Wednesday, October 26, 2016

Small Yet Mighty, Hurricane Seymour Quickly Strengthens

On Sunday night, the National Hurricane Center (NHC) correctly pegged a small tropical cyclone off the coast of Mexico as being a candidate for rapid intensification, and rapidly intensify it has. Just a minimal tropical storm as of 11 am EDT Sunday, and a minimal hurricane at 11 am Monday, Hurricane Seymour was a strong Category 3 storm as of 11 am EDT Tuesday, with top sustained winds now at 125 mph. Located about 600 miles southwest of Cabo San Lucas, Mexico, Seymour is moving away from land toward the west at about 14 mph. No reconnaissance flights will be going into Seymour, but satellite imagery suggests that the hurricane likely completed an eyewall replacement cycle on Monday night.

Seymour could strengthen even more in the next 24 hours, as wind shear remains very light (only about 5 knots), sea-surface temperatures are very warm (28 - 29°C), and the atmosphere remains fairly moist (mid-level relative humidities of 60 - 70%). NHC’s official forecast makes Seymour a Category 4 storm with 140-mph winds by late Wednesday. Beyond Wednesday, shear will increase and SSTs will decrease along Seymour’s path, and the hurricane will weaken fairly quickly as it begins curving toward the north-northwest. Seymour will dissipate long before it can threaten the Pacific coast of Mexico or the U.S.

Figure 1. Enhanced infrared image of Hurricane Seymour as of 10:30 am EDT Tuesday, October 25, 2016. Image credit: NOAA/NESDIS.

Figure 2. Visible satellite image of Hurricane Seymour as of 10:45 am EDT Tuesday, October 25, 2016. Image credit: NASA/MSFC Earth Science Office.

On Sunday night, NHC forecasters began noting Seymour’s potential for rapid intensification, which had become evident in both statistical and dynamical forecast models. NHC has gained skill at intensity prediction over the last few years, but the task remains much more difficult than track prediction, even at short time periods (see Figure 3 below]. Intensity changes are heavily influenced by complex, small-scale processes that are tough for computer models to accurately represent. This is especially true for rapid intensification, defined by NHC as an increase in a tropical cyclone’s maximum sustained winds of at least 30 knots (35 mph) in a 24-hour period. Models often disagree on whether a tropical cyclone might rapidly intensify. As an operational agency, NHC strives to avoid “windshield-wiper” forecasts, where the predictions lurch back and forth every few hours based on short-term trends. This means that NHC will often be cautious in predicting rapid intensification, to avoid having to pull back in a subsequent forecast.

Figure 3. Trends in intensity forecasts from the National Hurricane Center for Eastern Pacific tropical cyclones through 2015. Units are nautical miles (left) and knots (right); add 15% to obtain miles and miles per hour. Image credit: NHC.

As far back as its 12Z Sunday run, the HWRF model--on average, the best of the dynamical models at predicting intensity for an existing storm--was consistently predicting that Seymour would become at least a Category 3 storm by midweek. Singing a similar tune was the SHIPS statistical model, which takes into account various environmental factors to calculate the odds of a storm rapidly intensifying over various time periods. By 12Z Sunday, the SHIPS model gave Seymour (then just a 30-knot tropical storm) a 54 percent chance of gaining 55 knots of strength (i.e., becoming an 85-knot or 100-mph Category 2 hurricane) by 12Z Tuesday. With very consistent messages coming from these tools, NHC forecasters began to dramatically hike their intensity predictions for Seymour late Sunday. NHC’s early-Monday forecast called for Seymour to vault from 65-mph to 115-mph sustained winds in just 48 hours, which the storm more than accomplished.

The forecast for Seymour was made easier by its modest size. Smaller tropical storms and hurricanes can both strengthen and weaken more rapidly than large ones, since they contain less mass (air, water vapor, clouds) to spin up or slow down. As of Tuesday morning, Seymour’s tropical-storm-force winds extended out only 70 miles from its center, and hurricane-force winds extended out just 15 miles.

The signals were much more mixed, and the forecast more difficult, for Hurricane Matthew when it underwent spectacular intensification on September 29 - October 1. During that period, Matthew leapt from tropical-storm strength (70 mph) to Category 5 hurricane strength (160 mph) in just 36 hours. Matthew was an unusually large storm to undergo such rapid intensification: Matthew’s tropical-storm-force winds extended out 205 miles from its center even before the storm reached hurricane strength.

Figure 4.Tropical Storm Kyant as of 11 am EDT Tuesday, October 25, 2016. Image credit: CIRA/CSU/RAMMB.

Tropical Storm Kyant a slow grower in the Bay of Bengal
Tropical Storm 3, dubbed Kyant by the Indian Meteorological Department, is gradually strengthening as it moves west through the Bay of Bengal, with top sustained winds up to 50 mph as of Tuesday morning. SSTs are quite warm along Kyant’s path (close to 30°C or 86°F), and wind shear is light to moderate (10 - 15 knots), but the storm will be moving into relatively dry air (relative humidities around 40%), which will hinder its growth over time. Kyant is predicted by the Joint Typhoon Warning Agency to make landfall north of Chennai, India, on Friday night as a minimal tropical storm.

Debunking the 1913 global heat record at Death Valley
WU weather historian Christopher Burt has given us a masterpiece of meteorological detective work in his Monday post, “An Investigation of Death Valley’s 134°F World Temperature Record.” Chris joined forces with geographer and climatologist William Reid to dig into the background surrounding the 134°F reading taken at Furnace Creek, near Death Valley, California, on July 10, 1913. This was eclipsed as the world’s hottest air temperature by the 58.0°C (136.4°F) reading at El Azizia, Libya, on September 13, 1922. However, the latter was disqualified by the World Meteorological Organization in 2012, which re-throned the 1913 Furnace Creek report as the world’s hottest observed surface temperature. In his post on Monday, Chris lays the groundwork for why the 1913 reading ought to be disqualified as well. It’s a long but fascinating read--give yourself time to savor it!

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

Bob Henson

Figure 5. This photo of Death Valley, California, from Dante’s View shows the Badwater Basin just below (white area) and the Furnace Creek Ranch and the national park visitors area to the north where the green-shaded area is visible in the top right portion of this image. This photo is a still from the documentary film ‘Dead Heat’ produced by Weather Underground in 2012.

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