Dr. Jeff Masters ·

Above: Tropical Depression One-E as seen on Tuesday afternoon, May 9, 2017, by the VIIRS instrument on the Suomi satellite. At the time, the storm was the earliest tropical depression ever observed in the Eastern Pacific; the system became Tropical Storm Adrian later that evening. Image credit: NASA.

The Eastern Pacific hurricane season is off to a record-early start after the formation of Tropical Storm Adrian on Tuesday, May 9. The previous earliest appearance of a tropical depression in the Eastern Pacific since reliable satellite records began in 1970 was on May 12, 1990, when Tropical Storm Alma got its start. Three other systems got their start on May 13, according to NOAA’s Historical Hurricane Tracks website.

Satellite imagery on Wednesday morning showed that Adrian was poorly organized, with only a modest area of intense thunderstorms. The storm had not increased in organization since Tuesday night, and was suffering the effects of 20 – 25 knots of wind shear. The 12Z (8 am EDT) Wednesday run of the SHIPS model predicted that wind shear might fall to the moderate range (10 – 20 knots) over the next few days, which might allow some slow strengthening to occur. Ocean temperatures were a very warm 30.5°C (87°F), about 0.5°C above average for this time of year, so Adrian has the potential to intensify into a hurricane if the wind shear does drop.

Our four top models for predicting hurricane tracks, the European, GFS, HWRF and UKMET models, all predicted in their 0Z Wednesday runs that Adrian would continue on a west-northwesterly to northwesterly track well offshore of Mexico for the next few days, then encounter very weak steering currents that would give rise to slow and erratic motion this weekend. The location and timing of any potential landfall are highly uncertain.

Figure 1. According to NOAA’s Historical Hurricane Tracks website, there have been 39 tropical cyclones in the Eastern Pacific in May, but only two of them made landfall at hurricane strength in May: Category 1 Hurricane Agatha of 1971, and Category 1 Hurricane Barbara of 2013.

Is the Eastern Pacific hurricane season getting longer?

We might expect that hurricane season will start earlier and end later in coming decades, due to warming of the oceans allowing more storms to form when ocean temperatures are marginal for tropical cyclone formation. I’m not aware of any research that has been published on whether this is already happening in the Eastern Pacific, though. The only years since 1970 to see two named storms in May in the Eastern Pacific were 1984, 2007, 2012, 2013, 2015 and 2016, so it is possible that a statistical analysis would show that there is more early season activity there, but CSU hurricane scientist Phil Klotzback tweeted that a plot of Eastern Pacific hurricane season start dates shows no trend since 1970. 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 of the models—but not all—projected an increase in the length of the Eastern Pacific hurricane season in a future warmer climate.

The Atlantic hurricane season does appear to be getting longer in the region south of 30°N and east of 75°W, according to a 2008 paper by Dr. James Kossin of the University of Wisconsin in Geophysical Research Letters titled, "Is the North Atlantic hurricane season getting longer?" A 2016 analysis by Dr. Ryan Truchelut of WeatherTiger also supported this idea. However, Juliana Karloski and Clark Evans of the University of Wisconsin-Milwaukee found no trend in tropical cyclone formation dates when looking at the entire Atlantic, for the period 1979–2014.