28 January 2012

A quick look at 4/9/11 and why it produced numerous strong nighttime tornadoes in Iowa.

4/9/11 was one of the few days I was able to chase in 2011. I had made two targets this day.... one sure target north of Omaha, NE and another sleeper target in W. IL. The sure-thing target went nuts after dark while the sleeper target was a bust. Let's take a look at why so many tornadoes occurred after dark in NW IA.

All day conditions were favorable for the development of severe thunderstorms across IA/NE border region. By 5p the cap started to break and thunderstorms developed off to the northwest. Initially these storms produced large hail and numerous gustnadoes. While tornado warned, the initial storm wasn't getting it's act together until it reached Mapleton, IA. That is when an EF-3 touched down and hit the town producing major damage but no deaths. We missed the Mapleton tornado due to a poor choice of roads. I wasn't confident the storm would produce anytime soon so I made a play at going on the south road up and around a hilly region so we could emerge on the otherside just as the supercell was moving into the area. We missed the Mapleton tornado, but soon after caught the storm that was now producing another brief tornado. It was at this time where other storms started developing off to the west and south and we figured the tornado show was over as storms looked like they were lining out. On our way to Ft. Dodge I noticed a suspicious lowering off to the north and proceeded to head that way.... one of my chase partners Matt Cumberland said he spotted a wedge. We scanned the skies, and through the lightning flashes there was in fact a large wedge tornado in progress. Our data had been shady all day, but when we regained it we quickly noticed a training line of supercell thunderstorms all producing large tornadoes. I had the best night chase of my career and capped off an amazing day with lots of large night time tornadoes.

Let's take a look at a couple of reasons why Iowa went nuts that night....

*03z 0-3 EHI values over 9 in NW IA showing environment very conducive to rotating supercells*
*03z Effective Bulk Shear values in the 50kt range, most supercells thrive in 25-40kt bulk shear* 
*03z H85 winds/low level jet screaming into the area bringing a warm moist influx of energy for supercells*
*STP shows a major bullseye in the area the tornadoes occurred.
So with the maps I have provided I can tell you conditions were more conducive for supercells after dark with a strong LLJ aiding low level moisture and rotation. The big factor that came into play was a warm front draped across the MN/IA border. To the south of it warm moist air and to the north cold damp air. It was something like mid 30's north and mid 60's south. We feared that these supercells would cross over into the cool stable air and quickly become elevated. This simply did not happen, each supercell latched onto the warm front draped across the area. With the low continuing to lift across the area, storms continued to form in the area and latch onto the warm front and produce tornado after tornado. EHI values at 9 in April during the night are pretty significant. A southerly LLJ feeding into a storm that broke away from mean flow is just asking for trouble. Since bulk shear was at the 50kt+ range all that wind shear promoted supercells with low level rotation.

Pretty classic night time tornado outbreak situation that unfolded. Very good learning material. Warm front + strong low level jet + strong bulk shear = \__/

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