We’re traveling to Oklahoma tonight. It’s unusual to see a near textbook-perfect tornado setup somewhere in the country – it happens maybe once or twice a year. Tonight our computer models are showing just that in parts of Oklahoma and Kansas tomorrow.
A very powerful storm system over southern California and Arizona tonight is racing east. By Monday afternoon it will be moving through western Kansas and Oklahoma. Ahead of the storm is upward motion (important in storm formation), warm and moist winds at the surface, very strong winds aloft (95 knots at 500 mb), strong shear, and an elevated mixed layer (EML).
I’m going to break all these down as sort of a primer on severe weather forecasting.
Here’s the 18z NAM forecast for 18z Monday. It’s hard to miss the strong storm near the Oklahoma panhandle. Ahead of that storm is very strong lift or upward vertical motion in the Atmosphere. You can see a diffluent pattern to the height lines over Oklahoma and Kansas which is also indicative of upward motion. Lift is important because it can eliminate convective inhibition (known as CIN or a cap) and allow thunderstorms to develop along some type of focusing mechanism like a dry line or cold front. We’ll talk more about CIN in a moment.
Warm/Moist Winds at the Surface
The winds near the surface are forecast to be strong out of the south bringing north plenty of moisture off the Gulf of Mexico. Dew points will rise into the upper 60s and low 70s which acts as fuel for thunderstorms and can increase instability in the atmosphere.
Strong Winds Aloft/Shear
Strong southerly winds at the surface when coupled with strong westerly winds aloft lead to strong speed and directional shear. Shear is important for several reasons.
One reason is that strong shear tends to keep rain and hail out of a storms updraft (it literally blows it away from the updraft) which can keep the updraft strong and long-lasting. A second reason is that shear also can promote propagation of a storm in a certain direction which organizes storms as opposed to keeping their development somewhat random.
Tomorrow, with winds veering significantly with height (south at the surface and west aloft), the atmosphere will have big speed and directional shear that will promote supercells and possibly tornadoes.
Elevated Mixed Layer/Instability
The models have trended a bit away from a classic elevated mixed layer setup for tomorrow but are still impressive. An EML is present in many high-end severe weather outbreaks (for example the northeast 1985 and 1989 tornado outbreaks) because of several factors.
An EML develops when a well mixed boundary layer advects off the high terrain west of the Great Plains (Rockies or even northern Mexico) and feature steep mid level lapse rates (>8ºC/km). When these advect northeast and reside over a warm moist boundary layer you get what’s called a “loaded gun” sounding.
This is an example of a loaded gun sounding from Dulles, VA (IAD) on July 10, 1989 the day of the massive tornado outbreak in the northeast (including the Hamden, CT F4 tornado). You can see an inversion or “cap” at 800 mb with a very steep or left leaning temperature profile from 500mb to 800mb with lapse rates steeper than 8.0ºC/km. This can result in very high amounts of CAPE thanks to steep lapse rates and limited vertical mixing (keeps the boundary layer moist).
Once an area of lift approaches, the cap begins to weaken and storms can form. Keeping thunderstorms isolated at first allows them to realize the maximize amount of CAPE.
What Will Happen?
At this point it appears all the ingredients are coming together for a major tornado outbreak but there are always things that can go wrong. Clouds and ongoing thunderstorms in the morning could reduce the amount of forecast instability, for example.
It will be interesting to see how this event unfolds… but it’s always cool to see textbook-like examples show up in real life (from a distance, that is)!