Karl Approaches Bermuda; Trouble in the Caribbean Next Week?

After nearly a week as a lackluster system, Tropical Storm Karl is finally gaining strength as it heads toward a close encounter with Bermuda. As of the 11 am EDT advisory, Karl was located about 250 miles south of Bermuda, moving north at 12 mph. Karl’s top sustained winds were holding at 60 mph, its peak intensity thus far. Karl is continuing its multi-day struggle with vertical wind shear that’s tended to push its showers and thunderstorms (convection) east of its center. On Thursday night, the storm managed to consolidate a healthy core of convection around its center, but Karl remains somewhat asymmetric, with a comma-shaped structure and a large band of convection well to its east.


Figure 1. Latest satellite image for Tropical Storm Karl.

Outlook for Karl
A Hurricane Watch and Tropical Storm Warning are now in effect for Bermuda, and it is not out of the question that Karl will pass near the island as a minimal hurricane. Karl’s motion slowed dramatically on Thursday as the storm began rounding the west edge of a large subtropical upper-level high. The overall track forecast is straightforward: Karl will move north on Friday and begin arcing toward the north-northeast by early Saturday, bringing it near Bermuda by midday Saturday. Karl will then rapidly accelerate northeastward through the rest of the weekend and into early next week. The longitude of Karl’s center on Friday morning was only a few miles west of Hamilton, Bermuda, so it would be difficult for Karl’s center to pass directly over the island if it were to gain any eastward component to its motion as it approaches Bermuda. The 06Z Friday run of the HWRF model is a western outlier, bringing the center of Karl very close to the island on Saturday morning. The 06Z Friday GFS run is somewhat faster and keeps Karl about 50-100 miles southeast of Bermuda, as do the 00Z Friday runs of the UKMET and European models.


Figure 2. WU depiction of National Hurricane Center forecast for Tropical Storm Karl issued Friday morning, September 23, 2016. Bermuda is depicted as the tiny dot just north of the “8 PM Fri” label.

Models agree that Karl’s strength will peak later in the weekend, when it begins to merge with a midlatitude storm system over the North Atlantic while racing northeastward. Karl will most likely be strengthening on Friday night into early Saturday as it nears Bermuda, and NHC takes Karl to minimal hurricane strength by 12Z (8:00 am EDT) Saturday. The worst-case scenario would be for Karl to approach or reach minimal hurricane strength and pass very near Bermuda. The island has experienced many hurricanes over the years and is well equipped to handle a storm of that strength. A more likely scenario, and the one favored by NHC, is that Karl will pass about 50 to 100 miles southeast of Bermuda, keeping the island on the weaker left-hand side of the storm. That would still be close enough to bring tropical storm-force conditions, so Bermuda could experience a brief period late tonight or early Saturday with sustained winds of 40 - 60 mph, including higher gusts, along with very heavy rain that could total 3” - 5”. The island will also be buffeted by high surf and large swells. Karl’s accelerating motion will limit the main period of impact to just a few hours. Karl was already bringing heavy rain showers to Bermuda on Friday morning, as seen on Bermuda radar.

An African tropical wave that could be trouble
A  tropical wave located a few hundred miles west of the coast of Africa and about 350 miles southeast of the Cabo Verde Islands on Friday morning was poorly organized, with only a limited amount of heavy thunderstorm activity and spin. This wave is currently too close to the equator (near 8°N) to be able to leverage the Earth’s spin and acquire enough spin of its own to develop into a tropical depression, and is not likely to develop through this weekend as it heads rapidly west at 20 - 25 mph. However, the tropical wave may move far enough from the equator to be able to develop by early next week, when it reaches a point about halfway between the Lesser Antilles Islands and the coast of Africa. There was increased model support for development of this tropical wave in the Friday morning runs of the models compared to their Thursday morning runs. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Friday runs that this tropical wave would develop into a tropical depression or tropical storm between Monday and Thursday next week. About 60% of the 20 forecasts from the members of the 00Z Friday GFS ensemble showed development, and about 30% of the 50 members of the European model ensemble did so. Troublingly, a considerable number of the ensemble model runs showed this storm becoming a hurricane in the Caribbean. Working against development, at least in the next five days, will be the fast forward speed of the system—tropical waves moving at 20 mph or faster usually have trouble getting organized. 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.  In their 8 am EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 0% and 20%, respectively.





Figure 3. Forecasts out to ten days from the 00Z Friday European model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late next 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.

Tail-end development in the Gulf of Mexico looks unlikely
A cold front will move into Texas on Monday, and potentially stall just offshore of Texas on Tuesday. We’ll need to watch the tail end of this cold front for tropical development if it lingers over the Gulf of Mexico for a few days. However, there is less model support today than yesterday for this occurring, with fewer than 5% of the members of the 00Z Friday GFS and European model ensemble forecasts showing potential tropical development over the Gulf of Mexico next week.

Invest 94E off the Pacific coast of Mexico may develop
In the Eastern Pacific, satellite loops on Friday morning showed that an area of low pressure located about 800 miles south-southwest of the southern tip of the Baja California Peninsula on Friday morning (Invest 94E) was well-organized with plenty of heavy thunderstorm activity, and our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Friday runs that 94E would develop into a tropical storm or tropical depression over the weekend. The GFS model predicted that this storm would hit the southern portion of the Baja Peninsula on Wednesday, while the other two models predicted that the storm would stay well offshore of Mexico’s Baja Peninsula though the next five days. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave this system 2-day and 5-day development odds of 70% and 90%, respectively.


Figure 4. Tropical Storm Megi (center) is gathering strength more than 1000 miles west-southwest of Taiwan (upper left), as shown in this enhanced infrared satellite image collected by Japan’s Himiwari-8 satellite at 1330Z (9:30 am EDT) Friday, September 23, 2016. Image credit: CIMMS/SSEC/University of Wisconsin.

Yet another typhoon threat for Taiwan
Tropical Storm Megi is gaining strength in the Northwest Pacific and will be approaching Taiwan as an intensifying typhoon by early next week. The island has already dealt with the close approaches of Super Typhoon Meranti to its southwest (which killed two residents and left nearly a million without power) and Typhoon Malakas to its northeast, and it’s quite possible that Megi will strike the island head on. As of 15Z Friday (11:00 am EDT), Megi was located about 1100 miles east-southeast of Taipei, Taiwan, moving west-northwest at about 16 mph. Top sustained winds were just 40 mph, but Megi has the potential to become a powerful typhoon. Sea surface temperatures of around 29-30°C (84-86°F) along Megi’s near-term path are at near-record highs--“exceptionally warm,” according to JTWC’s Friday morning update. Wind shear will be very light over the next several days, below 10 knots, and the atmosphere will be fairly moist, with relative humidity of around 70-75% at middle levels of the atmosphere.

Our best track models are in close agreement on an unusually direct path for Megi, continuing west-northwest over the next several days and arriving on or near Taiwan by early Tuesday local time. The 00Z Friday runs of the ECMWF and UKMET models bring a formidable Megi toward south Taiwan, while the 06Z GFS solution is the outlier, aiming a less intense typhoon toward north Taiwan. These are minor differences considering that Megi is still about four days from the island. The Friday morning prediction from JTWC has Megi approaching central Taiwan on Monday night with top sustained winds of at least 105 mph. With a moderate amount of oceanic heat beneath the storm’s path, a period of rapid intensification that would lead to an even stronger Megi can’t be ruled out.

We'll be back with a fresh update by early Saturday afternoon.

Bob Henson and Jeff Masters


Figure 5. WU depiction of Joint Typhoon Warning Center track forecast for Tropical Storm Megi issued on Friday morning, September 23, 2016.