May 1-3, 2013 Historic Spring Snowstorm

An unprecedented spring snowstorm impacted the central United States from May 1 to May 3, dropping record-shattering amounts of snow up and down the western Mississippi River Valley. Some locations across the southern United States saw their first ever May snowfall and cities in Kansas and Missouri saw their highest snowfall totals in 106 years. Meanwhile, noteworthy May single day and storm total records for the states of Iowa, Minnesota, and Wisconsin were eclipsed by this event. South central Iowa up through southeast Minnesota and western Wisconsin bore the brunt of the snowfall with 10+ inch reports not uncommon. Warm road temperatures across Iowa helped to initially melt the falling snow, but conditions quickly deteriorated across the regions where heavier snow fell. This led to numerous accidents and spin outs. Sporadic power outages also occurred as snow-coated tree branches broke and took out power lines.

Radar loop of the snowstorm from the Des Moines WSR-88D Radar from 10am on May 1 to 6pm on May 3 (56 Hours!)	Snowfall totals map from May 1 to 3 across Iowa. The highest snowfall amounts were recorded in south central and north central Iowa.	Snowfall totals across the central United States from the storm.

May snowfall and temperature records across Iowa fell like dominoes during this historic event. Below is a listing of central Iowa and statewide records that were surpassed. Previous records are listed in italics.

It took a combination of several unseasonably strong and unusual features to produce this exceeding rare snow event.

The last few days of April were characterized by warm, summer-like temperatures with Des Moines reaching 86 degrees on the 30th. This was a welcome reprieve after what had been undoubtedly a cold and damp start to spring. This warmup came to an abrupt end late on Tuesday, April 30 as a strong cold front slid through Iowa, igniting several severe thunderstorms over the southwest corner of the state. Temperatures behind the front began falling on Tuesday night and kept dropping through the day on Wednesday (May 1), with many locations recording their warmest readings for the day shortly after midnight. Daytime temperatures on May 1 were a full 30-40 degrees colder than the previous day. The surface observation map animation on the left loops between the early morning hours of May 1 and May 2, showing the stark change in temperatures (in red) over the state after the passage of the front.

Just behind the surface cold front, a strong mid to upper level trough of low pressure slid southeastward from Canada. However, a large stationary ridge of high pressure, known as a omega block, was also taking shape across the eastern United States. Its forward progress inhibited, the upper level trough simply came to a grinding halt over Iowa. The trough eventually formed a cut-off low centered over the south central US by Friday morning (May 3), which meandered over the region for the next three days before drifting eastward. The system was also able to tap into a nearly continuous supply of moisture from both the Gulf of Mexico and western Atlantic Ocean during its stay over the Midwest. This moisture worked up into Iowa, where strong mesoscale and synoptic lift generated a wide swath of precipitation that fell for days straight. The synoptic setup for this event is shown in the image on the right.

The precipitation shield first took the shape of rain showers over southeast South Dakota and northwest Iowa late on April 30th. This band of rain lifted northwestward during the overnight hours as the trough stalled and began transitioning to snow over far northwest Iowa just before sunrise on Wednesday, May 1. It then began a slow, steady march to the southeast over the next 36 hours. The leading edge of the precipitation shield consisted of light rain with embedded heavier showers. The rain eventually transitioned to a rain/snow mix and eventually snow as continued cold air advection and melting ice crystals cooled temperatures aloft below freezing. The cross section on the left attempts to tie the type and location of forcing, vertical temperature/moisture profiles, and radar signatures to what was observed on the ground. The diagram shows that the precipitation-generating frontogenetical forcing on the eastern side of the precipitation shield was well within the low-level above freezing layer, leading to the rain (and lack of a melting layer aloft in CC). Meanwhile as one travels further NW the frontogenesis band becomes more elevated and broad, synoptic lift associated with the trough of low pressure becomes evident in the omega plot. This leads to the generation of large snowflakes aloft (note how the omega maximum is located in the dendritic growth zone) that, as they fall/melt through the 850-700mb warm layer, quickly cause the air to cool below freezing and allow the snow to reach the ground. This schematic almost perfectly matches what was seen in the CC radar product (top right), where the melting layer shows up as the area of orange and yellow values.

The cold air advection and cooling due to melting snow caused the rain/snow line to shift all the way to east central Iowa. The line then became stationary late on May 2 as the upper level trough transitioned to a closed low. The rain/snow line then retreated back to the west during the day on May 3 as a broad region of warm air wrapped north and west around the eastern flank of the low. This surge of warm air eventually brought the snow to an end by late in the day on Friday, but a widespread, cold rainfall continued into the weekend with the cut-off low spinning overhead.

The WSR-88D dual-pol radar in Des Moines captured many intriguing features and signatures during this snowstorm. As with the blizzard back in December, the new dual-pol products did a great job honing in on the transition zone between rain and snow, as well as discerning the different types of rain drops and snowflakes. Each screenshot below consists of a four panel image, with conventional reflectivity in the upper left, correlation coefficient (CC) in the upper right, differential reflectivity (ZDR) in the lower left, and specific differential phase (KDP) in the lower right. Click on each image for more information on what the radar was sampling. Show the definition of the dual-pol products.

Below are some of the amazing photos taken across central Iowa during and after the storm.

The table below contains storm total snowfall amounts from May 1 to 3, sorted by amount. These values are still considered preliminary and may be subject to adjustment.