Forecasters warn the storm could cause extreme flooding and mudslides. No tropical storm has made landfall in Southern California since 1939.
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Hilary could drop significant rainfall in Southern Nevada, Southern California and surrounding desert areas.
An unnamed hurricane off Baja California is drawing winds, clouds and moisture toward the valley.
The week is expected to start out hot and dry, but Wednesday and Thursday have a slight chance of thunderstorms.
The York Fire area and other rural roads on the eastern side of Mojave National Preserve were under a flash flooding warning Wednesday.
Summerlin, Red Rock Canyon and parts of the west and northwest valley were slammed by a severe thunderstorm on the first day of August.
The July record comes as the weather service said the 14 days ending Saturday were the hottest two weeks in Las Vegas history.
The Las Vegas area has a chance of isolated thunderstorms through Wednesday.
The average of the high and low each day for July stood at a record 97.6 degrees through Saturday. Some relief might come with a chance of rain through Tuesday.
Kingman, Arizona, was also under a flash flood warning Tuesday afternoon.
There was a small chance of rain in the valley forecast Tuesday, and temperatures are expected to go back up again starting Wednesday.
The high reached 110 on Sunday, but moisture and cloud cover may make it hard to extend the streak to 11 days, says the National Weather Service.
The airport unofficially reached 100 degrees by 8:45 a.m. Satuday and rose to 115 just at 2:30 p.m. as the heat streak boils on. The record for the date was 114 in 1937.
Burn and illness cases mount as summer of extreme heat nears — and may exceed — record of consecutive days at 110 or hotter.
Las Vegas could possibly tie or surpass its all-time stretch for consecutive days with 110-degree or higher temperatures.