Notes on tropical weather invests
June 1, 2011
It is my endeavour over the course of this season to create articles as background references for my postings on the ongoing hurricane season. A couple of years ago, a friend of mine, in the course of attempting me to persuade me to bring my blog back to its original purpose **, commented something to the effect “I read some of your stuff on hurricanes and it seemed good, but I didn’t understand one bit of it.” Indeed, it is the author’s (unwarranted) tendency to assume all of his readers know exactly what he is talking about and require no explanations of jargon whatsoever. Perhaps by having reference articles, the blog posts will be a bit more accessible to the lay reader of this blogs and those who encounter my Twitter stream.
I intend to write an article on the information that is available on disturbances as they develop into storms and beyond at some point in the near future and had hoped that nothing would be active in the Atlantic before doing so. Of course, that has not been the case. For the moment, I’ll cover a particular portion of that life-cycle, known as the invest.
The term invest refers to an area of weather that tropical weather forecasters are interested in monitoring the potential for tropical cyclone formation. The National Hurricane Center defines it in their glossary thusly:
- A weather system for which a tropical cyclone forecast center (NHC, CPHC, or JTWC) is interested in collecting specialized data sets (e.g., microwave imagery) and/or running model guidance. Once a system has been designated as an invest, data collection and processing is initiated on a number of government and academic web sites, including the Naval Research Laboratory (NRL) and the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS). The designation of a system as an invest does not correspond to any particular likelihood of development of the system into a tropical cyclone; operational products such as the Tropical Weather Outlook or the JTWC/TCFA should be consulted for this purpose.
As alluded to in the definition, there are a number of things that take place once an area has been tagged as an invest. Specialized satellite information appears on the Navy Research Laboratory’s tropical cyclone page. Personnel at the Satellite Analysis Branch of NOAA’s Satellite Services Division and the National Hurricane Center’s Tropical Analysis and Forecasting branch issue position and intensity estimates for the disturbance based on satellite imagery. Those estimates are fed to models specialized to tropical cyclones to give an idea of where a storm would go and how strong it would be if one were to develop from the disturbance. That in turn can be used to task the Hurricane Hunters to fly a mission to determine whether a tropical cyclone has formed.
Invests are numbered 90 through 99, with a one letter suffix to indicate the basin (for the western hemisphere, L indicates the Atlantic and E the eastern Pacific; each basin keeps its own set of numbers) and the numbers are recycled over the course of the season. If an area of weather tagged as invest becomes a tropical cyclone, it is re-numbered inside the Automated Tropical Cyclone Forecasting System to be a numbered storm, with the first tropical cyclone of the season being 01 and so on. If the area does not develop, the invest will be “de-activated”. Either way, if there are more than 10 invests in the course of a season, as is almost certain to be the case, the number will be re-used. As such, in a busy season there can be two, three, or more “Invest 90″s.
In the case of this season, there were three invests designated prior to the currently live Invest 93, which was designated last night. Invest 90 was for an area in the east Atlantic in March. It never got interesting enough to merit a special Tropical Weather Outlook to advise of the possibility of tropical cyclone formation. Invest 91, on the other hand was the basis of the first Special Tropical Weather Outlook of 2011 in April. Invest 92 was also the subject of a special TWO.
As far as I know there is not set objective guidance on what should be tagged as an invest and when. In some cases, it is for an area that merely “looks interesting” on satellite; probably nothing will become of it, but it’s investigated further just in case. In other instances, the area doesn’t look particularly interesting at the moment, but multiple global forecast models have suggested that it will be the place of tropical cyclone formation in the next few days.
It is likely that the next invest will fall in the latter category. Multiple global forecast models are suggesting tropical cyclone formation will take place this weekend from an area of low pressure currently off the coast of Nicaragua. The NHC is mentioning it in the Tropical Weather Outlook, giving it a 10% chance of cyclone formation in the next 48 hours. It seems likely to me this area will be tagged as Invest 94 in the next day or so.
It is important to keep the word “area” in mind when thinking about the location of invests. Unlike tropical depressions, etc, where there is a known and (at least somewhat) definite center of circulation,. Usually, unless tropical cyclone formation is imminent, there is no such definite center. While a position is fed to the forecast models, it is often a “notional center” For example, if the invest area consists of two clusters of thunderstorm activity with one 50 miles to the north of the other and neither apparently dominant, the “notional center used for the position would be in between the two. It is likely that any developing storm would form north or south of that point, but until it is clear, the difference is split to nominally account for both possibilities. It is not unusual for positions of invests to jump around as the particular feature that seems to likely to become the focus for cyclone formation changes.
** Despite the title, when I started this blog I didn’t have storm-blogging in mind at all. It was the surprise rapid intensification of Hurricane Charley as it bore down on my friend in Fort Myers that suddenly reminded me of my love of hurricanes and the realization that it would be a good subject for blogging. “Eye of the Storm” was meant to imply that it would be an area of calm and reasoned blogging (on politics and other sundry topics) amidst a storm of hysteria.
Edited on June 4 to add a tidbit on the numbers being suffixed with a letter to indicate the basin. E.G the Atlantic invest 94 I’ve been talking writing about is properly 94L.