Watching Kirk, Leslie, and 98L in the Atlantic, Trami in the Pacific

There is plenty of action in the Atlantic to discuss today, though nothing appears to be a threat of getting its name retired.

The most notable threat was Tropical Storm Kirk, which was speeding to the west at 21 mph towards the Lesser Antilles Islands as a minimal-strength tropical storm with 40 mph winds. Satellite images on Sunday afternoon showed that Kirk had only a moderate amount of heavy thunderstorms, which were clustered on the west side of the center of circulation, but the storm did appear to be growing more organized. Upper-level outflow was apparent on the north side, and Kirk had one decent-looking low-level spiral band on its northwest side, with another trying to develop on its northeast side. Wind shear was light, 5 – 10 knots, and the sea surface temperatures (SSTs) were adequate for strengthening, near 27°C (81°F).

Kirk's future: Will the Caribbean force field hold?

The 12Z Sunday run of the SHIPS model predicted that SSTs along Kirk's path would steadily increase to a warm 29°C (84°F) by Tuesday. During this time, the atmosphere was predicted to stay moist, and wind shear remain light. These conditions should allow Kirk to strengthen, though development may be limited by its rather low latitude (near 9°N), and rapid forward speed. By Wednesday, Kirk will begin to encounter the ever-present wall of high wind shear that has done in so many other tropical cyclones that have approached the Lesser Antilles Islands this hurricane season. The latest example is Tropical Depression Eleven, which was torn apart by high wind shear and dry air on Sunday, about 350 miles east-northeast of the Leeward Islands. If the 0Z and 6Z Sunday runs of the GFS, European, HWRF, and HMON models are correct, high wind shear and dry air should be able to destroy Kirk’s circulation just before or soon after it arrives in the Lesser Antilles Islands on Thursday. However, Kirk may still be capable of trouble in this scenario, as the storm could pack tropical storm-force winds and dump heavy rains of 3 – 6” in the islands, causing flash floods and landslides.

None of the 50 ensemble members from 0Z Sunday run of the European model predicted that Kirk would attain hurricane strength, and all but one predicted Kirk’s demise in the eastern Caribbean, or just east of the Lesser Antilles Islands. About a third of the 20 members of the 0Z Sunday GFS model ensemble predicted Kirk would intensify into a Category 1 hurricane this week, but only 30% of the ensemble members predicted Kirk would survive long enough to affect the Lesser Antilles. Kirk’s expected demise is not shared by two of our top three intensity models, though—both the DSHIPS and LGEM models predicted with their 12Z Sunday runs that Kirk would pass though the Lesser Antilles Islands on Thursday just below hurricane strength, with winds near 70 mph.

Figure 1. Predicted total precipitation for the period 8 am EDT Sunday, September 23, through 2 pm EDT Wednesday, September 26, from the 12Z (8 am EDT) Sunday run of the GFS model. The model predicted that 98L would bring 1 – 2” of rain to eastern North Carolina. Image credit: Levi Cowan, tropicaltidbits.com.

98L could bring 1 – 2” of rain to North Carolina Tuesday through Wednesday

A broad area of low pressure (98L) was located about 300 miles south-southwest of Bermuda on Sunday afternoon. Satellite loops showed that 98L was nearly devoid of heavy thunderstorms, thanks to very dry air with a mid-level relative humidity of 35%. Otherwise, conditions were favorable for development, with moderate wind shear of 10 – 15 knots and warm SSTs near 28.5°C (83°F).

The low was expected to track to the northwest towards North Carolina during the next few days, with the center passing very near Cape Hatteras Tuesday night or Wednesday morning. On this track, 98L could bring unwelcome rains of 1 – 2” to portions of eastern North Carolina ravaged by Hurricane Florence’s extreme rains a week ago. None of our top three models for predicting tropical cyclone genesis—the European, GFS, and UKMET models—predicted with their 0Z or 12Z Sunday runs that 98L would develop into a tropical depression, but development was supported by more than 50% of the 70 members of the GFS and European model ensemble forecasts from 0Z Sunday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 98L 2-day and 5-day odds of development of 20% and 30%, respectively.

Figure 2. Infrared-wavelength image of Subtropical Storm Leslie at 1600Z (noon EDT) Sunday, September 23, 2018. Image credit: NASA/MSFC Earth Science Branch.

Cannibalism in the Atlantic: Newborn Leslie may get consumed by another cyclone

It’s all storms for themselves in the central North Atlantic this week as an unusual pattern plays out. Subtropical Storm Leslie was christened by the National Hurricane Center at 11 am EDT Sunday with top sustained winds of 40 mph. Leslie has a very broad, distinct circulation, but with little in the way of strong showers and thunderstorms (convection) in the relatively dry air around it, except in a band pushed well to the east of Leslie’s center by 10 – 20 knots of westerly wind shear. SSTs are adequately warm for development, in the range of 27-28°C (81–82°F), which is around 1-2°C above average.

Five of this year’s 12 named storms to date originated as subtropical storms: Alberto, Debby, Ernesto, Joyce, and Leslie. All of these except Alberto—plus Tropical Storm Chris—were assigned their names while north of 30°N. (Former Hurricane Beryl also redeveloped as a subtropical cyclone on July 14 in the same latitude range, near Bermuda.) To get 5 of 12 named storms forming north of 30°N is an impressive feat, and a sign of the exceptional warmth of the midlatitude North Atlantic this year.

Figure 3. Weekly departures from average (degress C) in sea surface temperature (SST) across the North Atlantic as of September 15, 2018. The unusual warmth north of 30°N has persisted throughout hurricane season. Image credit: NOAA/NWS/NHC.

Debby, Ernesto, and Joyce all ended up taking on enough tropical characteristics to be reclassified as tropical storms. Leslie may not have enough time to make such a leap, although the NHC isn’t ruling this out should more convection develop near Leslie's center. The storm only has a couple of days to linger at minimal strength before an approaching midlatitude trough carves out a new, stronger low just to its north. The new low will eventually get cut off from the jet stream and is expected to absorb Leslie, becoming a powerful storm of its own. The NHC gives this system a 30% chance of becoming a tropical or subtropical cyclone by Friday. It could meander for a week or more (perhaps as a named system) in the vicinity of 30-40°N and 40-50°W, as recurrent zones of high pressure develop to its northeast and prevent a quick recurvature.

Figure 4. Infrared image of Typhoon Trami as of 1435Z (10:35 am EDT) Sunday, September 23, 2018. Image credit: RAMMB/CSU/CIRA.

Heads up, Asia: Trami could be the next super typhoon

Residents of Taiwan, Japan, and eastern China need to keep an eye on Typhoon Trami, which is rapidly gathering strength in the Northwest Pacific. Trami intensified on Sunday even more quickly than models had predicted. The typhoon was already a Category 4 equivalent with top sustained winds of 130 mph at 8 am EDT Sunday, according to the Joint Typhoon Warning Center. Satellite images showed a very well-organized typhoon with a central shield of intense thunderstorms and a sharpening 8-mile-wide eye, surrounded by a larger eyewall ring that will soon be taking over through an eyewall replacement cycle.

Over the next several days, wind shear affecting Trami will remain below 10 knots, and the typhoon will traverse SSTs of around 29°C (84°F)  amid a moist atmosphere. Thus, there is every chance Trami will reach super typhoon status (1-minute sustained winds of 150 mph) as soon as Monday, and it may become the equivalent of a Category 5 hurricane, as predicted by JTWC.

Trami will remain on a straightforward west-northwest course for the next couple of days, but there is some uncertainty in the longer-term track forecast. A weak dip in the midlatitude jet stream will try (unsuccessfully, it now appears) to recurve Trami to the northwest around Wednesday. Trami’s motion may become slower and a bit erratic later in the week before a more sustained recurvature kicks in around next weekend. It’s possible Trami will approach Taiwan late in the week, but based on the GFS and European models, the typhoon may linger across Japan’s Ryukyu Islands, then carry out a northeastward sweep across or near central Japan about a week from now. By the time it reaches Taiwan or Japan, it’s unlikely Trami would be a super typhoon, but it could still be a formidable, dangerous storm.

Bob Henson co-wrote this post.