Historical weather data, all-time temperature records, and monthly climate averages for Johnston, IA. Explore typical weather conditions by week for trip planning, and see how today compares to the historical record.
April 4 in Johnston sits in that classic Iowa spring in-between zone — average highs of 56° and lows of 34° mean mornings still bite. The date's range tells the whole story: it's hit 83° as recently as 2021, and plunged to 13° back in 2018. Precipitation is unlikely today, with just a 20% chance of any and a 3% shot at snow.
Johnston's weather has real teeth on both ends — the thermometer has swung from a brutal -27° in February 1996 all the way up to 104° in July 2012, a 131-degree spread. A single day in July 1997 dropped 4.45 inches of rain, and a March 2004 snowstorm buried the area under 15.6 inches in one shot.
Johnston is warmest in mid-July, when average highs peak around 85-86°, and coldest in mid-January with highs barely cracking 28°. If you're trying to avoid rain, skip late May — the week of the 21st averages 1.61 inches, the wettest stretch of the year. Spring and fall offer mild temps in the 60s-70s, but pack layers since the season swings can be dramatic.