Hurricane Watch for Hawaii’s Big Island; TDs 8, 9 May Become Tropical Storms

In a dual scenario unprecedented in hurricane recordkeeping, two major hurricanes are heading toward Hawaii, and both could affect the island with high surf, torrential rain, and potential high winds over the next week. Hurricane Madeline is the closer of the two, located about 630 miles east of Hilo, HI, as of the 5 pm EDT advisory from the Central Pacific Hurricane Center. Now moving west-northwest at 10 mph, Madeline has been rapidly intensifying, growing from tropical storm to Category 3 strength in just 24 hours. As of 5 pm EDT, Madeline’s top sustained winds were at 115 mph. CPHC is projecting Madeline to move on a leftward-arcing path that would take it just south of the Big Island as a Category 1 hurricane by Wednesday night (see Figure 1 below). A Hurricane Watch is now in effect for all of the Big Island (Hawaii County). Update (11:30 pm EDT Monday]: Madeline's top sustained winds had increased to 125 mph as of the 11 pm EDT CPHC advisory.


Figure 1. Tracking map for Hurricane Madeline as of 5:00 pm EDT (11:00 am HST) Monday.

Madeline could become the first hurricane on record in the Big Island
At its closest, the center of Madeline is projected to be roughly 100 miles south of the Big Island. Given that this is more than two days out, we cannot yet entirely rule out the possibility that Madeline will stay far enough north to produce the first-ever hurricane strike on the Big Island in records going back to 1949. The 12Z Monday HWRF and GFDL model runs bring Madeline into the Big Island at hurricane strength, while the European and GFS models keep Madeline south of the island. Even if the latter occurs, very strong northeast winds rotating around the hurricane could produce torrential rains, flooding, and huge surf on the east side of the island. Tropical-storm-force winds currently extend out up to 115 miles from Madeline, and that envelope may expand by Wednesday as the hurricane matures.


Figure 2. Resembling the eyes of some ghostly sea creature, Hurricanes Madeline (left) and Lester (right) were charging westward toward Hawaii (far left) at 1800Z (2:00 pm EDT) Monday, August 29, 2016. Image credit: NASA/MSFC Earth Science Office.

About 1000 miles east of Madeline, Hurricane Lester remains impressive, now packing minimal Category 4 winds of 130 mph as of the 5 pm EDT advisory from NHC. As its shield of thunderstorms grows larger, Lester is taking on more of the characteristics of an annular hurricane--the type that features a large eye and a single broad ring around that eye, as opposed to spiral bands. Annular hurricanes tend to be slow to weaken, which raises the odds of Lester remaining strong enough to affect Hawaii as a hurricane. NHC predicts that Lester will be roughly 150 miles northeast of Hilo on Saturday morning as a Category 1 hurricane, on a northwestward-angling track that could keep the storm just north of the islands. There is enough error in five-day tracks to put most of the Big Island within NHC’s “cone of uncertainty” for Saturday, and a direct strike from Lester on one or more islands cannot yet be ruled out.

One factor that could influence both tracks is the Fujiwhara effect, in which hurricanes within about 800 miles of each other begin to rotate around a center of gravity in between them. Lester is slowly catching up to Madeline as it moves west at 14 mph, vs. Madeline’s 10 mph motion. If the two hurricanes get close enough, the Fujiwhara effect will tend to angle Madeline’s path toward the south and Lester’s toward the north--in both cases, exactly what you would want to reduce the chance of a direct hit on Hawaii. Such an outcome is by no means guaranteed, though. Given Hawaii’s limited experience with tropical cyclones (see this morning’s post for more details), both of these systems 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.


Figure 3. Tracking map for Hurricane Lester as of 5:00 pm EDT (11:00 am HST) Monday.

TD 9 struggling with dry air and wind shear
Tropical Depression Nine continues to struggle with dry air and wind shear, and has not strengthened into a tropical storm yet, confirmed a NOAA hurricane hunter aircraft on Monday afternoon. TD 8 was headed to the west-northwest at 5 mph on a track just north of western tip of Cuba. The storm’s top winds remained near 35 mph, with a central pressure holding steady at 1007 mb.


Figure 4. MODIS visible satellite image of Tropical Depression Nine in the Florida Straits on Monday afternoon, August 29, 2016. Image credit: NASA.

Satellite images on Monday afternoon showed a slow increase in the intensity and areal coverage of TD 9’s heavy thunderstorms, though late in the day the circulation center became exposed to view—the telltale sign of a tropical cyclone struggling with high wind shear. Long-range Key West radar showed heavy rain over western Cuba, where up to 12” of rainfall had likely fallen, and a few scattered rain showers over the Florida Keys, but little in the way of low-level spiral bands. The main factor keeping TD 9 from developing was wind shear that was a moderate 10 - 15 knots. TD 9 was also struggling with dry air, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, though, near 30  - 30.5°C (86 - 87°F).

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest 12Z (8 am EDT) Monday runs of our top models continue to bring TD 9 to a landfall on the Florida coast north of Tampa on Thursday. There is significant spread in the timing of TD 9’s landfall in Florida, with the HWRF model predicting a 1 am strike, European model an 11 am strike, and the GFS model an 8 pm strike. In their 5 pm EDT Monday 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 43%, 36%, and 36%, 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 Orlando, The Villages, Daytona Beach, Gainesville and Jacksonville in Florida, and to Kings Bay in Georgia. The latest run of the European model showed TD 9 scooting along the coasts of Georgia and South Carolina early Friday morning, and it is possible the storm could spread sustained winds near 40 mph to the coast there.

One potential monkey wrench in the track forecast: TD 9 could undergo a relocation of its center to point more than 50 miles south of its current center, so that it is closer to the heaviest thunderstorms near the western tip of Cuba. If this occurs, a southward shift in the predicted track of TD 9 may be required.


Figure 5. The 12Z (8 am EDT) Monday run of the HWRF model predicted that TD 9 would have top winds of 50 mph at landfall, and would bring copious rains of 8 - 16” along its track across Florida. The HWRF rainfall amounts are likely too high, as the official NHC forecast at 5 pm EDT Monday called for 3 - 7” of rain along TD 9’s track, with isolated amounts of up to 10” along the coast near its landfall location. Image credit: NOAA/EMC.

Intensity forecast for TD 9 becoming clearer
Once TD 9 pulls away from Cuba, a round of steady intensification is likely, with the system reaching tropical storm strength by Tuesday morning. The SHIPS model on Monday afternoon predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots Monday afternoon through Wednesday. 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 - 70%. There is a significant amount of dry air at middle and upper levels of the atmosphere that may interfere with development, though. Our three best intensity models—the HWRF, DSHIPS and LGEM models—were in reasonable agreement with their latest runs available on Monday afternoon, with landfall intensities for TD 9 ranging from 50 - 75 mph. NHC is going with a forecast of a 65 mph tropical storm at landfall, noting that increasing wind shear in the final day before landfall may stop the intensification process. TD 9 could be a Category 1 hurricane at landfall, as suggested by the DSHIPS intensity model, and residents along the Gulf Coast of Florida should anticipate this possibility. This portion of the coast is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf water offshore that extend up to 90 miles from the coast. A worst-case Category 1 hurricane hitting at high tide can cause a storm surge that will inundate the Florida Gulf Coast north of Tampa to a depth to 9 - 10 feet, as seen in SLOSH model imagery available in WU’s storm surge pages.

Tropical Storm Warning for the Outer Banks of North Carolina
A Tropical Storm Warning is up for the Outer Banks of North Carolina as Tropical Depression Eight chugs northwest at 6 mph towards the state. TD 8 has not strengthened into a tropical storm yet, confirmed an Air Force hurricane hunter aircraft on Monday afternoon, as the plane found top winds in the storm near 35 mph, with a central pressure holding steady at 1011 mb.


Figure 6. Radar image of TD 8 from the long-range Morehead City, North Carolina radar at 4:39 pm EDT August 29, 2016. TD 8 was a spirally-looking thingy.

Satellite images and long-range Morehead City, North Carolina radar on Monday afternoon showed TD 8 had a vigorous circulation and a modest but growing area of heavy thunderstorms. Development was being slowed to very dry air (45 - 50% relative humidity at mid-levels of the atmosphere), combined with moderate wind shear of 15 - 20 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 northwest track through Tuesday morning, then make a sharp turn to the north and northeast on Tuesday afternoon after getting caught in the steering flow of a trough of low pressure passing to the north. 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 5 pm EDT Monday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 8 were 42% for  Cape Hatteras, North Carolina. Dry air and moderate wind shear will continue to affect TD 8 through Wednesday, and it is unlikely this storm will be stronger than a 50 mph tropical storm at the time of its closest approach to the coast on Tuesday night/Wednesday morning. As TD 8 accelerates away from the coast on Wednesday and Thursday, more significant strengthening may occur.

A new tropical wave worth watching is leaving the coast of Africa
A large tropical wave with plenty of spin was emerging from the coast of Africa on Monday afternoon, will move through the Cabo Verde Islands on Tuesday, and potentially develop into a tropical depression later in the week as it heads west at 15 - 20 mph across the tropical Atlantic. The latest 12Z (8 am EDT) Monday runs of two our three top models for forecasting tropical cyclone genesis, the GFS and European models, continue to agree that this wave will develop into a tropical depression late in the week. The UKMET model dropped development in its 12Z Monday run after supporting development in its previous few runs. The wave should remain on a fairly straightforward west to west-northwest path through the week, arriving near or just north of the Lesser Antilles Islands by Sunday evening. In their 2 pm EDT Monday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 50%, respectively.

Wunderblogger Steve Gregory has an excellent update on the tropics in his Monday afternoon post, TD 9 Getting Better Organized as TD 8 Heads for NC Coast. See our morning post for more on the other significant storm of the hour, Hurricane Gaston, which is spinning harmlessly in the middle of the North Atlantic.

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

Bob Henson and Jeff Masters