Convective Addiction

Yazoo City, MS Tornado – April 24, 2010

Posted on 25. Apr, 2010 by Jesse in Case Studies, Jesse's Chase Logs

The video that I have uploaded chronicles the monstrous supercell that struck Yazoo City, Mississippi on Saturday, April 24, 2010.  Convective Addiction compatriots Jesse Risley and Brad Goddard, along with fellow chaser Kevin Crawmer, intercepted the supercell several miles to the south of Yazoo City, MS.  At is became apparent that a tornado of elephantine proportions was enshrouding itself just behind the curtains of rain wrapping into the southern flank of the cell, our efforts turned from videographer to diverting oncoming motorists who were obviously unaware of the dangers that they were about to encounter as they proceeded northbound on U.S. 49.  Unfortunately, a few of them encountered the outer flanks of the wedge tornado and sustained vehicular damage just up the road from our location, though no one was seriously injured as far as we could tell.

The most disturbing aspect of this entire day was the fact that when we originally entered the community from the north about twenty minutes prior to the arrival of the tornado, a Tornado Emergency had already been issued by the NWS office in Jackson, MS several minutes prior to our  arrival on Hwy 49N.  However, it was business as usual in the community and no sirens appeared to have been activated at that time.  Witnesses later told us that they heard the sirens very briefly before the tornado hit (app. 1-2 minutes as best, others said 5-10 minutes).  You can hear a forecaster from the NWS in the background several minutes into the videotape reading a live on-air NOAA Weather Radio script of the Tornado Emergency.

As we entered the community again immediately following the passage of the twister, our efforts quickly turned to assisting with imminent search and rescue operations, as there were no emergency personnel on the scene of some of the worst damage that occurred in the eastern side of the city for 5-10 minutes after our arrival.  This was likely due more to the extent of damage and road blockages community-wide than it was a failure on anyone’s (EMS, LE or Fire) part to respond in a commensurate manner.  In a situation this drastic, the only first responders may very well be ordinary citizens and neighbors.

There is much to be said about the prudence and sanity of chasing HP supercells that are producing strong to violent tornadoes, wrapped in rain, while moving at hypersonic speeds of 55+ mph, and in the “jungles” of the Deep South nonetheless.  This is exactly why we positioned ourselves far enough south to ensure that we wouldn’t even be in imminent danger if the cell took a sudden right turn, yet still allowing respectably close viewing of the tornado, which was mostly wrapped in rain from our vantage point.

My kudos go out to the fellow chasers who also laid down the chaser hat and worked selflessly for hours with search and rescue efforts, helping numerous residents that were trapped in the remnant shambles of a neighborhood forever changed that Saturday afternoon.

The damage video that I shot isn’t even the tip of the iceberg of what I saw yesterday, but it does offer a glimpse at the destructive power of nature.  This was certainly a significant chase day, yet it will remain my most humble and tragic to date.  Most of the damage footage taken in the more sunnier conditions was shot about 2-3 hours after the passage of the twister, when we were finished helping with search and rescue efforts.  I have only included what I feel is appropriate to share, as this was an emotionally charged atmosphere and some individuals sustained serious injuries as well.

An ongoing damage assessment analysis from the NWS (KJAN) can be found here.

A few radar grabs from GR Level 2 are included below.  They were provided to us by chaser Jared Thompson of Texas.

A look back at some visual aides from April 24th

The Day 1 Outlook from the SPC is shown below:

Probabilist Tornado Graphic (within 30 miles of a point)

Storm Prediction Center's Day 1 1630z outlook

The EHI is a simple tool that is calculated by combining helicity values and atmospheric instability into one single unit for determining whether or not a particular environment is ripe for supercell development.  A 0-1 km EHI > 1-2 suggests that significant tornadoes may be possible; the values at 17z (12 PM CDT) are approaching 10 in central MS!

0-1 km EHI at 17z (12 PM CDT)

SPC's significant tornado parameter and mixed-layer convective inhibition at 17z (12 PM CDT)

The Significant Tornado parameter (see above) for the fixed layer is a composite index that comprises the 0-6 km bulk wind difference, 0-1 km SRH, SBCAPE, sbCIN (note no shading), and surface parcel LCL height. This mesoanalysis version of STP uses fixed layer calculations of vertical shear, and the surface lifted parcels, as an alternative to the “effective layer” version of STP.

Note the 300 mb jet maxima (see above) is approaching 120 kts from the southwest.  The animation shows how the upper-level system evolved and approached the region in the hours preceding the outbreak as it begins to take on a strongly negative tilt during the late morning, aiming that aforementioned stout upper-level jet core’s exit region right at the Lower Mississippi River Valley, enhancing lift and providing more than sufficient forcing for a major severe weather outbreak.

Note the 500 mb winds of 60-80 kts (see immediately above), when combined with the 300 mb jet streak (above)  approaching 120 kts, yielded effective shear magnitudes of 50+ kts.  As the mid-level perturbation approaches the region, we can see the front quadrant of the mid-level jet streak approach WC Mississippi by 17z (12 PM CDT), placing the area within the maximum DPVA.

The 700 mb chart (see top graphic above) reveals little in the way of a strong cap (CINH), as convection fired fairly early in the day as the strong forcing arrived from the WNW with the trough becoming more negatively tilted as the strong mid and upper-level jet cores took aim on the region.  Note the 55-65kt low-level 850 mb jet (see bottom graphic above) which promoted enlarged hodographs, lending SRH values of between 300 (W. MS) and 700 (NE MS), as noted in the SRH chart down below. At that time, 850 mb dewpoints sat near 14C over western MS.

On the 925 mb low-level profile (see above), note the southerly winds pooling warm, moist air northward from the Gulf of Mexico, turning increasingly S-SE progressing northward through Mississippi.

17z (12 PM CDT) Storm Relative Helicity (SRH) Parameters

A strong LLJ rendered 0-1 km SRH (see above) of between 300-700 m2/s2 over the entire region, providing a more than ample environment for rotating updrafts, especially in the lowest 1 km of the atmosphere, rendering that any supercell which developed had a more than adequate helical near-storm environment to enhance vorticity for tornadogenesis.

At 16z (11 AM CDT) SFC-1km vertical shear (see above) is already 60 kts across this region; > 15-20 kts is considered favorable for supercell tornadoes.  Vertical wind shear is a measure of the change in winds with height increase in the atmosphere.  In an environment characterized by strong vertical wind shear, the presence of winds greatly increasing or changing direction with height serves to enhance supercell thunderstorms, as such an environment promotes wind removal of precipitation from the upper regions of the storm’s updraft; it can also promulgate stronger rotation within the updraft core.

Effective Bulk Shear at 17z - 12 PM CDT (LPL to 50% EL)

Effective bulk shear values of 60-70 kts (see above) suggest that supercells are likely (>40 kts is more than sufficient).

It is now evident that the 850 mb low-level jet maxima (LLJ) is promoting higher THETA-E values (see above), noting that any region with an elevated THETA-E corresponds to more significant instability and thus higher SBCAPE indices.

17z (12 PM CDT) 850 mb low-level moisture transport, height and theta-e advection

Warming temperatures and increasing low level dewpoints are increase instability, as CINH erodes.  This map above is showing elevated THETA-E values over the region by 17z (12 PM CDT).

Surface-based (SB)CAPE at 17z (12 PM CDT)

At 17z most of Mississippi sits with SBCAPE AOA (see above) 2000 J/KG, with CINH eroded, lending an environment primed for severe weather with seasonably high atmospheric instability present across the region due to ample surface heating and moisture influx.

Normalized CAPE (see above) values sit at 0.15 m/s**2 at 17z, suggested average parcel accelerations when lifted vertically.  Note: Greater than 40 m/s**3 is considered high; values sit at 80 just east of where the Yazoo supercell developed at 17z.

Craven Brooks Sig. SVR at 17z (12 PM CDT)

The Craven Brooks Significant Severe WX Parameter is often used to differentiate between ordinary severe weather events (e.g., pulse type, low-end damaging winds or hail, and tornadoes of less than EF2 in magnitude) and significant severe events (hail larger than 2 inches, 65+ knot wind gusts and tornadoes of greater than EF2 in magnitude).   The Craven Brooks SigSvr parameters is calculated by taking the product of 100 mb mixed layer (ML) CAPE and the 0-6 km magnitude of the shear vector, which comprises both instability (CAPE) and shear parameters.  Values that exceed 20,000 m3 sˉ3  (depicted as “20″ or greater on the map) are often associated with significant severe weather events.  Notice that the majority of significant severe events occur when the combined CAPE*shear parameter exceeds about 20,000 m-3 s-3.   As a result, this parameter is sometimes called the Craven/Brooks Significant Severe parameter (e.g., at the SPC Forecast Tools web site).  Values on this day were already at 80,000 m3 s-3 across central MS at noon on April 24, 2o10.

Water Vapor Loop - 12z (7 AM CDT) to 18z (1 PM CDT)

Note that a negatively tilted shortwave trough is beginning to advect ENE from the ARKLATEX region, which is clearly evident on the water vapor imagery loop from 7 AM to 1 PM.

17z (12 PM CDT) VGP

The VGP parameter is used to estimate the rate of tilting and stretching of horizontal vorticity by a thunderstorm updraft.  Values in excess of 0.20 m/s**2 suggest that tornadoes are more likely; here they are reaching 0.60 m/s**2 near Yazoo City.

LFC Heights at 17z (12 PM CDT)

The LFC paramater is indicative of the level at which a lifted parcel begins a free acceleration upward to equilibrium.  Tornado potential increases when LFC heights are between 1500-2000m above the surface.  As we can see, STRONG support for tornadogenesis is noted with LFC levels at 1000m.

LCL Heights at 17z (12 PM CDT)

The LCL Heights are sitting at between 500 and 1000m, lending good support for tornadoes. This is the level at which a lifted partial saturates, and is also used to estimate cloud base heights.

The BRN shear value sits at over 150 m2/s**2 across the entire region.  The BRN Shear is the denominator in the formula for the Bulk Richardson Number, a ratio of CAPE values to deep layer shear (6km wind – sfc wind)^2.  This indicates that an incredible amount of shear is present over the area that would soon host a number of tornadic supercells Saturday afternoon.

SFC total deformation, dilitation and THETA K at 17z (12 PM CDT)

This chart gives us a glimpse of frontogenesis characteristics as an operational characteristic of kinematics.

JAN 12z sounding from early on April 24th; note that SFC-6 km shear is at 68 kts; storm motion is at 233 degrees (SW) and 57 kts!

Finally, both a satellite loop and a surface map composite from the region, between 16z (11 AM CDT) and 19z (2 PM CDT), appears below.  Watch as the thunderstorms ignite across a region of significantly backed winds out of the SSE in an environment ripe for tornadic supercells across the region.  The radar loop shows the deadly supercell as it tracks across central Mississippi and become a very long-lived tornadic beast.