Jamaica is running low on ganja. Heavy rains followed by an extended drought, an increase in local consumption and a drop in the number of marijuana farmers have caused a shortage in the island's famed but largely illegal market that experts say is the worst they've seen. “It's a cultural embarrassment,” said Triston Thompson, chief opportunity explorer for Tacaya, a consulting and brokerage firm for the country's nascent legal cannabis industry. Jamaica, which foreigners have…