Hurricane Watch Continues for TD 9; Hawaii Hunkers Down for Madeline

A Hurricane Watch continues for the Florida Gulf Coast from the Anclote River to Indian Pass, and Tropical Storm Watches have been issued for portions of the Florida and Georgia Atlantic coasts as Tropical Depression Nine sits nearly stationary in the Gulf of Mexico, deciding what to do next. Satellite images on Wednesday morning showed a significant change to TD 9’s organization, with a huge blow-up of heavy thunderstorms over the storm’s center, which created a central dense overcast (CDO) of high cirrus clouds. A CDO is typically the classic sign of a tropical cyclone undergoing significant intensification, but this particular cloud signature is different from the classic CDO, and is often associated with a storm in an arrested state of development (see the discussion of "Central Cold Cover" after Figure 7 in the original 1984 satellite classification paper by Dvorak, 1984.)

There have been no hurricane hunter missions into TD 9 since Tuesday night, so NHC elected not to upgrade TD 9 to Tropical Storm Hermine. The Hurricane Hunters were scheduled to fly a number of missions into TD 8 and TD 9 since last night, but no missions flew. They may be over-extended and suffering mechanical issues, due to the grueling requirements of flying three simultaneous storms over a multi-day period in both the Atlantic and Pacific. Starting this week, the National Center for Atmospheric Research is sending the NSF/NCAR Gulfstream-V aircraft to sample the large-scale environment around Atlantic storms through October while NOAA’s Gulfstream-IV undergoes maintenance. As of 1 pm EDT Wednesday, a NOAA hurricane hunter aircraft had begun a mission into TD 9.

Buoy 42003, located about 200 miles to the northeast of TD 9’s center, has seen a steady increase in wind speed on Wednesday morning. At 11 am EDT, winds at the buoy were 31 mph, gusting to 38 mph. The strong winds from TD 9 were already creating a two-foot storm surge at Cedar Key, Florida on Wednesday morning. A nearby ship measured sustained winds of 42 mph at 8 am EDT, but this measurement may have been made at a height much higher than the standard 10 meters above the surface used for classifying systems as tropical storms. Wind shear continued to be a moderate 10 - 15 knots on Wednesday morning, but water vapor satellite imagery still showed plenty of dry air to the storm’s north and west; the combination of these factors is likely slowing down the intensification process. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, near 30.5°C (87°F). 


Figure 1. The view of TD 9 from the cockpit of NOAA hurricane hunter aircraft N43RF (Miss Piggy) on an August 30, 2016 flight. Image credit: NOAA/AOML/HRD Twitter feed.


Figure 2. Radar estimated rainfall between August 29 - August 31, 2016 from the Tampa radar. Swaths of 2 - 4” of rain (yellow colors) were common over Florida.

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest Wednesday morning runs of our top models are in solid agreement that TD 9 will make landfall along the Florida Big Bend coast north of Tampa on Thursday afternoon or evening. In their 11 am EDT Wednesday 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 59%, 56%, and 55%, respectively, for Apalachicola, St. Marks and Cedar Key, Florida.


Figure 3. Projected 7-day rainfall from 12Z (8:00 am EDT) Wednesday, August 31, through 12Z Wednesday, September 7, 2016. Rainfall amounts of 5 - 10” are expected along TD 9’s path across Florida and along the Southeast U.S. coast. Heavy rains from the system may also affect Southeast Massachusetts on Labor Day. Image credit: NOAA/NWS Weather Prediction Center.

A Labor Day spoiler for the Mid-Atlantic and Northeast U.S.?
Once TD 9 crosses over Florida and southern Georgia and enters the Atlantic again, the storm will be directly over the axis of the very warm Gulf Stream current. This will make the storm resist weakening, despite the expected presence of high wind shear in excess of 20 knots. The storm will also begin transitioning to a powerful extratropical storm at that time, and begin deriving energy from atmospheric dynamics, rather than from the heat energy of the ocean. In their 11 am EDT Tuesday Wind Probability Forecast, NHC gave odds of at least 25% for tropical storm-force winds affecting the Southeast U.S. coast from Daytona Beach, Florida northwards to Cape Hatteras, North Carolina--including the entire coasts of Georgia and South Carolina.

The official NHC forecast at 11 am EDT Wednesday had TD 9 headed northeastward out to sea, caught in the steering flow of a trough of low pressure, after clearing the coast of North Carolina on Saturday. TD 9 is likely to become a powerful extratropical storm with 60 - 65 mph winds by Sunday. However, the extratropical storm expected to emerge from TD 9 may turn north and spoil the Labor Day plans of millions of people in the Northeast U.S. The latest 06Z (2 am EDT) and 12Z (8 am EDT) Wednesday runs of the GFS model predicted that the trough pulling TD 9 to the northeast would quickly be replaced by a ridge of high pressure, resulting in a northward turn by TD 9 followed by a stall after passing by North Carolina. This track would threaten the Mid-Atlantic coast with heavy rain on Saturday and Sunday, spreading into the Northeast coast on Sunday and Monday. In their latest 0Z Wednesday (8 pm EDT Tuesday) runs, the European and UKMET models did not go along with this idea as strongly--predicting that TD 9 would stay well away form the Mid-Atlantic coast--but they had the storm potentially threatening Southeast Massachusetts on Monday (Labor Day.)

The bottom line: there is an unusually large amount of uncertainty with TD 9’s possible track over the weekend, and the storm might be capable of bringing heavy rain and strong winds--potentially at tropical storm force--to the New England and Mid-Atlantic coasts.


Figure 4. Screen shot of NHC’s interactive Storm Surge Probability product from 5 am EDT Wednesday, August 31, 2016, showing the probability of inundation in excess of 4’ above ground level from TD 9. A 100-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 60 - 70% chance of getting a inundation in excess of four feet (orange colors). The odds were around 40% near Savannah, Georgia. 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
The SHIPS model on Wednesday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through landfall on Thursday afternoon. 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 Wednesday morning, with landfall intensities for TD 9 ranging from 55 - 70 mph. NHC is going with a forecast of a 65 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 Wednesday morning, NHC was calling for a maximum storm surge of 3 - 5’ above ground from TD 9 along a stretch of the Florida coast to the right of where the center is expected to make landfall. The other main 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. We also cannot rule out a few tornadoes from the storm.


Figure 5. Infrared satellite image of Hurricane Madeline at 1400Z (10:00 am EDT) Wednesday, August 31, 2016. Image credit: NOAA/NESDIS.

Weakening Madeline still on track to sweep past Hawaii’s Big Island
Hurricane Madeline is taking a turn for the better as far as Hawaiians are concerned. Madeline has weakened dramatically over the past 24 hours, losing its well-defined eye and becoming much less organized (see image above). After topping out with 135-mph sustained winds on Tuesday morning, Madeline’s top winds were down to 80 mph as of the 11 am EDT advisory from the Central Pacific Hurricane Center (CPHC). Now a Category 1 storm, Madeline is ingesting fairly dry mid-level air (relative humidities of 30-40%) in moderate wind shear of 15-20 knots. All of these factors should keep Madeline’s intensity on a downward path, even as it moves over steadily higher SSTs.

Another piece of good news: the much-anticipated leftward swing in Madeline’s path now appears more likely to keep the center from making landfall on the Big Island. All of the major computer models now keep Madeline’s track just south of the island. Madeline was only about 140 miles east-southeast of Hilo at 5:00 am HST (11:00 am EDT). The latest CPHC outlook brings Madeline within about 50 miles of the southeast coast of the Big Island this afternoon as a minimal hurricane. Since hurricane-force winds extend ony about 10 miles from Madeline’s center, such a track would keep those winds offshore, although gusts could still reach 80 - 100 mph in a few spots. The most likely outcome for the bulk of the Big Island is widespread tropical-storm-force winds sustained at 40 - 70 mph, strongest toward the south. A Hurricane Warning remains in effect for the Big Island, and a Tropical Storm Warning is now in effect for the islands of Maui, Molokai, Lanai and Kahoolawe.

Potential impacts from Madeline
The steep topography along the shore of the Hawaiian Islands tends to minimize storm surge while maximizing surf height. Surf as high as 25 feet can be expected along the Big Island’s east- and south-facing coastlines. A storm surge of 1-3 feet is possible along parts of Hawaii’s southern and eastern coast. Storm surge expert Hal Needham provided background on Hawaii’s surge and surf risk from tropical cyclones in a blog post on Tuesday afternoon. Heavy rains are already beginning to nudge into the Big Island, and torrential rains, landslides, and flooding are a distinct possibility. Localized rainfall amounts could exceed 15” along east-facing slopes as Madeline’s winds slam against the Big Island’s mountainous terrain. The best place to find frequently updated local statements on Madeline’s expected impact is at a dedicated website maintained by the National Weather Service office in Honolulu.

Overall, Madeline’s impact could match or exceed that of Hurricane Iselle (2014), another Category 4 storm that weakened quickly while approaching Hawaii. Iselle struck on the Big Island as a moderate tropical storm--the strongest landfall on record for the island--and caused 1 fatality and inflicted nearly $80 million in damage across Hawaii, qualifying as an agricultural disaster. This year’s Hurricane Darby came ashore on the Big Island as a 40-mph tropical storm on July 23, with widespread flooding but no deaths or major damage.

Hawaii’s hurricane history and its potential hurricane future
Apart from Iselle and Darby, only three other tropical cyclones have made landfall anywhere in Hawaii since records began in 1949:

--The state’s worst hurricane by far, Hurricane Iniki, which hit Kauai as a Category 4 hurricane, killed 6 and caused $1.8 billion in damage (1992 dollars.)

--Hurricane Dot, which hit Kauai as a Category 1 hurricane, caused 6 indirect deaths and $6 million in damage (1959 dollars.)

--An unnamed 1958 storm that brought sustained winds of 50 mph at landfall to the Big Island killed one person and caused $0.5 million in damage.

The region around Hawaii has seen a lot of tropical activity over the past four years, including a number of near-misses. Partly this is a result of El Niño, which warmed the waters of the tropical Central and Eastern Pacific where Hawaii-heading cyclones are born. However, the uptick may also be a harbinger of things to come. See the August 2014 post, Climate Change May Increase the Number of Hawaiian Hurricanes.

The winds at 13,000 feet on Mauna Kea
The weather on top of the highest point in Hawaii, the Big Island's Mauna Kea, elevation 13,796' (4,205 m), will be interesting to follow as Madeline passes the Big Island. Winds will rise steadily today, and there are six anemometers on top of the mountain to watch. However, beware of the data from the Canada - France - Hawaii Telescope (CFHT). The Mauna Kea webcam page says that those winds are highly exaggerated due to location of the anemometer tower between two large telescope domes. You can see this tower on Google Maps.

Links
Regional Hawaii radar
Weather on Mauna Kea
Live stream from KHON2 TV in Honolulu
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu



Figure 6. Infrared satellite image of Hurricane Lester at 1400Z (10:00 am EDT) Wednesday, August 31, 2016. Image credit: NOAA/NESDIS.

Lester still lurking farther east of Hawaii
While Madeline has lost ground in the past day, Hurricane Lester remains a powerful and well-organized Category 4 storm. Lester was located about 1085 miles east of Hilo as of the 11 AM EDT Wednesday advisory from NHC. (Advisories on Lester will be issued by CPHC once the hurricane moves west of 140°W.) Lester’s core of intense storms has shrunk and become more asymmetric over the last few hours, but its top sustained winds have dropped only slightly, down to 130 mph in the most recent advisory from a Tuesday night peak of 140 mph.

Computer models agree that Lester’s westward path will start bending toward the west-northwest by Thursday, with the hurricane gradually weakening as it encounters greater wind shear and waters churned up by Madeline. Computer models are in close agreement on taking Lester just north of Hawaii during the weekend on a path closely paralleling the island chain. The NHC outlook positions Lester about 100 miles north of Oahu early Sunday as a strong tropical storm. It’s still uncertain exactly how close Lester’s path will be to Hawaii, especially Oahu, so residents throughout the state need to keep tabs on this powerful storm. One good thing: assuming no major southward shift in the path, Hawaii will lie on the weaker left-hand (south) side of Lester, which would reduce the potential impact. Huge surf is a safe bet.


Figure 7. Visible satellite image of Hurricane Gaston at 1015Z (7:15 am EDT) Wednesday, August 31, 2016. Image credit: NOAA/NESDIS.

Spectacular Gaston heads toward the Azores
For a good contrast with the underperforming, disheveled TD 9, you need look no further than Category 3 Hurricane Gaston (above), which has become a marvel of persistence and organization. Gaston didn’t hit its stride until it reached the subtropics, around 30°N. Here, Gaston lingered for days in very weak wind shear atop unusually warm waters (1-2°C above average), gathering strength even as it churned up deeper, cooler water. Gaston became a Category 3 storm for the second time on Tuesday night, matching its peak sustained winds of 120 mph from Sunday night. Fortunately, Gaston has remained well northeast of Bermuda, posing a threat only to shipping thus far. At 11 am EDT Wednesday, Gaston was located about 1150 miles west of Faiail Island in the Azores with top sustained winds of 115 mph.

Gaston has become a classic example of an annular hurricane--the type with a huge eye surrounded by a solid ring of thunderstorms, with little or none of the spiral banding typical of strong hurricane. Annular storms develop when enough mixing occurs between the eye and eyewall to allow the eye to expand. Because of their resilient structure, annular storms are less vulnerable to disruption and often maintain intensity longer than their peers. Gaston could remain a major hurricane into Thursday, when it will begin accelerating east-northeast over cooler waters amid increasing wind shear. The latest NHC outlook brings Gaston across the Azores late Friday into early Saturday as a strong tropical storm. NHC will likely be issuing a Hurricane Watch or Tropical Storm Watch for parts of the Azores later Wednesday Given its annular nature and the rarity of hurricanes in the Azores--only about one per decade, on average--Gaston could pack a punch should it pass over or near any of the widely scattered Azores islands. This year’s first Atlantic storm, Hurricane Alex, struck the island of Terceira in the central Azores on January 15 as a bizarrely out-of-season tropical storm in January.

92L off the coast of Africa embedded in dry air
A large tropical wave with plenty of spin that emerged form the coast of Africa on Monday was designated Invest 92L by NHC, but they are no longer issuing their suite of model forecasts for the system, due to the system’s lack of potential for development. The wave was just west of the Cabo Verde Islands on Wednesday, and was embedded in a major area of dust and dry air from the Sahara Desert. 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 Wednesday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--did not show any development of the system over the next five days. 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 Wednesday 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 Wednesday afternoon.

Bob Henson and Jeff Masters


Figure 8. Surface winds around Gaston late Tuesday, August 30, 2016, as estimated by the Advanced Scatterometer (ASCAT) aboard the European MetOp satellite. Image credit: NESDIS/STAR, via Grant Wise, @wise_wx.