Case #2: 14 June 1998 Severe
Convective Windstorm / Tornado case
In this case we will focus our attention
on:
- Outflow boundary - convective line intersections
- Growth of vortices near the location of intersection
During the early morning of 14 May 1998; an intense bow echo moved just north of St. Louis and produced extensive widespread wind over 60 mph with local gusts exceeding 100 mph, and several non-supercell tornadoes of F0 - F1 intensity. Major damage occurred to several homes, buildings, numerous trees and power lines with damage estimated in the millions of dollars across Lincoln, northern St. Charles, northern St. Louis counties in Missouri and southern Calhoun, southern Jersey and northwest Madison counties in Illinois. In several instances, we uncovered classic intense microbursts damage patterns. The majority of the damage resulted from the intense straight-line winds.
Fig. 1. Damage map associated with the 14 June 1998 severe MCS. (W) represents locations of wind damage; (T) signifies location of tornado damage. Squall line positions are denoted every 30 minutes in UTC. (F1) F-scale devised by Dr. T.T. Fujita represents the degree of structural damage caused by wind speeds associated with a tornado. Damage survey was conducted by the SOO, the WCM and one NWS forecaster from KLSX the following day.
Fig. 2. Surface chart for 1100 UTC, 14 June 1998.
- A cold front extended from an
area of low pressure over southeast Nebraska through eastern Oklahoma and southwest into
northwest Texas. A warm front stretched from west-central Missouri through northeast
Arkansas and southeast into northern Mississippi.
- Surface trough extended across from near Kansas City through southeast Missouri. Such a
trough often reflects a region of strong low-level warm advection.
- Dewpoints from 68 to 70 degrees advected northward into southwest - west-central
Missouri.
Fig. 3. 500 mb chart for 1200 UTC, 14 May 1998.
- One shortwave trough extended from across southern Iowa through west-central Missouri while a second shortwave stretched from Nebraska through the Texas Panhandle region. A third shortwave trough was moving southeast from the northern Rockies. Note the 500 mb jet streak was advecting very dry air into Missouri. A thermal trough was also detected over Missouri - further steepening mid-level lapse rates over the region.
Fig 4. SHARP sounding for
1200 UTC, 14 May 1998 Springfield (KSGF) Missouri with select thermodynamic
and kinematic parameters.
- Degree of instability (P max) CAPE = 2006 J/Kg / CIN = -1 J/Kg
- Vertical Wind Shear (0 - 3 km) = 17 m/s
- Well defined inverted "V" profile was identified within the 800 - 550 mb layer.
