20 October 2011: What’s happening in the Tropical Atlantic?

Investment advisors always tell their clients that “past performance is no guarantee of future results”. The question that comes to mind today 20 October 2011, is: does the same hold true for weather? Why is this particular question  relevant today?

Satellite image in the afternoon of 20 October 2011 showing the low-pressure cell reaching from Central America to the Yucatan Peninsula over the western Caribbean

In looking at the tropical Atlantic today, particularly at the Caribbean sub-basin, current conditions have a strong resemblance to those that prevailed for most of the past week, as well as those that developed over the same region the week before that. I am referring to low-pressure systems that evolved over the Caribbean and eventually affected a large region from the Central America and the Yucatan Peninsula of Mexico to Cuba, the Bahamas and Florida bringing copious rain over several days to the area and points beyond. Most of us either experienced the actual events or remember the reports of large amounts of rain, gusty winds and numerous instances of flooding throughout the affected region. On this Thursday, 20 October 2011, there is a large low pressure system in the northwestern Caribbean, which is affecting Central America, the Yucatan, and portions of Cuba and has been generating quite a bit of rain over the area. This system is already interacting with a cold front and high pressure ridge that has descended over the Gulf of Mexico, most of Florida and the mid-Atlantic coastal region.

Color-enhanced infrared GOES satellite image on 20 October 2011 showing the region of disturbed weather over the northwestern Caribbean being discussed here

What will this system do? Given its location and characteristics it has become an area of interest to the National Hurricane Center, which is giving it a low probability of developing into a tropical cyclone mainly because of the unfavorable atmospheric environment to the west and north of the system. Will it be redirected toward the north or northeast by the high-pressure ridge and cold front, and will this eventually lead the system toward southern Florida where it could generate another bout of, by now unwelcome, rain? I believe the satellite image above tells the whole story; here we have the low-pressure disturbed weather system over the Caribbean [see #1 in the image] already generating large amounts of rain over a wide region. Toward the upper center of the image [see #3] there go the remnants of the system that dumped vast amounts of rain over most of Florida while also generating windy conditions, gusting to 100 kph in some locations, and tornadoes in Broward County. At the same time, as we look toward the east over ‘Hurricane Alley’ there is another cell of disturbed weather some 1,400 km east of the Lesser Antilles [see #2] showing some potential for further strengthening and perhaps tropical cyclone development over the next few days.

Image showing lower level winds over the North Tropical Atlantic, based on analyses by the Cooperative Institute for Meteorological Satellite Studies, Space Science and Engineering Center at the University of Wisconsin, Madison on 20 October 2011

To put current weather conditions over the larger Tropical Atlantic basin in perspective, and understand what the couple of systems discussed here, which are showing some potential for cyclone development, are encountering as an atmospheric environment that will affect their development and movement in days ahead, just take a look at the image above [courtesy of the Cooperative Institute for Meteorological Satellite Studies (CIMSS), Space Science and Engineering Center, University of Wisconsin at Madison] and see the convoluted tracks of lower level winds prevailing over the basin on 20 October 2011. The image clearly shows not only the various wind currents, but also ridges of high pressure, a cold front descending over the Gulf, and regions of dryer air, all of which will have an effect over these systems.

What will these effects be? Do vulnerable communities in Central America, the Caribbean, Florida and elsewhere in the coastal USA Gulf and Atlantic regions, have anything to worry about? Are either of these two systems tropical cyclones in the making? Answering these questions calls for a prediction not only if there is tropical cyclone genesis in the future of either of or both of these systems, but of what will happen with numerous atmospheric components ranging from the central and eastern Pacific Ocean, the western USA, to the Gulf and Caribbean, and the ridge of high pressure near the Bermudas over the next few days. An early prediction has already been made by the Regional and Mesoscale Meteorology Branch [RAMMB] of NOAA working in collaboration with the Cooperative Institute for Research in the Atmosphere [CIRA] at Colorado State University, which have identified both of these systems as areas of interest assigning each one a percentage of probability of tropical cyclone genesis based on a range of criteria including: current location [LAT/LON], barometric pressure, prevailing winds, surrounding coupled ocean/atmosphere environment etc. This is illustrated on the figure that follows:

showing several regions of interest in the eastern Pacific and Tropical North Atlantic, with assigned percentage probability of future tropical cyclone genesis as of 20 October 2011″]On a larger scale  several factors are affecting what happens weather-wise over the northern Tropical Atlantic and its various sub-basins. The onset of autumn over the northern hemisphere and early signals of the approaching winter have not only erased any vestiges of a lingering summer, but have also created conditions that may be conducive to clashes between warm and humid tropical components with cold and dry events, which result in stormy events over communities located in the transition regions. Tropical waves continue to be generated over equatorial Africa, but their route has shifted considerably to the south toward the equator as the tropical weather chases the Sun above. The composite satellite image below illustrates this larger environment and current conditions on this Thursday 20 October 2011:

Composite full-disk satellite image of Earth's Western Hemisphere on 20 October 2011 showing the 'belt of tropical activity' just north of the equator, and the cells of disurbed weather discussed in the article

So, there you have it! A snapshot of the tropical North Atlantic on this Thursday 20 October 2011. Numerous active weather systems and features over the tropics and beyond, a low potential for tropical cyclone development at this time, the threat of yet one more rain-maker causing flooding and damage over vast regions, which may include Florida; in summary, a lot of weather components to keep track of, and too many questions that need answers.

For us here in Florida, and residents of vulnerable communities in Central America and the Caribbean, and the coastal USA around the Gulf and mid-Atlantic regions the immediate task is to keep an eye on the system now over the northwestern Caribbean, to pay attention, be prepared and to practice mitigation! Also, keep monitoring the system now progressing along hurricane alley!

GOES satellite image showing the low-pressure system in the northwestern Caribbean beginning to interact with a cold front and ridge of high pressure over the Gulf on 20 October 2011

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