During this very early cold spell on August 7th, the radiosonde observation from Caribou, Maine reported an 850 mb temperature of only 2C.
As another cold air mass settled over the Great Lakes on August 17th, the temperature gradient was strong enough to generate a 140+ knot jet stream over Michigan and Lake Huron.
The southern hemisphere winter has been very harsh, particularly in South America. Cold air penetrated deep into the Amazon forest, reaching as far as 2N latitude in Brazil. In Peru's normally hot Amazon region, temperatures plummeted to 9C (48F). Argentina had its coldest winter in 40 years, and millions of fish were killed by the cold in Bolivia.
On August 28th, this West Coast trough produced conditions favorable for severe thunderstorms in Idaho and western Wyoming.
CAPE values exceeded 3000 J/kg in Idaho.
700 mb temperatures were already well below zero in the Pacific Northwest.
A 991 mb low developed over Salt Lake.
On August 29th, the West Coast trough deepened even further:
The next day, August 30th, a rare tornado hit the Big Horn Basin in north central Wyoming. Then on August 31st, accumulating snow fell on the Wind River Range, to the southwest of where the tornado struck: