PTC 6 Still Likely to Become Florence; Super Typhoon Jebi Eyes Japan

A strong tropical wave that emerged from the coast of Africa on Thursday morning, dubbed Potential Tropical Cyclone Six (PTC 6) by the National Hurricane Center, is likely to develop into Tropical Storm Florence by Friday night or Saturday morning. Favoring development of PTC 6 are warm sea surface temperatures (SSTs) near 28°C (82°F), and the fact that the dry air and dust of the Saharan Air Layer (SAL) lies well to the north of the tropical wave. Wind shear over the eastern tropical Atlantic is a moderate 10 – 15 knots, favoring development. Satellite images on Friday morning showed that PTC 6 had developed a well-defined surface circulation, but the associated heavy thunderstorm activity was still too disorganized for this system to be classified as a tropical cyclone.

PTC may bring tropical storm conditions to the southern Cabo Verde Islands on Friday evening and Saturday, which prompted NHC to begin issuing advisories on the system before it earned the name Florence. PTC 6 is likely to bring rainfall amounts of 2 - 4” to the southern Cabo Verde Islands through Sunday; some of the islands may also experience winds of minimal tropical storm force (40 – 45 mph) at the height of the storm. The European, GFS, and UKMET models all show development of PTC 6 into a tropical storm by Friday evening, and in their 11 am EDT Friday advisory, NHC gave the system 2-day and 5-day odds of development of 90%. Steering currents favor a west-northwesterly to northwesterly track over the next week, and PTC 6 is very unlikely to affect the Caribbean.

Figure 1. A tropical wave centered near Hispaniola, as seen at 9:30 am EDT August 31, 2018. Image credit: NOAA/RAMMB.

Tropical wave near Hispaniola has a low chance of developing in the Gulf of Mexico next week

A tropical wave located on Friday near Hispaniola was headed west to west-northwest at 10 – 15 mph. This system has a low potential to develop into a tropical depression next week in the Gulf of Mexico. Satellite images and Puerto Rico radar on Friday showed that the wave was generating a moderate amount of heavy thunderstorms over Hispaniola, western Puerto Rico, the southeast Bahamas and the surrounding waters, but the system had very little spin. Much of Puerto Rico and the Virgin Islands received over an inch of rain from the system since Thursday, with a few reports of 4+ inches. High wind shear of 15 - 25 knots was keeping the system’s heavy thunderstorm activity disorganized.

Yesterday's Rainfall - Preliminary Totals #prwx #usviwx pic.twitter.com/DERDnSgsGJ

— NWS San Juan (@NWSSanJuan) August 31, 2018

The wave is expected to continue to battle high shear through Sunday as it moves over Hispaniola, eastern Cuba and The Bahamas, and development is unlikely during this period. On Monday, the wave is expected to enter the eastern Gulf of Mexico, where a modest drop in wind shear may allow the wave to organize into a tropical depression later in the week. None of the operational versions of our top three models for predicting tropical cyclone genesis--the European, GFS, and UKMET models--predicted in their 0Z Friday runs that this wave would develop into a tropical depression. About 30 – 40% of the members of the 0Z Friday runs of the European and GFS model ensemble forecasts did support development, though. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 0% and 10%, respectively.
 

Figure 2. Infrared satellite image of Super Typhoon Jebi at 1510Z (11:10 am EDT) Friday, August 31, 2018. Image credit: RAMMB/CIRA/CSU.

Central Japan in the crosshairs of Super Typhoon Jebi

The odds are rising that central Japan will get a major hit early next week from Earth’s strongest tropical cyclone of the year thus far. Super Typhoon Jebi, a Category 5 equivalent, was packing top sustained winds of 175 mph as of 8 am EDT Friday, according to the Joint Typhoon Warning Center, with a central pressure estimated by the Japan Meteorological Agency at 915 millibars as of 11 am EDT Friday.

Jebi rapidly intensified on Thursday local time atop very warm SSTs of around 29°C (84°F) and deep oceanic heat content (see Figure 3 below), with wind shear on the light side (5 – 10 knots) and very healthy outflow at upper levels. These conditions will prevail through at least Sunday. The main factor affecting Jebi’s strength over the next couple of days will be one or more hard-to-predict eyewall replacement cycles (ERCs), each of which could dent Jebi’s intensity slightly for a day or so. One ERC appeared to be already under way on Friday. In addition, Jebi’s upper-level outflow may become more constricted by Sunday and Monday.

Check out Typhoon #Jebi in the Pacific from #Himawari - wow. May the force be with it pic.twitter.com/Q8D9zCpcTU

— Dan Lindsey (@DanLindsey77) August 31, 2018

Jebi is expected to carry out a classically recurving path that would take it to the coast of Japan early next week. It appears that Jebi will likely have enough of a northward component of motion to ensure a landfall on Honshu, Japan’s largest and most populous island. Some weakening can be expected as Jebi nears landfall, but the 06Z Friday runs of the GFS and HWRF models bring Jebi onshore as a Category 2 or 3 storm near Tokushima and Wakayama prefectures on Tuesday local time. The 00Z European model had a roughly similar scenario. A landfall in this vicinity could put Osaka and Kyoto near the storm’s center or in its front right quadrant, where the strongest winds typically occur. Strong winds could easily extend east into the Tokyo area if Jebi continues angling toward the northeast across Honshu after landfall.

Along with the risk of hurricane-force winds, widespread torrential rains of 5” – 10”, with localized totals of 10” – 20” or more, can be expected across central Honshu, perhaps extending as far east as the Tokyo area. Even though Japan is very typhoon-experienced, a landfall from Jebi could have major impacts. Only a slight eastward trend in the landfall location would raise concerns for the Tokyo area significantly.

Figure 3. Oceanic heat content will remain fairly high (greater than 50 kilojoules per square centimeter) along Jebi’s path through Sunday evening local time, the 48-hour position shown on this outlook. Jebi will encounter less heat content from that point until reaching Japan, although the surface waters will remain quite warm. Jebi passed over a zone of very high oceanic heat content (75-150 kilojoules per sq cm) on Thursday, fueling its rapid intensification. Image credit: RAMMB/CIRA/CSU.

Miriam and Norman continue to churn across Central and Eastern Pacific

Two hurricanes spinning between Hawaii and Mexico on Friday continued to push up the score on a remarkable August in the Eastern Pacific. The month’s place was already secure as the most active month on record between the International Date Line and the Americas for accumulated cyclone energy (ACE), according to Phil Klotzbach (Colorado State University). The ACE index incorporates tropical cyclone strength as well as longevity. Reliable records in the Northeast Pacific extend back through the satellite era to 1971. Through early Friday, according to CSU, the hyperactive hurricane season of 2018 had produced 160.7 units of ACE east of the Date Line, compared to an average at this point of 74.7. With the Eastern Pacific hurricane season only about 60% over, we've already seen 14 named storms, 8 hurricanes, and 5 intense hurricanes. An average Eastern Pacific hurricane season has 15 named storms, 8 hurricanes, and 3 intense hurricanes for the entire year.

Hurricane Miriam is staying well clear of Hawaii, although it intensified slightly overnight. Miriam’s top sustained winds were at 90 mph as of 11 am EDT Friday. The hurricane was located about 910 miles east of Hilo, moving north at 10 mph. Wind shear and cooler SSTs will take an increasing toll on Miriam over the next several days, and the hurricane will likely be a tropical storm by Saturday and a post-tropical low by Monday. As the shear rips the upper part of the storm away from its low-level circulation, the center will tack toward the northwest, still remaining far from Hawaii.

Figure 4. Infrared satellite image of Hurricane Norman (left) and a disturbance likely to become the next named storm across the Northeast Pacific (right) as of 1600Z (noon EDT) Friday, August 31, 2018. Image credit: NASA/MSFC Earth Science Branch.

On the heels of Miriam is Hurricane Norman, which peaked just short of Category 5 strength on Thursday. As of 11 am Friday, top sustained winds were down to 130 mph, but Norman remained a powerful hurricane at the low end of the Category 4 scale. Located about 825 miles west-southwest of Cabo San Lucas, Norman was moving toward the west-southwest at 8 mph, keeping it over warm waters of around 28-29°C (82-84°F). Wind shear of around 20 knots was pushing the bulk of Norman’s showers and thunderstorms southwest of its center on Friday, and it appears Norman is also going through an eyewall replacement cycle. Shear is predicted to relax by Sunday, but Norman will be moving over progressively cooler waters from then onward, so we’re most likely to see a gradual decline in Norman’s strength from here on out. Norman’s path will angle toward the west-northwest over time, heading toward the weakness in the steering currents close to Miriam’s current location. This track will likely keep Norman from affecting Hawaii directly.

In its Tropical Weather Outlook issued at 8 am EDT Friday, NHC gave 70% odds that a disturbance labeled Invest 91E, located several hundred miles southwest of Manzanillo, Mexico, would become at least a tropical depression by Monday. Models agree that 91E will become a tropical storm and perhaps a hurricane as it rolls toward the west away from Mexico. The HWRF is especially bullish, projecting that 91E will be a major Category 3 hurricane by Monday.

Dr. Jeff Masters wrote the Atlantic section of this post.