On April 16, 2012, Washington was 18 degrees warmer than usual. Two months earlier, unusual weather had settled over Bucharest and Kyiv. A computer noticed that such coincidences had recurred in the past, and it predicted the heat wave in Washington.
The rule is simple: if an anomaly in one city has been followed by an extreme in another at least four times, that pattern can be used for forecasting. For Washington, three such rules were found, and they correctly predicted nearly a fifth of all hot days over three years. Moreover, the sign of the deviation was always right: if the algorithm said "it will be hot," the heat did arrive.
This method does not require supercomputers or complex atmospheric models. It simply sifts through pairs of cities and waits for the echo to repeat. Even today, it could be used for early warnings of heat or cold in the energy and agriculture sectors.
The heat in Washington on April 16, 2012 was predicted from temperatures in Bucharest and Kyiv 60 days before the event, even though these cities are more than 7000 km apart.