Past Features

This page lists out the IEM Daily Features for a month at a time. Features have been posted on most days since February 2002. List all feature titles.

Features for Sep 2023

Fri Sep 01, 2023
Temp Ranges + Precipitation
01 Sep 2023 05:30 AM
With the three calendar month season of summer in the books, let us compare the season total precipitation with the average range between the daily high and low temperature. The featured chart does just that for an areal averaged value over Iowa. We see a very good and intuitive relationship with increasing precipitation leading to smaller diurnal temperature ranges. The reason for this is that more precipitation likely means more near surface soil water, higher atmospheric humidities, and likely more cloudiness. All of these factors will dampen heating during the day and cooling during the night time, leading to smaller ranges between the daily high and low temperature. The five most extreme years are labelled on the chart along with the 2023 value, which is obscured in the cluster of dots. It is always fun to see intuitive relationships show up in observational datasets! You can generate this chart for other variables of your choice and for months of your choice.
Voting: Good - 14 Bad - 0

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Mon Sep 04, 2023
Reaching 100 in September
04 Sep 2023 05:30 AM
Very warm daytime high temperatures have returned to Iowa over the weekend with the only saving grace being much less humidity than the heat wave two weeks ago. Waterloo reached 100 degrees on Sunday and that is a somewhat rare feat for September. The featured chart shows the warmest daily high temperature for each September for Waterloo. Yesterday's 100 degrees was the first such reading since 7 September 1939! The yearly bar color indicates if the value is above or below the long term simple average. It is interesting to see 12 of the past 13 years with all above average max daily high temperature. The 30 year trailing average has certainly increased over this period as a result. The ongoing drought this year certainly contributed to the very warm temperatures we are now experiencing as drier vegetation and soil heats more efficiently than wetter.
Voting: Good - 7 Bad - 0

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Tue Sep 05, 2023
Clouds and Temps for September
05 Sep 2023 05:30 AM
Labor Day was quite warm with high temperatures well into the 90s and even a few 100 degree observations found in the state. Ample sunshine along with dry surface conditions allowed temperatures to quickly warm during the day. The lack of clouds is the focus of today's feature plot showing the frequency of having overcast sky conditions reported by air temperature for Des Moines during September. A clear relationship is shown with both very warm and very cool temperatures having the least amount of overcast clouds present. For the warm temperatures, sunshine is needed to help push temperatures well above average. For the cool temperatures, clear skies allow for more efficient cooling and cooler temperatures. So clouds effectively help to moderate temperatures and temperatures with the highest frequencies are found at approximate values of common near surface soil temperatures for this time of year.
Voting: Good - 9 Bad - 0

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Wed Sep 06, 2023
95 Degree Hours
06 Sep 2023 05:30 AM
While the cold front that swept Iowa on Tuesday did not produce much in the way of needed rainfall, it did get rid of the hot and muggy air mass. Air temperatures were not quite as warm on Tuesday due to the extra clouds and elevated humidity, but heat index readings felt like air temperatures experienced on Labor Day. With hopefully the really hot weather now gone for the year, it is a good time to total up the number of hours spent AOA (at or above) 95 degrees. The featured map does just that and compares it against a simple past five year climatology. Positive values would indicate more hours this year vs an average of the previous five. This map tells a nice story of this year with large positive departures generally overlapping places under analyzed drought. The reason is that drier conditions changes the surface energy budget and allows for more heating. There is also a self fulfilling prophesy of having less rain implying less cloudiness and more solar heating. An additional map can be found here with the current US Drought Monitor overlain.
Voting: Good - 14 Bad - 2

Tags:   2023  
Thu Sep 07, 2023
Sioux City vs Des Moines
07 Sep 2023 05:37 AM
The afternoon high temperature for Sioux City was a couple degrees warmer than Des Moines. The featured chart compares the two sites for this temperature metric. The top panel shows a heat map by week of the year for this temperature difference with a line showing the weekly average value. The bottom panel plots the temperature difference by the afternoon average wind speed. So what's interesting about the chart is that the spring and fall seasons are the most favored times for Sioux City to warmer than Des Moines. This is likely due to Sioux City being a bit more of a high plain climate than Des Moines, so is able to warm more during the least humid and strongest solar heating times of the year.
Voting: Good - 9 Bad - 2

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Fri Sep 08, 2023
Thursday after Labor Day
08 Sep 2023 05:30 AM
With the first Thursday after Labor Day now the start of the NFL season and having this year's game in Kansas City, the featured chart today looks into the history of high temperatures on this date for the state. The high yesterday at the Kansas City airport was 83 degrees, which is practically the average value over the period of record. It can certainly be much warmer and even much cooler this time of year and it is one of the reasons outdoor football can take place in about any type of weather as we transition from late summer to early winter. Now focusing on another big football game on Saturday in Ames, the high temperature is currently forecast near 83 degrees. Go Cyclones!
Voting: Good - 16 Bad - 1

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Tue Sep 12, 2023
Change in 90 Day SPI
12 Sep 2023 12:01 AM
The seven day total precipitation ending this morning around 7 AM is considered for the weekly update to the US Drought Monitor issued on Thursday. This period has seen a couple of rounds of rainfalls hitting various parts of the state. The focus of today's daily feature is assessing if any of it was enough to move the drought needle? While there is no one-to-one association of drought classification to any precipitation index, a commonly consulted value is trailing 90 day standardized precipitation index (SPI). The left hand map shows the most recent US Drought Monitor depiction along with long term climate sites drought classification based on 90 day SPI. The top right panel shows the weekly change in 90 day SPI over the past seven days with the arrow showing the direction of change over that period. The bottom right shows a count of stations meeting various before and after SPI values. The first thing that may catch your eye are the very negative red lines in the upper right panel. These can be explained by the fact that the trailing 90 day window dropped some heavy rainfall events from 3 months ago as the window slid forward. This is a huge caveat when looking at a given number of day SPI value as the big rainfall events can introduce noise as they come in and leave the time window. Another caveat is that replacing a week of June with a week of September lowers the climatological accumulation, which is effectively a positive addition to SPI without it even raining! So getting back to the original question, the last week of rain didn't do too much to erase any of the drought that continues to plague the state.
Voting: Good - 11 Bad - 2

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Wed Sep 13, 2023
80+ in September
13 Sep 2023 05:30 AM
The weather this week has generally been a treat for early fall with crisp mornings and comfortable highs in the 70s. The forecast has a chance for a few upcoming days in the low 80s for highs, which is certainly something typical for this time of year as shown by the featured chart. The chart presents the daily frequency of having a high temperature of at least 80 degrees for Des Moines during September. There's a remarkable difference between the start and end of the month. There's also an interesting bump in frequency around 15th through 20th, which is difficult to easily explain away. The moral of this story is that such warm days should be enjoyed as we creep closer to October and such days increasing fleeting.
Voting: Good - 10 Bad - 0

Tags:   sep  
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Thu Sep 14, 2023
First Fall Dates
14 Sep 2023 05:30 AM
While some isolated locations in northwestern Iowa have dipped into the 30s, there has yet to be any sub 32 degree temperatures in the state so far. Having said that, we are now at the time of the year that such values are possible based on climatology. The featured chart presents the accumulated frequency and percentile dates of having the first fall low temperature at or below the given threshold by the given date of the year for Ames. The table lists the percentiles along with the earliest and latest calendar dates at the given threshold. For Ames, you can see the first 32 happened on 13 September 1902, which was yesterday for a calendar date! You can see that the frequencies rapidly increase by the day. Values well below 32 though don't start rapidly increasing until October. You can generate this chart for other locations and thresholds of your choice by following the "Generate this chart" button.
Voting: Good - 12 Bad - 0

Tags:   fall  
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Fri Sep 15, 2023
Last Fall Dates
15 Sep 2023 05:30 AM
Yesterday's Daily Feature presented the frequency of having observed a given low temperature by a fall season date. Today's plot shows the opposite with the frequency of having observed the last high temperature of a given threshold by the fall season date. For example, about 50% of the years observed the last day of at least a 85 degree high temperature by today. Or rewording, only about 50% of years on record experienced a 85+ degree high temperature later than today's date. It is interesting to note the relative distance between the 85, 80, and 75 degree lines. Certainly reaching 85 degrees is a bit more difficult this time of year than making it to 80. The right hand table shows the percentile dates along with the min and max dates. You may recall that 15 December 2021 (ob taken 7AM on 16th) event shown as the last 70 degree day when crazy warmth and a serial Derecho impacted the state!
Voting: Good - 16 Bad - 1

Tags:   fall  
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Mon Sep 18, 2023
Biggest Rainfall in February
18 Sep 2023 05:30 AM
The featured chart is a bit of a complicated look at precipitation metrics over the state for this year. The top panel estimates the areal coverage of Iowa receiving at least a daily one inch precipitation total for the given date. The colors partition the area by how much of that precipitation fell over locates that were below an inch total over the previous seven days. This is attempting to measure an effectiveness as you generally want precipitation events to fall on locations that need it. The bottom panel shows the areal coverage that needed rain and got the rain. Of interest is that the highest percentage shown was for an event on 27 February! The lack of big rainfall events during April through June is readily noticeable and a couple of events during July and August were likely saviors for the agricultural crops that were running short of moisture all season long.
Voting: Good - 12 Bad - 2

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Tue Sep 19, 2023
Benchmark Years
19 Sep 2023 05:30 AM
Drought continues to tighten its grip on the state. When accessing how bad this year has been, it is often interesting to compare against recent benchmark drought years that some folks may recall. The featured chart attempts to do this by presenting a heatmap of the year with the lowest precipitation total for a period between the given date and yesterday for some selected sites in Iowa. Note that the sites are labelled with the most recent drought monitor classification. Areas in green indicate that for the given date to 18 Sept period, 2023 is the driest on record for the given site. Of course, when you compute this over just a few days and it did not rain over those days, 2023 will show up on this map, hence the green slivers on the left hand side. That aside, some of the hardest hit areas this year show up with some green areas dating back to early summer, including Waterloo and Cedar Rapids. The good news is that there are considerable chances of precipitation in the forecast, including some good rains that have fallen overnight.
Voting: Good - 16 Bad - 1
Wed Sep 20, 2023
Smoothing Out Variability
20 Sep 2023 05:30 AM
Some training thunderstorms brought a highly localized heavy rainfall event to locations near and south of US-30 in central Iowa on Tuesday. As has been the case for other storm events this year, Story County (county of Ames) saw significant variability with totals ranging from about one to five inches! The featured chart is a bit of a thought experiment attempting to resolve if there is some period of time that washes this one event's variability out. The input data is provided by a high resolution precipitation product produced by the University of Iowa. There are just over 9,000 grid cells within the product covering Story County. The top panel accumulates the precipitation for each of these grid cells with the color representing the distribution of values on 19 September. The middle panel shows the range of values and the bottom panel shows the percentage of grid cells that are within 5% of the median value. So you can see that the initial four inch variability never goes away and increases with time. You may recall the big July event had a similar north/south gradient, so it certainly did not help to flip the gradient generated yesterday. This crude analyses only went back to 1 January, but it certainly would be interesting to extend it back to previous years to see how long it takes to tighten up the distribution.
Voting: Good - 10 Bad - 2

Tags:   variability  
Thu Sep 21, 2023
Iowa's Largest D4
21 Sep 2023 07:30 AM
The weekly update to the US Drought Monitor was just released and the analysis for Iowa has gotten worse. Please note that the analysis only includes precipitation up until 7 AM Tuesday, so the heavy rainfall events these past two days are not included! The most severe drought category "D4" was significantly expanded over Iowa with the coverage now the largest on record since the inception of the product in the year 2000. The featured chart presents a weekly time series of the category coverage. The plot starts in 2012 as that was the first occurrence of D4 in Iowa. Only 2012, 2022, and 2023 have seen such a drought category in the state.
Voting: Good - 9 Bad - 1

Tags:   d4  
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Fri Sep 22, 2023
Decorah Monthly Precip
22 Sep 2023 05:30 AM
Yesterday's Feature mentioned the expansion of "D4" classification drought over the state of Iowa. Far northeastern Iowa was one of the two locations that got a newly designated D4, which included Decorah, which is the subject of today's featured chart. The chart presents the monthly precipitation totals over the past 20 years along with the long term mean at the bottom. This chart nicely hints at what likely saved the crops from a total disaster in the area, that being a wet cold season prior to this dry summer. Totals for November 2022 through May 2023 were close to or above average. So with this year's crop done and significant drought in play, we will likely again need a wet cold season to recharge the soil moisture.
Voting: Good - 15 Bad - 0

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Mon Sep 25, 2023
Wettest Day in September
25 Sep 2023 05:30 AM
With 1.30" reported on Friday, Waterloo had its wettest day of the year so far. The featured chart presents some metrics on the wettest day of the year for Waterloo. The left panels show the amount on the wettest day and a simple histogram for the events by the day of the year. The right panels plot the max precipitation by year and a cumulative distribution function for the values. Having just a 1.30" total as the maximum yearly value is rather low compared to the period of record data. Having the event during September is not all that exceptional and there have been years with the data occurring during later months of the year.
Voting: Good - 10 Bad - 0

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Tue Sep 26, 2023
2023 Corn Harvest Progress
26 Sep 2023 05:30 AM
The USDA NASS Crop Progress report released yesterday estimated 9% of Iowa's corn grain crop harvest completed. The featured chart presents the yearly corn harvest estimates by day of the year based on weekly values linearly interpolated to daily. The white X denotes the location of the 9% value each year. The early start to this harvest season was no-thanks to some late heat and moisture stress that finished off the crop along with concerns about stalk health that provokes some farmers to make an early harvest to save on yield loss / harvest efficiency. Recent rains have slowed the progress down and typically farmers prioritize getting the soybeans out first.
Voting: Good - 3 Bad - 0

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Features for Sep 2023