Sept. 12 (UPI) — Tropical Storm Mario is small but strengthening off the west coast of Mexico Friday.

The latest update from the National Hurricane Center was at 4:23 a.m. Friday. It said Mario is a small tropical cyclone about 20 nautical miles off the coast of Guerrero, Mexico. The tropical depression was boosted to Tropical Storm Mario with maximum winds estimated at 40 mph.

Because of Mario’s closeness to the coast of Mexico, the Mexican government has issued a tropical storm watch for a small segment of the coast from Lazaro Cardenas to Punta San Telmo.

NHS said it’s having trouble predicting Mario’s trajectory because of its small size and closeness to land. Some models show the system moving inland and dissipating Friday, but others show Mario reaching hurricane strength. Mario is expected to reach colder waters by day five and become a post-tropical cyclone.

Mario has been moving faster toward the west-northwest at 14 mph, parallel to the coast of Mexico.

Heavy rainfall will affect southern Mexico through Sunday, which could result in flash flooding, particularly in areas of higher terrain.

Tropical storm conditions are possible along portions of the coast of Michoacan Friday. Gusty winds are possible elsewhere along the coasts of western Guerrero, Michoacan, and Colima through Friday night.