Case Study: May 10, 2010

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


This case should demonstrate the connections between the large-scale synoptic weather pattern and the weather that occurs on the mesoscale and microscale. By the end of this page, you should be able to define the criteria that classify a thunderstorm as "severe," and that classifies a funnel cloud as a tornado.

Case Study...

You'll see numerous examples of severe weather outbreaks in this course, but one common thread that they all have is the strong link between the mesoscale and synoptic-scale patterns. To briefly illustrate the connections between the spatial scales we've covered in this lesson, let's take a look at an outbreak of tornadoes across Oklahoma and Kansas from May 10, 2010.

The eruption of severe weather in this area was no surprise to forecasters who had studied the "big picture" synoptic-scale weather pattern ahead of time. In fact, on the morning of May 10, forecasters at the Storm Prediction Center (SPC) pinpointed this region as having a high risk for severe thunderstorms in their "Day 1 Convective Outlook." We'll examine SPC's convective outlooks a bit closer later on, but if you're interested in learning more now, check out the Explore Further section below for some links and brief discussion.

How was it so clear to forecasters that this area was primed for severe weather? For starters, take a look at the 18Z surface analysis on May 10, 2010 (below), which indicated a low-pressure system centered over the Colorado-Kansas border. In the warm sector (the region between the warm and cold fronts), warm, moist, maritime-Tropical (mT) air streamed northward from the Gulf of Mexico.

Surface analysis at 18Z on May 10, 2010 showing a mid-latitude cyclone over the southern Plains
The 18Z surface analysis on May 10, 2010, showed a low-pressure system centered over the Colorado-Kansas border. In the warm sector, warm, humid air streamed northward from the Gulf of Mexico.
Credit: Weather Prediction Center

Experienced forecasters know that widespread, organized severe weather events are usually linked to mid-latitude cyclones, because they can bring together the ingredients necessary for powerful thunderstorms. But, of course, there's more to a mid-latitude cyclone than just air masses and surface fronts. Meanwhile, the supporting shortwave trough was located over the Rockies at 12Z on May 10 (check out the 500-mb analysis at that time), which produced cooling near 500 mb, helping to destabilize the middle troposphere as it approached the southern Plains.

To understand this, recall from your previous studies that a 500-mb trough corresponds to an elongated region of low 500-mb heights. So, as a shortwave trough approaches, 500-mb heights typically fall. To confirm, check out the 18Z analysis of 500-mb heights, winds, and 12-hour height tendencies below. Fortunately, forecasters had access to such analyses in near real-time thanks to the hourly initializations of mesoscale models! To get your bearings on this analysis, the color-filled areas represent height falls (in meters) over the 12-hour period from 06Z to 18Z on May 10. Note that 500-mb heights fell more than 120 meters in 12 hours along the path of the approaching 500-mb shortwave trough over southeast Colorado, northeast New Mexico, and the panhandles of Texas and Oklahoma, which signified that the middle troposphere was cooling (remember that lower heights are an indication of colder air columns).

Analysis of 500-mb height tendencies showing strong height falls over the Rockies and western Plains
The 500-mb analysis at 18Z on May 10, 2010. The thin, dark contours represent 500-mb heights, and the color-filled areas represent changes in 500-mb heights during the 12-hour period spanning from from 06Z to 18Z. Blues indicate height falls associated with 500-mb troughs, and reds correspond to height rises associated with 500-mb ridges.
Credit: Storm Prediction Center

What's the practical significance of cooling in the middle troposphere? We'll explore this issue more deeply later in the course, but for now consider that, all else being equal, a cooler middle troposphere means that temperature decreases faster with height (on average) from the surface up to 500 mb. Recall from your previous studies that a rapid decrease in temperature with increasing height tends to make the atmosphere unstable, so mid-level cooling often goes hand-in-hand with destabilization.

With the environment becoming more favorable for thunderstorms, they erupted violently through the afternoon (check out this spectacular visible satellite loop spanning from early afternoon through early evening). By 23Z, supercell thunderstorms were raging across Oklahoma and Kansas (check out the 23Z radar mosaic), and severe weather was widespread across the region. Formally, what classifies a thunderstorm as severe? SPC christens a storm as "severe" if at least one of the following criteria are met:

  • the thunderstorm produces wind gusts of 50 knots (58 mph) or more
  • the thunderstorm produces hail with a diameter of one inch or larger
  • the thunderstorm spawns a tornado

Clearly the synoptic-scale weather pattern helped drive the development of these severe thunderstorms, which were ultimately meso-β and meso-γ features. Although large hail and gusty winds were reported over the southern Plains during the outbreak of severe weather on May 10, 2010, tornadoes (microscale features) made the news that day, particularly in Oklahoma. There were several confirmed tornadoes near Oklahoma City, including one twister southeast of Norman (see photograph below) rated EF-4 on the Enhanced Fujita Scale.

Photograph of tornado near Norman, Oklahoma on May 10, 2010
A photograph of the EF-4 tornado east of Norman, Oklahoma, on May 10, 2010. The photographer, Chad Snyder, was looking west-northwest as the tornado passed by. At this time, the twister was causing EF-2 / EF-3 damage in a hollow about three-fourths of a mile away. The twister went on to cause EF-4 damage (a twister's EF-rating depends on the most destructive damage it inflicts).
Credit: Chad Snyder

In the photograph above, the condensation funnel (a funnel-shaped cloud associated with rotation and consisting of condensed water droplets, as opposed to smoke, dust, debris, etc.) did not touch the ground at this time. Yet, the debris cloud indicated that a violently rotating column of air was indeed in contact with the ground, signaling that a tornado was present. As an aside, you've probably heard storm chasers or television weathercasters say (or yell) "tornado on the ground!" But, the definition of a tornado states that the rotating column of air must be in contact with the ground. So, saying "tornado on the ground" is redundant and silly. The phrase implies that tornadoes exist that aren't in contact with the ground, which isn't the case!

The bottom line of this brief case study is that the synoptic scale primed the atmosphere for thunderstorms (mesoscale features), which in this case produced tornadoes (usually microscale features). So, just because this is a course in mesoscale meteorology, we'll spend significant time connecting mesoscale weather to events on other spatial scales!

That wraps up our introduction to mesoscale forecasting. Up next, we'll start examining the tools that forecasters use to analyze and predict mesoscale weather.

Explore Further...

Forecasters at the Storm Prediction Center are always assessing the risk of severe thunderstorms, and they issue Convective Outlooks accordingly. They issue the "Day 1 Convective Outlook" several times per day, and even issue Convective Outlooks for several days into the future. But, what do the various risk categories really mean? To learn more about each of the categories, the issuance schedule, etc., I recommend studying SPC's Convective Outlook product description. Not only will it help you become familiar with the various categories used in the outlooks, but it will help you connect the categories to probabilities of various types of severe weather.

I encourage you to follow SPC's Convective Outlooks regularly. Not only will they help you keep up on where severe weather is possible, but the accompanying discussions can be a great learning tool!