Gulf of Mexico's 93L a Heavy Rain Threat; Ana Leaves Hawaii Alone

An area of low pressure over the Gulf of Mexico's Bay of Campeche (93L) contains moisture and spin from the Eastern Pacific's Tropical Storm Trudy, which made landfall near Acapulco last weekend. 93L will bring heavy rains to Mexico's Yucatan Peninsula, Western Cuba, and South Florida Wednesday through Friday. Satellite loops show the low has a moderate degree of spin and plenty of heavy thunderstorm activity, but these thunderstorms are poorly organized, due to high wind shear of 30 knots. Mexican radar out of Altamira does show at least one spiral band had formed near the coast Tuesday morning, though. Water vapor satellite images show there is dry air from Mexico flowing eastwards over the western Gulf of Mexico, which is likely slowing development. Sea Surface Temperatures (SSTs) are very warm, about 29.5°C. The 8 am EDT Tuesday run of the SHIPS model predicted that wind shear would fall to the moderate range, 10 - 20 knots, Wednesday - Friday, giving 93L a better chance to develop then. The Tuesday morning runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, all showed support for some slow development of 93L this week. The storm is likely to move slowly eastwards across the Bay of Campeche on Tuesday and Wednesday, cross over the Yucatan Peninsula on Thursday, move through the Florida Straits between Cuba and South Florida on Friday, then into the Bahamas on Saturday. Along its path, 3 - 6" of rain are are likely--with higher rainfall amounts to be expected if 93L ends up developing into a tropical depression. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 93L 2-day and 5-day development odds of 40% and 50%, respectively. A hurricane hunter mission is scheduled to investigate 93L Tuesday afternoon, but may be cancelled.

The prospects of 93L developing into a damaging hurricane are very low, and this storm is primarily a heavy rain threat. However, both the GFS and European models show the possibility that the trough of low pressure expected to pick up 93L and pull it northeastwards out to sea this weekend may leave behind an area of spin in the Western Caribbean early next week that would potentially have the capability to develop into a more dangerous tropical cyclone than 93L. It's too early to be sold on this model solution yet, but we should pay attention to the evolution of this storm system over the coming week.


Figure 1. Latest satellite image of Invest 93L in the Gulf of Mexico.


Figure 2. Predicted precipitation for the 5-day period ending Sunday, October 26, 2014. 93L is predicted to bring rainfall amounts of up to five inches to South Florida. Image credit: National Weather Service.

Eastern Atlantic disturbance 92L little threat
A large non-tropical low pressure system spinning in the Eastern Atlantic a few hundred miles south-southeast of the Azores Islands (92L) brought heavy rains and flash flooding that killed five people in the Canary Islands on Sunday. This low is headed slowly westwards into a region with higher wind shear, and should not affect any more land areas. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 2-day and 5-day development odds of 10%.


Figure 3. MODIS satellite image of Tropical Storm Ana brushing the Hawaiian Islands at 7:55 pm EDT on Monday October 20, 2014. At the time, Ana had top winds of 65 mph, and high wind shear had allowed the surface circulation to be exposed to view. Image credit: NASA.


Figure 4. Radar-estimated rainfall from the Molokai radar for Hurricane Ana shows that extreme rains of 15+" fell just 20 miles off the coast from Honolulu.

Tropical Storm Ana headed away from Hawaii
Tropical Storm Ana is headed northwestwards away from the Hawaiian Islands; all watches and warnings have been dropped for the main Hawaiian Islands. Heavy rains of 4.72" fell in Honolulu from Ana, and widespread rain amounts of 4 - 7" were reported on Oahu. The island was very fortunate, though, since a large area of 15+" of rain fell just 20 miles offshore, according to radar estimates. Satellite loops on Tuesday morning showed that Ana was having trouble with high wind shear, with the surface circulation partially exposed to view. Ana will turn north and then northeast over the next few days and gradually weaken over cooler waters, without affecting any other land areas.

Jeff Masters