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Hurricane Seymour Gains Steam in East Pacific

By: Bob Henson 3:48 PM GMT on October 24, 2016

More than three weeks of quietude in the Eastern Pacific has come to an end with the development of Hurricane Seymour, which could become the region’s sixth major hurricane of the year. Seymour gained hurricane status about 450 miles southwest of Manzanillo, Mexico, at 11 am EDT Monday, with top sustained winds at 75 mph. As noted by Dr. Phil Klotzbach (Colorado State University), Seymour is the Pacific’s first named storm east of the International Date Line since Hurricane Ulika, which dissipated September 30, and the first named storm in the Eastern Pacific east of 140°W since Tropical Storm Roslyn, which dissipated on September 29.

It was exactly a year ago (October 22-23, 2015) that Hurricane Patricia rocketed from tropical storm to Category 5 strength in just 24 hours over the northeast Pacific. Though Seymour doesn’t appear set to challenge Patricia, it is strengthening at a robust enough pace to be considered a rapid intensifier. In the 24 hours from 11:00 am EDT Sunday to Monday, Seymour vaulted from a 35-mph tropical depression to a 75-mph hurricane, and more growth lies ahead. Showers and thunderstorms have expanded around Seymour’s compact center in the last few hours, enhanced by very low wind shear (less than 10 knots) and a fairly moist atmosphere (mid-level relative humidities of 60 - 70%).

Figure 1. Hurricane Seymour as of 10:57 am EDT Monday, October 24, 2016.

Outlook for Seymour
For the next several days, Seymour will be traveling west to west-northwest over very warm SSTs above 28°C (82°F), while other conditions remain favorable. Seymour’s peak intensity should arrive late Tuesday or Wednesday, by which point it is likely to attain at least Category 2 or 3 strength as suggested by the HWRF model and statistical guidance. The National Hurricane Center (NHC) official forecast issued at 11 am EDT Monday has Seymour peaking as a 120-mph hurricane on Tuesday evening. The 12Z Monday output from the SHIPS statistical model gives Seymour a 68% chance of attaining 105-knot sustained winds (Category 3) by Tuesday night and a 31% chance of reaching 115 knots (Category 4) by Wednesday morning. By Thursday, higher wind shear and lower SSTs will begin sounding the the death knell for Seymour as the hurricane begins recurving toward the northeast. Seymour is expected to dissipate by next weekend while still far offshore from the U.S. and Mexico. However, some of Seymour’s moisture could get entrained into a strong midlatitude storm expected to plow into northern and central California late in the weekend, bringing welcome rains and mountain snows.

Figure 2. NHC’s outlook for Seymour as of 11 am EDT Monday, October 24, 2016.

It’s been another busy year in the East Pacific
Forecasters had expected 2016 to produce a bit more tropical activity than usual in the Central and East Pacific. Factors in play included the projected transition from El Niño to La Niña (the latter tends to suppress hurricanes over the northeast Pacific Ocean), and the possibility that the global SSTs patterns that favored Central and East Pacific hurricanes in 2014 and 2015 might remain. (2014 was the region’s fourth most active season on record, as measured by named storms, and 2015 was the second most active.) As it turned out, La Niña has been slow in materializing, and SSTs over the North Pacific and North Atlantic have remained favorable for eastern Pacific hurricanes.

Back in May, Mexico’s national meteorological service (SMN) predicted 17 named storms, 9 hurricanes, and 4 major hurricanes in the Central and Eastern Pacific, which is slightly above the 1981-2010 average of 15.4 named storms, 7.6 hurricanes, and 3.2 major hurricanes. Likewise, the NOAA outlook skewed above the long-term mean, calling for a range of 13-20 named storms, 6-11 hurricanes, and 3-6 major hurricanes. With Seymour now a hurricane, the 2016 season stands at 20 named storms, 13 hurricanes, and 5 major hurricanes, which is near the top end of NOAA’s range and considerably above the SMN outlook. The NOAA range is designed to encompass 70% of potential outcomes for seasons that share the same climate conditions and uncertainties.

This is the fifth year in a row with an above-average number of both tropical storms and hurricanes in the Central and Eastern Pacific. Counting Seymour, the region has seen a grand total of 105 named storms since 2012! If that whole five-year stretch had featured the same rate of activity as the period 1981-2010, there would have been 77 named storms.

Elsewhere in the tropics
For the first time in a long time, we’re looking at a quiet week across the tropical Atlantic. NHC expects no tropical cyclone formation through Wednesday. Among ensemble guidance from 00Z Monday, the GFS ensemble continues to favor the development of a tropical depression well east of the Lesser Antilles this week, but the European ensemble shows less than a 10% chance of this outcome, and none of the leading operational models (GFS, Euro, and UKMET) show any significant development. Climatology also leans away from development in the open tropical Atlantic by late October.

In the Bay of Bengal, a depression west of Myanmar dubbed Invest 99B (see Figure 3 below) could gain some strength as it moves west toward India this week, although models are not bullish on any major development.

We’ll be back with a new post on Tuesday.

Bob Henson

Figure 3. Invest 99B as of 3:18 am EDT Monday, October 24, 2016. Image credit: CIRA/CSU/RAMMB.


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