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Dr. Jeff Masters' WunderBlog

Tropical Storm Erika likely soon; a weaker Jimena powers towards Baja


Posted by: JeffMasters, 7:48 PM GMT on September 01, 2009

Hurricane Jimena has weakened to a low-end Category 4 hurricane with 135 mph winds, thanks to the collapse of its inner eyewall. The hurricane is undergoing a process common in intense hurricanes called an Eyewall Replacement Cycle (ERC), where the eye shrinks to a size too small to be stable, resulting in the disintegration of the eyewall, and the formation of a new eyewall from one of the outer spiral bands. Once the new eyewall becomes stable tonight, Jimena has an opportunity to re-strengthen.

The latest set of 12Z model runs show little change in Jimena's track. The hurricane is still expected to make landfall Wednesday along Mexico's Baja Peninsula. If the hurricane makes landfall over the southern portion of Baja, it would likely be at major hurricane strength. However, SSTs cool quickly to 23°C at the middle of the Baja Peninsula, and Jimena would likely weaken to a Category 2 or 1 hurricane if it makes a strike towards the middle of Baja. The computer models are split on what might happen once Jimena makes landfall on Baja, with one camp predicting a northeast turn bringing the storm over Mainland Mexico and eventually into Arizona, and the other camp stalling Jimena out over or just west of Baja. Regardless, the main threat from Jimena will be flooding and mudslides due to heavy rains.

Baja's hurricane history
The most powerful hurricane on record to hit the west coast of Baja occurred last year, when Hurricane Norbert made landfall on the central Baja coast with sustained winds of 105 mph (Category 2) . Norbert's central pressure of 956 mb at landfall made it the 3rd strongest hurricane to hit the Pacific coast of Mexico since record keeping began in 1949. Norbert killed eight, knocked out power to 20,000 homes, and damaged or destroyed 40% of the homes on the islands of Margarita and Magdalena. Norbert crossed the Baja Peninsula and made landfall on Mainland Mexico as a Category 1 hurricane with 85 mph winds.

Only two major hurricanes have made landfall on Baja since record keeping began in 1949. Both hurricanes hit the east (Gulf of California) side of Baja. The first was Hurricane Olivia of 1967. Olivia made landfall on October 13, 1967 as a Category 3 hurricane with 125 mph winds. Due to its small size and the unpopulated region of coast it hit, damage was minimal. The second major hurricane was Hurricane Kiko, which made landfall on August 27, 1989, as a Category 3 hurricane with winds of 115 mph (minimal Category 3). Kiko was a small hurricane and hit a relatively unpopulated area, resulting in no loss of life and only scattered reports of damage.


Figure 1. A plot of all the major hurricanes to pass within 200 miles of Mexico's Baja Peninsula since 1949. Image credit: NOAA Coastal Services Center.

Invest 94L almost Tropical Storm Erika
The Air Force Hurricane Hunters are investigating tropical wave 94L, about 200 miles east of the Lesser Antilles Islands. So far this afternoon, they have found a broad circulation with top winds of 45 - 50 mph in heavy thunderstorms on the east side. The circulation may be too broad for 94L to be considered a tropical storm, but visible satellite imagery shows that the circulation is getting better defined, and 94L's heavy thunderstorms are moving closer to the center of circulation. Recent satellite wind shear analyses by NOAA/Colorado State University and the University of Wisconsin show that wind shear is a moderate 10 - 20 knots over 94L, and this shear is keeping any thunderstorms from developing on the system's west side.

The forecast for 94L
The latest of 12Z model runs are more in tune with the idea that 94L will become a tropical storm, with the latest ECMWF run now on board. The storm will be steered west-northwest to northwest over the next three days thanks to the presence to two upper-level lows to the northwest of the storm. Since most of 94L's heavy thunderstorms and high winds are on the east side, the northern Lesser Antilles and Puerto Rico will probably be spared tropical storm force winds from this system. The best guess track for 94L, assuming it does intensify into a tropical storm in the next 24 hours, is on a northwest path between the Bahamas and Bermuda. If 94L stays weak, a more southerly path, along the northern edge of the Bahama Islands, is more likely.


Figure 2. Infrared satellite image taken at 12pm EDT, Tuesday Sep 1, 2009. Five major African tropical waves are apparent. Image credit: Navy Research Lab, Monterey.

Elsewhere in the tropics
It's September, the most active month for hurricane activity in the Northern Hemisphere. There's every indication that the 2009 Atlantic hurricane season will have an active first half of September, since SSTs are 0.5 - 1.0°C above average, wind shear is near average, and the African monsoon is sending a long parade of African waves spinning off the coast of Africa. An IR satellite image from noon today shows this activity well (Figure 2). We can see a line-up of five African waves stretching from the Lesser Antilles to eastern Africa. The GFS model develops the waves numbered "2" and "3" into tropical depressions next week, and the waves labeled "1" and "4" also have a chance to develop into tropical depressions, as well. The wave labeled "1" is mentioned on NHC's Tropical Weather Outlook as having a low (less than 30%) chance of developing into a tropical depression by Thursday. This wave is under a moderate 10 - 15 knots of wind shear, and is over sufficiently warm waters (27 - 28°C) that some development may occur this week. The wave is far enough north that it will be hampered by dry air from the Sahara Desert.


Jeff Masters