The calendar has mercifully turned to October and after the rains end this morning, there is hope for a much needed upcoming stretch of drier weather. Will likely be able to create a number of daily features over the coming days denoting how remarkable the weather of this past September was. The featured chart today looks into daily average relative humidity as computed for the Des Moines Airport weather station. The top panel plots the daily observations along with the climatology which has been smoothed to remove the day to day noise. The bottom panel plots the smoothed frequency of having the daily average relative humidity above 90%. Around 10 days for 2026 are shown having an average relative humidity of 90% or greater, which works out to 33% of the days! The bottom panel puts the climatology around 5% of the days. High humidity and the lack of a killing frost are contributing to significant grain quality concerns as a mature crop continues to wait to be harvested.
It would appear that the recent non-stop rainfall events have finally relented with dry weather lasting into next week along with plenty of sunshine currently forecast. The lack of dry weather during September has certainly delayed the fall harvest season for Iowa. The featured chart presents USDA NASS soybean harvest estimates for Iowa. The NASS reports are weekly, so daily values are linearly interpolated between the weekly reports. The most recent report valid on this past Sunday estimated just 3% of Iowa's soybean acres as harvested. The left hand chart plots the yearly process numbers with the 3% value denoted each year. The right hand chart lists out those yearly 3% dates. Only a couple of years over the past 15-20 have been more delayed than this year. Things will likely not have improved much when NASS releases its next report on Monday as significant rains fell this week and we will need a number of days to dry out!
The featured chart presents the combination of average September high temperature vs low temperature by year for a statewide averaged value for Iowa. The plot is separated into quadrants based on the averages for each. It is certainly most common to have both values either both above or both below average. The 2026 value is highlighted and while having one of the warmest average low temperatures on record, the average high temperature was not that exceptional. Such was the case this past September with ample cloudiness and elevated humidity preventing overnight cooling and thus buoying low temperatures. Much chillier over night temperatures have already occurred this October as clear skies and lower humidity have allowed temperatures to rapidly cool during the overnight hours.
Our just finished month of September was very wet and many folks are always curious if it was record breaking or not. The answer to this question is perhaps not as straight forward as many would hope it to be. The primary reason is that the vast majority of long term climate data provides 24 hour totals valid at some reporting hour during the daylight hours and not at midnight. This creates an accounting problem for how precipitation is included within a given calendar month since the observation on the first and then the first of the next month can both include partial precipitation totals that are inside and/or outside of the current month of interest. For Iowa and a number of other states, this situation was exasperated by a very heavy rainfall event on the evening of the 30 September into 1 October. In general, this significant rainfall event did not "count" into the September totals as the 24 hour reports came on 1 October. Now for a handful of reporting sites with 24 hours reports near midnight, it did, but for things like climate district and statewide estimated totals, it did not. Oh boy! So the featured chart presents a summary of the impact of this event on September precipitation ranks for the unofficial IEM computed climate district values. The first two data columns are generally the "standard" way with the 24 hour report on 1 October not included. The second set of columns include the 1 October report, but then drop the 1 September report. The final column presents the rank change between the two. Note that the ranks are computed for the exact specified days in each case. The top five largest positive and negative rank changes are shown along with the Iowa values. So two Iowa districts are shown as being second wettest, but wettest if that last big rainfall event was included.
After the rains ended on the first, the weather has thankfully turned drier this October and now warmer with high temperatures on Tuesday well into the 70s and even lower 80s for some. Des Moines hit 81°F, so the featured chart checks in on year-to-date accumulated days with a high temperature of at least 80°F. The green line shows the 2026 accumulation and back during early June, this year was not far off the record pace set by 2012. The few warm days during September has brought the accumulation closer to a simple long term average than the record total. The tan lines show each previous year on record and the intermittent jumps during October show how fleeting these days are this month and how rare they are by November. A couple more such days appear likely over the coming seven days, so try to enjoy them while they last!
After a very wet September delayed harvest progress, the weather has done a full turnaround with near ideal conditions after the rains stopped on the first of October. The featured chart looks into the frequency of a given day of the year participating within a streak of at least seven consecutive days with a precipitation total below 0.01 inches. Note that this threshold sweeps the long term complexity of how trace amounts are handled as the minimum accumulation possible is 0.01 inches. Rewording, no measurable precipitation would include trace reports and thus be considered "dry weather" for this chart. The frequency of these events do increase as the fall season progresses and the chart nicely shows the clear annual signal. Iowa's current stretch of dry weather may be briefly interrupted for some with scattered light rain later on Thursday and into Friday morning expected, but coverage and amounts will not be significant. More robust rainfall chances will return later next week.
The strong storms that developed early Thursday evening over Iowa were a bit of a forecast surprise with only very late runs of various forecast models depicting a possible robust environment to produce elevated thunderstorms capable of large hail. The NWS issued a number of warnings for the event and got hail reports to verify those warnings. The featured chart looks into the time of day frequency of having Severe Thunderstorm Warnings during the month of October. The chart presents the time of day frequency for Des Moines with the overall frequency plotted in tan and the values for October in blue. Of course, there are much fewer events during October so the plot gets a bit noisy vs the smooth full year values. Having such warnings during the night time is certainly more common than during the day as processes like the nocturnal "Low Level Jet" help fuel thunderstorms overnight.
