Thursday, September 20, 2018

Florence spawns a killer tornado near Richmond, Virginia on 9/17/18

Anyone following the news the past week knows that Florence was a devastating hurricane in North Carolina (NC), South Carolina (SC), and Virginia (VA), with at least 37 deaths so far.  The Carolinas deaths were mainly flood-related due to Florence's slow movement and deluge of rain, and with such a large and wet tropical system, tornadoes were probably the least of most people's worries.  But tornadoes did occur, particularly on Monday the 17th when one person died in a tornado that hit the Richmond, Virginia area (see above).

There were around 100 tornado warnings issued during Florence and her remnant's slow journey inland from Thursday 9/13/18 through Monday 9/17/18.  Yet the 17th turned out to be the only truly significant tornado day of Florence's 5-day assault on the mid-Atlantic states.  After my last post about tornadoes from the remnants of Gordon on September 8, I thought it would be interesting to look at possible reasons why the strongest and longest track tornadoes were on the 17th, several days after Florence's landfall.

The inland track of the center of Florence and her remnants from Friday 9/15/18 through Monday evening 9/17/15 is shown below, with times marked:
A few reports of brief weak tornadoes began coming in on the evening of the 15th near Wilmington NC, but the most tornadoes associated with Florence occurred on the 16th (brief and weak), and on the 17th (stronger and longer-lived), as indicated on the graphic above.  But again, why was Monday the 17th the most prolific tornado day?

First, from classic research on hurricane tornadoes, such as McCaul's August 1991 paper, the strongest and most numerous tornadoes with tropical systems are most likely in their right front quadrant (looking downwind along the direction of movement).  More recent studies, such as Verbout et al. (2006) also suggest that tropical systems recurving to the northeast inland over the eastern U.S. are more likely to produce tornadoes.

Such recurvature is usually due to tropical systems meeting and merging with westerly or southwesterly flow from a mid or upper level trough as the remnants move northward (this can increase the surrounding deep-layer wind shear to better support supercells and possible tornadoes -- more on than later).  It is interesting that Florence's remnants did indeed merge with such a trough, as seen on the 500mb charts below (roughly 18,000 ft above sea level) for both 9/16/18 and 9/17/18:

From our tracking chart earlier, notice that Florence's center by daytime on the 17th was moving faster and actually curving back to the northeast, a result of it merging with the westerly flow aloft and the 500mb trough.  Even though Florence's remnants were more spread out at this point, this change in movement and direction still put Virginia in the right front quadrant of the remaining tropical system center on Monday the 17th, a favored location for tornadoes from the research noted earlier.  It is notable that the Virginia tornadoes occurred during the period of Florence's remnants recurving northeastward on the 17th.

Second, looking at tornado forecasting parameters from the Storm Prediction Center (SPC) mesoanalysis, the environment associated with Florence's remnants over land appeared most favorable over Virginia on the afternoon of the 17th, particularly when compared to the 16th.  This can be seen from the Enhanced Energy-Helicity Index (EEHI) parameter, which combines low-level wind shear and instability (important supportive factors for supercell tornadoes), shown below:
Note that EEHI values most supportive of supercell tornadoes remained largely over water on the 16th.  But, on the afternoon of the 17th, large EEHI values came together inland over Virginia where thunderstorms were occurring with the spread out remnants of Florence as the remaining circulation center over West Virginia now moved northeastward.

Although low-level shear near ground (typically in the 0-1 km layer) is a key factor in hurricane tornadoes, deeper-layer shear (throughout the 0-6 km layer) also appears to be a factor for tornado development and strength with tropical systems over land (see my 2006 paper here).  Stronger deep-layer 0-6 km shear (larger than 30 kt or 15 m/s) seems to be important in such cases.  Comparing analyses of this deeper-layer shear (below) on the 16th with the 17th, notice how the 0-6 km deep-layer shear was rather weak (< 30 kt) over the Carolinas on the 16th, but was stronger (> 30 kt)  over Virginia on the 17th where the killer tornado occurred near Richmond. This was a result of Florence merging with the 500mb trough and westerly flow aloft, discussed earlier:
Stronger deep-layer shear helps strengthen and organize convective updrafts, and can help to support tornadoes when combinations of low-level shear and instability are also in place.

It is interesting that on the 16th, with weaker deep-layer 0-6 km wind shear, a supercell tornado over water (a 'tornadic' waterspout) came directly ashore in Myrtle Beach SC (see image below), but produced little reported damage over land (probably EF0 in intensity):

This is in contrast to the killer EF2 tornado near Richmond VA on the 17th (below) when low-level shear and instability combinations appeared more favorable (see the EEHI graphic earlier), and were  supported by an area of stronger deep-layer 0-6 km shear over Virginia (see the 0-6 km shear graphic earlier); this stronger deep-layer shear was absent over the Carolinas the day before.  Here's some more images of the large Richmond tornado:

Distinguishing tropical systems that produce stronger or more numerous tornadoes from those that don't produce tornadoes or are associated with brief weak tornadoes is difficult and certainly not that well understood.   But some of the factors discussed above can be helpful at times in forecasting such tornadoes.

If you can, please donate to a charitable organization to help with recovery from Hurricane Florence.

-  Jon Davies  9/20/18

Monday, September 10, 2018

September 8, 2018 Kentucky-Indiana tornadoes from remnants of tropical storm Gordon

It's the middle of hurricane season, with dangerous hurricane Florence bearing down on the Carolinas later this week.  Hurricanes and tropical systems can produce tornadoes, particularly in their right front quadrant.  So, I thought it would be interesting to look back at last week's much weaker tropical system (Gordon) that produced some tornadoes (see above) in Kentucky and Indiana on Saturday, September 8.

Gordon never quite made it to hurricane strength before landfall on September 4 in the Mississippi/Alabama/northwest Florida coastal area, and only produced one or two very brief weak tornadoes on the 4th and 5th with little or no damage.  But then, after a couple of days with no tornadoes, the remnants of Gordon produced several tornadoes on September 8 far inland in northern Kentucky (KY) and southern Indiana (see area indicated on the 2 pm CDT surface map below):

This included one tornado of EF1 intensity at Stanley KY, just west of Owensboro, shown at the top of this post.  Why would Gordon suddenly start producing tornadoes again four days after landfall?

By Saturday the 8th, after having moved northwestward in weak upper flow for three days, Gordon's remnants had merged with a non-tropical midlevel disturbance evident at roughly 20,000 ft MSL (dashed heavy red line on the 3 pm CDT SPC mesoanalysis 500 mb map below):

This caused Gordon's remnants to "recurve" to the east-northeast as a surface low along an east-west quasi-stationary front over the Ohio Valley (see the surface map earlier).  The 3-panel map below shows this recurvature of Gordon's path (red squares and dashed lines) during September 4th through the 8th:

This graphic also shows deep-layer wind shear (surface to 6 km above ground, in blue lines) on September 4th, 6th, and 8th.  Notice how the shear weakened and essentially disappeared as Gordon's remnants moved farther inland on the 6th and 7th.  But by Saturday the 8th, when encountering energy from the midlevel disturbance, the wind shear dramatically increased near Gordon's remnants over the Ohio Valley.  This is what helped to produce the tornadoes, because supercell tornadoes, along with an unstable environment, typically require sizable wind shear (a change in wind direction and increase in wind speed with height) to develop.

At 3 pm CDT on the 8th, instability and wind shear together (shown using an enhanced version of the energy-helicity index or EHI) highlighted an area supportive of tornado development on SPC graphics along the Ohio River border area of Indiana and Kentucky , which is where the tornadoes occurred:

Thankfully, the tornadoes weren't that strong, but the one at Stanley KY did do notable damage to several homes, and was widely photographed:

Hurricane Florence (category 4 as I write this) poses a much bigger threat than Gordon's localized tornadoes when it hits the Carolinas on September 13 and 14.   Wind driven storm surge, heavy rain, and flooding will likely be huge and potentially life-threatening issues, and I'm just hoping it won't be as bad as most forecasters are thinking.

-  Jon Davies  9/10/18