Grace on the Decline; 92L Forms near Bermuda

As we approach the peak of hurricane season, 2015 continues to underwhelm in terms of tropical cyclone intensity across the Atlantic, though we’re a bit ahead of schedule on the number of named storms. From 1966 to 2009, the median date for the seventh named storm was September 16. This year’s #7 system was downgraded to Tropical Depression Grace with the 11 am EDT advisory from the National Hurricane Center. Grace saw a modest flare-up of convection on Monday night, but by Tuesday morning, showers and thunderstorms were again on the wane around Grace’s center, struggling against dry mid-level air and persistent wind shear. At 11 am EDT Tuesday, Grace was located more than 1200 east of the Lesser Antilles, moving west at about 20 mph. Grace’s top sustained winds were down to 35 mph, and with conditions even more hostile down the road, Grace should degenerate into a post-tropical low or a tropical wave by Thursday, if not sooner.


Figure 1. Invest 92L (top) is healthier-looking than Tropical Depression Grace (bottom) in this water-vapor image from the GOES-East satellite, taken at 1445 GMT (10:45 am EDT) on Tuesday, September 8, 2015.

Convection was much more robust on Tuesday morning around Invest 92L, located about 300 miles east-southeast of Bermuda. Sea-surface temperatures throughout the northwest Atlantic north of 20°N are running 1-2°C above normal, which will provide 98L with ample oceanic fuel. Upper-level flow will remain weak above 98L for the next several days. NHC gives 98L a 40% chance of development over the next five days, and statistical models are in league with the HWRF in bringing Invest 92L to tropical storm strength over the next couple of days. By the weekend, 92L could be clipping southeast Newfoundland on a rapidly recurving trajectory.

Another area to watch is the far western Gulf of Mexico, where the tail end of a cool front may intersect with a weak upper-level low slated to drift westward during the week. Long-range forecasts from the GFS and European models continue to suggest that a tropical cyclone might develop in the far western Gulf over the weekend or early next week. Even if no such system develops, large amounts of deep tropical moisture are likely to be steered into the upper Gulf Coast region, possibly dumping heavy rain.

Linda surges to Category 3 strength
Making the most of a favorable environment that won’t last long, Hurricane Linda rapidly intensified to Category 3 strength on Tuesday morning, becoming the fifth major hurricane of 2015 in the Northeast Pacific. Linda’s top sustained winds were 120 mph in the 11 am EDT Tuesday advisory from NHC. Linda’s strength will be no match for the increasingly cool waters and stable air that lie ahead of the hurricane on its northwestward track, which will run parallel to and about 300 miles west of Baja California.


Figure 2. A visible-wavelength image of Hurricane Linda collected by the GOES-West satellite at 1430 GMT (10:30 am EDT) on Tuesday, September 8, 2015. Image credit: NOAA/NESDIS.

Meanwhile, Tropical Storm Jimena continues on a rare track that’s taking it westward across the Pacific several hundred miles north of Hawaii. Jimena’s upper- and lower-level centers of circulation have been hard to reconcile over the last 24 hours, but the storm has hung on in the face of westerly wind shear of more than 25 mph. “This seems to be the season for stubbornly resilient storms,” mused a forecaster at the Central Pacific Hurricane Center in a discussion of Jimena on Monday night. Jimena may survive for several more days before its remnants become absorbed in a midlatitude front.

Etau approaches Japan; Kilo heads for Siberia
Tropical Storm Etau will bring heavy rains and wind to the Japanese island of Honshu, including Tokyo, as it sweeps through on Tuesday night (Wednesday morning local time). More than 120,000 people have been advised to evacuate. Etau should make landfall near the city of Nagoya, more than 100 miles southwest of Tokyo.


Figure 3. The convective core of Tropical Storm Etau was visible on radar southeast of Japan at 0035 JST Wednesday, September 9 (11:35 am EDT Tuesday). Image credit: Japan Meteorological Agency.

In its 11th day as a hurricane-strength system and its 20th day as a tropical cyclone, Typhoon Kilo continues its prolonged traverse of the North Pacific. Kilo was at minimal typhoon strength on Tuesday morning (sustained 1-minute winds of 75 mph) and was moving to the west-northwest at an increasing pace. Kilo may intensify slightly over the next several days before rapidly recurving toward Siberia’s Kamchatka Peninsula as a weakening post-tropical storm by this weekend. Though an impressively long-lived system, Kilo is destined to fall well short of the record 31 days as a tropical cyclone set by Hurricane/Typhoon John in 1994.

Jeff Masters will be back with our next post on Wednesday.

Wunderblogger Steve Gregory has an update on the remarkable new El Niño numbers in his Tuesday afternoon post.

Bob Henson


Figure 4. Long-lived Typhoon Kilo remains an elegantly structured tropical cyclone, as evident in this enhanced infrared MTSAT satellite image from 1401 GMT (10:01 am EDT) on Tuesday, September 8, 2015. Image credit: Japan Meteorological Agency.