Weather/Climate Discussion of the March 3 Southeast U.S. Tornado Outbreak
Discussion: The severe weather outbreak occurring in the Southeast U.S. the late afternoon and evening of Sunday March 3 (Fig. 1) was caused by a set of unusual circumstances. Normally, any major severe weather event during the cold season (NOV-MAR) is attributed to an injection of tropical moisture from the deep tropics usually inspired by an intra-seasonal transitional convection event known as Madden Julian oscillation (MJO).
MJO events, born in the equatorial Indian Ocean shift east and navigate the global tropics on average in about 7 weeks. MJO events produce widespread thunderstorm activity in the tropics and the energy released from these convective storms is entrained into the jet stream/storm track causing extreme weather.
However, it does not appear the MJO had anything to do with this severe weather outbreak.
Interestingly, and there is probably some relationship the eastern equatorial Pacific warmed sharply the past 7 days and we’ve SUDDENLY zoomed into an El Nino climate as of March 1st.
What we do know is the prevailing jet stream across the southern U.S. turning toward the Northeast States averaged 155-170 mph at the core (Fig. 2) which is about 50% stronger than normal for this time of year. The jet stream buckled over the Southeast U.S. yesterday causing vertical motion in the atmosphere leading to the severe weather outbreak.
The best severe weather index identifying the potentially instability in the atmosphere was the convective available potential energy (CAPE) index. At 4PM EST Sunday afternoon CAPE values were 2000-3000 j/kg (Fig. 3). Values of 3000 j/kg support tornadic development.
Another catalyst was entrainment of moisture off the northern Gulf of Mexico where sea surface temperature anomalies are a ROBUST warm anomaly of +2C (Fig. 4). Gulf of Mexico surface temperatures are unusually warm for early March and definitely contributed to the intensity of the severe weather event.
Fig. 1: Severe weather storm reports in the Southeast U.S. March 3, 2019.
Fig. 2: Jet stream level winds were a ROBUST 155-170 mph across the South and East U.S. yesterday afternoon. Intense convection was capable of bringing some of this high wind aloft down to the surface as violent wind gusts and/or tornadoes.
Fig. 3: CAPE index was 2000-3000 j/kg at 4PM EST yesterday afternoon. 3000 j/kg supports tornadic activity.
Fig. 4: The Gulf of Mexico is unusually warm with widespread SSTA of +1.5C to +2.5C which helped inspire very wet vertical currents driving the severe weather episode.