HOUSTON – It has been a dreary day across the Houston area with widespread showers and thunderstorms dumping heavy rainfall across the area.
The possibility of severe weather and tornadoes was also predicted, but that has not come to pass. This is a good thing and we are going to explain why the severe weather threat didn’t end up being as high as it could have been.
There were several different ingredients in place that would promote the formation of supercells and tornadoes. An important piece was wind shear of which there was plenty. Wind shear is the changing of wind direction and speed with height in the atmosphere and Thursday there was a large amount of that in place.
If you look at the graphic above, winds all day Thursday were pumping in from the south/southeast at the surface. Meanwhile, a little higher in the atmosphere, a jet stream was crisscrossing the surface wind from the southwest.
The severe threat was that if any storm could start to build in vertical height, the winds would shift from the SE to the SW and eventually W with the upper jet stream. Plainly put, twisting of the winds with height is how thunderstorm supercells turn into rotating, potentially tornadic cells.
We also had plenty of moisture in place. In many areas across southeast Texas, dew points were in the middle to upper 60s, which showed the presence of a very moist atmosphere, a key component in the creation of severe storms.
With several key components in place, why is it that storms just couldn’t get their act together and produce tornadoes? One important ingredient which was lacking was instability. The persistent cloud cover and rain limited the amount of destabilization in the atmosphere. Instability is an important element in the formation of tornadoes. The more instability there is, the more likely storms can form robust updrafts, which is necessary to produce tornadoes and other forms of severe weather.
Thankfully, there was a cap or lid on the atmosphere for most of the area Thursday that served to block fast, vertical growth of any storm that tried to get going. As the storms would grow, they would hit that cap and were not strong enough to break through, Think of it like a Topo Chico bottle of sparkling water: shake up the bottle but leave the top on, nothing happens. Shake up the bottle and take off the top, BOOM! Storms can explode just like an overflowing bottle of seltzer!