Temperatures Friday afternoon were quick to warm, rising into the upper 70s! This, after starting out in the 30s. The range from the morning low to afternoon high is what is known as a diurnal temperature range. Yesterday, that range was 45 degrees and the most in a calendar day since April 3rd, 2016, when the temperature range was 49 degrees. There are a couple reasons for a big temperature swing during the day, either a strong front (cold or warm) has moved through, or the air mass is very dry. Friday, we had a very dry air mass overhead. Air molecules in a drier air mass will move around freely, heating up more efficiently than in an air mass that has more moisture. Those air molecules are not able to move around as much, therefore making it harder to get a big swing in temperatures.
Temperatures Saturday will be just as warm, if not warmer, thanks to a milder start. South winds held temperatures Friday night in the low to mid 50s; warmer than what our highs were earlier in the week. The warmer start will help springboard us close to 80 degrees during the afternoon. In fact, it’s likely that a few locations in northern Illinois will reach 80 degrees. The air is still dry which will make the combination of warm temperatures, gusty winds, and lower relative humidity a threat for quickly spreading fires.
On average, the last 80-degree day in October occurs around October 4th. The last time Rockford reached 80 degrees this late in the month was October 22nd, 1975. The latest 80-degree day on record was November 1st, 1933. According to Chicago National Weather Service, since 1905, Rockford has experienced only 14 years (12%) where 80 degrees has been observed this late in the year. All of those 14 years occurred between 1908 and 1975.
Highs on Sunday could also reach 80 degrees depending on the amount of cloud cover during the afternoon.