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Tropical Storm Richard is likely later today

By: Dr. Jeff Masters, 1:48 PM GMT on October 21, 2010

The Hurricane Hunters are in Tropical Depression Nineteen, and have found winds of tropical storm force that support upgrading the depression to Tropical Storm Richard. Between 8:15 - 8:30am EDT, the Air Force hurricane hunter aircraft measured surface winds of 40 - 70 mph with their SFMR instrument in the heavy thunderstorm region on the east side of TD 19's center. Winds at the aircraft's flight level of 1400 feet peaked at 46 mph. These measurements support upgrading TD 19 to at least a 40 mph tropical storm. Winds have been steadily rising this morning at NOAA buoy 42057, located about 50 miles west-southwest of the center of TD 19, on its weak side. Winds were 27 mph, gusting to 29 mph, at 7:43am EDT this morning. Recent satellite imagery shows that the surface circulation center of TD 19 is nearly exposed to view, thanks to moderate wind shear of 10 - 20 knots from upper-level southwesterly winds. TD 19 has a moderate and increasing amount of heavy thunderstorm activity which is getting more organized, with a curved spiral band forming on the storm's south side. The storm is bringing very heavy rain to Jamaica. Water vapor satellite loops show considerable dry air to the west and north of TD 19, and the southwesterly winds over the storm are bringing some of this dry into the core of the storm, keeping all the heavy thunderstorm development confined to the east side of the center. The waters beneath TD 19 are very warm, 29°C, but TD 19 will not be able to take full advantage of these warm waters until the shear relaxes and stops pushing dry air into the core.

Figure 1. Morning satellite image of TD 19.

Intensity forecast for TD 19
The latest SHIPS model forecast predicts that wind shear over the Western Caribbean will remain in the moderate range, 10 - 20 knots, through Friday morning. As the storm moves westwards on Friday, it will position itself beneath an upper-level high pressure system, which will allow shear to drop to the low range, less than 10 knots. With water temperatures a very warm 29°C and warm waters extending to great depth, TD 19 should be able to attain at least Category 1 hurricane strength by Saturday. NHC is currently giving TD 19 a 3% chance of becoming a major Category 3 hurricane. Given the latest data from the Hurricane Hunters and the latest set of computer models runs, I believe the odds are higher, near 30%. The main inhibiting factor for intensification will be the possibility of the dry air to the west of TD 19 getting wrapped into the core of the storm while it is trying to organize.

Track forecast for TD 19
Steering currents are weak in the Western Caribbean, and will remain weak through Friday morning, resulting in a slow, erratic movement for TD 19. A slow drift to the south is the most popular track expected by the major models. Thus, Jamaica can expect rains to increase and become torrential at times until Saturday, when the storm will finally move off. The Cayman Islands and possibly the north coast of Honduras can also expect very heavy rains of 3 - 6 inches Friday through Saturday. By Friday afternoon, a ridge of high pressure is expected to build in, forcing TD 19 to the west or west-northwest, bringing the storm to a landfall in Belize or Mexico's Yucatan Peninsula Sunday night. The models are divided on what might happen after Sunday, with the GFS, NOGAPS, and UKMET models indicating a continued west-northwest track taking TD 19 across the Yucatan Peninsula and into the Gulf of Mexico, where high wind shear would destroy the storm early next week before it could make landfall in the U.S. The other model solution, provided by the ECMWF, GFDL, and HWRF models, is for TD 19 to get caught up by a trough of low pressure moving across the Eastern U.S. early next week, which would take the storm to the northwest through the Yucatan Channel and into the west coast of Florida as early as Monday night. A band of very strong upper-level winds associated with the jet stream will be over the Gulf of Mexico early next week, so it is likely that if TD 19 follows this track, the storm will be weakening quickly as it approaches Florida. Either solution is possible, and we will have to wait to see what future model runs show will happen. The 5am EDT NHC wind probability forecast is giving the highest odds for tropical storm-force winds at Guanaja in Honduras and Cozumel in Mexico, at 34% and 37%, respectively. Key West is the only U.S. city being given odds, and these are just 3%. These odds will very likely rise with the 11am NHC advisory.

Invest 90L
A tropical wave that emerged off the coast of Africa yesterday (Invest 90L) has a modest amount of spin and some growing thunderstorm activity. NHC is giving the system a 10% chance of developing into a tropical depression by Saturday. This system is not a threat to cross the Atlantic and affect the Lesser Antilles or North America.

Typhoon Megi takes aim at China
Typhoon Megi continues it slow march towards China at 5 mph, and is expected to make landfall Saturday morning on the Chinese coast opposite from Taiwan. Megi has been gradually declining in strength as it heads north, due to steadily rising wind shear (now a moderate 10 - 20 knots) and cooling sea surface temperatures. Megi is moving slow enough and is large and powerful enough that it is probably upwelling cold water from the depths to the surface faster than it can move away, and these upwelling cool waters are keeping Megi from being a major Category 3 typhoon. Wind shear will increase dramatically to 20 - 40 knots on Friday as the typhoon makes its final approach to the coast of China, and this shear should be high enough to reduce Megi to Category 1 status before landfall. Megi will still be a very large and powerful storm capable of causing considerable wind and storm surge damage even at Category 1 strength. However, heavy rain will likely be the storm's main threat, since it is moving slowly and is a huge storm. I expect Megi will be a billion-dollar disaster for China, mostly due to flooding from heavy rains. The outer rain bands of Megi are already affecting the coast of China near Taiwan, as seen on China's radar composite, as well as Taiwan radar.

The clean-up continues in the Philippines from Megi, which hit northern Luzon island on Monday morning at 3:30 UTC as a Category 5 super typhoon with sustained winds of 165 mph and a central pressure of 914 mb. Severe damage was done to Isabela Province in northern Luzon, and 19 deaths are being blamed on the storm. Considering most major typhoon that have hit the Philippine in recent year have killed hundreds and sometimes thousands of people, the low death toll from Megi is a testament to the excellent efforts by officials in the Philippines to get people out of harm's way in advance of the storm.

Figure 2. Rainfall rate for Megi as observed by the F-17 polar orbiting satellite at 5:49am EDT October 21, 2010. Heavy rains in excess of one inch per hour (orange colors) were present on the south side of Megi's eye, and a region of heavy rain was also present in a spiral band approaching the coast of China opposite from Taiwan. Image credit: Navy Research Lab, Monterey.

Next update
I'll have an update 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.