HURRICANE Paul is strengthening in the Pacific off Mexico and a tropical storm watch has been issued for a portion of Baja California.

The US National Hurricane Center says Paul's maximum sustained winds were 130km/h early on Monday. Some additional strengthening is forecast during the day before weakening is expected on Tuesday.

It's centered about 850 kilometers southwest of the tip of the Baja California peninsula and moving north at 15 km/h.

Meanwhile, Tropical Storm Rafael is expected to reach hurricane strength in the Atlantic later in the day.

Rafael's maximum sustained winds are near 110km/h. The storm is centered about 465km north of San Juan, Puerto Rico, and is moving north-northwest near 17km/h.