Significant rains fell over the past two days primarily over southeastern Iowa with totals over two inches in areas. The statewide averaged total estimated by the IEM for Wednesday was just over an half inch, but not all of the state saw rain. The featured chart presents the frequency of areal coverage of precipitation to total amount for four months of the year. The colors represent a relative frequency with reds being the most common. You can see the most frequent areas slightly differ by month with July around 15%, April and October around 30% and January a bit more spread out. July is dominated by smaller scale storms than the other month, so their coverage tends to be less. Storms systems are much larger in January, so precipitation tends to be smeared out over larger areas. Rainfall intensities are higher in July. So as the coverage increases, the amount increases as well. This also means that widespread, light rainfall events are not common in July.

Sunday was a very wind day over Iowa thanks to a deepening storm system to our west. For Des Moines, the peak sustained winds over a three hour period was 23 mph during the afternoon hours. The featured chart presents the IEM computed daily maximum three hour sustained wind speed for Des Moines. Note, this value is based on hourly reports and not a direct measure of sustained winds over the hour. The chart shows the annual cycle with peaks in spring and fall along with the minimum during summer. While thunderstorms can bring momentary bursts of 70+ mph winds, the sustained winds are powered by strong and large storm systems feeding off the sharp contrasts in air masses. These types of systems are not found around here in the summer time as the cold air masses are relegated well north. The two days shown over 50 mph were 3 April 1946 and 5 May 1950.

Rain and even some snow over the northern counties looks very likely today over Iowa. These type of events do not simply rain for an hour and then stop, but persist over a number of hours as the large storm complex passes by. The featured chart presents the frequency of an hour with measurable precipitation for a given month being followed by another hour with measurable precipitation. The lowest frequencies are found during the summer months as thunderstorms tend to be more isolated in nature. The lower values during the winter are likely due to having less intense precipitation events with some hours barely measuring one hundredth of an inch.

Our first winter storm with significant snowfall for the season is in the books. The featured map presents an analysis of reports showing the heaviest totals approaching 4-5 inches in extreme northwestern Iowa with amounts quickly dropping off to the south and east. Much of the state picked up needed rainfall with half inch and greater totals common. The IEM generates featured analysis maps like these after snowfall events where totals were generally greater than an inch or two. The winter season sees other snowfall events that produce lighter amounts and are not included in this series for the season.

The Toro Company is having an interesting promotion whereby a newly purchased snowblower this year is free if the area gets less then 10% of normal snowfall for the season. Is such a low snowfall total possible for Des Moines? The featured chart shows the yearly (winter season) snowfall total and the percent of long term average. Only one year is shown below 10 inches, let alone 10%. 10% of normal would only be just over three inches and the chart would indicate there is a near zero chance of it happening. "So you're telling me there's a chance..."

The featured chart shows the daily time-series of four inch depth soil temperature for each of the past 25 years for the Ames ISUAG site. The series for this year and last year is highlighted. Soil temperatures this time of year are in the cooling down phase as solar input decreases with each passing day and night time hours increase. This year's series shows the cold temperatures through March and the very cold temperatures during the snowfall in early May. Air temperatures are expected to get very cold next week, which will help soil temperatures cool more as well.

Since dropping to 27 degrees in late October, the Des Moines Airport has not stepped down below that level since. The featured chart presents the number of days each end of calendar year season that the low temperature set a new low for the season. The tally is started once the temperature dips below 50 degrees after 1 July. The blue bars represent the number of events prior to breaching 32 degrees and the red bars are the total for all events below 50. The chart shows that it takes around 5 events for the temperature to first reach 32 after dropping below 50 and the season total is around 10-15 events. The total so far (not shown) for this year is just seven events, so a number of colder nights appear probable prior to the first of the year. The first of these arriving Monday night with the coldest air of the season and highs on Tuesday struggling near freezing!

Winter made an appearance on Monday in Iowa with rapidly dropping temperatures and a brief dusting of snow causing some travel difficulties. The air temperature for Des Moines at 6 AM was 46 degrees Fahrenheit and by 2 PM was only 28, which makes for an eighteen degree drop. The featured chart presents the weekly frequency of temperature change between 6 AM and 2 PM for Des Moines. For most days, the temperature warms for this period and the most frequent warming occurring during the strong heating of the summer season. Large negative drops, like that on Monday, are relegated to the cold season when fronts are stronger (larger temperature contrasts). Temperatures today will warm from their 6 AM value, but barely warm to just above freezing for some in the state!

The airport weather sensor for Spencer reported a sub zero Fahrenheit on Tuesday morning making for the first sub zero temperature of the season. The featured map displays the number of years whereby the long-term climate site had the first sub zero temperature of the fall/early winter season. If two or more sites tied for that year, they all got credit for having the earliest value that year. Northwestern Iowa, by far, is the favored location with some subtle topography effects over the Nishnabotna River valley in west- central Iowa. Warmer weather will be building back into the state for the rest of the week with a pleasant weekend in store and highs in the 50s.

A prominent meteorological forecast model, North American Mesoscale (NAM), run by the National Centers for Environmental Prediction produced what would appear to be a very bad forecast for near surface air temperature on Wednesday. The featured map shows the forecasted temperature bias for noon with many sites in the upper midwest around 10 degrees too cold. On the surface, this would appear to be a bad forecast, but there was a reason why the temperatures were too cold. The shaded areas on the map show analyzed snow depth that the model was provided to start the forecast. Of course, these snow depths are way off with much of Iowa shown in the 1-3 inch range, whereas reality was just a trace, if that. So the worst temperature predictions are shown over areas with incorrectly analyzed snow depth. This begs the academic question of if this was a bad forecast or an example of garbage in - garbage out?

This delightfully complex plot is an attempt to illustrate the changing seasonal contribution of yearly precipitation. Each column in the chart represents that year's worth of precipitation. The plot is normalized so the accumulated precipitation for that year is expressed in percent total of that local year. The color ramp provides breaks at the 20, 40, 60, and 80 the percent. For these four levels, a linear regression fit is shown with the slope of that line presented at the top in units of days per century. With me so far? :) So the question is what is the interpretation of having three of the lines with a negative slope and only the 40% line with a positive slope? One explanation could be that the relative contribution of precipitation during May and early June is increasing relative to the rest of the year. Rephrasing, more of our yearly precipitation is falling during this time period. Having this period (within first half of the year) as relatively wetter also shifts the yearly distribution to an earlier date, which is likely the cause for the three lines having negative slopes.

The forecasted severe weather threat, including a rare high risk issued by the Storm Prediction Center, on Sunday materialized with many tornadoes and damaging wind reported. The featured chart presents the number of active severe thunderstorm and tornado warnings at a given time issued by the National Weather Service. A total of 125 tornado and 220 severe thunderstorm warnings were issued for the event which lasted from early Sunday morning till early this morning. Much calmer weather is expected this week with a few days in the 50s to start off the week before much colder air arrives.

The featured chart presents a climatology of water vapor mixing ratio for Des Moines. This value is an expression of humidity, just like dew point and relative humidity is, but has a nice property of being on a linear scale so two values are easily comparable. For example, the difference between a 50 and 60 degree dew point is much different than the difference between 60 and 70 degrees. Anyway, this past weekend's surge in humidity shows up in the bottom graph as a large departure from climatology. The drought period during this year shows up as well during July and August with most of the departures drier than average. Very dry air is expected to arrive this weekend allowing for very chilly temperatures and highs well below freezing expected on Saturday!

Today's IEM Daily Feature is the 3,000th in the series! The featured chart presents a time series and linear fit of time each day the feature was posted and the percentage of total votes that were in the good category. The chart shows features getting posted earlier and getting a higher percentage of good votes! Are those two trends correlated? Anyway, I enjoy attempting to provide something informative and relevant each day. The weather is always interesting and the large archive the IEM has collected over the past twelve years provides an endless supply of things to show. Here's to the next 3,000 features!

For those of you that were up by noon this past Saturday may have noticed that many concrete surfaces were wet that morning. The featured image displays two RWIS webcam images from that day along with a timeseries of air temperature, dew point, and bridge deck pavement temperature. The highlighted period during the morning shows the dew point being slightly warmer than the bridge temperature. When this happens, water vapor will condense onto the surface and that is why the concrete surfaces were wet that morning. When this process happens at temperatures below freezing, rhime and frost occur.

The featured map presents a new dataset about to be released by the Iowa DOT and IEM. IDOT is collecting realtime location and telemetry from over 900 trucks! The feature map presents an IEM calculated heat map of truck activity over the state on Thursday along with the winter weather advisory that was issued by the National Weather Service. It is interesting and intuitive to see these two datasets align! These IDOT trucks are only operating on state and federal roads, so local county/city trucks would not be accounted for in this analysis. These trucks are also reporting current air temperature, state of their plows, and material application rates. Look for more applications built off this dataset soon.

The featured map presents an IEM analysis of reported snowfall totals from the winter storm late last week. The largest report was only two inches, but this storm brought freezing rain and sleet, making for difficult travel conditions. Very cold air also filtered in behind this storm system making for a cold weekend. Winter looks to have firm control on our forecast with below freezing temperatures expected for much of this week and a fresh dusting of snow this Monday morning.

Snowfall totals on Monday reached 5 inches just east of Waterloo as yet another winter storm impacted Iowa this November. The featured analysis combines NWS storm reports with CoCoRaHS and COOP reports. This snow looks to stick around for Thanksgiving as temperatures won't climb back above freezing until this coming weekend. The good news is that travel will not be a problem in Iowa, unlike the east coast which is dealing with a major winter storm.

Much of Iowa looks to be in line to experience a white Thanksgiving (having snow cover). The featured chart presents the observed frequencies of white Thanksgiving, Christmas, and years with both days having snow cover based on long term data for Des Moines. Please note that the Thanksgiving total is for the fourth Thursday of November for each year since 1893, but the actual thanksgiving was on a different date some years prior to 1941. Having snow cover on Thanksgiving does slightly elevate your chances for snow cover on Christmas, but its not an absolute.