WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

Irene pounds the Dominican Republic, heads for the Bahamas

By: Dr. Jeff Masters, 3:32 PM GMT on August 23, 2011

Hurricane Irene is pounding the north coast of the Dominican Republic this morning with tropical storm-force winds and torrential rains, as the storm continues to head west-northwest towards the Bahama Islands. Puerto Plata on the north coast of the Dominican Republic reported sustained winds of 58 mph at 5am local time this morning, with heavy rain. In the Turks and Caicos Islands ahead of Irene, winds have gusted to 42 - 49 mph this morning on Providenciales at personal weather stations at the Regent Grand and at Pine Cay. The latest hurricane hunter eye report at 10:38am EDT found a central pressure of 980 mb, and top surface winds of 85 mph using their SFMR instrument. The plane had not finished sampling the storm yet.

Yesterday, Irene hit Puerto Rico as a tropical storm with 70 mph winds, reaching hurricane strength as it emerged into the Atlantic northwest of the capital of San Juan. No deaths or major injuries have been reported thus far from the islands, though the storm knocked out power to 1 million residents of Puerto Rico, including the entire island Vieques; 28% of Puerto Rico had no running water Monday afternoon. Billionaire Richard Branson's 8-bedroom mansion on private Necker Island in the British Virgin Islands was hit by lightning during the storm and burned down, and Branson relates on his blog how actress Kate Winslett had to carry out his 90-year-old mother from the main house to safety. Haiti has thus far escaped heavy rains from Irene, though the main danger comes today as the storm makes its closest approach to Haiti.

Figure 1. Morning satellite image of Irene, showing a hint of an eye starting to pop out.

Track forecast for Irene
Yesterday's dropsonde mission by the NOAA jet helped significantly narrow the uncertainty in the 1 - 3 day forecasts from the computer models. Irene will track through the Turks and Caicos islands today, the central Bahamas on Wednesday, and northwestern Bahamas on Thursday. However, the models still diverge considerably on their 4 - 5 days forecasts. One reliable model, the UKMET, takes Irene into South Carolina, while several others have the hurricane missing the Southeast U.S. completely, passing just offshore of the Outer Banks of North Carolina on Saturday. The official NHC forecast of a landfall along the North Carolina coast is a reasonable compromise, though with the models trending more eastwards of late, I would favor a landfall farther east than NHC is predicting. Irene will continue north or curve northeast after its encounter with North Carolina, and the hurricane could be a dangerous and destructive storm for the entire mid-Atlantic and New England coast.

A research project funded by NOAA known as the Joint Hurricane Testbed has produced a remarkable number of tools now in operational use at the National Hurricane Center to improve hurricane forecasts and warnings. One of these projects, called "Prediction of Consensus TC Track Forecast Error and Correctors to Improve Consensus TC Track Forecasts", was an effort by Dr. Jim Goerss at the Navy Research Lab to improve the accuracy of the NHC "cone of uncertainty" (AKA the "Cone of Death") showing where a storm is expected to track 2/3 of the time. The radius of the circles that make up the cone are based on error statistics of the official NHC forecast over the past five years. We can expect in certain situations, such as when the models are in substantial disagreement, a consensus forecast made using these models will have much greater than average errors. Since the NHC typically bases their forecast on a consensus forecast made using a combination of reliable hurricane forecasting models, it is instructive to view the "GPCE" (Goerss Prediction Consensus Error) circles to see if the uncertainty cone should be smaller or larger than usual. The consensus forecast I'll look at is called "TVCN", and is constructed by averaging the track forecasts made by most of (or all) of these models: GFS, ECMWF, NOGAPS, GFDL, HWRF, GFDN, and UKMET. In the case of this morning's 12 UTC (8am EDT) runs of these models, here is what the radius of the "cone of uncertainty" should be, in nautical miles:

12 hours: 27 nm
24 hours: 44 nm
36 hours: 64 nm
48 hours: 81 nm
72 hours: 137 nm
96 hours: 201 nm
120 hours: 308 nm

And here is the radius of NHC's "cone of uncertainty" for their official forecast, based on the average errors for the past five years:

12 hours: 36 nm
24 hours: 59 nm
36 hours: 79 nm
48 hours: 98 nm
72 hours: 144 nm
96 hours: 190 nm
120 hours: 239 nm

So, the GPCE error estimates are showing that the latest forecasts for Irene are better than average over the 1 - 3 day time period, and worse than average for 4 - 5 days. Note the error estimate of 308 nm (355 miles) for today's 5-day forecast. That's more than the distance from New York City to Boston, suggesting that we really don't know what portions of New England might be at most risk from Irene. It is still quite possible the core of the hurricane could miss New England.

Intensity forecast for Irene
Latest microwave data suggests that Irene does not have full eyewall; a gap exists in the southwest side. With wind shear now a moderate 10 - 20 knots, Irene may have trouble intensifying today. The hurricane is embedded in a large envelope of moisture, and wind shear is expected to remain moderate, 10 - 20 knots, for the next four days. With water temperatures very warm, 29 - 30°C, these conditions should allow for intensification to a Category 3 storm sometime in the next two days. Satellite loops show that Irene is steadily growing in size, which will protect the storm against major disruption by wind shear. The storm is lacking much development on its southwest side, where the presence of Hispaniola is interfering with development. Once Irene pulls away from Hispaniola tonight, intensification is more likely.

Irene's impact on the Turks and Caicos Islands
Heavy rains from Irene have already reached the Turks and Caicos Islands, which form the southeastern portion of the Bahama Islands chain. The last hurricane to affect the Turks and Caicos islands was Hurricane Ike of 2008. Ike's northern eyewall devastated Grand Turk, Salt Cay, South Caicos, and a few other smaller cays when the storm was at Category 4 strength. Ike then weakened slightly to a Category 3 storm with 125 mph winds before making a direct hit on Great Inagua Island. Approximately 70-80% of the houses on Great Inagua Island sustained roof damage, and 25% had major damage or were destroyed. The Morton Salt factory on the island was forced to halt operations as Ike damaged its offices and loading docks. A few West Indian flamingos were killed by Ike but most of the 50,000 flamingos in Inagua National Park--the world's largest colony--survived by taking shelter within the park's mangroves or flying to other islands. Risk Management Solutions estimates that total damage costs are between $50 and $200 million (USD) for the Turks and Caicos and the Bahamas. Irene will be weaker than Ike, so will not do as much damage. The main threat from Irene will be wind damage.

Figure 2. The path of Hurricane Ike of 2008 took it through the Turks and Caicos Islands as a Category 4 hurricane with 135 mph winds (pink colors).

Irene's impact on the Bahama Islands
Irene will pass through the length of the Bahama Island chain and cause widespread destruction on those islands unfortunate enough to encounter the storm's right front eyewall. Currently, it appears that Crooked, Cat, Exuma, Eleuthera, and Abaco Islands are all in danger of experiencing the eyewall of Irene, which will be capable of bringing storm surges of 9 - 13 feet. The current forecast puts the Bahamas' two most developed islands--New Providence and Grand Bahama--on the weaker west side of Irene, where Category 1 hurricane winds are likely. These winds will likely cause considerable but not devastating damage. Irene will come very close to the capital city of Nassau on New Providence Island on Thursday morning, home to 70% of the population of the Bahamas. Nassau has received direct hits from three major hurricanes since 1851--the Category 4 Nassau Hurricane of 1926, which killed 287 people, a Category 4 hurricane in 1866 that killed 387 people, and a Category 3 hurricane in August 1949. The island is vulnerable to high storm surges--a ten-foot storm surge is theoretically possible on the south shore of Nassau in a Category 3 hurricane. However, the south shore of the island is relatively undeveloped, and the city of Nassau and Paradise Island are mostly higher than ten feet in elevation. A much higher storm surge of 20 feet is possible along the southwest shore of Exuma Island, but again, this shore is not heavily developed.

Figure 3. The height above ground that a mid-strength Category 3 hurricane with 120 - 125 mph winds would push a storm surge in a worst-case scenario. The image was generated using the primary computer model used by the National Hurricane Center (NHC) to forecast storm surge--the Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model. The accuracy of the SLOSH model is advertised as plus or minus 20%. This "Maximum Water Depth" image shows the water depth at each grid cell of the SLOSH domain. Thus, if you are inland at an elevation of ten feet above mean sea level, and the combined storm surge and tide (the "storm tide") is fifteen feet at your location, the water depth image will show five feet of inundation. This Maximum of the "Maximum Envelope of Waters" (MOM) image was generated for high tide and is a composite of the maximum storm surge found for dozens of individual runs of different Category 3 storms with different tracks. Thus, no single storm will be able to cause the level of flooding depicted in this SLOSH storm surge image.

Irene a potential multi-billion dollar disaster for New England and the mid-Atlantic
Though it is still possible the core of Irene will miss the U.S., the current NHC official forecast would mean that Irene would bring destructive flash flooding, significant beach damage, and widespread power outages due to tree damage along the entire U.S. coast from North Carolina to Maine, costing several billion dollars. If Irene ends up skirting the Outer Banks of North Carolina and not significantly weakening, then plowing through the mid-Atlantic and New England states as a Category 1 or 2 hurricane, it could become one of the ten most damaging hurricanes in history. The latest 06Z (2am EDT) run of the GFS model puts Irene ashore in Southeast Massachusetts on Sunday afternoon as a large storm with a central pressure of 974 mb. The latest run of the ECMWF model has Irene with a central pressure of 964 mb over Chesapeake Bay, and 972 mb over New Jersey. These central pressures correspond to strong tropical storm or Category 1 hurricane strengths, and are similar to what Hurricane Floyd or 1999 had when it moved up the mid-Atlantic coast after hitting North Carolina as a Category 2 hurricane. Floyd was the 14th most damaging hurricane in history, with total damages estimated at $9.2 billion (2010 dollars.) Most of the damage was in North Carolina, which experienced its worst flooding on record. If the GFS and ECMWF models are correct, Irene could easily be a $10 billion hurricane, causing widespread damage along a long section of heavily populated coast. The most damaging Northeast U.S. hurricane of all time was Hurricane Agnes of 1972, with damages estimated at $11.8 billion (2010 dollars.) Currently, it appears that Irene will hit North Carolina on Saturday, and New England on Sunday. I strongly urge all residents of the coast from North Carolina to Massachusetts to assess their hurricane preparedness immediately, and anticipate the possibility of hurricane conditions this weekend.

For those of you wanting to know your odds of receiving hurricane force or tropical storm force winds, I recommend the NHC wind probability product.

Wunderground has detailed storm surge maps for the U.S. coast.

Elsewhere in the tropics
There are two tropical waves far out in the eastern Atlantic, Invest 90L and Invest 98L, that NHC is giving 20% chances of developing into tropical depressions by Thursday. At present, the long-range models are showing that both of these disturbances will not be a threat to any land areas over the next seven days, and will probably move too far north to ever be a threat to land.

Texas/Oklahoma heat wave sets all-time 100° records
The unprecedented heat wave gripping Texas and Oklahoma set several new all-time heat records yesterday. The high temperature hit 101° at the Houston Intercontinental Airport yesterday, the 22nd consecutive day of 100°+ heat and 33rd day of 100°+ heat in the city. Both are all-time records for the city. Oklahoma City recorded its 51st day of 100°+ temperatures yesterday, breaking the record for most such days in year, set previously in 1980. Temperature records for Oklahoma City date back to 1891.

Jeff Masters
Hurricane Irene
Hurricane Irene
Ragged eye of Irene
This is the scene around Lake Sam Rayburn. Dead fish every 20 yards or so and dead fresh water clams everywhere. The lake is 11 feet low and is losing about an inch a day.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.