TD 9 Organizing in the Gulf of Mexico; TD 8 Headed Towards North Carolina

Tropical Depression Nine has not strengthened into a tropical storm yet, confirmed a NOAA hurricane hunter aircraft early on Monday morning, as the storm headed to the west at 7 mph on a track just north of western Cuba. The storm’s top winds remained near 35 mph, with a central pressure holding steady at 1007 mb. The strongest winds observed at a surface station on Monday morning were at Pulaski Shoals Lighthouse, located about 70 miles west of Key West, Florida, which recorded sustained winds of 31 mph, gusting to 35 mph, at 10 am EDT.


Figure 1. Latest satellite image of Tropical Depression Nine.

Satellite images on Monday morning showed a slow increase in the intensity and areal coverage of TD 9’s heavy thunderstorms. Long-range Key West radar showed heavy rain over western Cuba, where up to 12” of rainfall was expected, 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 moderately high 15 - 25 knots. TD 9 was also struggling with dry air, as seen on water vapor satellite imagery. Proximity to western Cuba was also interfering with development, as the storm’s counterclockwise flow of air pulled air across the mountains of Cuba into the storm. As this air descends to the ocean after crossing Cuba, warming and drying of the air occurs, robbing TD 9 of an important moisture source. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, though, near 30 - 30.5°C (86 - 87°F).


Figure 2. Projected 5-day rainfall from 12Z (8:00 am EDT) Monday, August 29, through 12Z Saturday, September 3, 2016. Rainfall amounts of 3 - 7” are expected over most of Florida. Image credit: NOAA/NWS Weather Prediction Center.

Track forecast for TD 9: a Florida Gulf Coast landfall
For several consecutive runs, there has been model consensus among the GFS, European, HWRF and UKMET models that TD 9 will move on a west-northwest track through Tuesday morning, slow down and turn north in the central Gulf of Mexico on Tuesday afternoon, then get caught in the steering flow of a trough of low pressure passing to the north on Wednesday. These steering currents should bring TD 9 to a landfall on the Florida coast north of Tampa on Wednesday or Thursday. There is significant spread in the timing of TD 9’s landfall in Florida, with the HWRF model predicting a Wednesday afternoon landfall as a Category 1 hurricane, and the GFS and European model predicting a Thursday morning or afternoon landfall as a 40 - 45 mph tropical storm. In their 11 am 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%, 39%, and 34%, 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 in excess of 30% to Orlando, The Villages, Daytona Beach, Gainesville and Jacksonville in Florida, and to King Bay in Georgia.

Intensity forecast for TD 9: more uncertain than usual
Once TD 9 pulls away from Cuba, a round of steady intensification is likely, with the system reaching tropical storm strength by Monday night. Satellite imagery late Monday morning showed that this process was already underway, with a notable increase in the storm’s organization. The SHIPS model on Monday morning predicted moderately favorable conditions for intensification, with wind shear falling to 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%. However, there is a significant amount of dry air at middle and upper levels of the atmosphere that may interfere with development, and the usually reliable European and GFS models showed little development of TD 9 in their 12Z Sunday and 0Z Monday (8 pm EDT Sunday) runs because of this dry air, NHC forecaster Stacy Stewart argued in his forecast discussion on Sunday night. Our best dynamical intensity model, the HWRF model, had TD 9 intensifying into a Category 1 hurricane just before landfall, though, and our two best statistical intensity models, the DSHIPS and LGEM models, had TD 9 as a borderline tropical storm/Category 1 hurricane at landfall. 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 our best intensity models, 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.

NOAA/RAMMB has some impressive rapid scan loops of TD 9 with images taken every minute.

TD 8 headed for the Outer Banks of North Carolina
A Tropical Storm Watch is up for the Outer Banks of North Carolina as Tropical Depression Eight churns northwest at 7 mph towards the state. TD 8 has not strengthened into a tropical storm yet, confirmed an Air Force hurricane hunter aircraft on Monday morning, as the plane found top winds in the storm near 35 mph, with a central pressure holding steady at 1011 mb.


Figure 3. Latest satellite image of Tropical Depression Eight.

Satellite images on Monday morning showed TD 8 had a vigorous circulation but only a meager amount of heavy thunderstorms. The depression was not developing due to very dry air (45 - 50% relative humidity at mid-levels of the atmosphere), combined with moderate wind shear of 10 - 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. 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 Monday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from TD 8 were 46% 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. 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 will emerge from the coast of Africa late on Monday, 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 runs of our three top models for forecasting tropical cyclone genesis, the GFS, ECMWF and UKMET models, continue to agree that this wave will develop into a tropical depression late in the week. 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 8 am EDT Monday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 50%, respectively.



Figure 4. Visible satellite image of Hurricane Gaston as of 1415Z (10:15 am EDT) Monday, August 29, 2016. Image credit: NOAA/NESDIS.

Powerful Gaston loiters over the remote Atlantic
Hurricane Gaston continues to aim its formidable power on the open Atlantic, about 600 miles east of Bermuda, rather than on people and structures. After peaking early Monday with top sustained winds of 120 mph, Gaston weakened slightly, but it remains a strong Category 2 hurricane with top winds of 110 mph in the 11 am EDT Monday NHC advisory. Gaston’s show of strength is remarkable given that it is practically parked over the remote subtropical Atlantic: it has moved less than 100 miles in the 24 hours ending at 11 am EDT. Hurricanes moving this slowly are often weakened as they churn up cooler water--but the western subtropical Atlantic is remarkably warm this summer, with SSTs of 29°C (84°F) near Gaston more than 1.5°C above average. Currently drifting north at just 2 mph, Gaston has another day or so of favorable conditions before a North Atlantic trough bumps up the currently-light wind shear and begins accelerating Gaston toward the northeast and toward cooler waters. It will likely hold its own for several days, though, perhaps remaining a hurricane till Thursday or Friday.

Gaston is on track to sweep over or near the Azores this weekend. The 5-day NHC outlook puts Gaston close to the northern Azores as a 60-mph tropical storm on Saturday, September 3. The Azores see a tropical cyclone landfall only about once a decade on average, although the nation was struck this past January by the bizarrely out-of-season Hurricane Alex. The islands have never recorded two landfalls in a single year.


Figure 5. GOES-West infrared image of Hurricanes Madeline (center) and Lester (right), both moving west toward Hawaii (left). Image credit: NASA/MSFC Earth Science Office.

Will the Fujiwhara effect keep Hurricanes Madeline and Lester away from Hawaii?
Amazingly, two hurricanes are rolling across the North Pacific, one behind the other, on tracks aiming toward the seldom-struck Hawaiian islands. Either one could make landfall in the next week--but it’s also possible that an obscure atmospheric mechanism will kick in just in time to steer one or both of them away from the 50th state.

Hurricane Madeline is the more immediate threat. As of the 11 am EDT Monday advisory from the Central Pacific Hurricane Center, Category 2 Madeline was located about 700 miles east of Hilo, HI, moving west-northwest at 10 mph with top sustained winds of 100 mph. Madeline has been getting stronger over the past day, with a broadening shield of convection and an eye intermittently visible. It’s not out of the question that Madeline will become a major hurricane by Tuesday, with wind shear very low (around 5 knots) and SSTs of 26-27°C. Somewhat drier air and increasing wind shear will begin affecting Madeline by midweek, with gradual weakening expected. The official CPHC track takes Madeline on a west-southwest arc, brushing the Big Island on Wednesday night as a Category 1 storm. At its closest, the center of Madeline is projected to be roughly 100 miles south of Hilo--which is smaller than the average three-day track error of 130 miles in this region. The upshot is that the first-ever recorded hurricane strike on the Big Island is within the realm of possibility. Even if Madeline stays south of the Big Island, very strong northeast winds rotating around the hurricanes could produce torrential rains and flooding on the east side of the island.

Hard on the heels of Madeline is even-stronger Category 3 Hurricane Lester, which on Monday become the fourth major hurricane of the 2016 East Pacific season. Packing top sustained winds of 125 mph as of the 11 am EDT Monday advisory, Lester is showing signs of evolving into an annular hurricane--the type that features a large eye and convection concentrated in a single ring around that eye, rather than in spiral bands. Annular hurricanes tend to be slow to weaken, and Lester shows no signs of weakening in the near future. Wind shear will remain less than 15 knots for the next several days, with SSTs around 26-27°C adequate to keep Lester going. NHC predicts that Lester will be about 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. Lester is now moving west at 14 mph, bringing it gradually closer to Madeline.


Figure 6. 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 angle Madeline and Lester away from Hawaii.

Hurricanes and tropical storms are getting more common around Hawaii
The prospect of two potential Hawaiian landfalls in one week is an exceptional event, since tropical storms are so rare in the state. Only five tropical storms have struck since records began in 1949, and two of those have been in the last three years:

--Hurricane Darby made landfall along the southeast shore of Hawaii’s Big Island on July 23, 2016, as a minimal tropical storm (top sustained winds of 40 mph). Damage was minimal and there were no deaths from Darby.

--Tropical Storm Iselle, which, like Darby, made landfall along the southeast shore of the Big Island, arriving as a 60-mph tropical storm on August 8, 2014. Iselle killed one person and did $79 million in damage.

--Hurricane Iniki, which hit Kauai as a Category 4 hurricane, killing 6 and causing $1.8 billion in damage (1992 dollars.)

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

--An unnamed 1958 storm that had sustained winds of 50 mph at landfall on the Big Island. The storm 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.


Figure 7. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2016. On July 25, 2016, Tropical Storm Darby made the closest approach on record by a tropical storm to Honolulu, passing just 40 miles to the south and west of Hawaii’s capital with sustained 40 mph winds. Darby brought torrential rains in excess of ten inches to portions of Oahu. Image credit: NOAA/CSC.

We'll be back with updates as conditions warrant.

Jeff Masters and Bob Henson