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 2022

Thu Sep 01, 2022
Is it fall?
01 Sep 2022 05:30 AM
The date that ends the summer season is always debatable. Some say 31 August, others say Labor Day Monday, and others say 21 September. Is there a way to quantify this? Well, if you consider the hottest 91 day and coldest 91 day stretch of days over a given period of time, what drops out in between those two periods can be assigned spring and fall. The featured chart does just that with the top panel showing the trailing 91 day average temperature for Ames, the middle panel showing the date of maximum trailing value each year on record, and the bottom panel showing the number of days between this start date and the start date of the coldest 91 days (the length of fall season). So the 2022 value appears to have presently peaked for Ames back on 29 August, but the near term forecast has temperatures that may allow this value to peak again over the coming days as early June was rather chilly (warmer days in Sept replacing colder days in June for the 91 day average). That ambiguity aside, the bottom panel is sort of bad news for fall season lovers as it indicates the average length is only 86 days, so less than an equally split season around 91 days.
Voting: Good - 10 Bad - 1

Tags:   fall  
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Fri Sep 02, 2022
JJA Average Temps
02 Sep 2022 05:29 AM
With the summer months of June, July, and August (JJA) now in the books, it is a good time to review how the summer fared. The featured map presents an IEM estimate of climate district average temperature ranks for the period. A rank of 1 would indicate that 2022 was the warmest since at least 1893 and 130 the coolest. Values in Iowa are in the 50 to 70 range, so most are slightly warmer than a median rank value of 65. Looking more broadly, we don't find too much for extreme values in the corn belt with the warmest areas off to the south and west. Looking forward in time, the seasonal forecast outlooks from the Climate Prediction Center continue to advertise an above normal chance at warmer than average weather for the period with the highest chances again off to our west.
Voting: Good - 19 Bad - 0

Tags:   summer  
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Mon Sep 05, 2022
2022 LSR Counts
05 Sep 2022 05:30 AM
The featured map presents the number of Local Storm Reports (LSR) issued per NWS office so far this year. There's quite a bit of variability shown on the map with values ranging from 27 to 7,099. While LSRs are issued for reports of severe weather received by the NWS, some offices also issue them for things like snowfall, wind, and rainfall reports. This local preference leads to the large variance shown on the map for neighboring offices. You can generate this map on this website for a time period of your choice and limit the LSRs to specific types of interest.
Voting: Good - 6 Bad - 0

Tags:   lsr  
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Tue Sep 06, 2022
Corn needs more time
06 Sep 2022 05:30 AM
This is always a nervous time of year, crops creep toward maturity and chances of the first fall freeze creep upwards as well. The featured chart presents the period between 2,200 and 2,500 accumulated growing degree days by planting date for Ames. Bars that are gray indicate 2,500 units have yet to be reached, the absence of a bar indicates 2,200 has yet to be reached, and a colored bar indicates that 2,500 has been reached with the color showing the number of days between those two levels. While some corn did get planted in April, much of it did not get going until the rapid warm up on the second week of May. The good news is that there is no chance of a freeze in the forecast for the next week or two, in fact, it looks very hot and dry. Getting even late planted corn to maturity seems probable at this point.
Voting: Good - 8 Bad - 0

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Wed Sep 07, 2022
Days till Half Inch
07 Sep 2022 05:21 AM
Rainfall patterns are often quite complex during the summer season as thunderstorms are more isolated and move slowly, which both lead to very fine scale gradients. The featured map is an illustration of this by showing the number of days one has to go backwards in time before accumulating at least a half inch of precipitation. The blue areas of the state generally saw rainfall back two weekends ago. The rest of the state one has to go further back in time. While the threshold nature of this plot will lead to some arbitrary gradients, there are still some very impressive localized differences such as Humboldt county ranging from 0-10 to 30-40 days and much of southeastern Iowa. Forecast rainfall totals for this coming weekend have crept upwards a bit for southeastern Iowa, so hopefully that will pan out!
Voting: Good - 12 Bad - 1

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Thu Sep 08, 2022
Spencer v Burlington
08 Sep 2022 05:30 AM
The featured chart compares daily high and low temperatures between Spencer (NW Iowa) and Burlington (far SE Iowa) for 2022. When the high or low at Spencer was warmer than Burlington, the bar appears in red. This chart nicely illustrates a couple of aspects of Iowa's climate. For high temperature, Burlington is often warmer during the non-summer months, but then often cooler during the warm half of the year. This is explained by Spencer being closer to a high plains type climate that features stronger atmospheric mixing and lower humidity which help to raise temperatures during the daytime summer season. The opposite is true during the night time as lower humidity allows more overnight cooling and you can thus see much fewer occasions that Spencer has a warmer low temperature regardless of the season.
Voting: Good - 16 Bad - 0

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Fri Sep 09, 2022
This would be nice, but
09 Sep 2022 05:36 AM
The featured map presents the current US Drought Monitor and the five day forecast precipitation from the Weather Prediction Center. The forecast is rather bullish for significant rainfall over the state, but the heaviest amounts are expected to fall over the areas that need it the least. The direst areas of NW and SE Iowa would mostly miss out, if this forecast verifies. At least it is going to attempt to rain, which is better than most of our recent days in Iowa. The bulk of this forecast rain will fall on Saturday.
Voting: Good - 18 Bad - 1

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Mon Sep 12, 2022
100 days of 50 and above
12 Sep 2022 05:19 AM
The coldest air of the season arrived over the weekend after the passage of a cold front that brought along much needed rainfall as well. A few isolated locations even dipped into the upper 30s on Sunday morning. For Waterloo, the lows on both Saturday and Sunday remained at or above 50 degrees which kept a streak alive of such days. The featured chart presents the longest such streaks each year for the site. This year's streak of 100 days bests the previous record 99 days way back in 1938.
Voting: Good - 12 Bad - 0

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Tue Sep 13, 2022
2022 Precipitable Water
13 Sep 2022 05:30 AM
This past weekend's moist airmass was swept out of the state via a cold front, which ushered in much drier air to start off this week. One measure of this change can be found by the precipitable water value derived from sounding profiles. This value is a measure of the water depth within a column of atmosphere if all water was changed to a liquid form. This value has many useful forecasting applications and is a direct measure of how humid the atmosphere is. The featured chart presents the climatology and 2022 computed precipitable water values for Minneapolis. The tail end of the 2022 values show the rapid drop in this metric with values well above the 75th percentile being related by values below the 25th percentile. This chart also shows the IEM computed monthly max and minimum values. This chart is also a nice illustration as to why it is difficult to get significant heavy rainfall events during the cold season due to having much less available water in the atmosphere available to make rain!
Voting: Good - 13 Bad - 0

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Wed Sep 14, 2022
Smoky Sunrise Sky
14 Sep 2022 05:30 AM
While not as bad as last year, the smoky skies have been noticeable lately with some very vibrant sunrises and sunsets as a result. The featured image is from the ISU AMS webcam on the Design Building at ISU yesterday morning. The orange colors and sun appearing as a smooth disk are tell-tale signs of a smoky atmosphere. Thankfully air quality near the surface here in Iowa has not been as bad as some places out west, which have recently seen dangerous air quality levels.
Voting: Good - 13 Bad - 0

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Thu Sep 15, 2022
2022 Severe Thunderstorm Tags
15 Sep 2022 05:30 AM
When the National Weather Service issues Severe Thunderstorm Warnings, they include "tags" that indicate the estimated and/or observed threat associated with the warning. The featured chart presents the frequency of a given wind gust and hail size combination for all Severe Thunderstorm Warnings issued for Iowa this year. Since these tags can change with the life cycle of a warning event, this chart provides the maximum value of each per each warning. The right hand and lower sides of the chart show the per magnitude totals. The chart shows that the vast majority of warnings are at the minimum severe thresholds of wind gust (~60 MPH) and hail size (one inch diameter). Only about 2% of all warnings reach thresholds of 80+ MPH and/or two inch diameter hail. You can generate this chart for other states and time periods of your choice!
Voting: Good - 12 Bad - 0

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Fri Sep 16, 2022
Full Moon and First Freeze
16 Sep 2022 05:30 AM
You ask most any old timer and they'll tell you that one needs to worry about frost on the first full moon during late September or October. The reason for this belief ranges from moon's gravity pulling humidity / clouds away allowing more rapid cooling or perhaps the moon helping to cool the atmosphere itself somehow, anyway. We just got past a full moon last weekend without a freeze, so perhaps we should worry when the next full moon comes around (more on this date later). The featured chart shows rather clearly that there isn't much of a relationship between the first fall sub 32 degree temperature for Ames and the number of days to the closest full moon date. What is interesting though is that the overall date average on this plot is around 9 October and you can likely guess when our next full moon is scheduled! Lock it in now? We certainly will have to wait at least a week or two more as temperatures look quite hot for the foreseeable future.
Voting: Good - 17 Bad - 0

Tags:   moon  
Sun Sep 18, 2022

View larger image
Data courtesy of National Lightning Detection Network
Lightning delays at Kinnick
18 Sep 2022 06:23 AM
While talking about University of Iowa football on a ISU website may get me in trouble, there was a significant weather angle to game Saturday night in Iowa City vs Nevada. The game experienced three lightning delays with the game finishing after 1 AM. These delays were due to having a strike detected within eight miles of the stadium, which then starts a 30 minute clock requiring no further strikes before play can resume. The featured chart presents the time since last strike within eight miles of Kinnick Stadium with the delays shaded on the chart. Getting the exact delay times was a challenge, so perhaps this plot will get updated later today once better timestamps can be found.
Voting: Good - 14 Bad - 2

Tags:   lightning   football  
Tue Sep 20, 2022
Warmest Day of September
20 Sep 2022 05:30 AM
Tuesday will be remarkably warm for the 20th of September with high temperatures in the 90s and lows near 70. For at least Des Moines, it is possible that both values will be the warmest this month. Is it common to have the warmest high or low temperature this late into the month? The featured chart addresses that very question with the daily frequency of the given day having the highest high or low temperature for that month in a given year. Overall frequencies are higher during the first week to ten days as one may expect as the period is closest to summer and features higher receipts of solar energy than the later portions of the month. There is kind of an interesting blip higher during the 17th to 21rst period for high temperature that may be related to the combination of sufficient heating, warm and dry soil temperatures, and lower atmospheric humidity due to senescence, but it could be a coincidence as well!
Voting: Good - 11 Bad - 1

Tags:   sep  
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Wed Sep 21, 2022
Latest 101
21 Sep 2022 05:30 AM
While it depends on your calendar date definition of if we are still in summer or not, the weather on Tuesday was undoubtedly summer-like with temperatures in the 90s and dew point readings in the upper 60s to near 70 degrees. A few locations in northwestern Iowa were even able to eek out a triple digit air temperature with the Spencer Airport hitting 101. Hitting 100 degrees in September is a rare feat and given this happened on September 20th makes it even more exceptional. The featured chart presents the latest into the year that a given high temperature threshold was at least observed. Just to note that a high temperature of 101 would count for that level and all levels below it within this chart accounting. So you can see that yesterday's observation was the latest date for a threshold of 98 degrees through 101.
Voting: Good - 9 Bad - 1

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Thu Sep 22, 2022
Minneapolis 70+ Streak
22 Sep 2022 05:30 AM
A week or so ago, the daily feature denoted that Waterloo had established a new record for consecutive days with a low temperature above 50 degrees. Today's feature denotes a similar type streak, but for daily high temperatures at or above 70 degrees for Minneapolis, MN. The high on Wednesday was just 70 and with a forecast high much cooler on Thursday, it appears the streak will end at 118 days. The featured chart presents the longest such streaks with the bar chart showing the longest such streak by day of the year. The frequency of having such warm high temperatures really drops off in a hurry during October and becomes nearly impossible to have more than a one day streak during the winter months. Having such warm weather this September will likely increase the shock once full blow fall season arrives in a few weeks.
Voting: Good - 9 Bad - 0

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Fri Sep 23, 2022
Accumulated Freeze Coverage
23 Sep 2022 05:27 AM
With about every accounting scheme now placing Iowa in the fall season, it is time to start worrying about when the first freeze will happen. Such temperatures are not far away from Iowa this Friday morning with sub freezing temperatures expected in Wisconsin. The featured chart presents an IEM analysis of the accumulated areal coverage over Iowa that has dipped below 32 degrees at least once by a given date of the fall season. This value is computed over a grid with the percentage of grid cells representing the coverage. The orange line represents the highest coverage by a given date and the green line the lowest coverage. The red line was last year's value and blue line is the simple climatology. So by the 23rd of September, at least some limited portions of the state have likely seen such cold temperatures. The 50 percent climatology value is about the second week of October, which nicely matches station climatology for a good chunk of the state.
Voting: Good - 16 Bad - 0

Tags:   firstfreeze  
Mon Sep 26, 2022
Relative Humidity vs Wind
26 Sep 2022 05:30 AM
Winds were gusty over the weekend thanks to stronger contrasts in air masses over the US with a more fall like weather pattern in place. On Sunday, these gusty winds were also helped by strong mixing of the lower atmosphere that transports higher momentum air to the surface during the daytime. You may have noticed the winds slacking after the stabilization of the atmosphere after sunset Sunday evening. This mixing of air aloft tends to also decrease humidity levels as near ground surface moisture is diluted with drier air. So it stands to reason that relative humidity would decrease as surface winds increase as is nicely shown by the featured chart of average relative humidity by wind speed during September for Des Moines. You may wonder though why values start bouncing around and even increasing with around 25 knot wind speeds and higher? This is due to a smaller sample size that is dominated by convective events (ie thunderstorms). These events will feature higher humidity values as it is typically raining with the storms!
Voting: Good - 8 Bad - 0

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Tue Sep 27, 2022
Iowa Hurricane Weather
27 Sep 2022 05:30 AM
Iowa is thankfully a long and safe distance away from any land-falling Hurricanes. These powerful storms still impact our weather as shown by the featured chart. The chart presents relative to landfall averages for temperature departures and precipitation frequency for the five days before and five days after for Des Moines. The two charts partition these events by those hitting the US Gulf Coast and those hitting the US East Coast. The impact on precipitation appears to be negligible, but the temperature impact is more pronounced with gulf coast hurricanes seeing higher than average temperatures prior to landfall. The likely reason is an increase in downward motion and upper level ridging over our area with the approach of the gulf hurricane. Both of which tend to warm air temperatures. So it is somewhat interesting that the opposite (cooler than average temperatures) will be happening with Ian, but Ian is expected to hit western Florida and be moving eastward, which may be a part of the difference for our weather this time.
Voting: Good - 10 Bad - 0

Tags:   hurricane  
Wed Sep 28, 2022
September 28th Percentiles
28 Sep 2022 05:32 AM
Clear skies and low humidity levels have allowed temperatures to really drop this Wednesday morning. The NWS has a frost advisory for almost the entire state with temperatures presently near or just below freezing, which makes for the first freezing temperatures of the season. September 28th is a bit early for such temperatures as shown by the featured map. It presents the percentile / percentage of years whereby a given site has seen a sub 32 degree temperature by today's date. For example, the Ames value of 9 represents just 9 percent of previous years on record having had such a cold temperature by today. So only the notoriously cold portions of the state in NE Iowa, NW Iowa, and the Nishnabotna River Valley have values in the 30 to 40% range. The big question is how much damage to the crops will this freeze cause. It depends on many factors including the maturity of the crop, how long temperatures were below freezing and how far below freezing the temperatures got.
Voting: Good - 13 Bad - 0

Tags:   freeze  
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Thu Sep 29, 2022
Yesterday's freeze
29 Sep 2022 05:30 AM
The coldest air of the season so far settled into the upper Midwest Wednesday morning. The featured maps present an analysis of available NCEP Real-Time Mesoscale Analysis (RTMA) hourly products. These hourly products are snapshots in time and not integrated over an hour, so they are not an exact accounting of minimum temperature nor hours below freezing. The top map presents the minimum temperature with some limited locations in northern Iowa reaching values below 30 degrees. The bottom map shows the number of hours below freezing. So it appears likely that this was a killing freeze over some portions of northcentral Iowa. Other places were likely more subject to micro-climate and highly localized effects that would not be well represented by this grid analysis product.
Voting: Good - 12 Bad - 0
Fri Sep 30, 2022
Daily Snowfall Climatology
30 Sep 2022 05:30 AM
With the calendar turning to October on Saturday, we can certainly now start taking about snow. The featured chart presents the simple daily average snowfall based on available data since 1951 for Des Moines. The plot has a lot of noise in it as individual heavy snowfall events will skew the day to day averages. The flat line of zero through September now starts to get a little life to it during October, but snowfall events are still rare for the month. The teeth of winter starts arriving in December for snowfall and so hopefully we can continue to enjoy nice fall weather until that reality arrives until we reach next April and May.
Voting: Good - 3 Bad - 0

Features for Sep 2022