TD 8 Forms in Atlantic; Still Watching 99L; Hawaii in Sights of TS Madeline

We’re now approaching the early-September climatological peak of the Atlantic hurricane season, and this week shows how active things can get. We may see up to three newly named storms between now and Labor Day in the Atlantic. Each of these could eventually affect the United States in some form or fashion. Another named storm could sweep over Hawaii in the next week--and Japan is bracing for its third tropical cyclone in a week.

TD 8 may swing past the Outer Banks as a tropical storm
In contrast to the prolonged saga of Invest 99L (see below), Tropical Depression 8 didn’t waste any time becoming a tropical cyclone. An impressive shield of showers and thunderstorms (convection) blossomed on Saturday night, and wind data from an airborne scatterometer (see Figure 4 below) showed that the system had a closed circulation. National Hurricane Center upgraded the system to TD 8 at 11 am EDT Sunday.

TD 8 was located about 400 miles southeast of Cape Hatteras, North Carolina, moving west at 9 mph. Moderate vertical wind shear from the southeast (15-20 knots) has pushed the convection toward northwest side of the low-level center of circulation, as shown in Figure 3 below. An Air Force Hurricane Hunter flight will investigate TD 8 on Sunday afternoon. TD 8 has some kinship to Tropical Storm Fiona: NHC notes that the latter’s remnants were absorbed into a larger circulation that ended up giving birth to TD 8.


Figure 1. Visible satellite image of Tropical Depression 8 as of 1508Z (11:08 am EDT)
Sunday, August 28, 2016. The low-level circulation is evident toward the southeast end of TD 8’s shield of showers and thunderstorms.


Figure 2. Infrared image of Tropical Depression 8 as of 1645Z (12:45 pm EDT) Sunday, August 28, 2016.


Figure 3. The Advanced Scatterometer (ASCAT) aboard the European MetOp-1 satellite detected the circulation around TD 8 (bottom of image) early Sunday, August 28, 2016. Winds of 20-30 mph were evident on the depression’s north side. Image credit: NOAA/NESDIS.

The outlook for TD 8
TD 8 is tucked into the south side of a compact upper-level ridge extending from the mid-Atlantic states into the northwest Atlantic. Computer models are in close agreement on a two-phase track that would bring TD 8 northwest to within several hundred miles of the Outer Banks of North Carolina by Tuesday before the ridge weakens and a sharp right-hand turn takes TD 8 out to sea. A tropical storm watch may be issued for the Outer Banks later Sunday, according to NHC. Tropical storm conditions could affect the Outer Banks even if TD 8 does not make landfall.

NHC projects TD 8 to become Tropical Storm Hermine by late Monday but keeps it at minimal strength (40 mph) thereafter. Wind shear should remain light to moderate (10-20 knots) for the next couple of days, and sea-surface temperatures (SSTs) will remain close to 29°C (84°F), roughly 1-2°C above average. Dry air (relative humidities of only around 40-50%) will be the main impediment to TD 8’s development, with some gradual moistening predicted over the next 3-4 days. By the time it makes its sharp right turn out to sea, TD 8 will be flayed by increasingly strong wind shear.


Figure 4. WU depiction of National Hurricane Center forecast for Tropical Depression 8 as of 11 am EDT Sunday, August 28, 2016.

99L remains a threat to the eastern Gulf of Mexico
The persistent tropical wave known as Invest 99L may yet become a tropical cyclone to contend with, although you might not guess it from satellite imagery. Once again, 99L appears less impressive in real life than some well-regarded computer models had predicted several days ago. On Sunday morning, 99L was grinding its way west-northwest toward the Florida Straits, with a weakly defined center located midway between South Florida and the coast of Cuba. This center was growing more defined early Sunday afternoon, though, with some rotation of the storm's shower activity apparent on Key West radar and some spiral banding near the center beginning to appear. Satellite loops showed that 99L's heavy thunderstorms were relatively modest in areal coverage and intensity, but with some increase in organization in the past few hours. 99L will pass across the Florida Keys late Sunday, bringing some heavy thunderstorms, gusty winds, and moderately high surf (up to 4-6 feet in the Florida Straits). The disturbed weather over the Keys and South Florida should extend into Monday and perhaps Tuesday, even though the center of 99L will have moved into the southeast Gulf of Mexico by that point.

After nearly a week of attention heaped on 99L--perhaps the most intensely covered “invest” in history--it may be tempting to let this system go, but now is not the time for complacency. Throughout the past week, computer models have leaned toward the Gulf of Mexico as the most likely spot for 99L to intensify, although the GFS model has been the most consistently skeptical. Now all three of our best models for tropical cyclone genesis--the GFS, ECMWF, and UKMET--develop 99L into at least a tropical storm as it makes its way through the Gulf over the next several days. The general tendency in these models is to keep 99L rolling west-northwest into the southeast Gulf for another couple of days, then to hook it fairly sharply by midweek to the north and northeast toward Florida’s Gulf Coast. Nearly all of the 50 members of the 00Z Sunday ECMWF ensemble run follow this scenario, with only a couple of runs taking 99L as far west as the Mississippi or far southeast Louisiana coast. The GFS ensemble track forecast from 00Z Sunday is quite similar, though about a day slower. It’s too soon to know which day 99L would make landfall; assuming it does, it could be anywhere in the latter half of the coming week.


Figure 5. MODIS visible satellite image of Tropical Depression Nine (formerly 99L) forming in the Florida Straits on Sunday afternoon, August 28, 2016. Image credit: NASA.

How strong could 99L get?
The major models have also converged on their strength outlooks for 99L. About 50-60% of the ECMWF and GFS ensemble members members bring 99L up to tropical storm strength over the Gulf. Fortunately, only about 10-20% of ECMWF ensemble runs, and none of the GFS ensemble runs, make 99L a Category 1 hurricane. The HWRF model has been consistently stronger--but although the HWRF is among the best intensity models for tropical storms and hurricanes that have already formed, it has a well-known extreme high bias on systems that have not yet developed, so it’s best to ignore HWRF intensity predictions at this point.

Conditions in the Gulf of Mexico will be moderately supportive of development for 99L. Wind shear will be dropping to the light range by Tuesday (5-15 knots). The atmosphere will be fairly moist (relative humidities should hold in the 65-75% range), and 99L will be passing over very warm sea-surface temperatures (SSTs) of around 30°C or 86°F. These warm waters do not extend very far down, however, so the amount of oceanic heat along 99L’s expected path in the eastern Gulf is not particularly large. Also, because 99L is entering the Gulf as a large, disheveled system, we can expect development to be slower than it might be for a smaller, better-organized circulation. In its 8 am EDT Tropical Weather Discussion, NHC gives 99L a 40% chance of development into at least a tropical depression by early Tuesday and a 60% chance by early Friday.

If 99L does become a tropical storm (which would be named Ian), it could have as little as 24-48 hours to organize further before making landfall, which reduces the odds of a significant hurricane. Nevertheless, 99L’s size gives it the potential to become a large-scale rainmaker. We can expect 4” - 8” amounts to be widespread along the Florida Gulf Coast and far South Florida over the next several days (see Figure 6). Moreover, even if it doesn’t make hurricane strength, 99L’s large size could enhance its ability to push water onshore and produce more coastal flooding than one might expect from a typical tropical storm.


Figure 6. Projected 5-day rainfall from 12Z (8:00 am EDT) Sunday, August 28, through 12Z Friday, September 2, 2016. The forecast assumes that 99L will make its way northward through the eastern Gulf of Mexico. Image credit: NOAA/NWS Weather Prediction Center.


Figure 7. Enhanced infrared image of Hurricane Gaston as of 1615Z (12:15 pm EDT) Sunday, August 28, 2016. Image credit: NOAA/NESDIS.

Category 2 Gaston spins through open Atlantic
Though it’s a powerful storm, with top sustained winds of 105 mph, Hurricane Gaston is a well-behaved storm in terms of impact. Located about 600 miles east of Bermuda as of 11 am EDT Sunday, Gaston is creeping northwest at just 5 mph en route to a very gradual recurvature. Gaston should be able to take advantage of favorable atmospheric conditions and unusually warm SSTs to gain a bit more strength before it begins accelerating east-northeast by midweek. By Friday or Saturday, Gaston could be passing just north of the Azores as a minimal hurricane, based on the latest NHC outlook.

An African wave worth watching
A powerful tropical wave will move off Africa this week and begin a long and potentially eventful trek across the tropical Atlantic. NHC gives the wave a 60% chance of development into at least a depression between Tuesday and Friday. The GFS, ECMWF, and UKMET models agree that this wave could be a significant tropical cyclone by next weekend. The wave should remain on a fairly straightforward west to west-northwest path through the period.

At least one landfall may be in the cards for Hawaii
Unusually high SSTs associated with El Niño and long-term warming have given Hawaii more than its historical share of tropical storm action in recent years. Two tropical cyclones moving from east to west became the first since Hawaii's statehood (1959] to make landfall in the Big Island: Tropical Storm Iselle (August 2014) and Tropical Storm Darby (July 2016). In addition, an unnamed tropical storm hit the Big Island in 1958. Now we have one tropical cyclone heading toward Hawaii--and another on its heels that could make a swipe at the islands just days later.

Tropical Storm Madeline continues on a course that is making a Big Island landfall increasingly plausible. Moving northwest at about 7 mph, Madeline was located about 1000 miles east-southeast of Hilo, HI, as of the 11 am EDT Sunday advisory from the NOAA/NWS Central Pacific Hurricane Center in Honolulu. Madeline is fairly small but well-structured, with top sustained winds of 50 mph. The storm is now over waters that are 1-2°C above average, around 28°C, and wind shear will remain very light (less than 10 knots) for the next couple of days. Working against strengthening will be fairly dry air, as mid-level relative humidities around 60-70% now will drop to around 40-50% by midweek. In addition, SSTs along its path will be dropping from around 28°C on Sunday to around 26-27°C by Tuesday, though they should increase later in the week. CPHC brings Madeline to minimal hurricane strength on Tuesday, with a slow decline as the rest of the week unfolds.


Figure 7. Enhanced infrared image of Tropical Storm Madeline (left) and Hurricane Lester (right) as of 1500Z (11:00 am EDT) Sunday, August 28, 2016. Image credit: NOAA/NESDIS.

Madeline is currently angling toward a weakness in the strong ridging to its north. By midweek, the ridge will re-strengthen, and Madeline should take a significant left turn, putting it on a course bending west and then west-southwest toward Hawaii. The 11 am EDT Sunday outlook from CPHC places Madeline near the south end of the Big Island by early Thursday. The average track error in a four-day forecast in this region is 185 miles, so it is too soon to know whether Madeline will actually strike Hawaii or which island(s) it might affect. It would most likely be a tropical storm by this point. If Madeline did pass toward the south end of the Big Island, very heavy rains could result, as strong east winds rotating around the storm would slam against the island’s high mountains. The only tropical cyclone on record to affect Hawaii from the east-northeast is Hurricane Orlene, which made landfall on the Big Island as a tropical depression in September 1994.

And then there’s Lester…
Next up in this unusual queue is Hurricane Lester, now churning well to the east of Madeline (about 1000 mile west-southwest of Cabo San Lucas, Mexico) with top sustained winds of 85 mph, down from peak winds of 105 mph on Saturday night. A strong ridge to its north should keep Lester on a strikingly direct east-to-west path between latitude 18°N and 19°N for the next few days. Lester’s intensity should also hold steady in the Category 1 hurricane range for at least the next several days, with shear remaining knots (10 knots or less) and SSTs holding above 26°C.

The big question concerning Hawaii is whether Lester will begin an expected gradual recurvature in time to avoid the islands. The 00Z Sunday runs of the GFS and ECMWF model take Lester about 100-200 miles north of the islands next weekend. It’s possible that Lester and Madeline will end up within 1200 miles of each other, enough to possibly trigger Fuijiwhara interaction—the process by which two hurricanes can gain a rotational element of motion around a common center. If this were the case, it would tend to nudge Lester further to the north and reduce its potential impact on the islands. The same effect could enhance Madeline’s potential arc toward the south.


Figure 9. The VIIRS instrument aboard NASA-NOAA's Suomi NPP satellite captured this image of Typhoon Lionrock at 0405Z Friday, August 26, 2016. Image credit: NOAA/NASA Goddard Rapid Response Team.

Heavy rains the big threat to Japan from Category 4 Lionrock
Fearsome Typhoon Lionrock is clawing its way through the Northwest Pacific with top sustained winds of 130 mph, as reported by the Joint Typhoon Warning Center at 12Z Sunday. This makes Lionrock the equivalent of a Category 4 hurricane. Lionrock is moving northeast parallel to Japan, about 400 miles south of Tokyo, but by midweek it will make a dramatic left turn into a mid-latitude storm system, not unlike the track taken by Hurricane Sandy in 2012. As with Sandy, the exact timing and angle of the turn will make a big difference in the impacts on northern Japan, which has been deluged over the last few days from Tropical Storms Mindulle and Kompasu.

Lionrock may have already peaked in intensity, with some degradation on its north side evident in satellite imagery over the last few hours. JTWC predicts that Lionrock will weaken to near minimal typhoon/hurricane strength by the time it makes landfall in northern Honshu. Torrential rains can still be expected across much of northern Japan, as well as along the far southeast Russian coast.

Jeff Masters and I will be back with a full update on Monday morning. We’ll also add updated information on 99L within this post on Sunday evening.

Bob Henson