A low off southern Mexico now has a 90% chance of becoming a tropical depression

South of the Gulf of Tehuantepec, a broad area of low pressure is forecast to develop, and the National Hurricane Center’s 11 AM PDT outlook on Sunday, October 4, 2026 put its seven-day chance of becoming a tropical depression at 90 percent. That is the “high” category in the center’s wording. The nearer-term number is far lower, at 20 percent through 48 hours, so the 90 percent describes a build-up across most of a week rather than something imminent.

The outlook expects the low to move slowly west-northwestward to northwestward, staying just offshore of the southern to southwestern coast of Mexico. The center places the most likely development in the middle to latter part of the week, a window that opens after Monday, October 5 and runs toward the weekend.

The Outlook Text and Its Issuance Time

The figure belongs to a specific product. The NHC’s eastern Pacific feed carried the outlook with a publication stamp of 1726 UTC on October 4, which is 11 AM Pacific daylight time, and the center’s graphical outlook page showed the same wording and the same two percentages. An earlier text outlook from 5 AM PDT that morning, signed by NHC forecaster Kelly, already carried the identical 20 and 90 percent values, which means the odds held steady across at least two issuances rather than jumping on a single run. The wording shifted slightly between the two products: the morning text said a low-pressure area was expected to form near the Gulf of Tehuantepec, while the 11 AM version said a broad area of low pressure was forecast to develop over the next day or so just to the south of the gulf, which points the same direction with a little more precision about timing.

Because these outlooks are reissued through the day, the percentage attached to this low can rise, fall or be replaced by an advisory on a named or numbered system at any update. Anything published after the 11 AM PDT Sunday issuance supersedes the numbers here, and the outlook page itself is where that change would appear first. A move to 100 percent, a drop toward the medium range, or a first advisory on a numbered depression would each replace the 90 percent attributed in this article.

Twenty Percent in Two Days, Ninety in Seven

Each outlook gives two probabilities for every area it flags: formation within 48 hours and within seven days. NHC’s text outlook lists them side by side for the system near Tehuantepec as 20 percent and 90 percent, labeled low and high respectively. The gap between the two figures is the forecast itself: the center expects the ingredients to come together slowly, not on Monday or Tuesday.

The wording matters here. A 90 percent chance is a probability that a tropical cyclone forms from the disturbance, and nothing in the outlook says one has formed. Until a closed circulation with organized thunderstorms exists, the area is a disturbance, and the product describes it as a low that is “forecast to develop.”

The 33-Knot Threshold for a Depression

The NHC glossary defines a tropical depression as a tropical cyclone whose maximum sustained surface wind, using the U.S. one-minute average, is 33 knots (38 mph, or 62 km/h) or less. That is the weakest rung on the ladder. A depression that strengthens past 33 knots receives a name, and the lowest winds that qualify as a tropical storm sit just above that threshold.

So the 90 percent chance for the Tehuantepec low is a forecast about reaching the first rung only. It carries no prediction of peak winds, rainfall totals or a landfall, and the outlook text offers none of those; it commits only to a slow west-northwestward to northwestward drift that stays offshore of the southern and southwestern Mexican coast. Whether it later becomes a named storm, and how close it passes to Mexico’s southern coast, are separate questions the outlook does not address.

A Busy Basin in Early October

The same outlook was tracking two hurricanes in the basin: Nolo, well to the west-northwest of the main Hawaiian Islands, and Rachel, a few hundred miles southwest of the southern tip of the Baja California Peninsula. The Tehuantepec low is therefore the third area drawing attention at once. For the Central Pacific, the same 11 AM issuance expected no additional tropical cyclone formation over the next seven days, a separate basin from the one where the Tehuantepec low sits. NHC’s climatology pages give the eastern Pacific hurricane season as running from May 15 to November 30, so an October disturbance sits well inside it.

The record to check is the issuance of 11 AM PDT on October 4, 2026: 20 percent at 48 hours, 90 percent at seven days, development expected mid-to-late week, with a slow west-northwestward drift offshore of southern Mexico.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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