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.
