Convective Addiction

Mesoscale Overview of New Years Eve Tornado Outbreak

Posted on 04. Jan, 2011 by Ryan in Uncategorized

Aside from the numerous storms which produced tornadoes, some significant on New Years Eve (SEE PREVIOUS POSTS), there were also storms that did not produce tornadoes despite impressive appearances on radar. This post will focus on two storms in Northeast Missouri and West Central Illinois in particular. Supercellular characteristics were first noted in the Pike County Missouri region before crossing the river into Pike County Illinois. These storms were less than 100 miles removed from cyclical supercells near Saint Louis which produced significant tornado damage across parts of that city.

10:39 AM St. Louis NWS Radar

On-going convection from overnight storms kept the entire area in clouds through 12z. These storms were in response to strong warm air advection from a strong overnight low level jet.

6am St. Louis Radar -- Showing on-going convection

These storms would go onto play a key role in the evolution of the New Years Eve outbreak. As the convection lifted north through the morning hours its outflow became the effective warm front. By 15z, parts of eastern Missouri broke through the cloud cover and were receiving sunshine. This only acted to enhance the thermal gradient along the warm front and also aided in increasing WAA and moisture transport.

9am Visible Satellite

17z Surface Analysis

17z RUC Dewpoint, Pressure and Wind Field Plot

By 16z strong storms began to redevelop over northeast Missouri. Tornado warnings were issued by 10:15am for these storms and by 10:30 the storms had strong supercellular characteristics. But after an hour and even with very strong couplets (on and off again as the storm pulsed), the storms failed to produce more than 1″ size hail. Why?

10:39am Pike County Storm

10:39 Pike County Storm Velocity

The environment over the Pike County storms was not completely void of favorable parameters. Sunshine had reached many parts of the area for 45min to an hour. This allowed low level instability to rise as evident by the 3km CAPE. Helicity was also incredibly strong. No doubt contributing to the large hail and supercelluar characteristics that these storms produced/obtained.

17z RUC 3km CAPE

17z RUC 3km Helicity

The position of the Pike County storm indicates it was riding right along the warm front. But yet failed to produce a tornado, or even a wall cloud based on chaser reports. It is possible the storm was slight north of the effective front.

While it is not entirely clear, the full extent of this low level instability very well may have never been released. Lifted Parcel Levels in vicinity of Pike County and to the north of the warm front were >2000m. But just a short jog south, with higher dew points and greater surface heating, LPL’s were significantly lower.

16z MU CAPE and LPL

While this author doesn’t feel its in a forecaster/nowcasters best interest to rely upon one parameter to determine tornadic potential in a storm there appears to be a strong correlation on this outbreak to tornadic storms and non-tornadic storms to the environments vorticity generation potential (VGP).

18z RUC 1km VGP

18z RUC 3km VGP

As eluded to above, VGP has been shown in previous studies (Rasmussen and Wilhelmson 1983) to have correlations to what type of storm can be generated. During the peak of a favorable environment near Pike County VGP only approached .1, while closer to Saint Louis the numbers were closer to .2. Using the box and whisker plot below, one can see that these numbers fall inline with what previous studies have shown.

VGP Box and Whisker Plot

What makes this scenario even more complex is that an EF-3 tornado occurred in Petersburg, Illinois — roughly the same latitude as the Pike Counties — and only an hour removed from when the Pike County storms failed to produce a tornado. It appears that the warm front had lifted far enough north by the time storms hit the area. Coupled with sunshine within an hour before the supercell hit and the low level thermodynamic atmosphere was much more capable of tornadic activity than just a few hours before.

18z RUC 1km VGP

Various soundings from the mid-Mississippi river valley show how the atmosphere significantly evolved over a space of 200 miles. (These soundings were creating using a parcel with a mixing layer of only around 25mb or so due to the short duration of surface heating.)

18z Keokuk, IA RUC Sounding (North of WF)

18z Pike Co, IL RUC Sounding

18z Petersburg, IL RUC Sounding

18z Saint Louis RUC Sounding

This was a day that turned very busy for meteorologist once it became apparent that a significant severe weather event was unfolding. It is highly unlikely that a forecaster trying to monitor radar and atmospheric conditions could have noticed these subtle difference. However, by being aware in the future how these interactions along warm front during off-season severe weather work, it may enhance a nowcasters situational awareness next time.

I would like the thank Adam Lucio for his help with graphics on this post.