4th Aug: Tropical Storm Update

Yesterday I spoke about an increasing chance of Tropical Storm Ernesto making his way into the Gulf and this still looks to be plausible with the NHC guidance giving a 20-30% chance of Tropical Storm Force Winds in the Gulf within 5 days.

NHC Wind Tropical Storm wind speed probabilities

This chart gives the probabilities of Tropical Storm wind speeds within the forecast period (5 days). It’s a really useful product available from the NHC here. It’s worth understanding how this chart works, we’re looking at the colours representing the probability NOT the intensity. If this was a swell or wind chart, which show a snapshot of size or strength at any one time, that spread of colour range in the Gulf would suggest the storm was weakening as it progressed. However this chart is the probability of a wind hitting the Tropical Storm threshold within 5 days and that fading colour scale simply reflects that as we move forward in time and distance the certainty of a particular outcome is less.

TS Ernesto is currently subject to very light wind shear (The changing direction of wind over altitude that disrupts storm formation and progress) and over an area of very warm sea (that feeds the warm core storm cycle) so there remains good reason to expect a major storm on a westerly track, although the NHC point out that some models aren’t seeing this strengthening at the same rate so as always there’s still significant spread in the forecast.

Tropical Depression Six

Tropical Depression Six. Courtesy National Hurricane Center

What was yesterday a 30% probability of an area of interest becoming a Tropical Storm is today a 70% chance as we see rapid intensification and the birth of Tropical Depression Six. The less favourable news is disruptive wind shear and ‘lukewarm sea temperatures’ mean that the storm is most likely to¬†dissipate¬†in the short to medium term, giving a significantly weakened outlook for surf for the US East coast than we’d have hoped. With warm seas vital in driving the formation and intensity of Tropical Storm activity this situation may dampen the outlook for surf making storms beyond just this one.