Potential Trouble in Tropical Atlantic; Karl Brushes Bermuda; Megi Eyes Taiwan

By Jeff Masters and Bob Henson
Published: 5:23 PM GMT on September 24, 2016

A tropical wave located a few hundred miles south-southwest of the Cabo Verde Islands on Saturday morning was poorly organized, with only a limited amount of heavy thunderstorm activity and spin. This wave was under low to moderate wind shear of 5 - 15 knots and was over warm ocean waters near 29°C (84°F), but was too close to the equator (near 9°N) to be able to leverage the Earth’s spin and acquire enough spin of its own to develop into a tropical depression. However, the tropical wave may move far enough from the equator to be able to develop by Monday or Tuesday, when it will be approaching the Lesser Antilles Islands. Working against development, at least in the next five days, will be the fast forward speed of the system. The storm will be driven west at 20 - 25 mph by the trade winds associated with an unusually strong Bermuda-Azores High; tropical waves moving at 20 mph or faster usually have trouble achieving the vertical alignment needed to intensify. However, the storm does not have as much dry air to contend with compared to other storms we have seen this year, and it would not be a surprise to see this system be close to tropical depression or tropical storm status when it begins moving into the Lesser Antilles Islands on Tuesday night or Wednesday morning. Once the storm enters the eastern Caribbean, long-range model runs suggest that the system might be very close to the coast of South America, which would interfere with development.


Figure 1. Satellite image of the tropical wave (left) located southwest of the Cabo Verde Islands at 1445Z (10:45 am EDT) Saturday, September 24, 2016. Image credit: NASA/MSFC Earth Science Office.

There was increased model support for development of this tropical wave in the Saturday morning runs of the models compared to their Friday runs. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Saturday runs that this tropical wave would develop into a tropical depression or tropical storm between Monday and Thursday. About 80% of the 20 forecasts from the members of the 00Z Saturday GFS ensemble showed development into a tropical storm, with 40% predicting a hurricane. The European model ensemble was less aggressive developing the storm, probably because of a predicted track too close to the coast of South America—about 30% of its 50 ensemble members predicted a tropical storm in the Caribbean. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 10% and 50%, respectively. The Hurricane Hunters are scheduled to make their first flight into the storm on Tuesday afternoon.





Figure 2. Forecasts out to ten days from the 00Z Saturday European model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots). The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future.


Figure 3. Enhanced infrared image of Tropical Storm Karl as of 1545Z (11:45 am EDT) Saturday, September 24, 2016. Image credit: NOAA/NESDIS.

Karl brushes Bermuda
Tropical Storm Warnings were lifted for Bermuda on Saturday morning after Tropical Storm Karl brought a night of heavy rains and squalls to the island. At 11 am EDT, Karl was located about 125 miles east-northeast of Bermuda, packing top sustained winds of 65 mph and moving northeast at 18 mph. Earlier on Saturday morning, a Hurricane Hunter flight measured top flight-level winds of 63 knots (73 mph) and detected peak near-surface winds of 47 knots (54 mph) with the SFMR instrument (stepped frequency microwave radiometer).

Karl’s core of showers and thunderstorms, already east of Bermuda, reintensified late Saturday morning after some fragmentation earlier in the day. Strong upper-level winds associated with a large upper-level low in the North Atlantic were accelerating Karl to the northeast. Karl could become the fifth hurricane of the 2016 Atlantic season on Saturday night or Sunday morning before it evolves into a powerful post-tropical storm in the North Atlantic, eventually getting swept up into the massive upper low east of the Canadian Maritimes.

Karl’s recurving path brough the storm’s weaker left-hand side to within about 50 miles of Bermuda. Sustained winds topped out at a mere 29 mph Saturday morning at Bermuda International Airport, with gusts reaching 43 mph. Heavy rainbands doused the island, though, as the airport racked up more than 4” from Friday afternoon to Saturday morning.

Lisa is a tropical storm again
After decaying to tropical depression status late Friday, Tropical Storm Lisa got a new lease on life Saturday morning, regaining its tropical storm strength thanks to a burst of convection that developed atop its low-level center. Those storms have now weakened and blown eastward in strong wind shear of 30-40 knots, leaving the center again exposed (see satellite loop at bottom], so Lisa's resurgence will be brief. The NHC outlook brings Lisa's peak winds back below the tropical storm threshold by late Saturday afternoon, and Lisa should be a post-tropical low by Sunday or Monday.

Invest 94E off the Pacific coast of Mexico may develop
In the Eastern Pacific, satellite loops on Saturday morning showed that an area of low pressure located about 900 miles southwest of the southern tip of the Baja California Peninsula on Saturday morning (Invest 94E) was well-organized with plenty of heavy thunderstorm activity. One of our top three models for predicting hurricane genesis—the GFS model—predicted in its 00Z Saturday run that 94E would develop into a tropical storm or tropical depression over the weekend, and remain offshore of Mexico’s Baja Peninsula though the next five days. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 80% and 90%, respectively.


Figure 4. Enhanced satellite image of Typhoon Megi as of 1641Z (12:41 pm EDT) Saturday, September 24, 2016.

Typhoon Megi taking aim on Taiwan
The last thing Taiwan needs is another approaching typhoon, but that’s exactly what is on the table with intensifying Typhoon Megi. With sustained winds at 65 mph as of 12Z (8:00 am EDT) Saturday, Megi was already a large and well-structured typhoon, with excellent outflow at upper levels helping to nourish its growth. Located about 900 miles east-southeast of Taipei, Megi was heading west-northwest at about 16 mph.

Models are in unusually close agreement on Megi’s continuing a remarkably straight west-northwest course, which would bring it to the vicinity of Taiwan by late Tuesday local time. That will give Megi plenty of time to strengthen, with unusually warm sea-surface temperatures of 29-30°C (84-86°F), a moist atmosphere (relative humidities around 70-75%) and low wind shear of 5 - 15 knots expected along Megi’s path over the next couple of days. The Joint Typhoon Warning Center projects that Megi will approach Taiwan as a Category 3 typhoon, with peak winds of 120 mph. There is a distinct possibility that Megi could strengthen more than predicted. The 00Z Saturday runs of the GFS, European, and UKMET models all bring Megi across southern or central Taiwan. The island has already dealt with the close approaches of Super Typhoon Meranti just to its southwest (which killed two residents and left nearly a million without power) and Typhoon Malakas just to its northeast. Only a few weeks earlier, Typhoon Nepartak hit Taiwan on July 7 as a Category 4 super typhoon with top sustained winds of 150 mph. Taiwan averages 3 to 4 typhoon strikes per year, according to the Central Weather Bureau.

On its predicted course, Megi would make a second landfall along the coast of southeast China, not far south of where Meranti claimed at least 29 lives and caused at least $2.6 billion in damage.

We’ll be back with our next update by early Sunday afternoon.

Jeff Masters and Bob Henson


Figure 5. WU depiction of Joint Typhoon Warning Center track forecast for Tropical Storm Megi issued on Saturday morning, September 24, 2016.



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About The Author
Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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