola at the time had relied on that single SLOSH map that was based on a perfect forecast (or, put all their eggs in one basket), they would have been woefully unprepared and may not have evacuated enough people away from the coast. Granted, such decisions would have been made more than 12 hours away from landfall, but at that time, forecast errors are even larger and make storm surge forecasting even more difficult.
If you’re going to put all your eggs in one basket, you might as well scramble them beforehand so that they don’t break when you drop the basket. In a sense, that’s what we do when trying to assess an area’s storm surge risk before a tropical cyclone. Instead of assuming one perfect forecast, we generate many simulated storms weighted around the official forecast–some to the left, some to the right; some faster, some slower; some bigger, some smaller–and then run each of those storms through SLOSH. We then “scramble” the SLOSH output from all storms together and derive statistics that tell us the probability of certain storm surge heights at given locations along the coast.
If we go back to our example from Hurricane Ivan, we can see the value of this method in assessing storm surge risk. The image below shows the probability that the storm surge would reach at least 8 ft. above the reference level (NGVD29) for Ivan from the NHC Tropical Cyclone Storm Surge Probability product. The first thing that should jump out at you is that the probability of at least 8 ft. of surge was just about equal in Mobile Bay (60-70% chance) and Pensacola Bay (50-60% chance). The probabilistic approach indicates that both areas were at a significant risk of storm surge, and both areas should have been preparing similarly for the arrival of the storm. Because we accounted for the uncertainty in the official forecast, we were able to assess the true storm surge risk for all areas near the coast.
The Tropical Cyclone Storm Surge Probability product provides the data that are used to create the Potential Storm Surge Flooding map that will be available experimentally beginning in the 2014 hurricane season. In other words, the Potential Storm Surge Flooding map accounts for the uncertainties associated with NHC’s tropical cyclone forecasts. In Part 3 of this storm surge series, we’ll talk more about the map itself and how it should be interpreted.
— Robbie Berg and Jamie Rhome
For many years NHC’s forecasts of tropical cyclone formation extended only 36-48 hours into the future. Recent advances in numerical modeling, however, as well as improved understanding of some of the physical triggers for genesis, prompted NHC to begin an in-house experiment to see whether its genesis forecasts could be extended. The four-year experiment showed, somewhat surprisingly, that a five-day tropical cyclone forecast could be made with nearly the reliability of the existing 48-hour forecasts, and NHC publicly extended the range of its Tropical Weather Outlook (TWO) text product to five days on August 1st of 2013.
In its present form, the text TWO describes areas of disturbed weather and their potential for development into a tropical or subtropical cyclone. This description normally includes discussion of the large-scale factors that could influence development, the general motion of the disturbance and any hazards that might affect land areas, and concludes with a quantitative forecast of formation likelihood for both the next 48 hours and the next five days.
The New 5-Day Graphic Explained
Beginning today, July 1, 2014, at 2 PM EDT (11 AM PDT), the text TWO will be accompanied by an experimental graphical depiction of the five-day potential cyclone genesis areas. These areas will appear as color-coded hatched areas (yellow, orange and red representing low, medium, and high risks of tropical cyclone formation, respectively). Although the areas or swaths don’t explicitly represent a track forecast, they do provide a general indication of where these systems are headed whenever the formation potential extends over several days.
If a hatched formation area is associated with a currently existing disturbance, the location of the disturbance is marked with an ‘X’ on the graphic. Arrows are used to link the location of a disturbance with its potential genesis area if the formation area is displaced from the current location of the disturbance. The overview graphic (above) can occasionally become crowded with disturbances, especially during the peak of the hurricane season, so separate graphics for each disturbance are created to ensure legibility.
The introduction of the five-day graphic on July 1st will be accompanied by an important change to the existing 48-hour graphic. Disturbances on this graphic will no longer be identified with circles or ovals; instead the location of current disturbances will be marked with an “X” for consistency with the five-day graphic.
In a future blog post we’ll be talking about how NHC’s Hurricane Specialists arrive at the formation probabilities appearing in the TWO, as well as some experimental guidance and ongoing research projects that might allow us to extend these genesis forecasts even further in time. In the meantime, we welcome user feedback on the new graphic, which can be provided at http://www.nws.noaa.gov/survey/nws-survey.php?code=FDGTWO
The following video also provides a description of the new 5-Day Graphical Tropical Weather Outlook: