Some of the wave energy will come from Tropical Storm Aletta, which reached hurricane strength off the west coast of Mexico last week. It's centered about 235 miles (380 kilometers) southwest of Manzanillo, Mexico, and is moving northwest at 10 mph (17 kph).
"Some strengthening is forecast today, and Bud could become a major hurricane later today", the NHC said.
Forecasters said Bud was expected to strengthen further before a slow weakening trend starts Tuesday.
A tropical storm watch was issued by the government of Mexico for Manzanillo to Cabo Corrientes, The Weather Channel reported.
Hurricane Bud is expected to deliver 3 to 6 inches of rain to much of southwestern Mexico, and as many as 10 inches in isolated pockets. A second system, Hurricane Bud, has since developed in the same region and is expected to send moist, humid air streaming into Southern California late this week.
The rain could cause life-threatening flash floods and mudslides, forecasters said.
The Hurricane Center said the storm could generate swells that cause life-threatening surf and rip currents over the coming days along Mexico's southwestern coast.
Meanwhile, Tropical Storm Aletta's winds were down to 40 miles per hour (65 kph) and it was predicted to dissipate by late Monday or early Tuesday.
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