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Little change to Hanna; Cat 4 Ike slowly weakening

By: Dr. Jeff Masters, 3:00 PM GMT on September 04, 2008

Tropical Storm Hanna is a large and very odd looking near-hurricane. In fact, it looks to me like it is more like a subtropical storm than a tropical storm. A subtropical storm tends not to have any heavy thunderstorm activity near its center. Instead, the heaviest rain is located in a band 100 or more miles from the center. Satellite loops show that this is the case with Hanna. Wind shear of 20 knots continues to interfere with Hanna's organization, and there is also plenty of dry air to its west that is causing the storm trouble. The amount and intensity of Hanna's thunderstorms has not changed much in the past day, and the central pressure has remained in the 985-990 mb range. The latest Hurricane Hunter report found a central pressure of 989 mb, with surface winds of 60 mph, at 6:33 am EDT.

Figure 1. Latest satellite image of Hanna.

Hanna hits Haiti hard
Hanna's flooding rains have killed at lest 61 people in northern Haiti. Hardest hit was the northern town of Gonaives, a city of 110,000 where over 2,000 people perished in 2004 due to rains from Hurricane Jeanne. Satellite estimates suggest 6-8 inches of rain fell on northern Haiti and the northern Dominican Republic due to Hanna. The death toll is likely to go higher, since rescuers have not been able to reach surrounding remote areas. Rains from Hurricane Ike are likely to make the misery worse of Sunday, when the hurricane is expected to pass north of Haiti and bring at least 2-4 more inches of rain. Northern Haiti is highly prone to flooding disasters, due to the steep mountainsides in the region that have had 98% of their forests chopped down.

The track forecast for Hanna
The computer models have come into a bit better agreement on Hanna's track, due in part to the data from a dropsonde mission flown last night by the NOAA jet. Hanna will come ashore in North Carolina or northern South Carolina. The exact landfall location is relatively unimportant in this case, since Hanna is a large storm with the winds well removed from the center. Both North Carolina and South Carolina will see winds near hurricane strength from this storm. Given that Hanna is so large and fast-moving, the entire mid-Atlantic and New England coast will see sustained winds of at least tropical storm force (39 mph) this weekend as Hanna zooms northeast.

The intensity forecast for Hanna
The wind shear is forecast to remain in the moderate to marginal range, 15-25 knots, over the remainder of Hanna's life. There is a large amount of dry continental air lying between Hanna and South Carolina, which will continue to cause problems for the storm. However, sea surface temperatures are a warm 29°C. The main intensity models, which yesterday thought that Hanna would not become a hurricane--have changed their tune. The HWRF the the most aggressive, predicting that Hanna will come ashore in northern South Carolina as a Category 1 hurricane with 90 mph winds early Saturday morning. The other intensity guidance is less aggressive, predicting top winds in the 55-75 mph range at landfall. Given the rather subtropical appearance of Hanna, with the heaviest thunderstorms well removed from the center, plus the rather high wind shear over the storm, rapid intensification is very unlikely. A tropical storm needs to have its heaviest thunderstorms close to the center in order to undergo significant strengthening. Hanna should intensify slowly, if at all, and be no stronger than a Category 1 hurricane with 80 mph winds at landfall in the U.S.

Figure 2. Latest satellite image of Ike.

Hurricane Ike has become a large and dangerous Cape Verdes-type hurricane much earlier than expected. With a remarkable burst of rapid intensification, Ike went from a tropical storm with 70 mph winds to a Category 4 hurricane with 135 mph winds in just 12 hours. We're lucky Ike wasn't bearing down on the Florida Keys, Tampa, or some other vulnerable populated area when this very rapid and unforecast strengthening occurred. There would have been no time to evacuate, resulting in heavy loss of life. It's situations like this that scare the bejeebers out of hurricane forecasters, and make us call for the very reasonable sums of money needed to be invested to improve hurricane intensity forecasts. We can do much better with intensity forecasts if we spend a few tens of millions per year more in such efforts. The payoff could well be the ability to foresee rapid intensification like Ike's, and prevent a major catastrophe.

Visible satellite loops of Ike show the classic signatures of a major hurricane--a well-formed eye, plenty of spiral bands, and well-established upper-level outflow. However, the storm has a squashed appearance, and is missing a chunk on the northwest side. This is due to 20 knots of wind shear impacting the storm, thanks to strong upper-level winds out of the northwest. However, Ike has formed a strong eyewall, and the high angular momentum of these eyewall winds are keeping the shear from disrupting its inner core.

Track forecast for Ike
The models are split into two distinct camps on the 1-3 day track of Ike. The UKMET/GFS/HWRF models allow Ike go go a bit further north initially, then show less of a southerly component of motion than the other models. The other camp of models, the NOGAPS/ECMWF/GFDN/GFDL, take Ike further south, and have more of a southward component of motion, with a threat to Cuba and Hispaniola by this weekend. The NHC forecast splits the difference between these extremes, and probably has higher uncertainty than average. Climatology, as seen in the latest historical comparison of similar hurricanes in the past, favors the more northerly track. Only one out of ten similar past storms has made landfall in the U.S. as a hurricane.

The longer term fate of Ike remains highly uncertain--as usual. If Ike follows the southern camp of models, it may hit eastern Cuba, as forecast by the latest (06Z, 2am EDT) runs of the NOGAPS and GFDL models. A hit on Cuba would severely disrupt the storm, weakening it to a Category 1 or 2. If Ike misses Cuba, South Florida can expect a highly dangerous major hurricane on its doorstep Tuesday. On Tuesday, a trough of low pressure is forecast by most of the models to turn Ike to the north. The timing and strength of this trough will be critical in determining the fate of South Florida. The GFS model turns Ike well east of Florida, sending the storm out to sea without affecting the U.S. The ECMWF model turns Ike directly over South Florida, while the NOGAPS model foresees recurvature just offshore, through the western Bahama Islands. It is impossible to know at this time when or if Ike will turn to the north, and whether Florida might be spared the full brunt of Ike. Ike may be a threat to North Carolina in the longer term, and one possible scenario for the hurricane would be a repeat of Hurricane Floyd of 1999. Floyd bore down on Florida as a borderline Category 4/5 hurricane before turning at the last moment, eventually hitting North Carolina as a Category 2 hurricane. Another scenario, which is suggested by the ECMWF model, is that Ike would recurve but not get pulled all the way out to sea. Instead, Ike might get trapped in a region of weak steering currents and wander for a few days, like Fay and now Gustav have done. This could occur offshore the East Coast, or over the Florida Peninsula.

Intensity forecast for Ike
Sea surface temperatures (SSTs) have warmed to 28.5°C and will warm an extra degree over the next two days. However, the shear is forecast to be unfavorable for intensification over the next two days, 20-30 knots. The high shear may be able to disrupt the inner core of Ike, reducing it to a Category 2 storm by Saturday morning. Re-intensification is likely beginning on Saturday night, when the shear is forecast to fall below 5 knots. On Sunday, when Ike may be approaching the southeastern Bahamas, it should encounter cooler waters stirred up by Hurricane Hanna. This may limit the re-intensification of Ike. The primary danger in the islands will be wind damage, since the storm surge typically flows around small islands. However, a Category 4 or 5 hurricane is capable of generating a maximum storm surge of 13 feet in the Turks and Caicos Islands on the right side of the eye, if it makes a direct hit on an island.

The first Hurricane Hunter mission into Ike is scheduled for Friday morning.

Tropical Storm Josphine is a long way out to sea, and it will be at least a week before it may threaten any land areas. Visible satellite loops show a moderately well-organized system that is having some struggles with dry air and wind shear.

Elsewhere in the tropics
The GFS model is forecasting that at least one more tropical wave will move off the coast of Africa over the next ten days and develop into a tropical storm.

My next blog entry will be this afternoon.

Jeff Masters

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

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