12 August 2008

Two easterly waves tracking across the basin...

Although there are no named storms at the moment, I wanted to point out a couple easterly waves.  One is centered near 54W, and the other near 31W.  Both have Lows associated with them, with approx 1008mb central pressures, and both are traveling W-WNW at 10-15kts.  You can track them in 12-hourly satellite images at:

The western one is forecast to gradually turn more westward, and be just north of Hispaniola in 4-5 days.  The eastern one is forecast to continue heading WNW-NW.  Both should strengthen, but the western one is favored... AND of more concern to the US.  By week's end, a 5-day forecast could be quite revealing: is it destined for a landfall or not?

And what exactly IS an easterly wave?  First of all, it's "easterly" because it travels from east to west, with the trade winds (the equatorial/tropical belt of winds that blow from east to west all year long).  The "wave" part has more interesting origins.  Large temperature differences between the desert and rainforest regions of continental Africa generate a strong low-level jet, or plume of enhanced winds.  This jet is unstable, and tends to break down into discrete waves, with ridges and troughs.  The troughs are what we then call "easterly waves", and are characterized by lower pressures and thunderstorms.  The jet is most active in the August-September timeframe, but is present from April through November.  As you've probably noticed, not all easterly waves develop into tropical cyclones.  Roughly 60 easterly waves track across the basin each season, but on average, only 6 or 7 of them might become tropical storms or hurricanes.

Please visit my tropical Atlantic headquarters.

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