Hot air and plenty of available moisture combined to produce the rare thunderstorm that swept across the central Kenai Peninsula on Monday afternoon.
The storm began around 2 p.m. over Skilak Lake, where temperatures temporarily reached 78 degrees Fahrenheit.
"We are having unseasonably warm temperatures, well above normal," said meteorologist Amy Bedal, from the Anchorage office of the National Weather Service.
More thunderstorm activity was predicted for early Tuesday evening. Normal central peninsula temperatures for June 28 would be expected to produce temperatures around 64 degrees, but the forecast was calling for highs in the low 70s, Bedal said.
Such conditions heat the surface, warm the air near the ground and turn the atmosphere unstable, exactly the opposite of the more stable warm-over-cool air situation, Bedal said.
Thunderstorms often begin as innocent-looking cumulus clouds. If moisture is available, they can grow into "towering cumulus" that may produce showers. Continued growth can lead to cloud formations called cumulonimbus.
"If the atmosphere is unstable, they can grow to produce thunderstorms," Bedal said.
That kind of instability is more common in the interior, where broad reaches of the dark ground absorb greater heat than the more reflective surface of bodies of water like Cook Inlet. While the inlet can contribute moisture for the formation of clouds, it actually has a moderating effect on local air temperatures, thus suppressing the formation of thunderstorms.
Hence, thunderstorms are more common in the Interior where that moderating influence does not exist, Bedal said.
Truly severe thunderstorms, defined as producing half-inch-sized hailstones and winds greater than 35 mph, are even more rare, Bedal said. There have been none on the peninsula this year and only one recorded near Talkeetna.
Recent summers have been warmer than usual, Bedal said, but that hasn't translated into more frequent thunderstorms. Last year's total statewide was below average, in part because of high pressures, which inhibit thunderstorm activity, she said.
On Monday and Tuesday, Alaska was experiencing a weak low-pressure condition over the state, and a weak ridge of high pressure along the north coast of the Gulf of Alaska, conditions ripe for noisy weather.
Is global warming contributing?
"We definitely are under a warming trend," Bedal said. "But what's causing that warming trend is a matter of debate."
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