|Above: Tropical Depression 15 on Tuesday morning, October 15, 2019, as seen by MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's Terra satellite. Image credit: NASA.|
Tropical Depression 15 was heading northwest at 9 mph on a track that would take it through the eastern Cabo Verde Islands on Tuesday afternoon and evening. With top sustained winds of just 35 mph at 11 am EDT Tuesday, TD 15 is primarily a heavy rain threat, with total accumulations of 1 - 3 inches expected in Cabo Verde.
Satellite images on Tuesday morning showed that TD 15 was a large and poorly organized system, with a modest amount of heavy thunderstorm activity. Sea surface temperatures (SSTs) were an unusually warm 27°C (81°F), and wind shear was moderate, near 10 knots, on Tuesday morning—favorable conditions for development. If TD 15 manages to take advantage of these favorable conditions and intensify into a tropical storm, it would be named Nestor.
TD 15 does not have a lot of time to develop, though, since wind shear is predicted to rise to a high 20 knots on Wednesday morning, and to 40 knots by Thursday morning. This wind shear is very likely to destroy TD 15 by Thursday. TD 15 will not be a threat to any land areas besides the Cabo Verde Islands.
|Figure 1. Tracks of all tropical cyclones (which includes all tropical depressions, tropical storms, and hurricanes) to form in the month of October or later, from 1851 to 2019, within the region depicted. Image credit: NOAA Historical Hurricane Tracks database.|
If TD 15 becomes Nestor, it will be the latest-in-the-year tropical storm in NOAA's historical hurricanes database to form east of Cabo Verde. The current record-holder is Tropical Storm Ginger, which was named on October 5, 1967, while tracking between Cabo Verde and Africa. Only one other tropical cyclone has been recorded east of Cabo Verde in October or later: an unnamed depression that developed near 11.5°N and 18.0°W on October 5, 1972. That depression survived until October 15 on a broad recurving path that took it into the central North Atlantic. The depression never became a tropical storm.
As usual, there are a couple of disturbances embedded within the #CAG envelope that have a chance to develop into TCs.— Philippe Papin (@pppapin) October 14, 2019
Regardless, heavy rain leading to flooding & mudslides over Nicaragua, Costa Rica, Honduras, El Salvador, Guatemala, ect. is the primary impact w/ this system. pic.twitter.com/nTjcnJObdX
Time to watch the Gulf of Mexico and the Central American Gyre
A seasonal large-scale circulation called a Central American Gyre (CAG) has made its typical autumn appearance. A CAG is a recurring, sprawling area of low pressure that often straddles the nations of Central America, leading to disturbed weather in both the Atlantic and Pacific. A CAG itself is not a tropical cyclone, but pockets of spin can develop within it and rotate out from it, sometimes leading to a tropical depression, tropical storm, or even a hurricane. Catgeory 5 Hurricane Michael of 2018, which devastated Mexico Beach in the Florida Panhandle, was spawned from an October CAG.
CAGs tend to form at the beginning and toward end of the Atlantic hurricane season, times when westerly low-level flow is especially common across the Northeast Pacific south and west of Central America. West winds in this location help drive the cyclonic circulation that makes up the CAG, as explained by Philippe Papin (Naval Research Laboratory).
NHC is watching two areas on either side of ths week's CAG. One compact disturbance is centered in the Caribbean roughly near Belize and moving northwest, a track that should put it into Mexico's Bay of Campache by late Wednesday or Thursday. Once there, the disturbance will have a shot at evolving into something apart from the CAG from late this week into early next week as it moves slowly northward across the western Gulf of Mexico. NHC gives this system a 30% chance of developing into at least a tropical depression in the western Gulf over the next five days, mainly between Wednesday and Saturday. A weak reflection of this disturbance appears in the 0Z Tuesday runs of our three top track models—the European, GFS, and UKMET. The system remains poorly organized in all models as it tracks toward the central Gulf Coast over the weekend. If nothing else, an uptick in showers and thunderstorms can be expected if the disturbance holds together.
|Figure 2. Infrared image of disturbed weather associated with a Central American Gyre at 1520Z (11:20 am EDT) Tuesday, October 15, 2019. A large area of showers and thunderstorms south and southeast of Mexico, and the smaller complex near and north of Belize, each have the potential to become at least a tropical depression. Image credit: NASA/MSFC Earth Science Branch.|
On the Pacific side, Invest 98E—a much larger and more intense complex of showers and thunderstorms, centered south of Guatemala—is likely to become a tropical depression as it moves roughly parallel to the Pacific coast toward southern Mexico. NHC gives this disturbance an 80% chance of development by Wednesday. Very warm sea surface temperatures of 29-30°C (84-86°F) and a very moist atmosphere (midlevel relative humidity of 80-85%) will support development, despite moderate wind shear of about 15 knots. The main question by midweek is how much land interaction will impede 98E as the system moves along or onto the coast of Mexico’s Oaxaca state. Longer-range models suggest 98E is unlikely to progress much further offshore toward Baja California.
Regardless of whether either system within the CAG becomes a tropical cyclone, heavy rainfall will continue across large parts of Central America, and flooding and mudslides are a real threat, especially across rugged terrain. Rains may intensify into far southern Mexico if the Pacific disturbance strengthens.
Bob Henson co-wrote this post.