Sunday, June 17, 2018

Our intense downburst experience on I-29 in southwest Iowa - June 11, 2018


Last week, on Monday, June 11, my wife Shawna and I were able to get away from family health and caregiving issues in Kansas City for an afternoon in eastern Nebraska, our first chase since May 1.  We saw what appeared to be a brief tornado northwest of Omaha, but more importantly, we witnessed a damaging downburst (see diagram and image above) crossing into southwest Iowa north of Nebraska City.  It's been an extremely busy week, so I'm just now getting around to doing a post about this interesting event.

We were heading south on I-29 around 7:00 pm CDT (0000 UTC) to get back to Kansas City, thinking we could outrun a supercell and developing squall line on the west side of the Missouri River.  As we drove south, we were surprised to see the storms rapidly and unexpectedly bow swiftly eastward, crossing I-29 in our path with winds in the 80-90 mph range (an 88 mph gust was recorded near Thurman, Iowa).  This wet bowing downburst felt very much like being in the eyewall of a hurricane, and several semi tractor-trailers were overturned on I-29.   Damage to trees, roofs, and buildings was also reported from near Union, Nebraska, to Thurman and Sydney, Iowa.

Here's an image from Shawna's video of the developing squall line as it overtook the supercell to our west shortly before 7 pm CDT (0000 UTC):





















And here are video capture images of the downburst racing across I-29 in front of us, looking south: 

These images were at roughly 10 second intervals during a 30 second period just after 7 pm CDT (0000 UTC), and the arrows mark the leading edge of the precipitation-laden downburst, showing just how fast it was moving and accelerating! 

These next two images shows how conditions became almost "white-out" as the rain and wind engulfed us:




































The downburst overturned 9 semi tractor-trailors on I-29 just ahead of us (3 miles west of Thurman), and one semi tractor-trailer rig on its side blocked the road in front of us:































In pouring rain and strong winds, Shawna jumped out to see if the driver was all right.  Anyone who is acquainted with my wife knows that she has a passion for helping out and knowing what to do after destructive weather and before first responders arrive. 

A level-headed Quik Trip truck driver also stopped beside us, and helped pull the semi driver out of his cab.  Shawna brought him over to our vehicle, where we could see a sizable gash on his arm and some cuts to his face from falling when his semi flipped over in the 80+ mph downburst winds.  

We dug out towels to wrap his arm, called 911, and waited for help.  After a bit, a state trooper began directing traffic along the narrow side shoulder to squeeze past the blocking semi, and told us to go to the next exit to wait for an ambulance.  To save time, we decided to drive on down to Nebraska City, where Shawna took the injured driver into a hospital emergency room.  Thankfully, his injuries weren't life-threatening, and we resumed our journey home, wishing him the best after he called his wife.

Going back to look briefly at data from a meteorological standpoint, the downburst appeared to develop when a line of storms was building after 6:00 pm CDT (2300 UTC) along the surface cool front back to the west near Lincoln (see radar images below, base reflectivity at left, base velocity at right for 6:13 pm):  
This was an area where there was both drier air aloft near the front, and a relatively dry layer below cloud base due to wide temperature-dew point spreads (see model sounding near Lincoln at 6:00 pm CDT / 2300 UTC below):  

Evaporation from rain falling through such dry layers can cool already precipitation-laden air even more to generate strong negative buoyancy and accelerate winds downward and forward to create a downburst.

This line began to show an area of stronger low-level winds (go back to the radar images above at 6:52 pm CDT / 2352 UTC and 7:05 pm CDT / 0005 UTC), overtaking the supercell that had earlier produced some brief weak tornadoes south of Omaha, near Louisville, Nebraska.  This area then bowed very fast across the Missouri River to intercept our path as the corridor of strong downburst winds accelerated eastward.

An additional note:  Earlier in the afternoon, we did see something that looked like a brief tornado near Fremont, Nebraska,  but we did not hear of any damage from this feature:

This possible tornado was from a tornado-warned supercell near the intersection of the cool front and a residual outflow boundary from convection earlier in the day. 

So, Monday, June 11 made for an unexpectedly interesting storm chase.  It sounds like there were no critical injuries (including the driver we took to the hospital), so we are very grateful!

- Jon Davies  6-17-18

Thursday, June 7, 2018

The awesomely photogenic Laramie WY Tornado June 6, 2018 - very difficult to forecast


It's been an active two weeks regarding tornadoes in Wyoming!  First, several tornadoes, including a large EF2, occurred northwest of Cheyenne on May 27.  Then an EF3 tornado injured a couple people near Gillette on June 1, the first EF3-rated tornado in Wyoming in over 30 years.  And yesterday (June 6, 2018) produced yet another EF3 tornado north of Laramie that was very photogenic and on the ground for around 50 minutes over mainly open country.

It's interesting that the environment supporting this long-lived and highly visible tornado was not much evident, even shortly before the tornado.  As a result, it was very difficult to forecast (for example, a 2% or less tornado probability on 6/6/18 SPC outlooks), making it a tornado case intriguing to examine regarding the contributing ingredients and setting.

The surface map at 2200 UTC (4:00 pm MDT, about an hour and 45 mins before the tornado) showed a surface front dipping into northern Colorado with a low near Denver, and then arcing sharply back to the northwest across Wyoming:  

Notice that the Wyoming portion of this front was located near Laramie, and acted much like a dryline.  Warm, dry air was southwest of the front (surface dew points in the teens deg F), while relatively moist upslope air was in place northeast of the front where dew points were in the upper 40s and low 50s (deg F) near and east of Laramie on southeastly winds.  

The tornadic cell started to develop around 2230 UTC (4:30 pm MDT, see arrow in satellite images below) in the convergence along this front near Laramie.  (The tornado developed at 2343 UTC, or 5:43 pm MDT.): 
The RAP model 1 hr forecast sounding at Laramie (LAR) valid at 2300 UTC (5:00 pm MDT), roughly 45 minutes before the tornado, showed plenty of mixed-layer CAPE (2500-3000 J/kg), but only marginal low-level storm-relative helicity (0-1 km SRH roughly 75 m2/s2):
























But with the sizable CAPE and no significant convective inhibition (CIN), and a very steep lapse rate/change in temperature (> 9.0 deg C/km) in the lowest 2 or 3 km, the stage appeared to be set for rapid upward parcel accelerations just above ground.  This would be particularly true given the high surface elevation (above 7000 ft MSL), and the mixed-layer moisture depth in the lowest 1 km, which would help reduce dry air entrainment and CAPE dilution as storm updraft parcels accelerated upward from near the ground.

In fact, checking surrounding model forecast soundings (not shown) at 2300 UTC, the Laramie profile had the best combination of steep low-level lapse rate, lack of CIN, and sizable CAPE over the southeast Wyoming and northern Colorado area near the surface boundary.  With nearly 40 knots of deep layer shear, and the frontal boundary likely enhancing the local low-level environmental shear, it seems that all these factors were able to support the Laramie storm as a well-structured and slow-moving tornadic supercell that was widely photographed:





















On the larger scale, the approach and passage of a short wave trough generating upward motion through the midlevels of the atmosphere helped with the generation and sustenance of storms near the boundary over Wyoming and Colorado.  This trough could be seen best on forecast models at the 700 mb level, marked below as a heavy dashed red line at mid-afternoon:































I'm hypothesizing here, but given the (at times) dusty "landspout" appearance of the tornado (see below), and the various ingredients/factors discussed above, the tornado may have resulted from a combination of both supercell (tilting and stretching of environmental shear) and non-supercell processes (upward parcel acceleration and stretching of vertical vorticity associated with a pre-existing sharp boundary): 










The SPC mesoanalysis graphics below, within an hour before the tornado, illustrate the combinations of ingredients and potential processes.  The first two panels show low-level lapse rate (red lines), and surface vorticity (light blue lines), and suggest that the developing cell just north of Laramie was within a gradient of steep 0-3 km lapse rates, as well as a zone of strong surface vertical vorticity along the aforementioned boundary.  These are ingredients typical of non-supercell tornado environments:



However, these next two panels show CAPE and SRH combinations via the original version of the energy-helicity index (EHI, green lines), and deep-layer shear (0-6 km bulk shear > 30 kt within the blue lines), ingredients typical of supercell tornado environments:
This convolution of localized ingredients suggests why this tornadic setting was so difficult to forecast in advance.  Most tornado settings that involve non-supercell processes are, by their very localized nature, essentially impossible to forecast, although an astute meteorologist may notice some of the ingredients coming together a short time before such an event.

A very interesting case to study!

A note: I haven't done any storm chasing or posted any blog cases in the past month, as my wife's mother has been in and out of hospitals and rehabs, and we're trying now to get her settled into a nursing home.  Family first.  But I'll keep an eye out for interesting severe weather cases to blog about as time allows.
  
- Jon Davies  6/7/18