Thankfully, July of 2011 is history, but the heat and muggy weather looks to continue. The featured chart presents the number of hours during July for the past 40 years that the heat index was above 100 degrees for Des Moines. This year's total comes in slightly higher than 1999. The average during this period is just over 28 hours as shown by the blue line.

Dew point temperatures once again soared into the upper 70s and lower 80s on Monday. These temperatures were remarkable for the calendar now being in August! The featured chart presents IEM calculated yearly maximum period where the dew point remains above 75 degrees and the total number of hours above 75 degrees. This year is still behind last year by those metrics, but there is still plenty of time to go in August to catch up. While not shown on the chart, many of these humid periods occurred during August.

The Register’s Annual Great Bicycle Ride Across Iowa (RAGBRAI) took place last week and was a quite warm for participants. This 7 day long trek across Iowa starts on the Missouri River and ends on the Mississippi River. Assuming that the weather reported by the Des Moines station is representative of the state, the featured chart presents the average heat index and westerly wind for the six day period each year that RABGRAI occurred on since it started in 1973. This year's average heat index was second to 1999. The bottom chart looks at a simple average of the westerly wind component during daylight hours. A wind from the east (negative values) would mean a head wind for bicyclists generally heading from the west to east. Easterly winds are typically associated with cooler weather, so you can pick your poison or a headwind or hot weather!

Our recent stretch of very warm weather saw some locations in the state hit the century mark. The official weather sensor for Des Moines is a the airport and the best it was able to do was 99 degrees on five different days. The featured chart presents the one minute interval temperature data for these five days. Our most recent chance on 2 August was our best one with temperatures reaching near 100 degrees early in the afternoon, but a front moved through just as the temperature was about to hit 100! The 20 July event saw temperatures hover at 99 for around three hours without hitting 100. Having five days hit 99 without hitting 100 for the year is most on record for Des Moines.

This growing season has seen both hot weather and above average rainfall for most places in the state. The featured chart presents Ames stress degree day and precipitation departures from long term averages. 2011 has seen the biggest stress degree day departure since 1988. The top chart clearly shows the years with excess heat having the least amount of rainfall.

The recent few days have been refreshing after a stretch of brutally high dew points. Dew point temperatures for Des Moines have yet to decline below 60 since the 3rd of July. This makes for the longest period above 60 for Des Moines since the early 1930s. The featured plot presents the yearly maximum streak of having dew point temperatures above 60 and the time period over which the streak occurred. The plot nicely shows most of these streaks happening over July and August. With August only a week old, there is plenty of time to increase our streak this year.

Getting timely rainfalls in July and August are critical for corn and soy production in Iowa. Some places in the southern plains have had crop failures this year because of the lack of rain and blistering heat. In Iowa, most places have seen timely rainfalls so far this summer. The featured chart presents the longest period during July and August in between quarter of an inch and half inch rainfalls for Ames. It is remarkable to note the lack of many above long term average periods during the past 20 years. This year looks to continue this trend of not seeing long periods of dry weather.

NOAA recently released estimates of July average temperatures and put this year as the 7th warmest on record for Iowa. The featured chart presents an analysis of hourly temperature departures and percentiles based on hourly data for Des Moines. The plot clearly shows the most exceptional temperatures occurring during the night time hours, which indeed tells the story of this month having very warm low temperatures. This was a result of the record amount of humidity in the state this July helping to keep temperatures warm overnight. Based on some IEM calculations: this July was the warmest on record for low temperatures statewide, but only the 40th warmest for highs (since 1893). The average of those two values gets us to 7th warmest.

It is hard not to keep featuring interesting stats from July after such an exceptional month! The featured chart displays the July high and low temperature departures from long term average for Des Moines. The bottom plot displays the monthly high versus low departure with a good correlation of 0.6 shown, which means warm highs usually mean warm lows and vice versa. The red dot on the scatter plot shows how exceptional this year was for low temperature.

The past few days have been a welcome relief from much warmer weather in July and the first few days of August. The featured chart compares the July average temperature departure from average against the following August departure. The low correlation and visual appearance would indicate that one does not necessarily beget the other. The departure so far this August is about the same as July. The forecast has temperatures near average for the next week or so.

August certainly started off on the hot side with highs near 100 degrees for Des Moines. The past week has been very nice though with highs in low 80s and much more comfortable humidity levels. This wonderful weather came just in time for the Iowa State Fair and it looks to continue this week.

This summer has seen extremes for lack of rainfall over the southern plains and daily rainfall totals. The featured chart is of one minute interval data from New York City's JFK Airport showing a rainfall total of 7.8 inches on Sunday. While this was the largest daily rainfall total for the airport site, it is not clear if this was the largest total for New York City as the official site is at Central Park.

Portions of southwest Iowa picked up heavy amounts of rainfall yesterday morning with totals approaching four inches. For Ames, the largest daily rainfall has been 2.65 inches so far this year, which is about average for the largest total of the year. The featured chart presents the largest daily rainfall total each year and the day of the year on which that highest total occurred. The three highest totals all occurred in August. It is also interesting to have the largest totals for some year occur in late February!

The Iowa State Fair will end this weekend and the weather so far has been better than "butter on a stick". The high temperature on Wednesday was the warmest so far at 88, but dew points were very comfortable. The top featured chart looks at the warmest temperature reported by the Des Moines sensor during past state fairs. The bottom chart presents the number of hours during the fair that the heat index was over 90 based on hourly data from the Des Moines Airport. This year has yet to see such a reading, which has helped to boost fair attendance. There are chances of rain in the forecast, but temperatures will remain on the cool side of average.

The Iowa State Fair ends this weekend and has seen about an inch of rainfall so far, which is close to long term average for the Fair. The featured chart presents the yearly rainfall totals during the fair and the frequency of having five hundredths or more of rainfall on a given day. The smallest value is on day three with a less than one in five chance. 1977 was the wettest year at over eight inches. This data is from the Des Moines weather station, which is not located at the fair, but should be a close approximation.

Today is the first day of school for many, so the featured chart presents a refresher of something you should learned in middle school math. There are many ways to statistically represent a distribution of values. Three of those ways are the mean, the median, and the mode. The featured chart presents these three values for all daily temperatures on a monthly and annual basis. In general, these three values are close to each other except during the winter and springtime. The last value is for the entire year and there is a large difference between the mode and median/mean. Our summertime can be very persistent thanks to high humidities holding temperatures around 60 over night and in the low eighties during the day. This is about what our weather has been like for the past week and will be like for most of the coming week.

The featured map presents an estimate of rainfall over the past two months and the current national drought monitor analysis (lines). Southwest Iowa has seen its share of heavy rainfall in the past few weeks and they are currently dealing with flooding. On the other end of the state, southeastern Iowa has been rather dry and the far corner is shown in "D1" (moderate drought) status. Heavy rainfall is falling this morning over north-central Iowa with the heaviest totals over a few inches.

Rapidly decaying storms last evening over southwestern Iowa produced what appears to be a heat burst event. The featured chart is a time series from the Atlantic AWOS showing a temperature rise above 100 degrees and a dew point drop to 7 degrees, which is a classic heat burst type signature. There is some doubt if the 7 degree dew point is legitimate as other sites in the area only dropped to as much as the mid 40s. Regardless, these events typically also bring damaging winds and a few trees were reported down. Heat bursts are not fully understood by scientists yet, but the common explanation is a decaying thunderstorm encounters a layer of dry air below the storm. Virga then falls into this layer, which quickly cools it by evaporation and the air mass rapidly sinks to the ground and warms along the way.

The muggy air that was with us on Tuesday has been vacated from the state in a hurry and replaced with a touch of fall. The dew point temperature for Ames is 25 degrees cooler now than the high of 77 on Tuesday. The forecast looks great with highs near 80 and lows near 60.

Besides dumping heavy rainfall last week, a cluster of powerful thunderstorms dropped large hail over parts of southwestern Iowa last Thursday. A week later and a clear sky overhead allowed the Terra MODIS satellite to get a good look at the result of the hail storm. These hail scars are easily visible given the contrast of green crops and the hailed on areas whose green leaves were stripped away exposing the brown ground. The featured image combines the satellite image with NWS collected reports of hail size on the 18th.

Hurricane Irene hit the east coast of the US this weekend bringing flooding rains and significant storm surge. The eye of the hurricane came ashore Saturday morning close to Morehead City, North Carolina. The featured chart is of one minute interval ASOS data from airport sensor in town. The one minute interval data was not available after about 11 AM local time. The traces nicely show the change of wind direction as the eye passes overhead along with the decrease in wind speed. This station was chosen for this plot as it had the lowest pressure reported among the automated sensors the IEM collects data from in the area.

The featured chart presents the largest accumulated precipitation departure during the month of August from the first of May. This chart attempts to show how wet or dry conditions were during the May to August period. The wet years of 1993, 2008, and 2010 immediately stand out along with the dry years of the 1930s. Since 1988, Ames has not seen a very dry growing season with the largest departures only approaching 5 inches. Some locations in Iowa are on the dry side of average this year. Rain is falling this morning over Iowa with some locations expected to pick up a much needed inch or so.

The featured map presents an IEM computed analysis of the percentage of days this summer (since 1 June) that the daily high temperature was above average for that day. Portions of Kansas are shown above 90%! Most of Iowa is depicted around 50-60% as we have had stretches of hot and cool weather. Our actual weather for today will hot and sticky again with highs well into the 90s on Thursday.