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Tropical Storm Rina forms

By: Dr. Jeff Masters, 1:49 PM GMT on October 24, 2011

Tropical Storm Rina formed in the Western Caribbean Sea just off the coast of Honduras last night, and is headed north-northwest towards Mexico's Yucatan Peninsula. Rina's formation brings this year's tally of named storms to seventeen, making it the 7th busiest Atlantic hurricane season since record keeping began in 1851. Only 2005, 1933, 1995, 1887, 2010, and 1969 had more named storms. However, 2011 has had an unusually low percentage of its named storms reach hurricane strength. Only 29% of this year's named storms have made it to hurricane strength (five), and normally 55 - 60% of all named storms intensify to hurricane strength in the Atlantic. The rare combination of near-record ocean temperatures but unusually dry, stable air over the Atlantic is no doubt at least partially responsible for this very unusual occurrence.

Visible satellite loops show that Rina has had a respectable burst of thunderstorm activity with high, cold cloud tops this morning. This "Central Dense Overcast" (CDO) is characteristic of intensifying tropical storms that are a threat to reach hurricane strength. Rina has been bringing sporadic heavy rain squalls to the Cayman Islands over the past day; George Town on Grand Cayman Island had received 2.28" of rain and a peak wind gust of 31 mph as of 9 am EDT from the storm. Wind shear is a moderate 15 - 20 knots due to strong upper-level winds out of the southeast, and these winds are injecting dry air into Rina's southeast side, inhibiting heavy thunderstorm development there. Water temperatures are very warm, 29 - 30°C, and these warm waters extend to great depth. We don't have any surface wind reports from the vicinity of Rina; the closest buoy is Western Caribbean buoy 42057, about 60 miles east of Rina's center, which had top sustained winds out of the ESE at 29 mph this morning. We'll have to wait for the next hurricane hunter flight, scheduled for 2 pm this afternoon, to get a better idea of Rina's intensity.

Figure 1. Morning satellite image of Rina.

Forecast for Rina
The intensity forecast for Rina has a high amount of uncertainty. Rina should be able to slowly intensify through Tuesday, becoming a strong tropical storm. On Wednesday, Rina will be approaching a dry airmass with high wind shear that lies over the extreme northwestern Caribbean. Since Rina is a small storm, these hostile conditions could cause the storm to dissipate on Wednesday as it nears landfall over the Yucatan Peninsula, as predicted by the ECMWF and NOGAPS models. The completely opposite scenario is predicted by the GFDL and HWRF models, which forecast Rina will stay just south of the high shear/dry air region, and attain major hurricane status. The official NHC forecast of a Category 1 Hurricane Rina late this week is a reasonable compromise between these extremes. The track forecast for Rina also presents difficulties. A west-northwest to northwest motion towards the Yucatan Peninsula is likely through Wednesday. A trough of low pressure is predicted to pass to the north of Rina late this week, but the models are increasingly suggesting that Rina will not be far enough north to get caught up in the trough, but instead will remain trapped in the Western Caribbean. None of the latest 06Z (2 am EDT) model runs of the GFS, NOGAPS, GFDL, or HWRF models are predicting that Rina's center will make it north of Cuba during the next five days. In any case, heavy rains from Rina should begin affecting Mexico's Yucatan Peninsula, northern Belize, and extreme Western Cuba on Wednesday.

A broad region of low pressure about 300 miles west of the southern Lesser Antilles Islands (Invest 97L), is moving west-northwest at 10 mph through the Caribbean Sea. This low has very limited heavy thunderstorm activity, due to dry air, and no signs of a surface circulation. NHC is giving 97L just a 10% chance of developing into a tropical depression by Wednesday. 97L is under low wind shear less than 10 knots, and this shear is expected to remain low through Thursday. By the time 97L reaches the region between Jamaica and Nicaragua in the Central Caribbean on Thursday or Friday, the storm should find a moister environment, and could develop into a tropical depression. The NOGAPS model predicts 97L could develop into a tropical depression by Saturday.

Jeff Masters


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