An Early Start to Hurricane Season in the Eastern Pacific and Atlantic?

The Eastern Pacific hurricane season officially starts on May 15, but for the second year in a row, we have the potential to see a record-early start to the season. Satellite imagery on Wednesday morning showed that a concentrated area of heavy thunderstorms in association with a broad area of low pressure located about 1200 miles southwest of the southern tip of the Baja California peninsula (90E) had acquired plenty of spin, but was not yet organized enough to be labeled a tropical depression.

Conditions were favorable for development, with low wind shear of 5 – 10 knots and sea surface temperatures (SSTs) a warm 28°C (82°F), about 1°F above average. The 6Z (2 am EDT) Wednesday run of the SHIPS model predicted that 90E had a relatively short window for development, though, with wind shear predicted to rise into the high range, 20 – 30 knots, by Thursday afternoon. In a special Tropical Weather Outlook issued at 12:15 am EDT Wednesday, the National Hurricane Center (NHC) gave 90E 2-day and 5-day odds of development of 70%. 90E is far out at sea, and will not affect any land areas. The first name on the Eastern Pacific list of storm names in 2018 is Aletta.

Figure 1. According to NOAA’s Historical Hurricane Tracks website, there were 39 tropical cyclones in the Eastern Pacific in May from 1950 - 2016, 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. Last year’s Hurricane Adrian is not shown here.

Is the Eastern Pacific hurricane season getting longer?

The Eastern Pacific hurricane season got off to a record-early start last year, when Tropical Storm Adrian formed on May 9. 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. With a record-early start to the season last year, and another potential record early start this year, the question naturally arises—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 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. Last year, 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.

Time to watch the Atlantic?

The Atlantic hurricane season officially starts on June 1, but if recent long-range runs of the GFS model are correct, the season could get off to an early start. The GFS model has been predicting that late next week, an area of disturbed weather over Central America will act as the seed to get a tropical storm spinning in the Western Caribbean. Water temperatures there are near 28°C (82°F)—about 0.5°C above average, and plenty warm enough to support a hurricane. The subtropical jet stream—which is typically located over the Caribbean in May, creating high wind shear that interferes with hurricane development—is predicted to lift northwards by late next week, creating conditions favorable for tropical cyclone genesis. However, the long range runs of the European model have not been supporting this idea, and the GFS forecasts of a tropical storm in the Caribbean late next week should be viewed as interesting, but improbable—worth lifting one eyebrow at, but not two.

Figure 2. Infrared satellite image of the unusual feature—possibly a subtropical cyclone—located several hundred miles west of the Chilean coast at 1845Z on Monday, May 7, 2018. Image credit: NASA/MSFC, via Levi Cowan (@TropicalTidbits).

Subtropical development in the far Southeast Pacific: a potential first

What may be the first subtropical cyclone ever documented in the far Southeast Pacific developed last weekend several hundred miles west of the Chilean coast, where a surface low consolidated beneath a strong upper-level low (see Figure 3). Despite seasonably cool sea-surface temperatures of 18-20°C (64-68°F), there was enough cold air aloft to result in instability, and pockets of shallow thunderstorms briefly wrapped around the low. 

The ASCAT scatterometer aboard the EUMETSAT satellite detected peak surface winds in the range of 35-40 knots (40-45 mph) on Tuesday. Phase-space diagrams from Florida State University indicate that the low qualified as a shallow warm-core cyclone for several days.

This is still pretty darn cool. Yes, that's Chile on the right-hand edge of the image. pic.twitter.com/NFz6rxTr5r

— Levi Cowan (@TropicalTidbits) May 8, 2018

This system won’t be officially designated as a tropical or subtropical cyclone by any weather agency, because none have been assigned to carry out operational tropical cyclone forecasting for this part of the world. In April-May 2015, a similar subtropical cyclone formed near Easter Island, nearly 2,000 miles west of this week's cyclone. Researchers posthumously dubbed it Subtropical Storm Katie. It developed over unusually warm water as the near-record-strong El Niño event of 2014-16 was intensifying, whereas the current cyclone is over much cooler water during a decaying La Niña event.

Tropical and subtropical cyclones do not occur on a regular basis in the Southeast Pacific or Southeast Atlantic, largely because of the cold surface waters normally brought up through upwelling currents. Because we’ve observed this region intensively by satellite only in the last several decades, it’s entirely possible that such events have occurred on rare occasions in the distant past—but clearly this isn’t an everyday happening. Jonathan Belles (The Weather Company) has more details on this very unusual cyclone.

Figure 3. The unusual subtropical cyclone west of Chile developed from an upper-level trough that became cut off from the polar jet stream encircling Antarctica. A surface low intensified beneath the cut-off low in an environment of cold air aloft and relatively low wind shear that favored symmetric development. Shown here are the heights of the 500-millibar surface (about four miles above the surface), in tens of meters (legend at right), together with the surface pressure (black contours) analyzed in the GFS model at 12-hour increments from 12Z Friday, May 4, to 0Z Monday, May 7, 2018. Image credit: tropicaltidbits.com.

Bob Henson wrote the section of this post about the storm off the coast of Chile.