3 Week Tropical Cyclone Formation Probability//09/29/26

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Last Updated - 09/29/26

Valid - 10/07/26 - 10/20/26

El Nino continues to dominate the global tropics, as supported by very strong and persistent convective and circulation anomalies in response to a highly anomalous warm water volume in the equatorial Pacific. MJO activity remains difficult to discern within the background state, however there is some support in the tools for an ongoing Western Hemispheric event. Dynamical models generally feature a westward retreating MJO signal towards the Western Pacific during week-1, followed by a resumption of eastward propagation over the Western Hemisphere with models being somewhat mixed as to when this occurs during week-2. An MJO propagating through the Western Hemisphere would favor increased chances for Tropical Cyclone (TC) development in the far Eastern Pacific and tropical Atlantic, with decreased chances over the western Pacific. However, with a secondary subseasonal signal moving through the western Pacific and the strong background state, additional TC development is possible.

It has been an active week for TCs with 7 separate systems forming across the Northern Hemisphere. In the Bay of Bengal, TC One formed on Sep 22 and moved into the eastern coast of India, dissipating by Sep 26. In the Western Pacific, TC Surigae formed in the Philippine Sea on Sep 23. Surigae is currently still active but is weakening, and is moving northwest parallel to and well away from the Japanese archipelago. For the latest information of Surigae please refer to the Joint Typhoon Warning Center. In the Eastern Pacific, three TCs formed. On Sep 24 Hurricane Nolo formed south of the Hawaiian Islands. Nolo quickly strengthened to a major hurricane, and tracked near the Big Island, bringing another round of heavy rain to the beleaguered region. Nolo is still active and is currently moving away from Hawaii. On Sep 29 TC Rachel formed south of the Gulf of Tehuantepec. Rachel is still active and has been tracking northwestward parallel to the Mexican coast. On Sep 29 TC 19-E formed near 15N-130W. 19-E is still active and is expected to track eastward over the next few days. For the latest information on Nolo, Rachel, or 19-E please refer to the National Hurricane Center (NHC). Two TCs also formed in the Atlantic over the last week. On Sep 25 TC Gonzalo formed south of the Cape Verde Islands. Gonzalo moved westward into conditions hostile to TC activity and dissipated on Sep 26. On Sep 28 TC Hanna formed east of Bermuda. Hanna is still active and is expected to weaken while moving northeastward. For the latest information on TC Hanna please refer to the NHC.

The East Pacific has been a major hotspot for TC activity, and model guidance indicates a continuation of this trend through at least week-2. Solutions from the ECMWF, GEFS, and Google AI models all indicate a surge of activity early in the forecast period, with potential genesis points appearing closer to the Mexican coast and further east than has been seen this year. Given favorable conditions and strong support from model guidance, a 40-60% chance of TC genesis is posted covering roughly 10-20N and 95-115W, and a 20-40% chance extending slightly further south and west. During week-3 model guidance becomes very noisy and difficult to interpret, while MJO phasing also becomes less favorable. Prevailing El Nino conditions remain favorable for TCs in the East Pacific though, so a 20-40% chance of TC genesis is indicated for much of the East Pacific basin. As noted above, the highlighted area in the Eastern Pacific is well to the east of what has been typical this year, and both the GEFS and ECMWF indicate a potential Central American Gyre formation early in the forecast period. Concurrently, both models show a modest signal for TC genesis either in the Bay of Campeche or the western Caribbean. Therefore, a 20-40% chance of TC genesis is posted for these areas. Model guidance portrays a similar, albeit noisier picture over the Western Pacific early in week-2. Various models depict potential TC spin-ups for a broad sector of the Western Pacific well east of the Philippines from the middle of week-1 into early week-2, warranting a 20-40% chance of TC genesis valid for week-2 over the described region. Beyond this model guidance quiets down noticeably, so no TC genesis is anticipated in week-3 for the Western Pacific.

The precipitation outlook for weeks 2 and 3 is based on potential TC activity, ENSO composites, and informed by GEFS, CFS, Canadian, and ECMWF ensemble mean solutions. Above-normal precipitation is noted in the southeast Pacific and into the coast of Chile, which may be problematic given very strong recent atmospheric river activity in the region. Above-normal temperatures are favored for much of the western Contiguous U.S. and into Mexico during week-2. For hazardous weather conditions in your area during the coming two-week period, please refer to your local NWS office, the Medium Range Hazards Forecast produced by the Weather Prediction Center, and the CPC Week-2 Hazards Outlook. Forecasts made over Africa are made in coordination with the International Desk at CPC.

3 Week Tropical Cyclone Formation Probability//09/29/26

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