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Tropical Storm Franklin forms

By: Dr. Jeff Masters, 4:41 PM GMT on August 13, 2011

Tropical Storm Franklin, the sixth named storm of the 2011 Atlantic hurricane season, formed this morning in the open Atlantic north of Bermuda. The usual date of formation for the 6th storm of the year is September 8, so we are well ahead of climatology. However, the usual date for the first hurricane of the season is August 10, and we still have not had a hurricane yet this year. All six of this year's storms have been tropical storms, making 2011 the first season since 2002 when that has occurred. Franklin is headed eastwards out sea, away from any land areas, and does not have long to live.

Figure 1. Noon satellite photo of Invest 94L, showing the well-defined swirl of its low-level circulation.

A low pressure system near 27°N 60°W, about 450 miles southeast of Bermuda, has developed a well-defined surface circulation, as seen on visible satellite images. However, this system, Invest 94L, has almost no heavy thunderstorm activity, due to dry air to its west being injected into its core by moderate wind shear of about 10 knots. The latest SHIPS model forecast is showing shear remaining in the moderate range and the atmosphere moistening in the vicinity of 94L over the next three days, which may allow the system to build enough heavy thunderstorms to become a tropical depression. None of the reliable models for predicting tropical storm formation (GFS, ECMWF, NOGAPS, and UKMET) show development of 94L, and NHC gave 94L a 30% chance of developing into a tropical depression by Monday morning in their 8am outlook. I'd put these odds higher, at 50%, given 94L's well-defined surface circulation. Bermuda is the only land area that needs to be concerned with 94L, with the storm making its closest pass on Monday. 94L should then curve north and northeastwards out to sea.

An African wave near 20°N 54°W, about 600 miles east-northeast of the northern Lesser Antilles Islands, is moving west-northwest at 20 mph. This system, Invest 92L, has only modest amount of heavy thunderstorm and is very disorganized, as seen on recent visible satellite loops. An ASCAT pass from 9:49 am EDT this morning showed no surface circulation and only a slight wind shift associated with 92L. Water vapor satellite loops show that a large area of dry air surrounds 92L, and this dry air is causing problems for the storm. The SHIPS model and University of Wisconsin CIMSS wind shear analysis are showing moderate wind shear near 10 knots affecting 92L.

Forecast for 92L
Light to moderate wind shear of 5 - 15 knots is predicted along 92L's path over the coming five days, which may allow the storm to organize if it can handle the dry air surrounding it. The latest 00Z and 06Z model runs of the four best models for predicting tropical storm formation (GFS, ECMWF, NOGAPS, and UKMET) show weak development or no development of 92L, and NHC gave 92L a 30% chance of developing into a tropical depression by Monday morning in their 8am outlook. A west-northwest to northwest motion for 92L is predicted by all of the models, which should bring the storm close to Bermuda on Tuesday. 92L should then curve north and northeastwards out to sea.

The disturbance Invest 93L, located near 12°N 35°W, midway between the Lesser Antilles Islands and the coast of Africa, has fallen apart over the past day. Recent visible satellite loops show almost no heavy thunderstorms and very little spin to the system. Dry air to the west and north, combined with moderate wind shear of 10 - 20 knots, have significantly disrupted 93L. The latest 06Z runs of the GFS and NOGAPS models do not show development of 93L, and neither do the latest 00Z runs of the ECMWF and UKMET models. NHC has reduced their odds of development of 93L to 10%. The system should find itself in an environment with lower wind shear on Sunday, and may be able to begin organizing again then. The storm should move through the Lesser Antilles Islands Tuesday or Wednesday.

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Jeff Masters


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