Severe Weather Analysis
Brian Pettegrew and Becky Ebert

0000 UTC 28 April 2002

Synopsis
For the cases study on April 28, 2002 we will first look at the 200 mb chart, where geopotential height has been overlayed with temperature. Notice a pocket of warm air located over Kansas, Nebraska, Northwest Missouri, and most of Iowa. This would be indicative of a stratospheric air intrusion or tropopause undulation. There is southwesterly flow located of most of the United State with a slight trough located along a line from South Dakota to Oklahoma and a ridge from the upper Great Lakes region down through Georgia. At 300 mb there is a jet streak located from California east into Illinois. Missouri is located in the left exit region of the jet core. Notice a deepening of the geopotential heights over South Dakota. Now looking at the 500 mb map, notice the trough that is located along an axis from Nebraska through Oklahoma. Near the center of the trough is an absolute vorticity maximum of 28. At 700 mb there is a closed low centered over the four corners of Nebraska, South Dakota, Minnesota, and Iowa. Notice the large pocket of moist air along and parallel to the low pressure. Also at 700 mb there is southwesterly flow that sweeps across the southern part of the United States and wraps around into the southeastern quadrant of the low pressure, this helps to create a strong moisture gradient along the Missouri-Iowa boarder. The closed low at 850 mb located over Iowa. The 850 mb map shows dew point depressions of less then five degrees Celsius encompassing a large portion of the Midwest and extending up from the Gulf of Mexico. Associated with this tongue of warm moist air is 50-knot low level jet core over Arkansas and Missouri, extending into Illinois. Strong areas of low level warm air advection is occurring in areas south and east of the 850 frontal boundary draping down from Iowa, across the Northwest corner of Missouri, and into Kansas. The center of the low at the surface is centered over the southwest corner of Iowa. Strong southerly flow at the surface is providing additional low-level moisture to the Mississippi Valley. Thickness contours indicate our surface frontal boundary, almost directly under the 850 boundary, across Kansas extending up to the low over southwestern Iowa. Also, behind the cold front is a fairly strong baroclinic zone. Temperature and dewpoint observations indicate the extent of the moist air feeding up through Louisiana where dewpoints are climbing above 70°F. STL radar indicate individual cells forming over Moniteau and Morgan Counties in Missouri with reflectivity of over 50 dBZ. These cells are seen better from the radar in EAX, with similar reflectivity values for the same cell and other cells developing south and west. Visible satellite imagery indicates clearly the center of the cyclone over Iowa. Water Vapor imagery shows the flux of ample moisture from the Gulf of Mexico wrapping around the cyclone. Notice the dry slot present over Southeastern Missouri at this time. IR imagery illustrates, again, the presence of the cyclone, but also, the development of convective cells over Central Missouri, supported by radar imagery.

Surface Analysis
(Color or Printable Version)

Upper Air Analysis
- 850 mb (Color or Printable)
- 700 mb (Color or Printable)
- 500 mb (Color or Printable)
- 300 mb (Color or Printable)
- 200 mb (Color or Printable)

Radar and Satellite
- 0000 UTC LSX (St. Louis, MO) Radar Image
- 0002 UTC EAX (Kansas City, MO) Radar Image

- 0015 UTC Visible Satellite Image
- 0015 UTC Water Vapor Image
- 0015 UTC Infrared (IR) Image

27-28 April 2002 MesoAnalysis

For this study we are going to focus on the tornadoes that struck southeast Missouri on 27 April 2002. The storm reports for such cases are as follows.

These storms hit southeastern Missouri the night of 27 April 2002. The reports above are provided by the Storm Prediction Center. The first of these storms in Missouri occurred at 9:40 PM, or 0440 UTC 28 April 2002, in St. Genevieve County, and the second occurred at 0545 UTC 28 April 2002, at Marble Hill in Bollinger County. RUC Initial Soundings in areas surrounding the storm locations for 0600 UTC 28 April 2002 provide a good idea of the atmospheric conditions at the time of the severe weather. Tracking soundings in eastern Kansas, around Topeka, and western Missouri, around Kansas City and Springfield, during the first 12 hours of the event provide conditions of the volatile atmosphere prior to their advent on the eastern side of Missouri. As time progresses, notice how sounding analysis to the west become more docile, while those to the east become more active. Sounding analysis at MEM at 0000 UTC 28 April 2002 indicate CAPE values of over 1000 J/Kg and negative CIN values. Although CAPE values remain high on the western side of Missouri at this time, they decline rapidly over the next 6 hours as areas such as STL, MEM, and SPI increase to over 1000 J/Kg with no cap. By observing the vertical sounding profiles, notice also that conditions become very unstable. LI values reach minimums of -8 over MEM at 0600 UTC with Total Total values in the area over 50, indicating an unstable atmosphere. Notice the profile for MEM at 1100 UTC 28 April as the storm passes through Missouri. This sounding is representative of a "loaded gun" profile. These types of profiles are commonly associated with the outbreak of severe weather events. Also notice the strong amount of vertical wind shear associated with the profiles.

Skew-T Analysis

Model Analysis
- 1200 UTC 27 April 2002

Starting with the initial run of the Mean Sea Level Pressure (MSLP), notice that the cyclone at 995 mb is centered over the Colorado-Kansas boarder. The cyclone takes a northeasterly track over the Central United States throughout the forecast period. At the end of the forecast run the position of the cyclone is over Lake Michigan at 992 mb. Through its track the cyclone continues to develop and intensify. As the cyclone propagates across the Central United States the surface temperatures indicate the passage of the warm front at the 12-hour forecast time and then the beginning of the cold frontal passage at the final forecast time. At the 850 mb heights and dew point depression notice the ample moisture associated with the closed low. There is also the presence of a dry slot slightly ahead and along the cold front. This would indicate that as the Low continued to propagate to the east, the ingredients for severe weather is present. The 700 mb Heights/Relative Humidity forecast re-emphasis the presence of a developed cyclone and a deep layer of low-level moisture. Referring above to the MEM sounding at 0600 UTC, notice the strong vertical wind shear. This compares to the MesoEta analysis for upper and low level jets, which indicates the presence of shear during the time of active weather. This analysis also shows the presence of strong low-level convergence coupled with strong upper-level divergence. This is supported by the divergence zone of the upper-level jet, seen on the 300 mb isotach maps, and areas of lower level moisture convergence and upper level moisture divergence, best viewed at 400 mb. The 500 mb heights reaffirm the presence of a deepening cyclone. At the beginning of the model run a vorticity maximum exists at the base of the trough. As the model time-steps through the time of the event, notice areas of vorticity advection on the outskirts of the cyclone producing small vorticity maxima swinging across southeastern portions of Missouri. The 850 mb Omega is not impressive in the area of concern during this forecast period. There is the presence of omega, or forcing for ascent mostly in regions where our vort. max occur. Looking at convective energy and inhibition (CAPE/CIN), notice the progression of high CAPE values across the region through the model period and the lack of CIN providing no CAP on the amount of strength that these storms could develop. This is a very religious region. The areas of high CAPE are located in regions of strong low-level convergence and strong upper-level divergence mentioned previously. These also correlate to the values documented above in the RUC initialization soundings. Associated with areas of high CAPE, were areas of strong Q-vector convergence in the 1000:700 mb layer. Q-vectors point from cold to warm air, and can be used to identify frontogenetic processes. Notice the 18-hour Q-vector forecast: an area of Q-vectors converge in Southeast Missouri, the location the event. With Q-vector convergence near the surface, there must be upper-level Q-vector divergence in a layer. This is supported by the appearance of Q-vector divergence maxima in the 400:700 mb layer over Southeast Missouri during the time of the event as presented by the model forecast (*Note that this is a 12Z MesoEta Analysis out to 24 hours. Some inconsistencies are expected for areas of cyclone development and storm progression).

- MesoEta 24 hr. analysis

Forecast Hours

What Really Happened
This case study was associated with a well-developed low that took a northeasterly track through the Central plains. Associated with this Low were warm surface temperatures, a deep layer of moisture, and strong low-level convergence and divergence aloft associated with the lower and upper-level jet. This created vertical shear through the layer, which helped to destabilize the atmosphere. This destabilizing was assisted by the formation of a dry slot indicated in Water Vapor imagery seen at 0015 UTC 28 April 2002. Also, the forcing for vertical motion was enhanced by Q-vector convergence and divergence. In summary, several severe storms were reported, including the incidence of tornadoes, as discussed above, as well as large hail and damaging winds. To see the severe storm summary, follow the link below.

- SPC Event Map