Sunday brought a fair amount of severe weather to the mid west US considering that the calendar now says August. The featured map displays the local storm reports sent by the National Weather Service. Iowa's severe weather was isolated to areas south of I-80, whereas Michigan had lots of reports.

The month of July is past us now, so it is a good time to look back and compare it with previous years. The featured plot is known as an aridity plot, which compares the normalized departure of temperature with precipitation. The normalization is accomplished by computing the standard deviation each data set and using that value as the unit of measure. The value for 2015 is highlighted with a circle around the origin, with some of the years outside of that circle labelled. The flood year of 1993 sticks out clearly as the historic precipitation totals well surpass any other July. This year came in on the cool and wet side of long term averages.

Conditions this summer have been very warm on a few days and very nice on others as shown by the featured chart of daily high temperatures for Ames since 1 June. The high temperature there for the past two days has only made it to 80. Outside of the fourth week of July, we have seen at least one day each week in the 70s since the first of June. Will that trend continue this week?

After seemingly endless rounds of rainfall in May, June, and even parts of July, a quick look at precipitation departures since 1 July show that many parts of the state are an inch or more below average. The driest location being northeastern Iowa. There are more chances upcoming to end this week, so hopefully they can address these dry areas.

The featured chart presents yearly number of hours that the heat index was at or above a given temperature. The hourly observations reported by the automated weather stations are assumed to persist for that local hour of time. The blue dots represent yearly averages while the red dots are actual totals for this year. Of course, this year has plenty of summer left, but totals so far are about half of normal. For this chart, the heat index is only included when its computed value is equal to or greater than the actual air temperature. You can generate this chart for other sites in the country to compare with Des Moines shown in the featured chart.

The long term climate record is mostly daily observations that do not represent calendar day summaries, but values representing 24 hour periods ending sometime during the mid morning or mid afternoon. While this time may not seem significant, there are very important climate implications. The featured chart displays the result of computing 24 hour summaries based on the hourly Des Moines temperature reports. If the values computed when the 24 hour period is split at midnight (actual calendar day summaries) is considered the 'truth', the bias can then be computed for the other 23 possible hours of the day that the temperature record can be split over. The result of this plot is known as a time of observation bias, with the largest biases for high temperature appearing in the mid afternoon. How can this be? Consider a hypothetical Sunday with each hourly temperature of 95 degrees, then the following day has each hourly temperature of 75 degrees. If you compute the 24 hour totals at midnight, the high on Sunday is 95 and on Monday is 75. But if you take the observation at 4 PM, the high reported on Sunday would again be 95 and the value reported at 4 PM on Monday would also be 95! The 24 hour period ending at 4 PM on Monday would contain those 8 hours of 95 degree reports from Sunday evening, thus the high is 95!

Folks in Ames woke up Sunday morning to a surprising amount of rainfall produced by very efficient and rather quiet storms during the early morning hours. The featured chart presents the one minute interval rainfall reports from the Ames Airport. Between about 2:30 and 4:30 AM, a majority of the reported four inches fell. The automated weather sensor only briefly reported "thunder in the vicinity" during this event, so the quiet nature of the storm likely meant that most people slept through it and awoke to a full rain gauge!

For farmers, this is about the time of the year when one worries about the progress of crop maturity and the hope that the crop reaches maturity prior to the first fall frost. The featured chart presents the period of days this summer in-between the growing degree day accumulation levels of 1135 and 1660, which are important levels for corn development. The thought is that having a longer period is a positive for corn yields. Of course, this period is going to vary depending on the planting date, which is why this analysis indicates that corn planted after 11 May has yet to reach 1660 units.

The featured map displays precipitation departures from average since 1 June for NWS tracked sites. Much of the southern half on this map is well above average with pockets of drier than average areas existing over the northern half. For Iowa, places like Des Moines and Burlington are shown well above average while northeastern Iowa is below.

The featured chart presents a histogram of the change in temperature over a 24 hour period for Des Moines. For example, the 5 AM temperature yesterday was 68 and today is 71, so the change would be 3 degrees on this chart. Temperature variability is lower in the summer time as warm soils and elevated humidity levels work to modulate temperature differences. It is interesting to note the some of the highest frequencies are at slightly negative values during the summertime. This represents the most common mode in the atmosphere, which is for cooling to occur. The frequencies of this level decrease outside of summer as our day to day temperatures are dominated by air mass movements (cold and warm air advection).

To date, the average temperature for August is warmer than the average for all of July. The featured chart presents the frequency of one month being warmer than another month for the same calendar year. The August warmer than July combination is not that uncommon with 31 years meeting that criterion for Ames. The forecast for this week has much cooler conditions with highs below 80 until this coming weekend.

A look at a nationwide composite of NWS NEXRAD RADAR information these past few days would show lots of scattered activity over the continental US. The featured chart looks at the climatology using the past 20 years of composite images of reflectivity returns at or above 35 dbZ. At this level and higher are your typical very rain showers and thunder showers. Summer clearly has the highest values with coverage values 3-4 times that of what we see during the winter season. The slope downwards is very steep as we start transitioning into fall during September. Thundershowers are currently visiting Iowa with more expected later today.

A Tornado Watch was issued Tuesday afternoon for portions of eastern Iowa. Thankfully, only a possible brief and weak tornado has been reported so far in Iowa. For some counties in northeastern Iowa, this was the first tornado watch of the season. The featured map displays the number of Tornado Watches issued by county so far this year. There are nine counties shown without a watch this year! The highest totals are found in southeastern Iowa with Scott and Des Moines county at seven. You can generate this map online for other warning types, other states, and even other WFOs in the country.

On Tuesday, the IEM Daily Feature was a plot of average coverage of 35+ dbZ radar reflectivity averaged by day of the year. Today's feature displays the average coverage by week of the year and hour of the day. The top image being for the continental US and the bottom image for approximately just Iowa. For the CONUS, the peak time is clearly in the late afternoon hour during the summer, this represents the storms driven by the heating by the sun. For Iowa, there is a much different story to tell with the vast majority of peak coverages occurring during the early morning hours. These storms are thunderstorm complexes that roll into the state from the high plains and are feed by a phenomenon known as the low level jet.

Last week, a featured plot presented the bias associated with computing 24 hour high and low temperature by hour of the day. Today's feature does the same analysis, but with precipitation totals. The biases with precip are very small due to the nature of accumulating precip over the 24 hour period, which is a conserved amount when aggregated over time. The chart displays the bias with the total number of 24 hour periods with precip for the year ("rainy days") and the average 24 hour precip total when there is precip that day. The peak with number of 'days' is interesting at around 7 AM and 7 PM, the interpretation is that these are common hours for precip, so it is likely that the next hour will be precip as well. So splitting the day at that hour, will spill precip into a second 'day'. The values shown are small regardless, so the differences are likely negligible!

The Iowa State Fair wrapped up on Sunday and after a warm first few days, the weather turned much cooler. The featured chart displays the yearly average high and low temperature at the fair using the Des Moines Airport weather data as a proxy for the fair weather. The average high this year is one of the cooler in the past few decades. The color of the bars present years that were above (red) or below (blue) long term average for each plotted metric.

The strengthening El Niño over the Pacific Ocean is currently generating many headlines as the current forecasts have the strength reaching levels not seen before since records began in 1950. In fact, one forecaster referred to this situation as a "Godzilla El Niño." The featured chart looks at the combination of August Southern Oscillation Index (SOI) and September plus October average temperature and total precipitation for Iowa. The color of the dots represent the SOI index. The relationship shown is not clear cut. You can generate this chart online and compare the SOI value from some number of trailing months to a summary number of months of your choice.

Our recent stretch of weather has seen some cool temperatures with temperatures this morning in the mid 40s! The featured chart displays the coldest temperature to date since 1 July for Ames with the climatology of this accumulated value plotted for comparison. The slope of the line really starts to steepen in September as the fall season settles in. This analysis also nicely shows the timing of the first fall 32 degree temperature.

The low temperature yesterday for Ames dipped to 46 degrees. How does one compare that value to other months? Having a low of 46 in January would be very balmy. One way to compare is to construct a quantile - quantile plot as shown by the featured chart. You effectively sample the percentiles from each distribution and then plot them against each other. You can then compare one temperature against another and so the 46 in August becomes a -18 in January! There is your comparable!

A few weeks ago, the featured chart displayed the frequency of August being warmer than July and at the time this August was doing just that. Much cooler weather has been the norm since and our average temperature for the month has taken a nose dive as shown by the featured chart. If you tack on the observations for each previous August to this August, these scenarios are not enough to lift this month above last month. So this August is highly likely to be colder than July!

Slow moving and very efficient storms were able to dump tremendous amounts of rain over portions of central Iowa. The featured map displays the combination of MRMS precipitation estimates and Local Storm Reports of precipitation relayed by the NWS. Two reports of 9+ inches of rain were received just to the northwest of Ames. All of this rain does not bode well for flooding in Ames, which is forecasted to occur today.

The past two mornings have seen Dense Fog Advisories issued by the National Weather Service. How common is this advisory issued? Are there favored places in the state for these conditions to warrant issuance of the product? The featured map attempts to answer both of those questions by displaying the average number of dense fog advisories issued each year. A clear east to west gradient is shown with averages ranging from 5 to 10 per year.