Multiple Threats in the Atlantic, and a Super Typhoon in the Pacific

Storms are being classified and declassified at a snappy pace in the Atlantic, as several weak systems have been fighting off dry air and wind shear. We may yet see one or more of these systems strengthening as the week unfolds—and there is no question about the ferocity of Super Typhoon Trami in the Northwest Pacific (see below).

Figure 1. Saharan Air Layer (SAL) analysis for 11 am EDT September 24, 2018. Dry air (orange colors) was affecting ex-Tropical Storm Kirk, Subtropical Storm Leslie, 98L, and the remains of TD 11. Image credit: University of Wisconsin/CIMSS.

The Caribbean’s invisible wall of protection that has been present during much of the hurricane season of 2018 bashed up Tropical Storm Kirk overnight, reducing the storm to a wave of low pressure without a surface circulation. Kirk entrained some dry air overnight, and this disrupted the storm enough that it could not maintain its closed circulation, due to its fast forward motion to the west at over 20 mph. Kirk’s demise may also have been aided by the passage of a suppressed Kelvin wave, as explained in a tweet by Michael Ventrice of The Weather Company.

Satellite images early Monday afternoon showed that Kirk was growing better organized, with low-level spiral bands rebuilding and a surface circulation re-developing. Wind shear was moderate, near 10 knots, and the sea surface temperatures (SSTs) were very warm, near 29°C (84°F). It would not be a surprise to see Kirk regain tropical storm status later on Monday. In its Tropical Weather Outlook issued at 2 pm EDT Monday, the National Hurricane Center gives Kirk a 50% chance of redevelopment into at least a tropical depression through Friday.

Figure 2. Visible-wavelength satellite image of ex-Tropical Storm Kirk at 12:05 pm EDT Monday, September 24, 2018. Image credit: tropicaltidbits.com.

Kirk's future

The 12Z Monday run of the SHIPS model predicted that SSTs along Kirk's path would remain a warm 29°C (84°F) for the remainder of the week, and the atmosphere moist. Wind shear was predicted to remain moderate, 10 – 15 knots, and these conditions are favorable for development. However, development will be limited through Wednesday by Kirk’s rapid forward speed. On Wednesday, Kirk will begin to slow down, though, potentially allowing it to grow more organized. But by Wednesday night, Kirk will 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. Wind shear should increase even further on Thursday as Kirk (or its remnants) arrive in the Lesser Antilles Islands on Thursday. In short, Kirk is not likely to be a tropical storm on Thursday when it moves through the islands, but it could still pack tropical storm-force winds of 40 mph and dump heavy rains of 2 – 4”.

Figure 3. Predicted total precipitation for the period 8 am EDT Monday, September 24, through 2 pm EDT Wednesday, September 26, from the 6Z (2 am EDT) Monday run of the GFS model. The model predicted that 98L would bring 1 – 2” of rain to parts of eastern North Carolina. Image credit: Levi Cowan, tropicaltidbits.com.

Figure 4. GOES-16 visible satellite image of 98L at 10 am EDT September 24, 2018. The coast of southeastern North Carolina is visible at upper left. Image credit: NOAA/RAMMB.

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

A broad area of low pressure (98L) was located about 500 miles southeast of the coast of North Carolina on Monday morning, and was headed west-northwest at about 10 mph towards the Southeast U.S. coast. Satellite loops showed that 98L had a broad area of rotation but no well-defined surface circulation, and only a modest area of heavy thunderstorms. This convection was located on the north side of the center, where the warmest waters were. Very dry air with a mid-level relative humidity of 40% was inhibiting development, and had caused a noticeable decrease in 98L’s spin on Monday morning. However, light wind shear of 5 knots and warm SSTs near 28.5°C (83°F) were favorable for development. These waters were considerably cooler than they were two weeks ago, thanks to the lingering cold-water wake left behind by the passage of Hurricane Florence.

The low was expected to turn more to the northwest towards North Carolina by Tuesday, with the center passing very near Cape Hatteras Tuesday night or Wednesday morning. On this track, 98L will likely bring unwelcome rains of 1 – 2” to portions of eastern North Carolina affected by Hurricane Florence’s extreme rains a week ago. The rains from 98L over North Carolina will begin on Tuesday afternoon. Fortunately, the heaviest rains will be limited to the Outer Banks, and lighter rains are expected over the region near the NC/SC border where river flooding is at its worst. The prolonged period of onshore winds that will accompany 98L will bring waves of 5 – 8 feet to the coast of North Carolina on Tuesday, with the waves subsiding to 4 – 6 feet on Wednesday.

Figure 5. Natural-color visible satellite image of Subtropical Storm Leslie as of 1520Z (11:20 am EDT) Monday, September 24, 2018. Image credit: tropicaltidbits.com.

Leslie’s future: Intriguing but murky

In its second day as a named system, Subtropical Storm Leslie continued to amble across the remote central North Atlantic, awaiting more dramatic developments to come later this week. Leslie has a weak but sprawling circulation: its top sustained winds of 40 mph extend up to 205 miles out from its center. Showers and thunderstorms have gradually increased in coverage on the east side of Leslie’s circulation, but they remained scant on its west side, a common pattern with subtropical storms.

The big question is how Leslie will interact with an infusion of upper-level energy heading its way from a developing dip in the midlatitude jet stream. Forecast models agree that the result later this week will be a large, intense cyclone cut off from the jet stream and marooned in the central North Atlantic. What remains to be seen is whether a new circulation will develop and engulf Leslie, or whether the infusion of energy will instead go into strengthening Leslie. In either case, the resulting cyclone (with a central pressure predicted to be below 980 mb) will most likely be classified as subtropical or non-tropical because of a sharp increase in wind shear from the infusion of jet-stream-level winds. However, phase-space graphics from Florida State University based on GFS models output indicate that the emerging cyclone will eventually gain a symmetric warm core that strengthens over time, which implies that it could transition into a tropical storm or hurricane.

Water temperatures in the vicinity of Leslie are in the vicinity of 27°C (81°F), which is adequate to support a tropical cyclone if the wind shear decreases. This complex storm—which poses no threat to land for at least the next few days—will be a fascinating one to watch.

CPS diagram assoc. w/ SS #Leslie. Current position (C) indicates Leslie is symmetrical, straddling line between warm & cold core. Leslie is fcst to interact w/ another trough, becoming asymmetric warm-core before undergoing warm seclusion & becoming even more symmetric warm core. pic.twitter.com/EXJptEj9Dh

— Alicia M Bentley (@AliciaMBentley) September 24, 2018

Super Typhoon Trami heading toward Japan’s Ryukyu Islands

UPDATE: Trami was updated to Category 5 status with the 5 pm EDT Monday advisory from the Joint Typhoon Warning Center (JTWC), with top sustained winds of 160 mph. Trami now qualifies as a super typhoon. Trami bolted to Category 4 strength over the weekend, then took a brief pause to reorganize after an eyewall replacement cycle.

Wild #eyewall replacement in Super Typhoon #Trami pic.twitter.com/T21DXEJHlj

— Stu Ostro (@StuOstro) September 24, 2018

Trami is now restrengthening around its new, larger eye, as it traverses very warm waters (29°C or 84°F) in very low wind shear (less than 10 knots) for the next several days. JTWC predicts that Trami will become a Category 5 equivalent by Monday night local time, and it could stay in the Cat 5 range for a couple of days. Watch for some spectacular satellite imagery of this very well-structured storm.

Super Typhoon #Trami in the Northwest Pacific at sunset, still traveling toward #Tawain but with a more northerly forecast track that now includes #Japan's Ryukyu islands. https://t.co/qGdHTcDE3G pic.twitter.com/3eye2D0tPU

— UW-Madison CIMSS (@UWCIMSS) September 24, 2018

A dip in the mid-latitude jet stream will try to pull Trami northward on Tuesday, after which the typhoon should resume a west-northwest or northwest track. There is some model disagreement on how far Trami will get during its northward jog, but it appears the jog will be big enough for Trami to steer clear of Taiwan later this week. The most immediate threat from Trami is to the Ryukyu Islands of southernmost Japan, which include Okinawa island. Unfortunately, long-range runs of the European and GFS model agree that Trami will recurve toward the northeast around this point. Such a track could take Trami over a large number of the Ryukyu Islands by this weekend and across the heart of Japan around Sunday or Monday. Trami should weaken below super typhoon strength by the time it reaches the islands of Japan, but it is projected to remain a dangerous storm.

Post-Florence problems: Some rivers are still rising in South Carolina

Most rivers across North and South Carolina are slowly receding after the extreme rains dumped by Hurricane Florence, although some remain above their previous records. More than a week after Florence, though, the highest waters have yet to arrive along the Waccamaw River in far eastern South Carolina. Unlike most of the region’s rivers, which head southeast from higher elevation into the Atlantic, the Waccamaw is a broad, slow-moving channel that originates northwest of Wilmington, North Carolina, and runs southwestward, parallel to the South Carolina coast, until it reaches Winya Bay near Georgetown.

Water from some of Florence’s heaviest rain is gradually working its way down the Waccamaw, and the Pee Dee River system is funneling additional water into the channel just upstream from Georgetown. Emergency shelters opened on Monday at two high schools in Georgetown County, where thousands of residents received phone calls on Saturday warning that evacuations may be needed.

Catastrophic #flooding is happening right now in the Pee Dee region. SCDOT crews are shoring up major roads as best they can. Now is not the time to be distracted by what roads may be open and what roads may be closed... we don’t know and won’t know until they flood. #sctweets pic.twitter.com/xl3Lrnf2gf

— SCEMD (@SCEMD) September 24, 2018

Figure 6. The Pee Dee River system. Image credit: Karl Musser/Wikimedia Commons.

Conway, a city of about 22,000 along the Waccamaw, was hit hard by floods from Matthew in 2016. That hurricane produced a crest along the Waccamaw of 17.87’ in Conway—the highest in more than a century of recordkeeping. The Waccamaw surpassed its record from Matthew on Friday, and it’s been rising ever since. Already at 20.77’ on Monday morning, the Waccamaw is predicted to crest at around 22’ in Conway on Wednesday before very gradually subsiding.

Figure 7. The Waccamaw River at Conway is predicted to remain above its previous record crest through this week. Image credit: NOAA/NWS/AHPS.

Water-blocking barricades were rising across Conway, according to the Charleston Post and Courier. Nearby, the government-owned Santee Cooper utility was trying to protect a pit that held 200,000 tons of potentially toxic coal ash, a concern in other areas hit by Florence.

One resident of Conway told Myrtle Beach Online, ““They keep saying ‘100-year flood,’ ‘500-year flood,’ ‘1,000-year,’ whatever year it’s on…It’s about every other year now.” Conway’s experience this week serves as a painful reminder that a 100- or even a 500-year flood can strike more often than those numbers suggest—especially in a warming climate where extreme rain events are becoming more extreme.

Dr. Jeff Masters co-wrote this post.

First it was flooded by #Florence. Now, a lingering stink along Interstate 40. https://t.co/uNswHbrert (Photo: Penderlea Fire Department) pic.twitter.com/MnD0Hdi6LQ

— The Weather Channel (@weatherchannel) September 23, 2018