Saturday, August 17, 2019

Tornadoes in Kansas in mid-August! 8/15/19 in northeast Kansas

Supercell tornadoes undoubtedly can occur in Kansas in early to mid-August, but they don't happen that often, and even as a native Kansan, I've never seen one.  That changed on Thursday, August 15 when a well-established supercell (see Shawna's photo above) in northwest flow produced tornadoes southeast of Manhattan and southwest of Topeka.

Here's a map showing the approximate location of three tornadoes that occurred with the supercell pictured above in the 7:00-9:00 pm CDT time frame:
(A fourth brief tornado occurred with a separate trailing supercell after 9:00 pm CDT.)

I was chasing with James Skivers and my wife, following the southernmost supercell that developed northwest of Manhattan and moved sharply southeast across Wabaunsee County, Kansas (KS).  We missed the first brief tornado (EF0) from this supercell near the western Wabaunsee County line south of I-70 around 8:12 pm CDT when we were driving south to stay ahead of the storm through a low area with trees.  Here's Brian Miner's cool photo of the full supercell base and this first tornado, all in the same shot:

The second tornado (rated EF1 by Topeka NWS) was longer-tracked near dark, southwest of Alma, KS and northeast of Alta Vista.  The image of this tornado below is from Shawna's video, backlit by lightning looking east-northeast, with the rear-flank downdraft (RFD) and gust front visible:

A third brief tornado (EF0, below, looking north and also backlit by lightning) occurred near the time the second tornado was ending, back to its northwest under an occluded mesocyclone behind the one that generated the EF1 tornado:

A separate supercell back farther to the northwest produced a brief EF0 tornado after 9:00 pm CDT east of Alta Vista (not shown).

The satellite image below, with relevant features superimposed, shows the setting just before 7:00 pm CDT (0000 UTC 8/16/19):

Note the stationary front and outflow boundary (from earlier storms that moved into northern Missouri) close together over northeast Kansas near Manhattan and Topeka. Winds from the low-level jet (LLJ, around 850 mb or 5000 ft MSL) were impinging on and overrunning these boundaries, helping to initiate storms northeast of a "cap" that was inhibiting convection to the southwest.

Three different supercells are visible in the image above (black arrows show their southeastward motion), but only the southernmost cell near Manhattan was able to access the most unstable air along and south of the boundaries by virtue of its location and motion, probably a big factor in its ability to produce tornadoes about 90 minutes after the time of the satellite image.  The low-level jet was also increasing near dark, as is typical, enhancing low-level shear supportive of tornadoes.

The SPC mesoanalysis depiction of the effective-layer significant tornado parameter (STP) at 8:00 pm CDT (shortly before the tornadoes) showed large values supportive of tornadoes along and near the aforementioned boundaries in northeast Kansas (the enhanced energy-helicity index, an experimental parameter, is also shown):

Notice that, even though these tornado forecasting parameters appeared "favorable" over a rather large area extending north of the boundaries, the tornadoes were limited to the air mass south of and near the boundaries.  This suggests the importance of following the location and evolution of relevant boundaries in forecasting/nowcasting tornado potential, rather than just relying on parameter "bulls-eyes".

Also, here's the NAM model 500 mb forecast (winds at roughly 18,000 ft MSL) showing the northwest flow driving into Kansas, a little unusual that far south in mid-August, and the reason the supercells moved southeastward to the right of the midlevel flow:

Two shortwave troughs are marked on the forecast above (thick dashed red lines) showing a lead shortwave associated with the morning and early afternoon storms that moved from northeast Kansas into northern Missouri, and a fairly strong trailing shortwave not far behind crossing Nebraska.  Often, morning and early afternoon storms leave subsidence (sinking motion) and a "worked-over" air mass in their wake, reducing the chances of severe storms behind them.  But that was not the case here, with another shortwave immediately behind generating upward motion and intensifying low-level winds converging on and overrunning the surface boundaries.

All in all, an extremely interesting case for mid-August in Kansas!

- Jon Davies  8-17-19

Thursday, August 8, 2019

Updated map and images of May 28, 2019 EF4 tornado in northeast Kansas

It's been over 2 months since the long-track EF4 tornado in northeast Kansas (KS) on May 28 that just missed the main population of towns like Lawrence, Linwood, and Bonner Springs KS.

Some interesting video has surfaced since May 28.  I thought it would be useful to correlate some images of the tornado with a newer map I put together (above) that shows the relative width of the tornado path (> 1 mile near Eudora & Linwood) at various points from my own survey.  The smaller tornado that occurred earlier and merged with the larger one as it formed southwest of Lawrence is also shown.

The map above shows image locations (in red) by image number as discussed below, along with direction of view.

Image 1 below (from Quincy Vagell's video) shows the wet portion of the storm in southwest  Douglas County looking west around 5:57 pm CDT where a small EF1/EF2 tornado was embedded that struck Silver Lining Tours (SLT).   The new mesocyclone from which the large EF3-EF4 tornado developed about 10 minutes later is also visible north of the rain-wrapped area concealing the smaller tornado:

Image 2 below (from video by Robert Reynolds) shows this smaller EF1/EF2 tornado emerging out of the rain near Lone Star Lake around 6:05 pm CDT, looking east about 1/2 mile away.  This is the same tornado that struck SLT 2 to 3 minutes earlier:

Image 3 below (from video by Dalton Coody) shows the large tornado shortly after forming southeast of the town of Lone Star (in Coody's video, it is only visible briefly before rain wraps around it).  This was at roughly 6:10 or 6:11 pm CDT, after the merger of the smaller tornado with this larger one as it was developing:

Image 4 below (from video by Jack Miller) shows the HP supercell at about 6:12 pm CDT looking southwest from the south side of Lawrence.  The tornado, visible in the Coody video, is hidden by rain wrapping in from the southeast (a wet rear-flank downdraft or RFD):
Image 5 below (from video by Matt Grantham) jumps ahead to near Linwood KS around 6:35-6:40 pm CDT, looking west along Highway 32 west of Linwood.  The tornado was about a mile wide (!) at this point, but somewhat visible even though still wrapped up in rain:
Image 6 (from video by Cybil Walters) is from a housing development north or northeast of Linwood, and shows suction vortices (smaller intense "swirls" within a larger tornado) clearly visible.  This was around the time of EF4 damage near Linwood, and suggests why some houses north of Linwood were totally demolished, while others were still standing with mainly roof and outer wall damage:

Image 7 (from video by Matt Grantham again), looking southwest from northeast of Linwood around 6:40-6:45 pm CDT, also shows the tornado at about the time of EF4 intensity, although it is still somewhat difficult to see from some rain-wrapping:
The more I look at the path of this large tornado, the more I'm struck by how very fortunate it was that it did not directly hit any population centers, including the Kansas City metro area!

The images in this post also show how difficult it was to see this tornado (and the smaller one before it), as they were both associated with a large high-precipitation (HP) supercell.

Spotters and chasers BEWARE such storms, especially when they are moving faster than 30 mph... give them a very wide berth!

Thanks to Robert Reynolds, Quincy Vagell, Dalton Coody, Jack Miller, Matt Grantham, and Cybil Walters for their video images, and thanks to Rick Schmidt and Eric Lawson for pointing me to additional documentation and information.

- Jon Davies  8-8-19