Twin Major Hurricanes Menace Hawaii; Little Change to Atlantic's TD 8 and TD 9

Powerful Hurricane Madeline continues edging toward Hawaii’s Big Island, where a Hurricane Watch remains in effect. An astounding 36-hour burst of intensification peaked early Tuesday, with top sustained winds of 135 mph at 5 am EDT making the hurricane a Category 4 on the Saffir-Simpson Scale. As of 11 am EDT Tuesday, Madeline’s top winds were down to 120 mph. The formerly distinct eye has become obscured in infrared imagery over the last few hours, another sign of weakening. Wind shear will increase to the moderate range (15 - 20 knots) by Wednesday, so we can expect at least gradual weakening to continue.


Figure 1. Hurricanes Madeline (left) and Lester (right), as captured by the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi NPP spacecraft on Monday, August 29, 2016. Image credit: NASA.

Located about 450 miles east of Hilo as of 11 am EDT, Madeline was moving west at 10 mph. Computer models agree that a strengthening ridge to the north of Madeline will help induce a leftward bend in Madeline’s track. This bend, plus interactions with Hurricane Lester (see below), may be enough to put Madeline’s track just south of the Big Island and well south of the rest of Hawaii, but there is still some uncertainty about this. The 00Z UKMET and the 06Z HWRF and GFS model runs keep the hurricane about 100-200 miles south of the island, while the 00Z European and 06Z GFDL run suggest a landfall on the Big Island at hurricane strength. No hurricane has ever struck the Big Island in records going back to 1949. The official track from the Central Pacific Hurricane Center brings Madeline within 50 miles of the south tip of the Big Island on Thursday morning, putting the island on the more dangerous right-side (north) side of the storm.


Figure 2. Tracking map for Hurricane Madeline as of 11:00 am EDT (5:00 am HST) Tuesday.

Although models diverge on how quickly Madeline will weaken, the 11 am EDT Tuesday outlook from CPHC projects that Madeline will be a Category 1 hurricane by the time it reaches the vicinity of Hawaii. Even a brush just to the south of the Big Island at this strength would push very strong tropical-storm-force winds into the east side of the island, slamming upslope against towering mountainsides. The result would be torrential rains of 5” - 15” or more and the potential for flooding and landslides. Winds could be strong enough to produce considerable damage to trees, power lines, and roofs. Surf is expected to reach 15 to 25 feet on east-facing shores by Wednesday, with significant damage to roads and coastal properties possible. For more details, refer to the local statements that are compiled on a CPHC website; these will be updated as Madeline approaches.


Figure 3. Infrared satellite image of Hurricanes Madeline (center) and Lester (right) as of 1200Z (8:00 am EDT) Tuesday, August 30, 2016. Image credit: NASA/MSFC Earth Science Office.

Next up for the weekend: Hurricane Lester
The 50th state has another storm to keep an eye on: Hurricane Lester. Peaking on Monday night with top sustained winds of 140 mph, Lester weakened slightly overnight, but on Tuesday morning it was rebuilding a very solid and well-structured core of convection. Located about 1350 miles east of Hilo and about 900 miles east of Madeline, Lester was packing top sustained winds of 120 mph as of the 11 am EDT advisory from NHC.

Lester’s current westward track is expected to bend to the west-northwest as it approaches Hawaii, which should produce a track over the weekend roughly parallel to the island chain and most likely 100-200 miles to its north. Model guidance is tightly clustered around this track, which is reflected in the official NHC outlook. Among the 00Z Tuesday runs of the leading models, only the UKMET produces a landfall in Hawaii, and that occurs with a much-weakened Lester. This gives us increasing confidence that Hawaii will dodge the Lester bullet, although there is still enough error in 4- and 5-day outlooks to keep most of the state in the “cone of uncertainty” for now (see image below). Lester is expected to weaken very gradually over the next several days, with more significant weakening possible as wind shear increases toward the weekend. Lester will also be moving over waters churned up by Madeline, which could further dampen its strength.


Figure 4. Tracking map for Hurricane Lester as of 11:00 am EDT (5:00 am HST) Tuesday.

Only five tropical storms or hurricanes have made landfall on a Hawaiian island since records began in 1949, and two of those have been in the last three years (see our Monday post for details). Given Hawaii’s limited experience with tropical cyclones, both Madeline and Lester need to be taken very seriously. An unprecedented deployment of hurricane hunter resources to Hawaii kicked off on Monday afternoon, when three Air Force Hurricane Hunter aircraft were sent to the islands. Beginning on Tuesday afternoon, these aircraft will provide regular fixes every 12 hours on Madeline, and will also begin flying into Lester when it draws closer to Hawaii.

Near-perfect storms
The remarkable strengthening of both Madeline and Lester since Sunday could serve as textbook examples of hurricane intensification. SSTs along the paths of both storms have been on the low side--just above the 26°C benchmark for tropical development—yet both storms have taken full advantage of exceptionally supportive atmospheric conditions, including very low wind shear and very strong upper-level outflow channels. According to output from the SHIPS model, the maximum potential intensity (MPI) for both Madeline and Lester as of Tuesday morning was about 130 knots (150 mph). MPI values take into account the ability of a tropical cyclone to create a “heat engine” based on SSTs and the atmospheric temperature structure. Most hurricanes never reach their MPI value, so that fact that two hurricanes so close to each other have managed this feat testifies to the extremely favorable atmospheric dynamics in play.

Figure 5. The Fujiwhara effect causes two tropical cyclones near each other to rotate around a common midpoint. This motion is on top of the preexisting movement of each cyclone. Image credit: Hong Kong Observatory.

Ironically, the coexistence of Madeline and Lester may help keep either one from a direct landfall on Hawaii, thanks to the Fujiwhara effect, which was discovered nearly a century ago by Japanese researcher Sakuhei Fujiwhara. When two tropical cyclones get within about 800 miles of each other, the interaction tends to make the pair rotate around a common point in between, with the effect superimposed on the storm’s preexisting motions. In a case like this, the easterm storm (Lester) would angle northward and the western storm (Madeline) would angle southward. Both effects would tend to drive Madeline and Lester away from Hawaii.

TD 9 still battling dry air and wind shear
The endless struggle of Tropical Depression Nine with dry air and wind shear continued overnight. The storm’s winds were below tropical storm strength on Tuesday morning, confirmed a NOAA hurricane hunter aircraft, though the central pressure was down 3 mb since Monday morning, to 1004 mb. TD 9 brought heavy rains to Cuba on Monday; In Central Cuba, Trinidad reported 252.4 mm (9.94”) in the 24 hours ending at 8 pm EDT Monday, while Santa Lucia in Western Cuba reported 228.3 mm (8.99”). Additional heavy rains of 2 - 4” are likely over western Cuba on Tuesday. Rainfall amounts of 1 - 2” were common over South Florida on Monday, with a few spots of 3+”.

Satellite images on Tuesday morning showed TD 9’s heavy thunderstorms building, though the circulation center was still partially exposed to view—the telltale sign of a tropical cyclone struggling with wind shear and dry air. Wind shear was a moderate 10 - 15 knots, and water vapor satellite imagery showed plenty of dry air to the storm’s north and west. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, near 30 - 30.5°C (86 - 87°F).


Figure 6. Projected 5-day rainfall from 12Z (8:00 am EDT) Tuesday, August 30, through 12Z Sunday, September 4, 2016. Rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida, with up to 12” possible near the coast where the storm makes landfall. Image credit: NOAA/NWS Weather Prediction Center.

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest 0Z Tuesday (8 pm EDT Monday) runs of our top models continue to bring TD 9 to a landfall on the Florida coast north of Tampa on Thursday. The models have converged on their timing of this landfall, which is expected to occur late morning or early afternoon on Thursday. In their 11 am EDT Tuesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 9 along the Gulf Coast of Florida were 49%, 39%, and 33%, respectively, for Cedar Key, Tampa, and Apalachicola, Florida. Tropical storm-force winds may also occur on the east coast of Florida near where the storm exits the coast after crossing the state: NHC gave odds of tropical storm-force winds of 25% or higher to the Florida coast from Cocoa Beach to Georgia, and to all of the Georgia coast. Lower odds were posted for the South Carolina coast. It is possible the storm could turn north over the weekend just offshore of the Mid-Atlantic coast, as suggested by recent runs of the GFS model. This would bring high surf to beaches from North Carolina to Massachusetts, but the heaviest rains would stay offshore.


Figure 7. Screen shot of NHC’s interactive Storm Surge Probability product from 5 am EDT Tuesday, August 30, 2016, showing the probability of inundation in excess of 4’ above ground level from TD 9. A 30-mile stretch of the Florida Gulf Coast to the right of where the center of TD 9 is expected to make landfall is predicted to have a 50 - 60% chance of getting a inundation in excess of four feet. The graphic is based upon an ensemble of Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model runs created using the current National Hurricane Center (NHC) official hurricane advisory. Storm surge probabilities depend on the historical accuracy of NHC's forecasts of hurricane track, and wind speed, and an estimate of storm size.

Intensity forecast: TD 9 likely to stay below hurricane strength
TD 9 has defied predictions that it would intensify its entire way across the Atlantic as tropical wave 99L, and now as TD 9, so there is reason to be dubious that the storm will ever significantly intensify. Indeed, the latest 00Z Tuesday runs of the GFS and European models show TD 9 as a tropical depression or weak tropical storm with 45 mph winds at the time of landfall on the Florida Gulf Coast on Thursday morning, likely due to the continued presence of dry air at middle and upper levels of the atmosphere. However, we cannot totally dismiss the possibility that TD 9 might put on a sudden show of intensification into a strong tropical storm before landfall on Thursday. The SHIPS model on Tuesday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through Thursday morning. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models—the HWRF, DSHIPS and LGEM models—were in reasonable agreement with their latest runs available late Tuesday morning, with landfall intensities for TD 9 ranging from 50 - 65 mph. NHC is going with a forecast of a 60 mph tropical storm at landfall. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast. On Tuesday morning, NHC was giving a 50 - 60% chance that the maximum height of the storm surge above ground from TD 9 would exceed four feet along a 30-mile stretch of the Florida coast to the right of where the center is expected to make landfall. The other hazard from TD 9 is heavy rain—rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida, with 15” possible near the coast where the storm makes landfall.

TD 8: Tropical Storm Warning continues for the Outer Banks of North Carolina
A Tropical Storm Warning continues for the Outer Banks of North Carolina as Tropical Depression Eight chugs north-northwest at 5 mph towards Cape Hatteras. TD 8 has not strengthened into a tropical storm yet, confirmed an Air Force hurricane hunter aircraft on Tuesday morning, as the plane found top winds in the storm near 35 mph, with a central pressure holding steady at 1011 mb. Satellite images and long-range Morehead City, North Carolina radar on Tuesday morning showed TD 8 continued to struggle to hold onto its heavy thunderstorms in the face of very dry air (45 - 50% relative humidity at mid-levels of the atmosphere), combined with moderate wind shear of 10 - 15 knots. Sea surface temperatures (SSTs) near TD 8’s center remained favorable for development, though, near 29°C (84°F).

Forecast for TD 8: grazing the Outer Banks of North Carolina
The computer models are in excellent agreement that TD 8 will continue on its current north-northwest track through Tuesday afternoon, then make a turn to the north Tuesday night, and then turn to the northeast on Wednesday. These steering currents should bring the center of TD 8 very close to Cape Hatteras, North Carolina on Tuesday night and Wednesday morning. A storm surge of 1 - 2 feet can be expected along the Outer Banks, along with heavy rains of 1 - 3 inches. In their 11 am EDT Tuesday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 8 were 43% for Cape Hatteras, North Carolina. Dry air and moderate wind shear will continue to affect TD 8 through Wednesday, but there is still the possibility it could become a weak tropical storm by Wednesday.


Figure 8. MODIS visible satellite image of Invest 92L on Tuesday morning, August 30, 2016. The tropical wave was embedded in a large area of African dust to its west and north. Image credit: NASA.

92L emerges from the coast of Africa
A large tropical wave with plenty of spin emerged from the coast of Africa on Monday evening, and was immediately designated Invest 92L by NHC. The wave will move through the Cabo Verde Islands on Tuesday and potentially develop into a tropical depression later in the week. While the latest run of the SHIPS model predicted that SSTs and wind shear would be favorable for development during the coming five days, 92L emerged from the coast at the same time that a major pulse of dust and dry air from the Sahara also left the coast, just to the north. This dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic.The latest 0Z Tuesday (8 pm EDT Monday) runs of our three top models for forecasting tropical cyclone genesis, the GFS, UKMET and European models, did not develop 92L over the next five days—though the more recent 06Z (2 am EDT) Tuesday run of the GFS model did develop 92L five days from now. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through or just north of the Lesser Antilles Islands on Sunday. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 40%, respectively.

We’ll be back with a new post late Tuesday afternoon.

Bob Henson and Jeff Masters