(Updates with cancelled flights at Phoenix airport)
By Dan Whitcomb
LOS ANGELES, June 19 (Reuters) - In Death Valley in eastern California and in the town of Needles near the Arizona border, temperatures are expected to reach 127 degrees Fahrenheit (53 Celsius) this week, National Weather Service meteorologist Tom Fisher said on Monday.
The extreme heat, brought on by a high-pressure system camped over the Four Corners region, has already boosted temperatures above normal across much of the Southwest, with the worst forecast for Tuesday and Wednesday.
The heat wave was expected to push temperatures even higher by mid-week, taxing power grids and prompting airlines to warn that flights could be disrupted. American Airlines cancelled seven flights on Monday and 43 for Tuesday.
Fisher said the sweltering weather could break records in some areas but was part of a cyclical pattern and not brought on by climate change or any other unusual phenomenon.
"It's not unprecedented at all, things oscillate around," he said. "We've had a fairly cool spring, had 'May Gray' and 'June Gloom', this our first real warmth of the season."
Temperatures were expected to drop by five to 10 degrees F (about 2.5 C to 5 C) later in the week but in the meantime the National Weather Service issued excessive heat warnings and advisories across the Southwest. Even in typically cooler San Francisco, residents baked in sultry 88 degrees Fahrenheit (31 C).
The National Park Service in Death Valley, known as the hottest place in America, cautioned tourists about the "EXTREME SUMMER HEAT" in a warning on its website.
"Expect high temperatures of 100°F to over 120°F (38°C to over 49°C). Drink plenty of water and carry extra. Avoid hiking (after 10 am). Travel prepared to survive. In the case of a heat related illness, get to a cool place and seek help ASAP!" the park service said.
"Temperatures like this happen less than once per year on avg. High risk of heat illness if doing strenuous activity outdoors," the Phoenix office of the National Weather Service said in a tweet on Monday morning.
There were no reports of severe heat-related illnesses as of Monday afternoon. Power grid operators and utility companies urged customers to conserve electricity at peak times to avoid the possibility of blackouts.
American Airlines advised passengers flying in or out of Phoenix Sky Harbor International Airport that the intense heat, which can affect the operation of jet airline engines, could cause flight delays or cancellations.
The airline said that Phoenix passengers with flights scheduled to arrive or depart between 3 p.m. and 6 p.m. local time would be allowed to change their travel plans without incurring additional fees.
A study in the journal Nature Climate Change published on Monday said that the number of extreme heat waves was expected to climb to affect half of the Earth's population by the year 2100. (Reporting by Dan Whitcomb; Additional reporting by David Schwartz in Phoenix; Editing by Sandra Maler)