When it comes to diagnosing the upper air pattern (jet stream), the rule of thumb is:
1) Upper level lows and low pressure troughs bring cold air.
2) Upper level highs and high pressure ridges bring warm air.
At the end of last week, a strong trough (for August) brought in a cold front at the surface. This upper trough became a "cut off" low as the cool temperatures continued.
Later this week, that will be replaced by a strong ridge of high pressure that will bring 90 degree temperatures back to the mid state.
August will return!