Earliest Tropical Depression on Record in Eastern Pacific

April 25, 2020, 5:45 PM EDT

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Above: Enhanced infrared satellite image of Tropical Depression 1E at 1655Z (12:55 pm EDT) Saturday, April 25, 2020. (tropicaltidbits.com)

The improbable became reality at 11 am EDT Saturday, April 25, when the National Hurricane Center began advisories on Tropical Depression 1E two weeks earlier than for any other tropical cyclone on record in the Eastern Pacific. TD 1E is not expected to affect any land areas, or become anything more than perhaps (perhaps!) a minimal tropical storm, but it’s already earned a place in the record books.

TD 1E was centered far out to sea at 11 am EDT, about 730 miles southwest of Cabo San Lucas, Mexico, and moving northwest at 7 mph. Top sustained winds were 35 mph.

The earliest start on record to the Eastern Pacific hurricane season was on May 9, 2017, when a tropical depression became Tropical Storm Adrian just a few hours later. The previous earliest appearance of a named storm 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.

The earliest tropical cyclone in the Central Pacific was Hurricane Pali, which formed on January 11, 2016. Pali benefited from unusually warm SSTs related to an intense El Niño event. (Arguably, Pali was a laggard storm from the 2015 season rather than an early storm in the 2016 season.)

TD 1E began as a robust tropical disturbance that gradually organized in the open Eastern Pacific, a region where strong wind shear and cool waters normally inhibit development so early in the year. Computer models correctly and consistently foresaw several days in advance that this disturbance had a chance to become a tropical cyclone.

Persistently light wind shear (less than 10 knots) allowed prominent upper-level outflow to develop to the north of 90E, helping to ventilate the system. The depression also drew on unusually warm water, with sea surface temperatures of 27-28°C (81-82°F) running about 0.5 to 1°C above average. A broad swath of convection (showers and thunderstorms) developed on Thursday night, waned on Friday, and then redeveloped in more compact fashion Friday night around TD 1E’s sharpening center of circulation.

If TD 1E has any chance of nudging up to tropical storm strength, it’ll have to happen soon. Much cooler water is lurking just to the north, as is much stronger wind shear. NHC concurs with model consensus in keeping TD 1E near its current strength into Sunday and then weakening it to a post-tropical remnant low.

Jeff Masters: Is the Eastern Pacific hurricane season starting earlier?

WU co-founder and Cat 6 founder Dr. Jeff Masters provided his thoughts Friday on whether human-caused climate change might be at work in the development of TD 1E.

“With a record-early start to the season in 2017, and another potential record early start this year, the question naturally arises—is the Eastern Pacific hurricane season starting earlier due to climate change?

“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 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.

“There has not been any research published thus far showing a change in the length of the Eastern Pacific hurricane season. In 2017, CSU hurricane scientist Phil Klotzbach tweeted a plot of Eastern Pacific hurricane season start dates showing no trend in the start date since satellite data became available in 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.

“In summary, we don’t yet know if climate change is leading to an increase in Eastern Pacific tropical cyclone activity, and will have to wait for another decade or so of data to see.”

Dr. Jeff Masters contributed to this post.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Bob Henson

Bob Henson is a meteorologist and writer at weather.com, where he co-produces the Category 6 news site at Weather Underground. He spent many years at the National Center for Atmospheric Research and is the author of “The Thinking Person’s Guide to Climate Change” and “Weather on the Air: A History of Broadcast Meteorology.”


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