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Rina still a Cat 2, and may have peaked in intensity

By: Dr. Jeff Masters, 1:48 PM GMT on October 26, 2011

There is little change to Category 2 Hurricane Rina, which continues slowly west-northwest at 4 mph towards Mexico's Yucatan Peninsula. The most recent hurricane hunter mission was at 3:58 am EDT this morning, and the next aircraft is not due into the storm until this afternoon, so we'll have to rely on satellite estimates of the storm's strength until then. Recent satellite intensity estimates suggest Rina has peaked in intensity, and may have weakened slightly since the last hurricane hunter mission. The eye is no longer visible on visible satellite loops, and the storm has a distinctly lopsided appearance, which are both signs that Rina may be weakening. Recent microwave images (Figure 1) suggest that the southern portion of Rina's eyewall may have a gap in it. Wind shear is a moderate 10 - 20 knots due to upper-level winds out of the south. Water temperatures are very warm, 29 - 30°C, but Rina will be leaving a region of high oceanic heat content late tonight, and will not have as much high-energy fuel to sustain itself. Rina has brought sporadic heavy rain squalls to the Yucatan; Cozumel Island airport reported 9.10" of rain yesterday and another 3.25" as of 9 am EDT today. I'm not sure this is correct, since two personal weather stations on the island reported only 1 - 2" of rain over the past two days. The outer rain bands of Rina are visible on Cancun radar and the La Bajada, Cuba radar.

If Rina does make it to Category 3 strength, it would join a very short list of major hurricanes that have occurred this late in the year. Since record keeping began in 1851, there have been only twelve major hurricanes in Atlantic on October 26 or later. Rina is a medium-small hurricane, with hurricane-force winds that extend out 25 miles from the center. The latest estimates of Rina's Integrated Kinetic Energy (IKE) made yesterday afternoon by NOAA's Hurricane Research Division put the damage potential of Rina's storm surge at 2.5 on a scale of one to six. This is a relatively low number for a Category 2 hurricane, and means that storm surge damage will be confined to a relatively small area in the right front quadrant of Rina's eyewall. For comparison, the storm surge damage potential for Hurricane Irene when it was a strong tropical storm approaching Long Island, New York on August 28, 2011 was 4.1 on a scale of 1 to 6, since Irene was a huge storm that put vast areas of the ocean into motion.

Figure 1. Microwave satellite image of Hurricane Rina taken at 7:15 am EDT October 26, 2011. Rina appears to only have the northern half of its eyewall intact. Image credit: Navy Research Lab, Monterey.

Forecast for Rina
The hurricane hunters found that Rina had a large elliptical eye with a diameter of 23 - 34 miles with a gap in the south eyewall during their 3:58 am EDT eye penetration this morning. Rapid intensification usually requires a circular eye with no gap in it, and thus I expect only gradual intensification of Rina can occur today. Wind shear is not expected to increase to the high range until Thursday afternoon, so Rina still has a day and a half to potentially intensify. Given the storm's inability to close off its eyewall so far, I expect that a Category 3 storm is the strongest that we will see. It is more likely that Rina has peaked in strength, and will begin to weaken. On Thursday, Rina will encounter high wind shear associated with upper-level westerly winds associated with the jet stream and a trough of low pressure moving through the Gulf of Mexico. These conditions should weaken the hurricane, and turn Rina more to the northwest by Thursday and northeast on Friday. Cozumel Island off the coast of Mexico's Yucatan Peninsula appears at the highest risk of receiving a direct hit from Rina; the 5 am EDT Wind Probability Forecast from NHC gave the island a 27% chance of receiving hurricane force winds, the highest chance of any land area in the forecast. If Rina hits Mexico, it would most likely be at Category 1 strength, with Category 2 strength also quite possible. NHC is giving Rina a 21% chance of being a Category 3 or stronger hurricane on Thursday afternoon, and I don't expect the storm will be a major hurricane at landfall. The chief threat from Rina is probably its rains; the hurricane is moving slowly and has the capability of dumping 8 - 16 inches of rain over the coastal Yucatan. Western Cuba is also at risk of receiving heavy rains of 5 - 10 inches.

After Rina makes its closest pass by Cozumel, it is uncertain if the storm will be strong enough to fully "feel" the steering influence of the trough, and be swept to the east-northeast into Southwest Florida and the Florida Keys. If Rina makes landfall in the Yucatan Peninsula and weakens significantly, the storm will likely be too weak to get caught up by the trough and will remain trapped in the Western Caribbean. This is the solution of the latest runs of the ECMWF and HWRF models, with the usually reliable ECMWF model continuing to predict that Rina will dissipate over the Yucatan. However, if Rina grazes the Yucatan and remains strong through Friday, it is more likely to get caught up by the trough and drawn into the Florida Keys or Southwest Florida near Marco Island, as a weakening tropical storm. This is the solution of the latest 2 am EDT run of the GFDL model. The latest 2 am EDT runs of the GFS and NOGAPS models are in-between, predicting that Rina will get very close to the Keys on Saturday, but then weaken and sink southwards towards Cuba. There is high degree of uncertainty which set of model runs will be correct, and the threat to Florida depends strongly upon how much of a blow Mexico receives from the hurricane.

97L in the Central Caribbean not expected to develop
A broad region of low pressure in the Central Caribbean south of Haiti and the Dominican Republic, (Invest 97L), is moving west at 10 - 15 mph. Heavy thunderstorm activity has decreased since yesterday, and is disorganized. The storm is surrounded by a large region of dry air, and this dry air is the main impediment to development. Wind shear is a low 5 - 10 knots, and is expected to remain low through Thursday. None of the reliable models are predicting that 97L will develop, and NHC gave 97L just a 10% chance of developing into a tropical depression by Friday, in their 8 am EDT Tropical Weather Outlook. NHC is no longer interested enough in 97L to generate computer model forecasts of its track. Heavy rains from 97L should reach Jamaica on Thursday, and the Cayman Islands by early Friday morning.

There will be a new post on Rina late this afternoon.

Jeff Masters


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