Tropical Storm Alvin forms off the Pacific coast of Mexico » Yale Climate Connections

The first named storm of the 2025 eastern Pacific hurricane season, Tropical Storm Alvin, is gathering strength in the warm waters about 700 miles offshore of southwestern Mexico. Alvin formed at 11 a.m. EDT on Thursday, May 29, and it is poised to intensify into a moderate-strength tropical storm with 60 mph winds on Friday before wind shear and dry air reduce it to a remnant low over the weekend, before Alvin reaches the coast of Mexico. No watches or warnings are posted for Mexico.

The tropical depression that became Alvin formed over unusually warm waters of 30 degrees Celsius (86°F) – about 1 degree Celsius above average – conditions that were made more likely because of human-caused climate change by a factor of 60-80, according to Climate Central’s Ocean Shift Index (see skeet above).

At 11 a.m. EDT Thursday, Alvin was located about 670 miles south-southeast of the southern tip of the Baja Peninsula of Mexico, moving northwest at 10 mph with top sustained winds of 40 mph and a central pressure of 1005 mb. Through Friday, Alvin will have favorable conditions for intensification, with warm waters of 29-30 degrees Celsius, light to moderate wind shear of 5-15 knots, and a reasonably moist atmosphere. As Alvin approaches the coast of Mexico on Friday night and Saturday, though, wind shear is expected to rise dramatically, and the atmosphere will dry out, resulting in Alvin degenerating into a remnant low by Saturday.

The remnants of Alvin are expected to move into southwestern Mexico on Sunday and Monday, bringing up to an inch of rain. Some of the remnant moisture will also make it into the U.S. Southwest by early next week, enhancing scattered showers and thunderstorms across the drought-stricken region. No hurricane hunter missions in Alvin are planned.

No evidence Eastern Pacific hurricane season is getting longer

Alvin’s formation date of May 29 comes nearly two weeks earlier than the average June 10 formation date of the eastern Pacific’s first named storm of the season (for the period 1991-2020). And back in 2021, the basin experienced its earliest-ever first named storm, Tropical Storm Andres, on May 9. So, is the season getting longer?

One might expect that hurricane season will start earlier and end later in coming decades as warming of the oceans allows more storms to form when ocean temperatures are marginally warm for tropical cyclone formation. However, hurricane genesis also requires low wind shear, high levels of moisture at mid-levels of the atmosphere, and something to get the low-level atmosphere rotating. In some ocean basins, climate change may inhibit early-season genesis events by decreasing these other factors needed for a hurricane to get started. Looking at the longer-term statistics for the eastern Pacific, there is no evidence that hurricane season is starting earlier (Fig. 1).

No research has been published thus far showing a change in the length of the Eastern Pacific hurricane season. A 2015 study of how climate change might be expected to influence season length in climate models, led by MIT’s John Dwyer, yielded mixed results for the Eastern Pacific, depending upon which model was used to simulate hurricane activity. Most models, but not all, projected an increase in the length of the Eastern Pacific hurricane season in a future warmer climate.

NOAA predicts a below- to near-average hurricane season in eastern and central Pacific

In its May seasonal forecast, NOAA predicted that the eastern Pacific hurricane season (for storms affecting Mexico) would most likely be below-average (50% chance), with a 30% chance of a near-average season, and a 20% chance of an above-average season. NOAA called for a 70% probability of 12-18 named storms, with 5-10 hurricanes, 2-5 major hurricanes, and an ACE index at 60-130% of the median. Using the midpoint of these ranges, NOAA called for 15 named storms, 7.5 hurricanes, and 3.5 major hurricanes. This is near the 1991-2020 averages of 15 named storms, eight hurricanes, and four major hurricanes.

NOAA’s central Pacific seasonal forecast called for a 50% chance of a near-average hurricane season, 30% chance of a below-average season, and a 20% chance of an above-average season. NOAA predicted 1-4 tropical cyclones (which includes tropical depressions, tropical storms, and hurricanes). A near-average season in the central Pacific has 4-5 tropical cyclones. The central Pacific is located north of the equator between 140°W and the International Date Line, and includes Hawaii.

Below-average hurricane activity observed in eastern Pacific in 2024

Last year, as is often the case when the Atlantic has an active hurricane season, tropical cyclone activity in the eastern Pacific was below average. The 2024 tally was 13 named storms, five hurricanes, three major hurricanes, and an ACE index of 82, which is less than 70% of average. There was one Cat 5 in the basin in 2024: Hurricane Kristy, which peaked with 160 mph winds on Oct. 21. One major hurricane hit Mexico: category 3 Hurricane John, which made landfall in the state of Guerrero, Mexico, on Sep. 24. John was blamed for 29 deaths and $2.5 billion in damage. According to EM-DAT, only three other eastern Pacific hurricanes have caused more damage in inflation-adjusted dollars in Mexico: Otis (2023, $12 billion); Manuel (2013, $5.6 billion); and Odile (2014, $3.2 billion).

Two named storms were observed in 2024 the central Pacific in the waters west of 140°W. Hurricane Hone formed in the central Pacific on Aug. 22, becoming the first tropical cyclone to form in that basin in five years. Hone dropped very heavy rainfall over the Big Island of Hawaii after passing just offshore at Category 1 strength on Aug. 25. And after peaking as a category 4 hurricane with 130 mph winds in the Eastern Pacific, Tropical Storm Gilma crossed into the central Pacific, dissipating in the waters well east of Hawaii on Aug. 29.

Alvin is the Northern Hemisphere’s first named storm of 2025

Although Alvin arrived on the early side for the first tropical storm in the Eastern Pacific, typically there has been a named storm somewhere north of the equator by this point – usually over the Northwest Pacific. However, Alvin is the hemisphere’s first named storm of 2025. As noted by meteorologist Boris Konon, only one year in data going back to 1950 has seen the hemisphere’s first named storm take longer than this to arrive. That year was 1973, when Ava developed in the Eastern Pacific on June 2.

Bob Henson contributed to this post.