The start of a new month is always a good time to review what happened for the previous month. The featured chart reviews hourly aviation flight rules computed from the Des Moines airport weather station. These rules consider cloud ceilings and horizontal visibility among other things to restrict aviation traffic. The featured chart only considers the weather station horizontal visibility distance and vertical distance to cloud coverage either in "broken" or "overcast" state. The chart is a nice first guess to indicate periods where there was likely poor flying weather conditions due to visibility reductions. The stormy start to April is most noticeable with reductions more isolated after the first few days.

After a chilly start to Saturday morning, temperatures quickly recovered and made for an excellent weekend to start off May. How cold it got Saturday morning is the subject of today's featured map displaying computed NCEP RTMA minimum temperature between 3 and 8 AM. So-called local micro-climates can introduce a fair bit of uncertainty into a map like this as subtle topographic and soil type differences can cause major differences in temperature over short distances. Normally, such an event on the second day of May would not be of much concern, but as was mentioned by the daily feature a few days ago, the significant early season warmth has gotten vegetation off on an early start and early May freeze events will continue to be of concern.

A front pushed through the state on Monday and while some portions of southeastern Iowa received rainfall, near surface dry air won out for the majority of the state. The featured chart presents dew point temperature climatology by wind direction based on period of record observations from Des Moines. The statistics are produced by computing vapor pressures first and then back-computing the dew point from those values so to better account for the non-linearity of dew point. The observed values for Monday are plotted as well for comparison. All of the values plotted are below the mean value with a few approaching the 5th percentile.

For Des Moines, the high temperature on Tuesday was just 59°F. The rest of the state did not fare much better with most below 60°F for the day. The featured chart presents the day of May frequency of having a high temperature below 60°F for Des Moines. The overall frequency for a single May is about 3.5 days, so these events are not terribly rare. The chart does nicely show the near linear decline in frequency as the month progresses, but another repeat looks to be in store today before warmer weather returns starting Thursday.

The start to May has seen a mix of warm and chilly days, but the lack of humidity has been consistent so far. The featured chart presents a daily climatology of humidity for Ames as expressed by mixing ratio (ratio of water to air). The top panel shows the daily range of observed values with the red line showing the 2026 values to date. The bottom panel shows the daily departure from the simple average. The start of May has seen values near the bottom of the observed range with the departure from average being the largest so far this year. Such abundant dry air is not conducive to precipitation, but the very wet April and cooler air temperatures have limited any substantial impacts so far. While the near term forecast does have precipitation chances, totals do not appear to be all that substantial.

Temperatures were rather pleasant on Thursday with mid 60s common over much of Iowa during the afternoon hours. Ames reported a temperature of 66°F at 3 PM. The featured chart looks into the frequency of having such a temperature by month for Ames. The bars present the frequency of having a temperature within the inclusive range of 63 to 67°F at 2 PM LST (3 PM CDT) with the blue dots showing the frequency below that range and red dots above. May does have the highest frequency of such temperatures, but only by a slim margin to April. It is kind of interesting to denote the near identical frequencies between February and July!

It is difficult to imagine the weather on Mothers Day being any better with ample sunshine, a cool breeze, low humidity, and high temperatures around 70°F. The featured chart presents the yearly Mothers Day high temperature as recorded at Des Moines. The previous two years had quite warm temperatures with highs above 80°F, but this year's high of 72°F was just a degree or so above a simple long term average value. While most mothers probably would have preferred it not to rain yesterday, much of the state is starting to need rain again as the growing season is now fully underway and it has been about two weeks since Iowa's most recent widespread and significant rainfall. The near term forecast is not very optimistic about upcoming rainfall events.

It has been a rather strange start to the growing season. The month of April was quite warm and wet, leading to an early start to the growing season with plenty of growing degree days accumulated prior to the start of May. May has now started off rather dry with low humidity levels allowing overnight temperatures to cool below freezing over a number of days so far this May. USDA NASS released their weekly crop progress report yesterday and estimated 72% of Iowa's corn crop having already been planted, a 30 percentage point increase over the estimate last week thanks to the mostly dry start to May. The featured chart looks into the weekly percentage point change in crop planting progress as estimated by NASS. The left hand labels show the year and the maximum weekly change value for each year. A 30 percentage point change is about at the middle of possible yearly maximum values.

Our mostly dry and lack of humidity start to May continued on Tuesday with dew point temperatures in the 30s and lower 40s. The featured chart presents a climatology of daily maximum dew point as calculated by available hourly observations from the Des Moines airport. The 2026 observations are shown as blue dots on the top panel. Each of the daily dots for May is found below a simple long term average for the day. This is in stark contrast to the elevated humidity levels that were common for much of April.

For Iowa State students, this is Finals week. For K-12 students, just a few weeks of school remain until summer break. So it is a good time to think about summer and present a metric to compute when the summer season begins. If you consider the summer season as the warmest consecutive 91 day period (1/4th of the year) each year, the featured chart plots the start date each year for this period for Ames. The dots are colored by the average temperature departure for this period over all years. A simple linear trend line is plotted as well. The overall average is around 7 June, which just over three weeks away, but plenty of years got started in just over week from now.

After a very wet April, May has been the exact opposite with very little rainfall to speak of. An unofficial IEM Iowa statewide estimate indicates the first 14 days of May being the driest on record since at least 1893. Thankfully, this situation is about to change and the subject of today's featured map. The map depicts Weather Prediction Center's seven day precipitation forecast along with the latest US Drought Monitor released on Thursday. The entire state is shown in the 1.5 to near three inch range.

The featured chart presents a time series of hourly dew point temperatures for Ames since 25 April. The lack of humidity during the month of May has dramatically reversed over this past weekend with even a 70°F dew point observation on Sunday. The return of humidity has certainly helped the return of spring time thunderstorms and severe weather with more of both expected on Monday.

Storms developed along a frontal boundary over southwestern Iowa on Monday afternoon with muggy air ahead of the front helping to fuel the storms. Cooler and less humid air has worked into the state overnight with dew point temperatures 10 to 15 degrees cooler. The featured chart looks into six hour dew point temperature change based on hourly data from the Des Moines airport. The data is partitioned by one degree Fahrenheit and by week of the year with the frequency units being average number of hours per week per year. Certainly values near zero are the most common as dew point fluctuations are less than air temperature. It is interesting to denote the largest frequencies at values just above zero during late June into late August. Some of this is the effect of evapo-transpiration processes adding moisture to the air during daytime hours, but it is also a tightening of the possible distribution of dew point temperatures during the most humid time of the year.

For mid May, Tuesday was a bit chilly with high temperatures struggling to get out of the 50s. Such chilly temperatures imply a meager growing degree day accumulation, which is ever important this time of year to promote crop development. The featured chart presents daily growing degree days for Des Moines with the observed value on Tuesday plotted along with two model forecasts, a crude daily average accumulation (short daily lines) and the associated accumulated values. The NWS NDFD forecast does go out as far as the GFS deterministic model, so that is why the red bars stop on the 26th. The chart nicely shows that we'll have four more days to go with below average GDDs prior to about a week of warmer weather taking us into June.

After a very dry start to May, this past weekend saw numerous rounds of severe weather that also brought much needed rainfall. Such is the conundrum this time of year, needed and significant spring season rains often come with thunderstorms that are sometimes severe. The featured chart attempts to look into how much precipitation is associated with severe weather by totaling up one minute interval precipitation totals from the Des Moines airport when either a severe thunderstorm or tornado warning is activate for the airport. The left two panels report the precipitation partitioning percentages during, within an hour, and outside of severe warnings. The upper right panel shows the frequency of a given one minute intensity happening during a severe weather warning. The bottom right panel is a bit of a downer, but denotes how much of the one minute precipitation is likely missing within the one minute observations (long story, but NCEI's collection and distribution of one minute data has become very unreliable). So the overall percentages are not that large with precipitation totals near 10%, but it certainly accounts for a good portion of the most intense rainfall totals.

The featured map presents May to date precipitation departures courtesy of PRISM. There is plenty of red (below average departures) to be found on the map. Outside of the rainfall events of last weekend, there has not been much to speak of for rain during May. Much of Iowa is surviving from the very wet April and the periods of reduced water demand like what was found this week with the cool and cloudy conditions. The precipitation forecast for the rest of May is not that bullish and temperatures will be increasing, which will start to materialize impacts over the driest parts of the state this May. For this time of year, it roughly takes about an inch of rain per week just to keep up with climatology, so departures can rapidly build when we get dry periods during the wettest portion of the year!