After the epic warmth of March, some probably wondered if we would pay for it with a cold April. We ended up with the first statewide snow- less April in 122 years! The featured map presents an IEM analysis of the difference the average temperature during April was from March. Areas shaded in red had a warmer March than April. Areas around Chicago and other locations to our east were remarkably warmer in March by about 3-4 degrees. The start of May looks to bring us warmer temperatures and numerous chances at thunderstorms along with severe weather.

Severe weather season kicks into high gear this week for Iowa with chances just about each day this week. The Storm Prediction Center has a moderate risk of severe weather for today for portions of western Iowa. The featured map presents the number of severe thunderstorm and tornado watches issued so far this year. Portions of western Oklahoma have had up to 19 watches issued so far, while not all counties in Iowa have seen a watch yet.

Parts of northern Ames were hit hard early Wednesday morning by a thunderstorm complex producing straight line winds upwards of 80 mph. The KCCI SchoolNet8 site in Ames sampled this event at about 6 second resolution and its data is presented in the featured graph. Data for only a seven minute period is shown between 1:49 and 1:56 AM. The peak wind gust of 61 mph occurred very briefly with wind speeds a few seconds later back down to around 20 mph. The pressure trace shows the jump in pressure as the colder thunderstorm air rushes by. The wind direction also shows an organized change from south to westerly winds. While not definitive proof, this plot would not indicate the presence of a nearby tornado as there probably would have been a pressure oscillation and more pronounced change in wind direction.

Some locations in Iowa just to the southeast of Des Moines near Pella experienced a somewhat rare phenomena known as a heat burst. The signature for these events can easily be identified when high resolution in time observation data is available. They are marked by a period of rapid increase in temperature, decrease in dew point, and typically gusty winds. They are caused by rain showers that rapidly decay causing very warm and dry air to rush to the ground. The featured chart presents a time series from the Twin Cedars KCCI-TV SchoolNet site near Bussey. The period shaded in blue is roughly the length of the event, just 18 minutes. Other observation stations in the vicinity recorded this event as well with a wind gust topping out at 78mph (Pella RWIS) and a temperature rise of 14 degrees Fahrenheit (Pella SchoolNet). With once per hour observations, these events typically go unnoticed. Our period of active weather will continue into the weekend!

Today (Sunday) will end our streak of having the Storm Prediction Center forecast slight risk of severe weather in Iowa. The featured image displays the categorical outlook issued each day at 7 AM. We have one more round of severe weather to go this afternoon and evening before the weather straightens out some this coming week. It would be good to see a dry stretch of weather to help get the agricultural crops planted in the state.

Warm and moist air surged into Iowa this past Saturday pushing dew point readings over 70 degrees Fahrenheit for Des Moines. The featured chart presents the period between the first and last 70 degree dew point each year. Saturday was only the 5th of May making for the earliest 70 degree reading since 1955!

The featured chart presents the year to date rank and departure this year's average temperature for Ames compares with previous years. Since the end of March, we have been in first place and have not relinquished the spot yet. The forecast for the rest of this week keeps temperatures near average values and mostly dry.

If you live in Iowa, you may have noticed that the clouds yesterday afternoon were very thin (having short vertical extent). The featured chart presents a vertical sounding of the atmosphere over Ames based on model data from the RAP model. The red line is temperature and green line is dew point. As the sun heats the ground, the lower part of the atmosphere warms and mixes vertically until it reaches air that is relatively much warmer and unwilling to mix down. The sounding shows that this level was very well defined with a sharp increase in temperature and rapid decrease in moisture. This caused the vertical extent of the clouds that developed at the top of the mixed lower atmosphere to be very flat.

The featured chart presents daily values of precipitable water from the NAM forecast model provided by BUFR data analysis for Des Moines. This value represents the depth of liquid water if all the water in a column of air was converted into a liquid form. In general, higher values allow showers and thunderstorms to become more efficient rain producers. The past few days have seen relatively low values of precipitable water and the result has been dry weather and pretty blue skies. Values for this variable increase during the summer reaching totals above two inches.

The featured map is forecast precipitation for this week until Friday evening by the Hydrometeorological Prediction Center. Only very small amounts of rain are shown over northeastern Iowa with the rest of the state remaining dry. A week of dry weather is exactly what most farmers in the state need to help complete the spring field work.

With the growing season well under way, a useful metric to look at our progress is called growing degree days. The featured chart presents the departure from climatology for GDD accumulations over time periods since the first of the year. Each bar represents the departure from that date until now. So since the beginning of the month, we are up 50 units and since the beginning of the year, we are up over 300 units. The right hand axis expresses these departures in terms of average 15 May days. So since the first of April, we are about a week ahead of schedule.

Temperatures warmed very nicely on Tuesday with Des Moines tieing their record high for the date of 91 degrees. The heat was accompanied by rather dry air as dew point temperatures were well below 50 degrees. The featured chart presents the dew point temperature by day of the year when an air temperature at or above 90 degrees was observed by the Des Moines weather sensor. Typically, hot weather in Iowa is accompanied by muggy conditions with dew point values well above 60 degrees.

May is an active month for severe weather in Iowa. The featured chart presents the number of county based warnings issued in Iowa by day of May for each year since 1986. The beginning of May this year certainly started off active with severe weather each day, but has recently been very quiet. Humidity values have been on the dry side recently, which among other factors have prevented storms from developing over the past week. Thunderstorms are an important part of our climate as they bring a majority of the yearly rainfall to the state, so we have to take the good (needed rainfall) with the bad (severe weather).

Our mostly dry stretch of May weather looks to continue into the next week with only slight chances of showers on Sunday. It has been a while since we have seen a daily rainfall over half of an inch as shown by the featured chart. Portions of east-central Iowa have gone over a month without such a rainfall based on estimated rainfall totals. While the dry weather has been beneficial to help get the crops planted, we certainly could use some rain to help the plants get off to a good start.

The clouds cleared out in time for most Iowans to get a glimpse of the partial solar eclipse this evening. The featured image is from the ISUAg Farm webcam showing an internal webcam reflection of the eclipse of the sun. I was not expecting to get this kind of view, but will certainly take it!

Precipitation has been difficult to come by for most of May for most in the state. For Des Moines, 15 straight days have passed without even a trace of precipitation. This is the longest such streak to occur within the month of May for the site. According to the NWS, the longest streak to occur during the spring season is only a few days away at 23 days during March of 1910. This should be our rainy season with precipitation every three to four days. While the dry weather has allowed spring planting to mostly finish, some of the seeds are sitting in dry soil waiting to germinate.

Getting the crops planted each spring in Iowa is not an arbitrary task. Threats of frost, rain, drought, and soil temperatures are among the issues farmers need to work through each year. The featured chart presents USDA weekly estimates of corn acres planted in Iowa since 1979. A few of the extreme years are highlighted in the upper chart. This year got off to a very early start, but wet weather arrived causing the planting rate to slow. We recently finished a bit ahead of schedule thanks to our recent stretch of dry weather. The lower chart presents the largest weekly jump each year in the percentage of acres planted. It is interesting to note that this value hasn't increased over the years. While individual farmers are easily able to plant more acres in a day, there are many fewer farmers and they need to work more days to cover the larger farm sizes that currently exist.

One of the reasons for our recent lack of rainfall has been the dearth of precipitable water. Precipitable water is a measure of depth of water in a column of air if all phases found were converted to liquid. The featured chart presents the combination of preciptable water analyzed by a weather forecast model and the total rainfall for that day. The upper chart presents the frequency of having rainfall on a given May day partitioned by preciptable water value. Increasing water content in the atmosphere increases our chances of rain. The lower chart presents the combination of daily precip observations against the precipitable water value. The one to one line shows that often the preciptable water value provides an upper bound to the amount of rainfall we may receive. Advection processes help to replenish atmospheric water content so we can get rainfall totals greater than the amount of water in the column of air ("in this house we obey the conservation of mass law").

While rainfall has been somewhat difficult to come by this May in Iowa, Miami, Florida had a significant heavy rainfall event on Tuesday totaling 9.70 inches! The featured chart presents the one minute interval precipitation data for that event. The actual hourly rate rate peaked at 4.50 inches! This is an intensity that we don't see in Iowa due to our continental climate. In fact, the 9.7 inch daily total, while only the second largest for Miami in May, is larger than the recorded history records for 95% of the sites in Iowa. Rain chances for Iowa are in the forecast for this Memorial Day weekend along with very warm temperatures into the 90s on Sunday.

Mike Bettes and Dr Greg Forbes of the Weather Channel have been roaming the plains this May on a hunt for tornadoes. The featured plot shows their driving track for each day they have chased and reported their location in realtime to the spotternetwork project. Most of the severe weather this month has been to the west of Iowa with strong storms again over Nebraska and Kansas this Sunday evening. Some severe weather is expected over night in Iowa, but it will probably not come in the tornado variety.

With May now winding to a close, it appears this month will come in well below average for most of the state. May is an important month to the yearly contribution of precipitation as shown by the featured chart. The chart shows the frequency of having a month with the same direction of departure as the year long total. For example, having this May be below average precipitation along with having the year of 2012 be below average precipitation. Interestingly, May has the largest frequencies in both directions. Precipitation in May can help "set the table" for the rest of the summer, as a wet May leads to wetter soils and more precipitation chances for the summer. A dry May can have the opposite effect. Most concerning is that this chart would indicate a 75% chance of having below average precipitation for 2012.

May is going to end on a cool note with high temperatures in the 60s for most Iowans and low temperatures into the 40s! This may be the coolest high and low temperatures for the month. The featured chart presents the frequency of a day in May having the coolest high or low temperature. The earlier part of the month is certainly more frequent than the latter part and it has happened previously that the end of the month was the coolest.

The last day of May is at hand and while more rain is failing today, the current difference between May rainfall totals for a location in far NW Iowa is compared against far SE Iowa in today's featured chart. Based on preliminary data, Rock Rapids has seen just over four inches more precipitation than Keokuk so far this May. This would be the largest difference since the early 1970s. Typically, the largest rainfall totals are over SE Iowa in May with the smallest over NW Iowa. So this May has seen a reversal of that, but it is not uncommon.