at 5:17 PM by Pierce Larkin

Recap: The May 10th Severe Weather Outbreak

In Kentucky, severe weather is fairly typical and expected in May. May is the peak time for severe weather across the US, and it is fairly typical for us to see some strong cold fronts and synoptic systems to roll through here and give us some severe weather.

The likelihood of severe weather occurring at a given spot on May 13th. h/t SPC
The likelihood of severe weather occurring within 25 miles of a location on May 13th. h/t SPC

Recently, however, we have been seeing a fairly quiet and calm pattern across the eastern United States. For the month, the US as a whole is running below normal on tornado numbers for the month, and is running below normal on severe weather reports as well.

The preliminary tornado numbers for May vs the 3 year average for tornadoes in May. h/t SPC
The preliminary tornado numbers for May vs the 3 year average for tornadoes in May. h/t SPC

We are over half way through the month, and are 174 tornadoes below our 3 year national average. Despite that, we found ourselves in an active pattern, favorable for severe thunderstorms last week. The state got exactly that. So why did we see one of our more active severe weather/tornado days since March 2, 2012?

Overall Setup

I will eat crow for this day forever. At WKU, we have a student forecast discussion group on Facebook (we’re all nerds, come on people) and I was very against the idea that anything was gonna happen on May 10th. I, obviously, was very wrong about this.

h/t giphy.com
h/t giphy.com

My forecast for that day on WxOrNot that day called for, “Mostly cloudy, with severe thunderstorms possible in the evening hours.” In retrospect, that was a good forecast. I got it right, but it was difficult for me to believe that we were going to see severe weather. Why? There was very little synoptic forcing.

h/t giphy.com
h/t giphy.com

Forcing mechanisms are of the utmost importance in terms of creating an environment capable of severe thunderstorms and tornadoes, and the forcing that was present that day (on the synoptic level) was very weak. We had a weakening shortwave move through the region during the morning hours, and move to our north, weakening as it propagated to the northeast.

The weakening 500 mb trough. h/t SPC
The weakening 500 mb trough. h/t SPC

However, associated with this was one of the more important ingredients in creating this event: a westerly 50 knot jet streak over much of the state. This set up well to have at least good speed shear, and with good winds that became southwesterly and south-southwesterly at the 700 and 850 mb levels (respectively), this created an atmosphere that had favorable shear parameters for tornadoes and severe weather. But how would storms form across the region?

500 mb flow by mid afternoon May 10th. The flow is predominately westerly across the western part of the state. h/t SPC
500 mb flow by mid afternoon May 10th. The flow is predominately westerly across the western part of the state. h/t SPC
700 mb flow by mid afternoon on May 10th. The flow is predominantly southwesterly across the western portion of the state. h/t SPC
700 mb flow by mid afternoon on May 10th. The flow is predominantly southwesterly across the western portion of the state. h/t SPC
850 mb flow by mid afternoon on May 10th. The flow was predominantly south-southwesterly across the western portion of the state. h/t SPC
850 mb flow by mid afternoon on May 10th. The flow was predominantly south-southwesterly across the western portion of the state. h/t SPC

Well, that is a complicated answer. There are several reasons that this forecast was as complicated as it was, and one of those reasons was the presence of morning rain showers. The night before, showers and thunderstorms formed in associated with the shortwave trough axis and moved towards our region by early on the 10th.

This could have done two things: one, it could have kept instability down, and hindered out storm threat; two, it could laid out boundaries across the region that would serve as forcing mechanisms. Almost always, the former happens and the latter cannot be used efficiently, and we are left without severe storms. However, this was a unique day, as the clouds cleared and instability built, and boundaries were laid that allowed for storms to fire along them. By noon, the region had plenty of instability for storms to work with.

The mixed-layer CAPE built up across the region by noon on May 10th. This provided plenty of energy for thunderstorms to work with as they developed. h/t SPC
The mixed-layer CAPE built up across the region by noon on May 10th. This provided plenty of energy for thunderstorms to work with as they developed. h/t SPC

Additionally, noon, a monster supercell associated with a 500 mb vorticity max propagating northeastward moved into central KY and strengthened rapidly as it approached Owensboro, KY.

This is the supercell that moved across the western portion of the state around noon. h/t GRLevel2
This is the supercell that moved across the western portion of the state around noon. h/t GRLevel2 and NWS Archived Radar Data

This produced an outflow boundary that would stretch to its south-southwest and would help to initiate thunderstorms as it moved into higher instability in central Kentucky. Additionally, this laid even more boundaries to work with as thunderstorms began to initiate along differentially heated boundaries over western KY.

 

h/t UCAR Archive
h/t UCAR Archive
The storm circled in red is the initial supercell that produced the EF-3 in western KY. h/t GRLevel3 and NWS Archive Radar Data
The storm circled in red is the initial supercell that produced the EF-3 in western KY. h/t GRLevel3 and NWS Archive Radar Data

As this all occurred, wind shear parameters across western and central KY became much better, and aligned with the instability parameters.

Mixed Layer CAPE that had developed by the time supercells were established and strengthening. h/t SPC
Mixed Layer CAPE that had developed by the time supercells were established and strengthening. h/t SPC
The 0-6 km wind shear. The shear was at and above the threshold considered "good" shear. h/t SPC
The 0-6 km wind shear. The shear was at and above the threshold considered “good” shear. h/t SPC
The effective shear by early afternoon was good enough for severe storms and supercells to develop. h/t SPC
The effective shear by early afternoon was good enough for severe storms and supercells to develop. h/t SPC
0-1 km Storm Relative Helicity (SRH). Helicity measures the potential an updraft has to rotate, and is a good measure of how strong the wind shear is. The values across western KY were good enough for supercells and tornadoes. h/t SPC
0-1 km Storm Relative Helicity (SRH). Helicity measures the potential an updraft has to rotate, and is a good measure of how strong the wind shear is. The values across western KY were good enough for supercells and tornadoes. h/t SPC
0-3 km SRH. The values of 200 m2/s2 and greater are good enough for supercells and tornadoes. h/t SPC
0-3 km SRH. The values of 200 m2/s2 and greater are good enough for supercells and tornadoes. h/t SPC

These storms developed, and flourished in an environment that was very favorable for severe thunderstorms, supercells and eventually tornadoes. Even without a major synoptic mechanism for lifting, the combination of mesoscale boundaries and the strong supercell helping to force other storms with its outflow, supercells developed and dropped 7 tornadoes across the state.

Results

As I said before, this was a day that features 7 tornadoes forming across the central and western portions of the state. Since March 2, 2012 (not including it), this was tied for the most tornadoes confirmed across the state of KY in one day. This day was very impressive, and produced an EF-3 tornado across western KY, and an EF-2 in the central portion of the state. Both were long tracked tornadoes that produced damage along their paths. The radar and satellite images from the day are quite impressive.

The satellite images from May 10th across the region. h/t CIMSS satellite
h/t NWS Louisville

Plenty of pictures were taken of the tornadoes, and I will leave you with them. This day is just an example of how difficult and complicated severe weather forecasting is (and how much I have left to learn), and how being prepared for severe weather all the time is a good idea.