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The calm before the storms

By: Dr. Jeff Masters, 8:24 PM GMT on August 19, 2005

The tropics are quiet today. The only area worth mentioning is the large tropical wave pushing off of the coast of Africa, which the UKMET model develops into a tropical cyclone by Tuesday. However, the GFS, Canadian, and NOGAPS models are all pretty ambivalent about the system and do not develop it much.

No hurricane season has ever gone at full tilt all the way from July to October. Active seasons have always had quiet periods when the large scale wind patterns alternate to a different mode, creating more wind shear over the tropics. This year, that quiet period is here now, and is forecast to continue for at least a few more days. However, this may change next week. The GFS model predicts that the high shear values that have dominated the Caribbean for the past few weeks will finally relax. And beginning Friday August 26, shear values may relax over the rest of the primary hurricane genesis area in the mid-Atlantic. The GFS shows an endless succession of tropical storms developing from tropical waves moving off of the coast of Africa beginning late next week. And from August 30 continuing through the end of the GFS's 16-day forecast period, the model predicts two and sometime three simultaneous tropical cyclones in the Atlantic. This prediction has been maintained over at least the past four runs of the GFS model, so it is not a fluke one can blame on a single bad model run.

Keep in mind that computer forecasts of specific tropical storms developing are VERY unreliable--particularly out seven days and more from now. The GFS is likely to be dead wrong about the specific timing of the tropical waves moving off the coast of Africa, which waves might develop into hurricanes, and where in the ocean they may develop. What is believable is the GFS's forecast of a fundamental shift in the general atmospheric circulation leading to an enhanced period of hurricane activity. If the GFS model is correct, the current time of calm will transition to a time of storms late next week.

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.