After a mild opening 10 days of October, temperatures have averaged nearly 5 degrees below normal. This weekend’s 60-degree weather interrupted an eight-day stretch of temperature deficits. Mild air will persist until a cold front sweeps through the area on Monday, accompanied by gusty winds and possible thunderstorms. Deep low pressure is to evolve over the upper Great lakes Monday and Tuesday, producing a large dip in jet stream winds and sending colder air into the Midwest. Forecast models are in close agreement in keeping a cool weather pattern in place heading into early November. Temperatures during the period from Oct. 23, through Nov. 2 are expected to average 6 to 8 degrees below normal. On average, Chicagoans observe their first snowflakes of the season on or about Oct. 30.