July 8, 2001 Storm Analysis
Two derechos formed in the same day in different places, both hitting Indianapolis. A derecho is a widespread convectively induced windstorm. The first derecho spawned around 9:00am in Eastern Illinois and had trekked through Indianapolis by 11:00am. It then made a beeline for the Cincinnati and northern Kentucky area, re-intensifying near Cincinnati. It weakened somewhat in Kentucky and then moved into Virginia and Tennessee, eventually dissipating entirely in North Carolina. The maximum DBZ, or reflectivity, in the radar pictures I printed was 65 DBZ, which indicated that the rain was quite heavy at approximately 16 inches per hour. However, this derecho moved fast and therefore, no affected area received anywhere near 16 inches of rain.
The second derecho also spawned around 9:00am, but in Central Iowa. This storm crossed into Illinois around 12:30pm causing extensive wind damage. This derecho was probably at its most dangerous stage around 4:30pm when it spawned four tornadoes in Eastern Illinois. It then moved on, hitting Indianapolis at around 6:00pm, meaning there was seven hours of separation between the two derechos. The second derecho then followed the same track as the one earlier, dissipating sometime well after sundown.
Many people know what’s happening in the lowest levels of the atmosphere, such as the current temperature. However, they do not realize what’s going on in the upper levels of the atmosphere. To forecast thunderstorms, you have to look in the upper atmosphere first. BUFKIT is a program used by the National Weather Service to analyze and forecast various conditions in the upper atmosphere, such as CAPE, CINS, LFC, and Helicity.
CAPE stands for Convective Available Potential Energy. It is basically a measure of how much energy is available for convection. Convection in meteorology usually refers to vertical motions. Therefore, it is related to the maximum wind speed in an updraft. A good CAPE level is in excess 1500 joules per kilogram. An excellent level would exceed 2000 j/k. Extreme amounts can reach 4000 j/k.
CINS stands for convective inhibition. CINS is a measure of the amount of energy needed in order to start convection. In other words, the lower the CINS level is, the easier it is for thunderstorms to form. A good CINS level is lower than 30 joules per kilogram. An excellent CINS level is lower than 15 joules per kilogram. These values reflect the strength of the cap. The cap, or capping inversion, is a layer of warm air aloft, which stops or delays thunderstorm development. This is because when a warm, moist air parcel is rising it needs to be warmer than the surrounding air. When it reaches the cap, the warm air inhibits the air parcel from rising any higher and therefore, no thunderstorms can develop.
The LFC, or level of free convection, should be low in order for thunderstorm development to occur, usually less than 7500 feet. This is because when moist, warm air is rising it has to fight upwards against warmer air or a weak cap until it reaches the LFC, where it will rise uninhibited. The lower the LFC is, the less an air parcel has to fight against warmer air. If the LFC is too high, the air parcel will lose much of its energy by the time it get to that level.
Helicity is not a key element in normal thunderstorm development. It instead tells us how favorable the conditions are for atmospheric rotation. The higher the helicity, the better chance for the formation and development of mesocyclones and tornadoes. Helicity is basically showing you the amount of vertical wind shear and turning, or vorticity, in the atmosphere. Vertical wind shear means winds move in different directions at different levels of the atmosphere. A helicity level that is favorable for rotation is anything near or above 150 meters per second.
For horizontal rotation to form, there has to be vertical wind shear and the winds must shift in a counterclockwise direction as you go up in the atmosphere. It is very much like rolling a pencil between your hands. Once a storm forms, the horizontal rotation is in place and begins to suck in warm, humid air to feed the thunderstorm. This strong inflow causes the gust front to be held closer to the storm, thus increasing chances for a long life span. The strong inflow also helps create strong downdrafts and strong updrafts. The strong winds of the updraft and downdraft help tilt the rotation into a vertical position increasing probabilities of a mesocyclone or tornado.
Unfortunately, I could only get BUFKIT data from Iowa where the second derecho formed. Therefore, I can only analyze one derecho using BUFKIT data. Des Moines CAPE levels at 7:00am were relatively low at 640 joules per kilogram. This reading was not high enough to generate severe weather. A good CAPE level would be a number exceeding 1500 joules per kilogram. An excellent number would be exceeding 2000 j/kg. So 640 j/kg is far off of what is needed for thunderstorm development. Later on, at 1:00pm, CAPE levels rocketed up to 2249 j/kg which was good enough to support thunderstorm development.
Levels of CAPE were also low at 7:00am in Dubuque at only 868 joules per kilogram. They did improve between 7:00am and 1:00pm, however they were still relatively low just breaking 1500 j/kg to reach 1511 j/kg. The storms hit Dubuque at approximately 1:45pm. They began to dissipate as they traveled east, then re-intensified over central Illinois. The average CAPE conditions near Dubuque may have contributed to the dissipation.
It appeared from the CINS levels that there was a relatively strong cap in place over much of Iowa due to the fact that CINS levels were over 100 j/kg. CINS levels in Des Moines were extremely high at 245 j/kg at 7:00am. However, the CINS levels dropped all the way from 245 j/kg to 38 j/kg in just six hours! This meant there was energy building up underneath the cap and when the cap was weakened, the energy was released in the form of convection and thunderstorms.
It was a different story in Dubuque and the Quad Cities as CINS levels only dropped from 141 j/kg to 102 j/kg in six hours meaning that the warm, moist air was held back, another reason why the storms lost their punch in western Illinois.
The LFC was relatively high across Iowa, but around Des Moines there was enough energy built up to produce thunderstorms. HELICITY levels were around 150 meters per second, which favored the development of rotation, but no tornadoes developed in Iowa.
The conditions in the upper atmosphere are very vital to thunderstorm development. The day the derechos developed, conditions were in place that favored thunderstorm development all the way from Iowa to South Carolina. To analyze upper air data, I reviewed upper air charts provided by the Storm Prediction Center, which are based on the atmospheric pressure in millibars. Each point on the upper air charts may not be at the same altitude.
For favorable thunderstorm development, the surface chart should show high moisture levels and a source of lift. The dew points were in the seventies across the area ravaged by the derechos and there was a moisture boundary, a front of some sort, over Minnesota and South Dakota moving ESE. The conditions in the upper atmosphere barely changed at all throughout the day. In the 925mb and 850mb maps, the same traits of high moisture levels and a source of lift should appear just as they do in the surface chart. The 850mb map shows an extensive area of moisture almost exactly where the derechos occurred. The 700mb and 500mb maps indicated strong winds, which are vital to downburst speeds. At 7:00am, the winds reached around 35 mph (30 knots) across Iowa, Illinois, and Indiana. At 7:00pm, winds had increased to about 58 mph (50 knots) over the same area. In the 300mb and 250mb maps, there was a strong jet stream, which helped drive the storm in a southeasterly direction. Divergence, which is separating winds, is another thing to look for in the 300mb maps. There was no significant divergence anywhere. Divergence works like a vacuum, helping to suck moisture in to initiate thunderstorm development and also helping to weaken the cap. There was already plenty of moisture in place over that area and none had to be ‘sucked’ in.
IV. Damage Reports
The damage report generated by the Storm Prediction Center was reviewed. Five of the most severe damage reports were as follows:
Winds pushed an empty grain train down a track in Linn Co, Iowa.