Friday, March 15, 2019

Tornadoes in New Mexico & Ohio Valley on March 12 & 14, 2019 with record-breaking low pressure system

The so-called "bomb cyclone" that caused hurricane force winds and record low pressure in the Plains on March 13, 2019 also brought tornadoes (see photos above) to southeast New Mexico (NM) on March 12 and to the Ohio Valley on March 14.  The tornadoes at Dexter and near Malaga NM on March 12 were the earliest EF2 tornadoes ever recorded for the state of New Mexico.

Interestingly enough, no tornadoes occurred on the day the surface low was the strongest over Colorado and Kansas (March 13, not shown).  This was because the system was so large and dynamic that it pushed associated surface fronts too far north and eastward, away from moisture and instability.  But that moisture recovered again as surface fronts slowed for severe weather on March 14th over the Ohio Valley.

This system was a good example of how warm fronts are efficient tornado producers. The surface maps below on March 12 & 14th show that the locations of several tornadic storms were near warm fronts: 

Warm fronts offer temperature gradients, increasing moisture, and enhanced low-level wind shear, which is why tornadoes seem to occur so often with storms near such boundaries. (Other tornadoes - not shown - did occur in the warm sector over Alabama during late afternoon and evening on March 14, away from boundaries.)

Note how instability and low-level wind shear combinations using forecasts of the 0-1 km energy-helicity index (EHI) from the RAP model were strong near warm fronts and tornadic storms on both days, offering good support for low-level storm rotation:

The 500mb NAM model forecasts below (roughly 18,000 ft MSL) also show how strong this system was, with a massive full-latitude midlevel trough moving out into the Plains.  A "spreading" jet stream pattern (white arrows) ahead of this large trough generated large-scale lift and ascent over the areas where daytime and evening severe weather occurred on March 12 and 14 (yellow ovals):

Thankfully, no deaths occurred from any of the tornadoes with this system.  But the Colorado blizzard associated with this powerful system claimed at least one life, and flooding in Nebraska from rapid snow melt and heavy rain caused at least one other death.

It is good to see that the week ahead will be much quieter weather-wise across the United States!

- Jon Davies 3/15/19

Monday, March 4, 2019

Many dead from huge Lee County, Alabama tornado 3/3/19

Sunday March 3, 2019 saw the largest death toll in the U.S. from a single tornado since the 2013 Moore, Oklahoma tornado.  As I write this, there are 23 deaths so far from this EF4 intensity tornado in the Beauregard/Marvyn, Alabama area, and more may be found in the next day or two.  (*** UPDATE: As of March 6, all those missing have been accounted for, and the death toll from this tornado stays at 23, with 90 people injured. ***)  With only 10 total deaths in the U.S. last year from tornadoes, the 3/3/19 event is certainly very tragic.

The tornado came from a supercell that entered southwestern Lee County in east-central Alabama around 2:00 pm CST, and the National Weather Service had both a tornado watch and a tornado warning in effect for Lee County when the tornado hit.  But with only 5-8 minutes warning lead time for the Beauregard/Marvyn area, and because this was essentially the first big tornado of a sizable outbreak and was moving fast (around 50 mph!), it probably caught some people off guard.  Also, as is so typical with southeastern U.S. tornadoes, visibility wasn't particularly good (low cloud bases, haze, and trees -- see the photos above and below), which was probably another contributing factor:

Below is a map showing the approximate path of the supercell's tornadic circulation on radar, and also two radar reflectivity images around the time of deaths in the Beauregard/Marvyn area of Lee County.  Note the large classic hook on the inset close-up radar image.

Later images showed a reflectivity "debris ball" (not shown), and correlation coefficient products from radar (not shown) indicated a distinct area of debris aloft as the tornado moved eastward toward the Georgia border.

The killer tornadic supercell occurred in a fairly classic position east of a surface low (see surface map below at 1:00 pm CST), with the storm moving roughly parallel to and just south of a stationary front.  Severe weather forecasters know this to be a rather favorable location for tornadoes.

Combinations of CAPE (instability) and wind shear were also favorable along and south of the front over east-central Alabama at early to mid-afternoon (see significant tornado parameter graphic below  from SPC mesoanalysis at 2:00 pm CST):

Computer model forecasts from the morning of 3/3/19 also did a good job of highlighting parts of central, east-central, and southeast Alabama regarding combinations of CAPE and low-level wind shear that might support tornadoes.  For example, the RAP model forecast of 0-1 km energy-helicity index (EHI) from mid-morning showed sizable EHI values for March near and ahead of the Alabama surface low at 2:00 pm CST (2000 UTC), suggesting good environmental support for supercell tornadoes:

The NAM model forecast of winds and contours at 500 mb (roughly 18,000 ft MSL) for midday on 3/3/19 showed a midlevel trough disturbance aloft (thick red dashed line below) moving toward the southern states.  A "spreading" of the jet stream pattern (white lines/arrows) ahead of this trough provided ascent over the area where the 3/3/19 outbreak took place (yellow oval).  Winds at this level were also forecast to be quite strong, approaching 70 kt, offering good support for supercells near the Alabama surface low.

Lastly, here's a RAP model 1 hour forecast sounding near Columbus, Georgia (Fort Benning, LSF) valid at 2:00 pm CST (2000 UTC), located 25-30 miles east-southeast of the tornadic supercell:

Notice the strong low-level wind shear (around 350 m2/s2 of 0-1 km SRH) and the large looping hodograph (upper left).  Winds at just 1 km above ground (roughly 3000 ft) were 50 kt/58 mph, which suggests impressive wind shear for supporting low-level rotation in storms.  Although the total CAPE wasn't terribly impressive (around 1000 J/kg), notice how much CAPE was centered around 3 km above ground.  This sizable low-level CAPE relatively close to the ground likely helped accelerate air parcels upward in a mature storm supercell updraft, assisting with tilting and stretching of horizontal vorticity from the strong low-level wind shear to support low-level rotation in the Lee County supercell.  

From all this information, sadly, it isn't a surprise that a violent and deadly tornado developed on Sunday March 3, 2019 in this setting.

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- Jon Davies  3/4/19