The National Hurricane Center gave a disorganized area of showers and thunderstorms off southwestern Mexico a 90 percent chance of becoming a tropical depression within 48 hours, according to the Eastern Pacific Tropical Weather Outlook issued at 11 p.m. PDT on Sept. 19, 2026. The system sat several hundred miles south-southwest of Manzanillo, Colima, and was drifting generally northward, parallel to the coast, as forecasters watched it consolidate. By the following evening, the agency’s next bulletin confirmed the system had done exactly that.
The 90 percent figure placed the system in the Hurricane Center’s highest formation-likelihood category, reserved for disturbances forecasters expect to organize almost immediately rather than over several days. Interests along Mexico’s southwestern and west-central coast were advised to track the system’s progress as it developed.
A 90 percent formation window off Manzanillo
The Sept. 19 outlook described environmental conditions as conducive for development, with the disturbance’s shower and thunderstorm activity already showing signs of organization as it tracked north off the coast. Forecasters attached the 90 percent figure specifically to the 48-hour formation window, the shortest and most confident of the windows the agency issues alongside every tropical outlook.
That kind of near-certain call is rare enough in Eastern Pacific outlooks that it typically signals a system already showing a defined circulation on satellite imagery, rather than one still in a purely speculative stage. The same bulletin covered other, far less mature disturbances elsewhere in the basin at formation odds closer to 20 percent, underscoring how far along this particular system already was.
The system organizes into Tropical Depression Seventeen-E
A follow-up outlook issued at 5 p.m. PDT on Sept. 20 confirmed the forecast had held. The National Hurricane Center reported it was “issuing advisories on Tropical Storm Odalys, located over a thousand miles west-southwest of the southern tip of Baja California, and on newly formed Tropical Depression Seventeen-E, located a few hundred miles south of Manzanillo, Mexico.” The depression’s position matched the system flagged the previous night almost exactly, confirming the earlier 90 percent call rather than describing a separate development.
Once a system reaches tropical depression status, the National Hurricane Center begins issuing full public advisories and forecast tracks rather than the general outlook language used beforehand. That shift means the loosely defined disturbance watched on Sept. 19 became, by Sept. 20, a numbered system with its own advisory schedule.
The “E” in Tropical Depression Seventeen-E marks it as the seventeenth tropical depression of the 2026 season in the Eastern Pacific basin, a sequential count that resets every year and runs independently of the Atlantic’s own numbering. Seventeen depressions by late September is a sign of an active season in a basin where development can continue well into November, though not every depression that forms goes on to earn a name as a tropical storm.
How the National Hurricane Center rates formation odds
Every Tropical Weather Outlook assigns each disturbance a formation chance across two windows, 48 hours and seven days, each labeled low, medium or high alongside a specific percentage. A near-zero percentage means forecasters see no organized circulation at all; a reading in the 90s means most of the ingredients for a depression are already in place and the main uncertainty is timing rather than whether formation happens. The system tracked here carried that top-tier reading in its final pre-formation outlook.
The distinction matters for coastal residents because the categories translate directly into how much warning time to expect. A system sitting at a 20 percent seven-day chance might dissipate entirely, while one already rated 90 percent through 48 hours is functionally treated as a depression in waiting. Forecasters revise every outlook as new satellite passes and reconnaissance data come in, so a system can climb from a 20 percent long-range chance to a 90 percent near-term one within a single day once its structure becomes clearer.
Tropical Storm Odalys and a crowded Eastern Pacific season
The Sept. 20 outlook was not tracking Tropical Depression Seventeen-E in isolation. The bulletin also covered Tropical Storm Odalys, positioned more than a thousand miles west-southwest of the southern tip of Baja California and far enough from land that it posed no immediate coastal threat, unlike the depression forming much closer to Manzanillo. The same outlook flagged the remnants of a separate former depression east-southeast of Hawaii, given a 70 percent chance of redeveloping within a week, and a third area of low pressure that could form south of the Gulf of Tehuantepec in the coming days at a much lower 20 percent chance. The outlook was signed by NHC forecaster Papin.
Three separate systems moving through the basin within days of each other is consistent with a Pacific season still generating storms into late September, and the Hurricane Center’s Eastern Pacific Tropical Weather Outlook is updated every six hours as each system’s odds shift.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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