Scorchers captain Meg Lanning’s 70 from 56 deliveries wasn’t enough for the hosts, as Jonassen’s unbeaten 28 saw her team chase down Perth’s 5-136 with four balls remaining.
Sent in to bat first by Heat captain Kirby Short, Elyse Villani gave her team a fast start before falling for 23, but Lanning was able to carry on where she left off.
She brought up a second consecutive half-century from 50 deliveries, clearing the rope twice before being run out in the 19th over, a dismissal that cost the Scorchers crucial momentum late in their innings.
Nicole Bolton’s four-over spell of 2-12 wasn’t enough for the Scorchers, who now have a 4-5 record after nine matches.
Scorchers Head Coach Lisa Keightley said several dropped catches proved costly for the home side.
“We can’t miss that many opportunities in the field and expect to win games, it’s disappointing,” Keightley said.
“We gave each batter nearly two opportunities and it’s just not good enough, we didn’t support and back up our bowlers unfortunately.
“With the bat we needed another partnership but we just kept losing wickets, during the middle overs our run rate slowed due to the loss of consistent wickets so in the end we just didn’t score enough runs.
“We definitely need to try and bounce back this weekend and string two wins together, which won’t be easy against an in form Sydney Thunder.”
Perth host the Thunder at Lilac Hill on Saturday and Sunday with both matches starting at 10am AWST.