We explain why weather forecasts sometimes “get it wrong”

Why is it that, more and more often, we come across “wrong” weather forecasts, which bring quite a few problems to sea-goers? Intrigued by this phenomenon of “fallibility,” we asked an internationally renowned meteorologist, Paolo Gemelli, for explanations.

THE MEDITERRANEAN WEATHER OF LATE SUMMER
We are used to thinking of the Mediterranean as a “big lake” with no particular pitfalls. The question then arises as to what are the “most dangerous” weather phenomena one might encounter while sailing in the Mare Nostrum in late summer. What advice to give to those who find themselves in the middle of it? “The Mediterranean presents some pitfalls,” Paolo Gemelli explains, “especially where air masses of oceanic or continental origin make their entry into the basin: namely Strait of Gibraltar, Gulf of Lion and northern Adriatic. Especially in August, when the sea has absorbed much of the summer heat, the incursion of relatively colder air masses can give rise to intense phenomena such as strong thunderstorms and related wind gusts. One must try to avoid such phenomena, but once in the midst of a severe thunderstorm, it is best to stay offshore away from other boats.This is a choice against one’s instincts: in the presence of a strong thunderstorm the tendency to rush into port is understandable, however, the limited space and speed of the phenomenon can expose us to serious risks.”

WHY DO PREDICTIONS “GO WRONG”?
Looking at the Meteomar general bulletin, its forecasts are often unrealistic because they cover too wide an area to be reliable in every area. Let’s take a practical example. I have been listening to the Meteomar bulletin, and the situation describes an area of high pressure facing the Mediterranean from the Atlantic and a cold front transiting eastward from France. The forecast over the Ligurian Sea will give SW winds and rising swell. What to do, being along the coast, in western Liguria, facing a tense NE wind? Often in a situation like this we rail against “wrong” predictions, but what is actually happening? “This is,” Gemini further explains, “the classic Mistral situation: the barometer will start to rise because we are west of the center of the depression over the Ligurian Sea. In some cases, the Mistral over the Gulf of Leo will not subside but strengthen further, sparing the Ligurian Sea but violently hitting the western coasts of Corsica (the south-central) and Sardinia. For obvious reasons, the bulletin must summarize with a value (SW in this case) the most significant wind direction in a certain area leaving the necessary adjustments to the expertise of the reader. The situation described is one of the most classic in the Mediterranean. Over the Ligurian Sea the Mistral takes a SW direction offshore, but very often along the west coast there is a NE rotation due to a small secondary depression that is created in these circumstances between Liguria and Corsica.”

WHO IS PAOLO GEMELLI
Paolo Andrea Gemelli has been a contributor to GdV for years. He has been involved in marine meteorology as a meteorologist for the most important Italian skippers (Giovanni Soldini, Mauro Pelaschier, Pietro D’Ali) participating as a routier in various editions of the Route du Rhum, Transat Jaques Vabre, Ostar and Global Challenge.