Our brief taste of summer is over and cold air is invading the state this morning. Snow is already falling over Sioux Falls and will likely accumulate over portions of Iowa during the next two days. It remains to be seen how far south the snow will get, but suffice it to say that accumulating May snowfalls are rare in Iowa. The featured top chart presents the last date in the springtime each year on which at least one inch of snowfall was reported. The bottom chart shows the maximum temperature reported between 1 January and the date of the last snowfall. Des Moines hit 86 yesterday, so if it does snow this would be the second warmest temperature on record prior to the last snow.

May Day felt more like Mother Nature yelling "mayday" as very cold air along with snow entered the state. The cold front still bisected the state by mid afternoon on Wednesday leaving the far eastern portions in the warm air. Temperatures soared well into the 80s while northwest Iowa was near freezing. The featured top chart displays the hourly temperature difference between Burlington and Sioux City since 1 March. The largest difference was 48 degrees with Burlington reporting 82 degrees while Sioux City was at 34. Based on IEM archives, this was the largest temperature difference between the sites on record for May. The largest difference for any month is 61 degrees on 14 Dec 2008.

Needless to say, we are witnessing history and rewriting the snowfall record books for a storm in May! The featured map is an attempted analysis of the snow that has fallen so far over the past three days. The difficult part is that ground and pavement temperatures are warm, so the snow is melting as it falls. The iterative method of measuring snowfall every hour or so will also tend to inflate the totals as compaction is an important part of snowfall measurement. Regardless, a lot of moisture has fallen in the form of rain, sleet, and snow. This will be an event to remember for a long time! Enjoy witnessing history!

On 2 May, the high temperature for Des Moines set a record for coldest high temperature for that date. It was the first cold record (minimum high or low) record set at the site since early 2010! Des Moines had a streak of 55 straight hot records being set or tied without a cold record. This is easily the largest streak experienced at the site since 1900. The featured chart presents the largest hot and cold streaks of records. It is remarkable to see a period of three years without a single daily cold record being set. There is no other period comparable to this.

The USDA released their weekly update on corn planting progress and the featured map shows the estimated percentage that has been planted by state along with the departure from average for the first week of May. Our wet and mostly cold April has put the Midwest states well behind average. Typically, about half of the corn acres should have been planted by now. Some of the eight percent estimated to have been planted in Iowa may have to be replanted as cold rain and snow might have damaged the seed. The good news is that the weather appears to have straightened out with warmer temperatures and mostly dry conditions.

The featured chart shows the 30, 90, and 365 day trailing precipitation departures for statewide areal averaged precipitation since 1 April 2012. While the short term metrics are around +4 inches above average, the long term deficit has only improved by two inches or so. The reason is that late April and early May 2012 was a wet period before the drought got started, so very wet days this year are replacing wet days last year resulting in little net change. The long term deficit will not be erased until either we have a wet June and July, or we simply reach August and that period drops out of the 365 day window. These time scale interactions are what makes drought analysis difficult.

The featured chart compares the number of severe thunderstorm warnings issued in Iowa against the statewide precipitation for the period of 1 April to 7 May. The IEM's archive of warnings goes back to 1986 and for that period, this year is the wettest. One might expect that with more severe weather, precipitation should be more intense and accumulate more. It turns out that severe weather often brings localized intense precipitation, but it tends not to be widespread. More heavy rain is visiting the state this morning.

The Des Moines NWS Office released a statement yesterday noting that a tornado has not been reported in the state since 24 May 2012. Since 1950, this is the second longest streak on record for Iowa and only a few days away from the longest tornado-less streak back in 1955-1956. The featured map presents the number of days since the last tornado warning by NWS Office. For the Des Moines and Davenport offices, it has been over a year since the last tornado warning. Interestingly, this drought extends southwest to offices like Pleasant Hill (Kansas City) and Topeka, which are certainly in the traditional "Tornado Alley".

Low temperatures on Sunday morning were very chilly as nearly the entire state was below 40 degrees and some locations even well below freezing. High temperatures on Tuesday are expected to push 90 degrees for much of the state. How common is it to go from having lows in the 30s to having a high 90 or above two days later? The featured map plots the number of events per climate site that this situation has occurred since 1951. The largest numbers are over Western Iowa. The reason is that the soils are sandier, which allows for more rapid heating. It is also closer to the high plains climate and Rocky Mountains, where intense pushes of hot and dry air are more common.

The warmest weather so far this year is expected today with high temperatures pushing 90 degrees over most of the state. The featured map displays an analysis of the average date of the first 90 degree temperature of the year. The general gradient is from west to east with extreme western Iowa seeing its first 90 degree temperature around this time of year and far northeastern Iowa around the end of June. It will also be windy today, which will help dry out fields in the state allowing further progress on planting.

Extremely warm air aloft was able to mix down to the surface yesterday and drove temperatures to record levels. The century mark was breached over northwestern Iowa with Sioux City topping out at 106! It certainly felt like summer. For Des Moines, the high was 94. Has summer already started? The featured chart presents the percentage of daily high temperature reports that occurred within the warmest 91 day period of the year or during meteorological summer (Jun, Jul, Aug). When Des Moines experiences a 94 degree high, ~95% of the time this day was also within the 91 warmest days of the year. So perhaps we have gone from snow to summer within two weeks.

High temperatures on Tuesday exceeded 100 degrees over most of northwestern Iowa. The featured map displays reported high temperatures from the ASOS + AWOS network and the period since as warm a temperature was reported. For some locations, this was the warmest weather in nearly 60 years! This is remarkable considering that it is only the middle of May and we are still two months away from the climatological warmest time of the year.

The featured chart is a timeseries of one minute interval observations from the Sioux City ASOS on Tuesday. The high temperature topped out at 106 degrees after a morning low temperature of 54. The reason it was able to warm so quickly was a very hot layer of air just above the surface that mixed down. The bottom chart clearly illustrates the mixing process as surface wind speeds increased until about noon and then flattened out for the next three hours along with temperatures. This is indicative of having a deep depth of the atmosphere begin mixed, so heated surface air parcels can ascend for a large distance and there is no higher momentum air to mix back down to the surface. Anyway, the noon temperature of 102 degrees was warmest noon temperature the IEM has on record for any site in the state on any date! It was 104 degrees at 1 PM, which was the warmest temperature for that time as well.

The featured map presents IEM estimated precipitation departures since the first of April for the Midwest. This past weekend saw more rounds of heavy rainfall visit the state after a relatively dry stretch of weather. Eastcentral Iowa up into southeast Minnesota and western Wisconsin have seen the largest departures from long term average. Further rounds of heavy rain and severe weather are expected today and on Tuesday.

A cold and wet April put Iowa and the rest of the corn belt well behind schedule for planting corn. The weather did straighten out enough last week for major progress to made as shown by the featured chart. The USDA weekly crop progress report estimated 71% of the state's corn crop was planted by this past Sunday. This was up 56% from the previous week! The featured chart shows the weekly progress reports since 1979 in the top panel and the maximum week over week change in the bottom panel. Recent rains this week will slow progress again and our weather looks to turn a bit cooler.

The featured chart displays an analysis of water vapor surface mixing ratio values for Des Moines. This value is a measure of the amount of water in the air. It is not temperature dependent and can be simply averaged whereas dew point temperature can not. For all the rain and snow we have had, humidity levels have not been often to the extreme high end on a daily basis. Yesterday's levels were about average based on long term data at the site.

Growing Degree Days (GDD) are a measure of heat units that have typically been associated with plant development. The equation to compute them is simple and only uses commonly available data variables of daily high and low temperature. The featured chart displays the daily distribution and average accumulation of GDDs for Ames. This year's values are the blue dots. This year has seen near zero GDDs up until near the start of April and a number of very cool days since then. The cool weather looks to continue until a warmup next week.

Thursday was a very pleasant day in Iowa with high temperatures near 70 and dew points in the 40s. The featured chart presents the frequency of having a sub 50 degree dew point during the afternoon hours for Des Moines. The top chart is the frequency at a given air temperature, so roughly 50% of the time when the air temperature is 70 degrees in the afternoon, the dew point temperature is below 50. The bottom chart shows the frequency by day of the year. The summer months are certainly the most humid and a sub 50 degree temperature is rare.

San Antonio, Texas had its second largest daily precipitation accumulation on Friday picking up just under 10 inches for the day. This was after picking up 2.2 inches on Thursday! The featured chart is of one minute interval precipitation observations from the automated weather station showing the intense rates experienced. It is remarkable to note this all fell between 3 and 11 AM. Iowa has experienced extreme rainfall events as well this weekend with significant flooding occurring and expected to continue this week.

Memorial Day weekend has been a cold washout for most of the state with flooding problems growing with each round of heavy rainfall. The featured map presents the past 3 days worth of precipitation estimates from the NMQ project. While the 10 inch totals shown may be slightly overdone, the general pattern of Northwest to Southeast heavy rainfall is problematic as the majority of Iowa's river basins are oriented in that same direction. To make things worse, severe weather looks to be a possibility today and later in the week.

A majority of Iowa's summer time rainfall comes during the overnight hours. During the drought of 2012, this contribution was largely missing. So far, 2013 has been a different story with multiple heavy rainfall events happening during the overnight hours. The featured chart presents the hourly precipitation totals for Des Moines and the long term climatology for May. The 3-8 AM totals have far exceeded climatology. The totals shown were up until 12 AM this morning and another half inch was fallen today.

The featured chart displays the monthly precipitation departures for Ames along with the El Nino 3.4 index value (determines El Nino vs La Nina). This month has seen over six inches of excess precipitation as compared with long term averages. The departure is the largest seen since the floods of 2010 and there are still 3 days to go in May and plenty of rainfall chances. The El Nino index is provided as our monthly weather is often correlated with it. Do you see any interesting relationship?

The meteorological spring season includes the months of March, April, and May. All of the rain and snow these past three months have contributed to the what appears to be the wettest spring on record for Iowa as shown by the featured graph. The numbers shown are IEM estimates and differ from the official values from the State Climatologist due do differences in the areal averaging technique. Regardless, the preliminary estimates from the State Climatologist have this year as the wettest on record as well. We still have two days of May left and there are more chances of rain and severe weather.

The featured map presents NOAA's "stage IV" multi-sensor precipitation estimates for the past six days along with the automated weather station reports. Marshalltown displays the highest value at just under 9 inches! The stage IV estimates show 9-10 inch estimates just west of that location as well. This all adds up to what is a record river level for the Iowa River at Marshalltown.