Rapid Intensification Watch: Orlene and Meranti Spin Up in Pacific

While the North Atlantic is on the tepid side this weekend in terms of tropical cyclones, we have two potentially fearsome storms in the North Pacific. One is unlikely to hit land; the other is taking a bead on Taiwan. The latter is Typhoon Meranti, located about 900 miles southeast of Taipei as of 12Z (8:00 am EDT) Sunday. Meranti’s top winds, as reported by the Joint Typhoon Warning Center using the 1-minute U.S. standard, jumped from 40 to 85 mph in the 24 hours leading up to 12Z Sunday. This meets the National Hurricane Center (NHC) definition of rapid intensification: an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph) in 24 hours.

Meranti’s central core of showers and thunderstorms (convection) is expanding and consolidating quickly, with excellent upper-level outflow evident on satellite. Sea surface temperatures will be holding near 30°C (86°F) along Meranti’s path. Mid-level relative humidity will climb from around 70% toward 80%, and wind shear will be dropping to or below 10 knots. All of these factors point toward Meranti becoming a formidable typhoon.


Figure 1. Enhanced infrared image from the Himiwari-8 satellite of Typhoon Meranti at 1500Z (11:00 am EDT) Sunday, September 11, 2016. Image credit: NOAA/NESDIS.

The outlook for Meranti
JTWC predicts that rapid intensification may continue over the next 24 - 48 hours, with an expected Category 4 strength of at least 125 knots (145 mph) by the time of Meranti’s projected landfall in Taiwan on Wednesday local time. On average, Taiwan gets a landfalling typhoon this strong about once per year, with 14 such landfalls occurring between 2000 and 2015. The 00Z Sunday runs of the GFS, European, and UKMET models all predict that Meranti will be a major typhoon passing across or near southern Taiwan, with the UKMET keeping Meranti just offshore. Although Taiwan’s largest city, Taipei, is located near the north tip of the island, the second-largest city, Kaohsiung, is in the far southwest. Even in this well-prepared nation, a major typhoon can cause significant damage and loss of life. Meranti should recurve along or near the coast of eastern China later in the week, perhaps dumping 10” or more of rain as far north as Shanghai and across parts of Japan.

East Pacific: Orlene gathers strength
The Northeast Pacific got its 16th tropical depression and 15th tropical storm of the year this weekend with the christening of Tropical Storm Orlene. This is the earliest occurrence of a year’s 16th tropical cyclone (including depressions) in the East Pacific since the frenetically busy 1992 season, which used the same list of names as 2016. In that year, Hurricane Orlene became a tropical storm on September 3. Nine additional named storms followed, culminating in Tropical Storm Zeke on October 26.


Figure 2. Latest infrared satellite image of Tropical Storm Orlene.

This year’s Orlene is off to a healthy start, with a solid convective core and top winds up to 50 mph as of the 11 am EDT Sunday advisory from NHC. Located well out to sea--nearly 700 miles southwest of Cabo San Lucas, Mexico--Orlene is in an environment primed for rapid growth. The 12Z Sunday SHIPS model shows very low wind shear (5 - 10 knots), warm SSTs of 28-29°C (82-86°F), and a reasonably moist middle atmosphere (relative humidity of 55-60%). Working against Orlene will be cooler waters lurking just below the surface, which are increasingly likely to be churned up as Orlene’s northwestward motion slows to a crawl by Tuesday. The SHIPS model gives Orlene a 44% chance of gaining at least 25 knots of strength by Monday morning, and a 39% chance of gaining 45 knots of intensity by Monday evening. The latter would push Orlene into the Category 2 bracket. A strengthening upper-level ridge should push Orlene westward after its midweek stall, nudging it away from land and toward cooler waters.


Figure 3. Visible satellite image of Invests 92L, 93L, and 94L as of 1615Z (12:15 pm EDT) Sunday, September 22, 2016. Image credit: NASA/MSFC Earth Science Office.

Atlantic: Three invests all in a row
Three tropical waves continue to vie for attention in the North Atlantic, although only the easternmost one shows any immediate sign of development. Invest 94L features a large circulation, with access to very warm SSTs (around 28°C) and a moist atmosphere. However, 94L’s convection has waxed and waned, and much of it is strung along a north-south axis rather than consolidating around a center. NHC gives 94L a 70% chance of developing into at least a tropical depression by Tuesday and a 80% chance by Friday. Its window of opportunity may close long before then, as the 12Z Sunday SHIPS model shows wind shear increasing to 20-30 knots by Monday, with significant mid-level drying. Our leading models for hurricane genesis, the UKMET, GFS, and ECMWF, insist that 94L has a very good shot at becoming a tropical depression if not a tropical storm by Monday, though only about half of the ECMWF ensemble members from 00Z Sunday bring 94L to tropical storm strength. I’m highly skeptical that 94L will develop into a named storm; if it does, I suspect it will be a highly sheared, asymmetric system rather than a classic, well-formed tropical cyclone. Upper-level troughing will steer 94L toward the north-northwest, keeping it well out to sea.

The Atlantic’s other two systems of interest are barely clinging to life. In fact, both were downgraded on Sunday morning from their “invest” (investigative area) status, the rating that calls for dedicated track models and other specialized attention from NHC. Across the eastern Bahamas, former Invest 93L remains highly disorganized, with no surface circulation associated with it. Ex-93L should limp west-northwestward across the Bahamas and into Florida over the next several days with some heavy showers and thunderstorms, but no major models are making it even a minimal tropical storm. In the southeast Gulf of Mexico, just north of western Cuba, former Invest 92L is even more poorly organized, with very little convection evident on satellite. Computer models agree that ex-92L is likely to dissipate in the open Gulf over the next day or so. NHC gives both systems a near-zero chance of development through the next five days.

We’ll be back with our next update by midday Monday. Happy birthday, Dr. Masters!

Bob Henson