Wow, I can’t believe it’s already December! I need to put up my Christmas tree before January arrives in a couple of days! I hope everyone had a terrific Thanksgiving weekend full of food, family, friends, and fun. I was in Hilo for all of that, along with getting hissed at by my sister’s cat. She (the cat, not my sister) despises me with a passion, though I have to admit that it’s mostly deserved. The end of the Thanksgiving weekend also marked the end of the official 2025 tropical cyclone (TC) season. Looking back, how did it all turn out? In the Central North Pacific (CNP) basin (north of the equator, from 140 West longitude to the International Dateline), there were four tropical cyclones. Use of the term “tropical cyclone” means that I’m including tropical depressions, tropical storms, and hurricanes. Definitions for each type of tropical cyclone can be found at the National Hurricane Center’s (NHC) glossary page on their website.
On average, the CNP has four to five TCs every year, though the number can vary considerably from year to year, as can be seen in the graph below. The distribution is positively skewed with some years having large spikes in activity, such as 2015 (16 TCs, record year), 1992 (12), 1994 (11), and 1982 (10). Active years tend to be associated with El Niño, though there are other large-scale climate features, such as the Pacific Decadal Oscillation (PDO), that help to enhance or suppress activity in addition to the El Niño Southern Oscillation (ENSO) cycle. Chu and Murakami (2022) provide a discussion of various climate scale phenomena that modulate TC activity in the CNP basin.
Graph of the annual number of TCs in the CNP from 1970 through 2025. Red bars indicate El Niño during July-August-September (JAS), blue bars indicate La Niña, and yellow bars indicate ENSO neutral. Data provided by the Central Pacific Hurricane Center. Bar colors based on ONI data from the Climate Prediction Center.
The map below from NHC shows the tracks of all four TCs in the CNP basin. But wait! There are five lines! The map incorrectly includes Gil, which became a “post tropical cyclone” (i.e. no longer considered to be a tropical cyclone) east of 140W longitude after losing all of its organized thunderstorm activity over cooler waters and within a drier air mass. NHC is aware of the issue and will be amending the map in the near future. Iona and Keli developed in the CNP during an active phase of the Madden-Julian Oscillation (MJO). Check out my August 1 blog post for more details on the formation and development of Iona and Keli. Henriette and Kiko moved into the CNP basin from the east.
Map from NHC of TCs that formed within or moved into the CNP during the 2025 TC season. Gil was inadvertently plotted on the map.
Iona and Keli were both “west movers” and were too distant to directly affect weather over the Hawaiian Islands in a significant manner. According to the Honolulu Forecast Office’s (HFO) weather discussions, Iona likely produced some short period swells that arrived along the south-facing shores of the state, but they were drowned out by a larger long period swell from the South Pacific that arrived concurrently. The loop below shows Hurricane Iona and Tropical Storm Keli on the night of July 28 (HST) and into the early morning hours of July 29. Iona reached peak intensity at the end of the loop with maximum sustained winds estimated at 115 knots (130 mph). Keli was at its peak intensity of 45 knots (50 mph) at the start of the loop, and started to weaken after the end of the loop. Another point of interest in the loop is that it’s not too often you see two fully developed TCs with less than 10 degrees of longitude separating them. Keli’s small size likely allowed this to happen.
GOES infrared image loop from the CIRA/RAMMB 2025 archive. The annotations at the start are mine. With this color enhancement curve, blues, greens, reds, and yellows are the coldest and highest cloud tops. The red (very cold/high cloud tops) surrounding Iona’s clear eye indicates that it’s a strong hurricane. Keli also has very cold cloud tops near its center but it lacks an eye and its deep convection is less organized and consistent.
About a week later, Henriette approached the CNP basin from the east. It was a tropical storm for several days as it struggled to develop over cooler waters and within a relatively dry air mass far east of the Big Island. It entered the CNP on the night of August 7 barely at tropical storm intensity and was declared to be a post-tropical remnant low on the morning of August 8 due to a lack of significant deep convection. NHC kept advisories going even after Henriette reached post-tropical status because sea surface temperatures were expected to increase along the forecast track and vertical shear was expected to be low. The forecast proved to be correct as Henriette regained tropical cyclone status late that night. Henriette steadily intensified and reached its peak intensity of 75 knots (85 miles per hour) maximum sustained winds in the early morning hours of August 11.
Henriette passed well northeast of the main Hawaiian Islands. Its main effect on the island chain may have been to worsen the drought over the state as it cut off the trade winds. Hilo Airport had its lowest August rainfall on record, and the period from August 9th through the 11th only had 0.01 inches. The GOES IR loop below shows Henriette as it passes northeast of the state during the afternoon of August 10.
Loop from the same archive as above, except for Hurricane Henriette at 75 knots (85 mph) maximum sustained winds. Note that there are no greens, reds, and yellows, indicating cloud tops that are warmer and lower than the ones in the Iona/Keli loop. Henriette’s eye is also cloud-filled and more ragged than Iona’s. These factors indicate a weaker system.
Kiko, the last TC of the season, probably generated the most interest across the state. It was the most intense TC of the season for the CNP, with maximum sustained winds peaking at 125 knots (145 mph). As it hit peak intensity, some of the forecast guidance suggested potential impacts to the main Hawaiian Islands, so it got the attention of emergency managers and the media across the state. It was also enough of a threat that NHC called out the “Hurricane Hunters” from the 53rd Weather Reconnaissance Squadron. Kiko ultimately remained northeast of the state, but its center got within 150 miles of the island chain. It was close enough for the western half of Kiko’s circulation to produce a slight boost in rainfall to portions of windward Kauaʻi and Oʻahu. Kiko also generated warning-level surf that affected the east-facing shores of the state. The first GOES IR loop below shows Kiko at peak intensity on the afternoon of September 3. Note how the cloud tops are cooler (yellow colors) than with Iona. The second loop shows a much weaker Kiko at its closest point of approach to the Hawaiian Islands. The coldest cloud tops are well separated from the center and are limited to the northern quadrant of its circulation due to the strong vertical shear affecting the system. The light and medium gray shades indicate low level clouds.
Loop from the same archive as above, except for Hurricane Kiko. This loop shows Kiko at its peak intensity of 125 knots (145 mph) on the afternoon of September 4.
Same as above, except for Kiko near its closest point of approach to the Hawaiian Islands on September 3. Kiko is a weak tropical storm at this time.
I’m always grateful when we get through another hurricane season without significant impacts. While I’m generally prepared for them, I don’t ever want to put my preparedness plans into action. There are better ways to get drought relief than a full blown tropical cyclone!
Reference
Chu, P.S., and H. Murakami, 2022: Climate Variability and Tropical Cyclone Activity. Cambridge University Press, 303 pp.