Hurricane Warning for Hawaii, a Watch for Florida; TD 9 Headed Towards NE U.S.?

NHC has issued a Hurricane Watch for the Florida Gulf Coast from the Anclote River to Indian Pass, and a Tropical Storm Watch for the Florida Gulf Coast west of Indian Pass to the Walton/Bay County line. You’d wouldn’t guess from Tropical Depression Nine’s appearance on satellite imagery, though, that the storm could become a hurricane by Thursday. TD 9 struggled with dry air and wind shear all day Tuesday, and a NOAA hurricane hunter aircraft on Tuesday afternoon found that top sustained winds had remained near 35 mph, and the central pressure had remained constant at 1004 mb. TD 9 continued to bring heavy rains to western Cuba during the day Tuesday, though; Santa Lucia in Pinar Del Rio province reported a 36-hour rainfall total of 317.4 mm (12.50”) ending at 8 am EDT. Additional heavy rains of 3 - 5” are likely over western Cuba before TD 9 finally pulls away on Wednesday.


Figure 1. MODIS visible satellite image of Tropical Depression Nine in the southern Gulf of Mexico on Tuesday afternoon, August 30, 2016. Image credit: NASA.

Satellite images on Tuesday showed little change to TD 9 during the day, with only a small amount of heavy thunderstorm activity near the center of circulation, and a circulation center that was still partially exposed to view—the typical look of a tropical cyclone struggling with wind shear and dry air. Wind shear was a moderate 10 - 15 knots, but 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).

Track forecast for TD 9: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest 12Z Tuesday (8 am EDT) runs of our top models are a little father to the north with their landfall locations, bringing TD 9 to the Florida Big Bend coast north of Tampa on Thursday afternoon. In their 5 pm 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 52%, 51%, and 50%, respectively, for Cedar Key, Apalachicola and St. Marks, Florida.

A Labor Day spoiler for the Mid-Atlantic and Northeast U.S.?
Once TD 9 crosses over Florida (and potentially 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 of 20 knots. In their 5 pm EDT Tuesday Wind Probability Forecast, NHC gave odds of at least 30% 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 5 pm EDT Tuesday 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 Friday. However, that outcome is in doubt.


Figure 2. Wind forecast for TD 9 made at 12Z (8 am EDT) Tuesday, August 30, 2016. Both the European model (left) and GFS model (right) were predicting that TD 9 would be a tropical storm just off the Northeast U.S. coast over Labor Day weekend.

For over a day, the GFS model has been predicting 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 and Northeast U.S. coasts on Sunday and Monday with heavy rain and high surf. In their latest 12Z (8 am EDT) Tuesday runs, the European and UKMET models have also jumped on this bandwagon, with the European model showing a very strong ridge building over TD 9 that forces a sharp left turn by the storm, back towards New England, after TD 9 approaches the Canadian maritime provinces. Thus, it looking increasingly possible that TD 9 could bring heavy rain and strong winds—potentially at tropical storm-force—to a large swath of the U.S. East Coast over the busy Labor Day weekend. Even if the storm’s rains and winds don’t do any damage, the lost business from a holiday weekend washout would run into the tens of millions of dollars. The uncertainties are high at this point, though—the diameter of NHC’s cone of uncertainty for their 5-day track forecast is about 550 miles.

Intensity forecast: TD 9 likely to stay below hurricane strength
TD 9’s failure to organize on Tuesday gives support to the thought that the storm will never significantly intensify in the Gulf of Mexico, but the latest 12Z (8 am EDT) Tuesday runs of the GFS and European models show a little more development of TD 9 compared to their previous ones. These latest runs show TD 9 as a tropical storm with 45 - 60 mph winds at the time of landfall on the Florida Gulf Coast on Thursday afternoon. The SHIPS model on Tuesday afternoon 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 afternoon, with landfall intensities for TD 9 ranging from 55 - 75 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 afternoon, NHC was calling for a maximum storm surge of 2 - 4’ above ground from TD 9 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.

92L off the coast of Africa embedded in dry air
A large tropical wave with plenty of spin that emerged from the coast of Africa on Monday evening was designated Invest 92L by NHC. The wave was moving through the Cabo Verde Islands on Tuesday and will potentially develop into a tropical depression later in the week. SSTs and wind shear would be favorable for development during the coming five days, but 92L is embedded in a major pulse of dust and dry air from the Sahara that left the coast at the same time, just to the north of 92L. 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 12Z Tuesday (8 am EDT) runs of two of our three top models for forecasting tropical cyclone genesis—the GFS and UKMET models—developed 92L, but not until 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 2 pm EDT Tuesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 0% and 40%, respectively.


Figure 3. Enhanced infrared satellite image of Hurricane Madeline as of 2100Z (5:00 pm EDT) Tuesday, August 30, 2016. Hawaii is outlined at far left. Image credit: NOAA/NESDIS.

Hurricanes Madeline and Lester barreling toward Hawaii
A Tropical Storm Warning and Hurricane Watch is in effect for the Big Island of Hawaii in advance of Category 3 Hurricane Madeline. A Tropical Storm Watch is also in effect for the islands of Maui County (Maui, Molokai, and Lanai). As of the 5:00 pm EDT advisory from the Central Pacific Hurricane Center (CPHC), Madeline was located about 370 miles east of Hilo, moving due west at 10 mph. Madeline’s top sustained winds were 115 mph, down from their peak of 135 mph on Monday night, and its once-clear eye became obscured by clouds on Tuesday afternoon. An Air Force Hurricane Hunter mission on Tuesday afternoon measured top flight-level winds of 100 knots (115 mph), with 115-knot (132-mph) surface winds detected by radiometer. As vertical wind shear increases to the 10-20 knot range, some fairly dry air will be pushed into the storm (mid-level relative humidities around 40-45%). Thus, Madeline’s strength should gradually drop over the next couple of days, despite SSTs warming slightly to 27-28°C. Madeline is expected to make its closest approach to Hawaii as a Category 1 hurricane.

Madeline should angle toward the west-southwest over the next couple of days, which may keep the center just south of the Big Island on Thursday morning. The official CPHC forecast brings Madeline less than 50 miles south of the southern tip of island. This track will put the island in the more dangerous right-hand (north) side of Madeline, slamming moisture-laden air and very strong winds against high mountains. Because of the inherent uncertainty in a 36-hour track forecast, there remains a possibility that Madeline will make landfall as the Big Island’s first hurricane in records going back to 1949.


FIgure 4. Tracking map for Hurricane Madeline as of 5:00 pm EDT (11:00 am HST) Tuesday, August 30.

Potential impacts from Madeline
Madeline’s area of hurricane-force winds is quite small, only about 25 miles in radius, but tropical storm force winds (sustained winds of 39 mph or greater) extend out to 125 miles. Thus, whether or not Madeline makes landfall as a hurricane, we can expect much or most of the Big Island—and perhaps parts of neighboring islands—to experience tropical storm conditions from late Wednesday into early Thursday, with very high winds, torrential rain (up to 15” in localized parts of the Big Island, with up to 4” in Maui County), and the potential for flooding and landslides. On the Big Island, some winds could be strong enough to produce considerable damage to trees, power lines, and roofs. Surf there is expected to reach at least 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.

The steep topography along the shore of the Hawaiian Islands tends to minimize storm surge while maximizing surf height. The highest storm surge on record for Hawaii was produced by Category 4 Hurricane Iniki (1992), which pushed waters 6 feet above normal astronomical tides and produced 30-foot surf. In a blog post on Tuesday afternoon, storm surge expert Hal Needham noted:

“On the Big Island, where Madeline’s impact will be felt the strongest, Hurricane Diana in 1972 generated surf levels that also reached 30 ft along the Puna coast, while storm surge levels at Hilo ranged from 4 to 5 ft (Central Pacific Hurricane Center 1972). Although this may not sound impressive, modest storm surges combined with tremendous wave heights can still be destructive, because waves are riding on top of the storm surge, enabling them to push a destructive force of water inland, well beyond the storm surge water limit. Diana’s combined storm surge and high surf swept four homes from their foundations and eroded 200 feet of a private road (Central Pacific Hurricane Center 1972).” He added: “The biggest concern for coastal flood potential will be on the southeastern shores of the Big Island, near the villages Naalehu and Pahala, where hurricane-force winds may wrap around the ‘back side’ of Madeline after its closest approach. Such winds come on strong from the opposite direction than they were previously blowing, quickly changing a low-water event into a sudden storm surge event, which can be surprisingly destructive.”


Figure 5. Satellite image of Hurricanes Madeline (left) and Lester (right) as of 2100Z (5:00 pm EDT) Tuesday, August 30, 2016. Image credit: NASA/MSFC Earth Science Office.

Next up for the weekend: Hurricane Lester
Hawaii has another storm to keep an eye on: Hurricane Lester. Located about 1280 miles east of Hilo and about 900 miles east of Madeline, Lester had top sustained winds of 120 mph as of the 5:00 pm EDT Tuesday advisory from NHC. Very favorable atmospheric conditions and modestly supportive SSTs around 27°C have allowed Lester to maintain its strength. From Thursday into Friday, Lester will be rolling directly over cooler waters churned up by Madeline, which could put a dent in its power. Wind shear is predicted by the SHIPS model to remain very low for at least the next 2-3 days before gradually increasing. The official NHC outlook brings Lester’s sustained winds down to minimal Category 1 strength (65 mph) by Saturday and strong tropical storm strength (70 mph) by Sunday.

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. The official NHC outlook continues to keep Lester 100-200 miles north of the islands on Saturday and Sunday. However, there is an increased note of uncertainty in the latest (12Z Tuesday) model guidance on Lester’s track. The UKMET and the averaged GFS ensemble runs (GEFS) bring Lester across the islands over the weekend, while the Euro’s path runs just north of Oahu and Kauai. Other models keep Lester farther north by various margins. The increased spread among model solutions is a reminder that paths can change by a large amount over a 4- or 5-day period. An upper-level high to the north of Lester will become quite strong later this week, implying that a southward track closer to the islands is certainly plausible. Tropical storm-force winds are predicted to extend outwards 90 miles to the southwest of Lester this weekend, putting most of the island chain at at the edge of getting sustained 40-mph winds should Lester follow the path predicted by CPHC.


Figure 6. Tracking map for Hurricane Lester as of 5:00 pm EDT (11:00 am HST) Tuesday, August 30.

The tracks of both Lester and Madeline may be influenced by the Fujiwhara effect, which causes two hurricanes that get within about 800 miles of each other to rotate around a common point in between, with that motion superimposed upon the primary storm motion. In a case like this, the easterm storm (Lester) would angle northward and the western storm (Madeline) would angle southward.

Wunderblogger Steve Gregory has an excellent update on the tropics in his Tuesday afternoon post, TD 9 Gets Better Organized/TD 8 to Skirt NC Coast.

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

Jeff Masters and Bob Henson