MEXICO CITY (Reuters) -Category 4 Hurricane Hilary was rushing toward Mexico's Baja California peninsula on Friday morning, threatening to bring "significant flooding" to parts of the region as well as the Southwestern United States, a U.S. government agency said.

Hilary was moving west-northwest at nearly 10 miles per hour(17 kph), packing maximum sustained winds of nearly 145 mph (230 kph), after being upgraded to a Category 4 overnight, the U.S. National Hurricane Center said in its latest advisory.

It will still be a hurricane when it approaches the west coast of Baja California over the weekend, but is expected to weaken to a tropical storm before reaching Southern California on Sunday afternoon, it added.

NHS issued a tropical storm watch for California, which is the first-ever for the state according to U.S. National Weather Service.

A storm surge could produce coastal flooding and destructive waves along the western Baja California peninsula of Mexico, the agency added.

Rainfall of 3 to 6 inches (7.6 to 15 cm) is expected across portions of Southern California and southern Nevada, which "would lead to significant and rare impacts."

Hurricane Hilary would bring soaking rains to Southern California and neighboring Nevada and Arizona. The region has suffered a brutal, record-breaking heat wave this summer.

Through July, Phoenix, Arizona, endured a month-long stretch of temperatures above 110 degrees Fahrenheit, (43.4 degrees Celsius), a record heat wave caused by a "heat dome" of stagnant air that was parked over the western United States for weeks, the National Weather Service reported.

The heat dome put tens of millions of Americans under heat watches and warnings. It pushed the temperature in California's Death Valley desert to 128 Fahrenheit (53 Celsius) in mid-July, among the highest temperatures recorded on Earth in the past 90 years.

(Reporting by Valentine Hilaire in Mexico City and Rich Mckay, Timothy Ahmann and Frank McGurty in Atlanta; Editing by Isabel Woodford, Jonathan Oatis)