With the calendar turning to September, it is a good time to look back on the summer season that was! The featured map presents IEM unofficial climate district ranks for total precipitation over the June, July, and August months. A value of one would indicate the wettest on record since 1893. There are some large contrasts indicated over the Midwestern US with the most noticeable one over northeastern South Dakota to northern North Dakota or even that of south-central South Dakota! Iowa's summer season was rather wet with only southwestern Iowa indicated to be only slightly above a long term average. The rest of the state was at about the 85th percentile or higher for total precipitation.
Temperatures this fine Tuesday morning are in the 50s as our recent stretch of pleasant late summer / early fall weather continues. The featured chart presents the hourly frequency of having a sub 60°F temperature for Des Moines partitioned by ISO week of the year. For the first week of September, such cool temperatures are still mostly relegated to the overnight hours. In takes until mid October until such temperatures are more common than not during the afternoon hours. The forecast for the remainder of this week looks to continue the very pleasant temperatures.
Way back in 2003, the IEM started generating NWS NEXRAD Mosaics for the contiguous US. Over the years, the archive was back-filled using data from NCEI to the start of 1995. The featured plot presents the mosaic product valid 5 AM each 3 September since 1995. These plots and certainly the mosaic itself is not necessarily meant to be a scientifically rigorous product, but something that can be used as a quick screening and assessment tool. These days there are much more advanced mosaics available, including NOAA MRMS and GridRad. Here's to the next 31 years of generating this product by the IEM!
A tell-tale sign of the seasons changing is the passage of robust cold fronts with legitimate air mass changes as what happened on Wednesday. For Estherville (NW Iowa), the high temperature for Wednesday was set during the early morning hours well before sunrise. This situation is somewhat rare for September with only a handful of other dates since 2010 having a similar occurrence. The featured chart looks into the hourly frequency by week of the year for the high temperature to occur at based on NWS CLI-mate reporting sites (read: mostly automated airport weather stations). The chart uses a logarithmic scale as the frequency counts during the mid-afternoon hours dwarf any other hour of the day. During the cold season, the influence of air masses can be seen with increased overnight frequencies than during the warm season. Additionally, you can see enhanced frequencies near midnight during the cold season as frontal passages will lead to multi-day warming or cooling events. The lowest frequencies are found during the pre-dawn hours during the warm season as solar heating is the strongest and cold-air advection during the daytime is not sufficient to counter.
The featured chart plots a histogram of observed high and low temperatures reported on 5 September for all IEM tracked long term climate sites within Iowa. The solid blue and red lines are observed frequencies with the dashed variants representing an idealized normal distribution based on the denoted statistics. The chart nicely shows the wide range of possibilities this time of year for Iowa with high temperatures above 100°F and low temperatures approaching 32°F. There is also considerable overlap shown between the two distributions with low temps and high temps in the 60s and 70s possible. The forecast for today has the high temps within this range and lows down into the 40s.
The first full weekend of September featured a considerable taste of the fall season with temperatures dipping into the 30s over most of northern Iowa. The featured map presents a minimum temperature analysis of 15 minute interval NOAA RTMA data covering Saturday and Sunday morning. Iowa avoided any freezing temperatures, but a number of record low temperatures were set, including Sioux City and Ottumwa. Temperatures are forecast to rebound this week with a change of reaching 90 degrees by the end of the week!
Sioux City set a record low Sunday morning of 36°F, breaking the previous record of 38°F set in 1986. This was also the earliest date for such a cool temperature for the site on record with archives going back to 1889! The featured chart presents the yearly period between the last spring season and first fall season date with a low temperature below 37°F. The yearly time series of dates is fit with a trend line, simple average, and trailing 30 year average. The recent trend and fit lines don't indicate much change for this metric over the period of record.
Starting around mid-June, much of the state began receiving significant amounts of rainfall and it did not take long to remove all analyzed drought classifications within the US Drought Monitor for Iowa. The wet pattern continued into July, but some parts of the state missed out during August and now with a very dry start to September, drought is again back on many folks minds. One of the driest locations since early August is southeastern Iowa. The featured chart presents trailing 31 day percent of average precipitation totals evaluated each day since the start of 2005. The bottom row of data tells the 2025 story with well above average during early August being replaced with a below average value by September. Rainfall totals over the next seven days look very meager and with increasing temperatures, additional drought impacts will likely show up shortly.
Very warm temperatures are forecast this Friday and into the weekend. Portions of Iowa will be pushing 90°F each day. The featured map presents the percentage of years that the given long term climate site recorded at least one daily high temperature of 90+ degrees between 12 and 30 September. Maps like these tend to be a bit noisy with varying periods of record and micro-climate effects, but the general pattern over the state is clear with the highest frequencies over southern Iowa and lowest over northeastern Iowa. Overall, percentages shown are below 50% for practically the entire state, so having such a warm temperature happens less often than not. Certainly, it would be the most exceptional to have such a warm temperature over northeastern Iowa, where shown frequencies are in the 10-20% range (one out of every 5 to 10 years).
It was quite warm for September this past weekend with highs into the 90s for much of the state both days. For Waterloo, a high temperature of 95°F on Saturday was followed by a high of 93°F on Sunday. As measured by average high temperature over a two day period, this was the warmest such period so far this year. The featured chart presents the warmest two days each year for Waterloo. The bottom left panel shows the day of year frequency with participating within such a period. The 13-14 September period this year comes in at the end of the observed range for such an event to occur. While not quite as hot as this past weekend, the warm temperatures will continue throughout much of this week.
An increase in humidity along with very warm temperatures helped to fire afternoon thunderstorms over Iowa on Monday. Most of these storms stayed below severe levels (less than 58 MPH gusts and one inch hail). The National Weather Service issued a number of Special Weather Statements (abbreviated SPS) to cover these sub-severe storms. The NWS SPS product can be issued for a specific polygon region to highlight a sub-county region under the highest threat from the storm. The featured map presents an IEM accounting of the number of NWS issued SPS products containing a polygon on Monday. The IEM offers a number of tools for these SPS products, including a GIS Download, a search tool, and the raw text which can be downloaded in bulk as well.
While the current weather feels more like summer than early fall, farmers have gotten started with the fall harvest season. The most recent weekly update from USDA NASS estimates 3% of the corn grain crop harvested within Iowa. The featured map presents the statewide estimates and a percentage points departure from a simple ten year average. The corn crop is generally planted to maximize yield potential from the available growing season, so crops tend to only reach maturity just before climatology says killing frosts occur. The corn crop also needs time to dry down prior to harvest, so only so much progress is possible this early in the season for Iowa.
Showers and thunderstorms have frequented Iowa over these past few days. Those of you that frequently monitor NWS RADAR may have noticed how strangely these storms have moved, or rather barely moved. The lack of movement is due to very weak winds aloft as illustrated by the featured chart. The chart presents wind speeds from the Wednesday evening NWS Davenport sounding (0 UTC 18 Sep is 7 PM 17 Sep CDT) expressed as percentile values compared against other 0 UTC soundings from the site during the month of September. For example, the 850 hPa (a few thousand feet aloft) wind speed value of 1.5 m/s (~3 MPH) ranks at the 1.2 percentile, so roughly the lowest value over a three year period. So while the atmosphere over Iowa has been unstable with plenty of available moisture, the weak winds aloft have caused the storms to be pulse and not move much at all. Some of the storms were able to attach to some surface or boundary aloft, which allows them to organize a bit more and produce gusty winds and/or hail. The slow movement has also lead to localized heavy rainfall events. You may notice the larger wind speed values above 100 hPa (stratosphere) and wonder why it is different. In some regards, what happens above the tropopause (boundary between troposphere and stratosphere) is detached from what happens below. Additionally, these values can be misleading when the troposphere is anonymously warm or cold and the mandatory levels occur at significantly different altitudes aloft. Anyway, the moral of the storm is to blame the weak winds for the strange storm activity!
