A late summer heat wave had many in its sweltering grasp Saturday and may have been been responsible for a six-car fire in Livermore, but that was just the beginning.

According to Steven Anderson with the National Weather Service, the heat will persist through Monday before Bay Area residents see a slight cool down Tuesday. Temperatures Saturday soared into the triple digits in the Tri-Valley Area, with Livermore reaching 100 degrees mid-afternoon. Hayward, Fremont and Concord did not fare much better, hovering in the low 90s. While the temperatures were not a record for the area, it was still a shock given the mild weather much of the East Bay experienced throughout the summer, Anderson said.

Night temperatures, however, were set to drop well over 30 degrees, with Oakland and Livermore hitting a cool 58 late Saturday.

The heat will return Sunday and Monday, however, with cities posting similar hot temperatures during the day before any reprieve at night.

On average, this weekend's temperatures were about 2 to 3 degrees warmer than predicted, Anderson added, but the overall heat trend was expected.

"We had a cooler summer, but even average temperatures for this time of year feel hot," he said.

The heat was not only tough on residents, however.

Late Saturday morning, six vehicles were damaged by fire sparked by intense heat in the parking lot of the Livermore Premium Outlets Saturday morning, sending smoke over the Tri-Valley for miles, fire crews reported.

The cars were not occupied at the time and no one was injured.

Damage to all six vehicles and their contents was estimated at $34,000, said Livermore Pleasanton Fire Department Battalion Chief Joe Testa.

Four vehicles sustained major damage, one was moderately damaged and one had minor damage. The fire likely started in the engine compartment of the Camry, Testa said.

It was a busy Saturday at the 120-store outlet mall and a large crowd gathered in the parking lot and watched the fires burn as crews work. Before the firefighters arrived, a security guard attempted to put the fire out with a fire extinguisher, but was unable to due to the size and location of fire, Testa added.

While a Spare the Air alert was issued for Saturday, the Bay Area Air Quality Management District did not issue an alert for Sunday, despite the continued heat projects.

Cooler temperatures -- in the 80s, according to Anderson -- are expected Tuesday.

Contact Kristin J. Bender at kbender@bayareanewsgroup.com and follow her at Twitter.com/kjbender. Contact Katie Nelson at knelson@bayareanewsgroup.com and follow her at Twitter.com/katienelson210.