New depression forming?

A tropical wave near 13N 39W, midway between Africa and the Lesser Antilles Islands, appears to be developing into a tropical depression. This wave was declared "Invest 90L" last night by NHC. The wave has a pronounced spin, and plenty of heavy thunderstorm activity surrounding it. This activity is fragmented and not concentrated near the center of circulation, but it gradually getting better organized. At the current rate of organization, formation of a tropical depression seems likely by tonight or Monday morning.

Three of the major models--the GFS, UKMET, and GFDL--do develop this system into a strong tropical storm or Category 1 hurricane by six days from now. The system is moving west-northwest at 10-15 mph, and these models all indicate that the long-range path of the storm will be north of the Lesser Antilles Islands. The GFS predicts the storm will become a powerful hurricane that will recurve a few hundred miles off the U.S. East Coast without hitting land. Wind shear over the system is low, about 10 knots, and the wave is over warm SSTs of 83-86F (28.5-30 C). Wind shear is forecast to remain low over the next few days. A large area of dry air and African dust to the wave's north may be an inhibiting factor, but this is probably to far away at present to be a problem. The large area of thunderstorms about 400 miles west of 90L, formerly designated "Invest 98L", is being absorbed into 90L.


Figure 1. Preliminary model tracks for Invest 98L. These models are described at the NHC web site.

Lesser Antilles tropical wave
A small tropical wave surrounded by a cloud of African dust is moving through the eastern Caribbean this morning. This wave, which NHC has designated "Invest 99L", is tracking west at 15 mph and has a small area of heavy thunderstorms associated with it. A large upper trough of low pressure over Cuba and Hispaniola that is creating about 10-20 knots of shear over 99L, preventing significant development. The trough is expected to weaken and move west over the next few days, potentially creating a low shear environment over most of the Caribbean. This could allow intensification of 99L into a tropical depression by Tuesday at the earliest. NHC has not run any preliminary models for this storm since yesterday.

New wave coming off the coast of Africa
A strong new tropical wave is emerged from the coast of Africa yesterday and is just south of the Cape Verde Islands. The wave has pronounced spin, but the thunderstorm activity associated with it is very disorganized. The wave is under 10-20 knots of wind shear, and has some potential for slow development over the next few days.

Tropical Storm John
Hurricane John spared the most heavily populated areas of Mexico's Baja Peninsula significant damage when it came ashore Friday afternoon on the sparsely populated eastern side of the Peninsula as a Category 2 hurricane with 110 mph maximum winds. Many roads were washed out in the resort towns of Cabo San Lucas and San Jose del Cabo, and helicopter airdrops of food and water were needed for 6000 residents cut off by washed out roads near where the center made landfall. However, no one was killed by the storm. Radar from Guasave shows John is still moving up the Baja Peninsula, spreading heavy rains. John will continue to weaken as it moves northwest along the Baja Peninsula, and should die on Monday. Moisture from the hurricane will likely bring flooding rains to portions of the Southwest early next week.

Typhoon Ioke
Typhoon Ioke is now barely a major storm, with its top winds of 115 mph barely qualifying it as a Category 3. It is forecast to recurve to the north and miss Japan.

Lessons learned from Ernesto
Ernesto may be gone, but its rain, winds, and flood waters still linger over the Eastern U.S. Rain from the storm penetrated all the way to may home in Michigan yesterday, a sure indication of a storm with a major far-reaching impact. Let's summarize two important lessons from the storm:

Lesson 1: This is not the Hurricane Season of 2005! By this time last year, we were already up to the 13th named storm of the season, Maria. Pretty much anytime something could develop, it did, and it usually took the worst possible path. This year, we are only up to the 5th named storm, and the storms have been taking some fortuitous paths. Certainly, we got very lucky with Ernesto--I was convinced that Haiti was in for a major disaster with heavy loss of life, and Ernesto would get his name retired. Hurricane Jeanne (2004) and Hurricane Gordon (1994) were both about the same strength as Ernesto when they hit Haiti, and both storms killed thousands. Yet Ernesto only dumped 2-8 inches of rain on Haiti, an unusually low amount for a Category 1 hurricane. Ernesto also took the longest possible path over land, given its general track, cutting across the Florida Straits without spending much time over water. I'll be full of much less doom-and-gloom for the next storm of this season that threatens land. This is not the Hurricane Season of 2005!

Lesson 2: The forecast track cone is often not big enough. The retiring director of NHC, May Mayfield, says that he wants the epitaph, "Don't look at the center track forecast line, look at the cone of possible tracks" engraved on his tombstone. Well, sometimes even the cone isn't big enough. The cone is based on the average track error in NHC forecasts over the past few years. Nearly half the time, the actual track of a storm will fall outside the cone. If you looked at those early forecasts of Ernesto going into the Gulf of Mexico, the cone did not quite extend all the way to the eventual landfall point at the extreme southern tip of Florida. Track forecasts are getting better--last year's 5-day forecasts were about as accurate as a 3-day track forecast 15 years ago--but there are many situations where the computer models and the human forecasters do poorly.

Next update
I'll have an update Monday morning.

Jeff Masters