Significant Weather Events

April 2 Tornadoes and Flooding

Analysis by Tim Humphrey, General Forecaster

Figure 1. Weather Prediction Center 2 April 2017 12Z surface analysis zoomed in on the Gulf of Mexico.

During the early morning hours of 2 April, an area of low pressure developed over south central Texas and lifted to the northeast. By 12Z 2 April, the surface warm front extended along the Gulf Coast from San Antonio, TX to Mobile, AL. South of the warm front, temperatures were in the low to mid 70s and dewpoints were in the upper 60s to low 70s (Fig. 1). These surface conditions combined with mid-level lapse rates near 8.0 C/km resulted in a very unstable atmosphere across much of southern Louisiana. Per the Storm Prediction Center's sounding climatology, the 2331 J/kg mixed layer CAPE sampled in the morning Lake Charles sounding was nearly triple the 90th percentile of mixed layer CAPE sampled in early April 12Z soundings. Additionally, a strengthening low level jet over southeast Texas and southwest Louisiana resulted in effective bulk wind shear around 40 knots across much of the region (Fig. 2). The combination of strong instability and wind shear supported the development of organized convection, including supercell thunderstorms.

Figure 2. 2 April 12Z upper air observation for Lake Charles, LA.

As the warm front lifted northward during the morning hours, a cluster of strong thunderstorms developed over south-central Louisiana and progressed to the east-northeast. The southern most cell became dominant and the multicell cluster transitioned into an isolated supercell thunderstorm as it entered Lafayette Parish (Fig. 3). The supercell produced quarter to ping pong ball sized hail across much of western Lafayette Parish while it continued to intensify. Effective layer helicity in excess of 200 m2/s2 and LCL heights less than 500 meters above ground level sampled in the 12Z sounding indicated the environment was conducive for the development of tornadoes (Fig. 2). As the supercell moved into eastern Lafayette Parish, the mesocyclone intensified and produced two brief tornadoes. Tragically, the second tornado destroyed a mobile home and resulted in the death of the mobile home's two occupants. The initial supercell continued off to the northeast and likely produced hail over the northern portion of the Atchafalaya basin (Fig. 3).

Figure 3. Multi-Radar Multi-Sensor (MRMS) reflectivity from 2 April 12Z to 2 April 18Z.

Figure 4. 1 minute GOES-16 visible satellite imagery from 14Z-23Z 2 April 2017.

By late morning, the warm front had lifted into central Louisiana and skies began to clear across the region, allowing for the atmosphere to destabilize further (Fig. 4). Around 18Z, two attempts were made to conduct a special sounding to sample the atmosphere within the warm sector. Both flights were caught in the updrafts of developing convection north of Lake Charles and did not provide a representative temperature profile for the warm sector. However, the special sounding collected valuable wind measurements that revealed substantial low level wind shear. The Ft. Polk VAD Wind Profile near the same time indicated that 0-1 km storm relative helicity had increased to around 400 m2/s2. In addition to being favorable for the development of mesocyclones, the low level wind field was also conducive for the development of significant tornadoes.

Shortly after 18Z, a second supercell thunderstorm developed over northern Allen Parish and moved off to the northeast (Fig. 5). Over the next three hours, the supercell's mesocyclone underwent cyclic tornadogenesis that resulted in at least four tornadoes occurring across central Louisiana. The first tornado occurred between Glenmora and Forrest Hill. The storm cycled north of Forrest Hill and produced the second tornado. This significant tornado produced a textbook Tornadic Debris Signature with high reflectivities, lower differential reflectivities, lower correllation coefficients, and a tornado vortex signature collocated west of Woodworth. The mesocyclone cycled again south of Alexadria and resulted in a weaker tornado that crossed the western portion of the city. The supercell then cycled between Ball and Pineville before it continued to the northeast and produced another tornado west of Jena in the National Weather Service Shreveport's county warning area.

Figure 5. MRMS reflectivity from 2 April 18Z to 3 April 00Z.

Figure 6. 3 April 00Z upper air observation for Lake Charles, LA.

By 21Z, a solid line of convection had developed across southeastern Texas and were moving into central Louisiana (Fig. 5). The southwest to northeast oriented line of storms slowed as they entered central Louisiana and began to train over portions of Rapides and Vernon Parishes. Meanwhile, the 3 April 00Z Lake Charles sampled a daily record precipitable water value of 1.95" and a warm cloud depth exceeding 12,000 feet (Fig. 6). Therefore, the environment remained favorable for convection capable of efficient rainfall production. A widespread 8 to 11 inches of rainfall fell over Rapides and Vernon Parishes over the period of several hours and produced flash flooding during the early morning of 3 April. The runoff from the heavy rainfall also resulted in moderate to major flooding along downstream rivers and creeks over the next several days.