Easter storms may become severe

Published 7:00 am Saturday, April 11, 2020

By Skip Rigney

There is an enhanced risk of tornadoes and damaging winds from severe thunderstorms on Easter Sunday. The highest risk for Pearl River County appears to be sometime between midday and early evening Sunday.
Have you ever cooked something, taken a few bites, and said, “It’s missing something,” then remembered that you forgot to add one of the spices in the recipe?
Just like certain food dishes, severe thunderstorms and tornadoes need certain ingredients. If any of those ingredients aren’t present in the right amount, the thunderstorms aren’t likely to have the pop needed to produce damaging straight line winds or tornadoes. Unfortunately, the computer weather models predict that all the needed ingredients will mix together across the Deep South on Sunday.
Ingredient #1 is rapidly rising air. Air from the lower atmosphere needs to rise miles high in order to form thunderstorm clouds. This ingredient will be in abundance on Sunday thanks to a combination of atmospheric instability and forced lifting of the air.
Instability will increase throughout the day as the circulation around an intense low pressure system streaking across Arkansas pulls warm surface air northward from the Gulf. At the same time, cold air associated with the low in the upper atmosphere will cool temperatures several miles above us. Add the usual afternoon heating near ground level, and the atmosphere Sunday afternoon and evening will likely be very unstable.
An area of strong winds up where the jets fly will be carrying away more air at those high levels than will be coming in from the region upstream. That will create a suction effect, lifting the air upward from below.
Ingredient #2 is moisture. Even with rising motion, you have to have an abundance of moisture to form thunderstorm clouds. We will have more than enough humidity Sunday courtesy of those strong southerly winds coming off of the Gulf. As air loaded with invisible water vapor rises, it cools, and condenses into cloud and rain droplets. Heat released in the condensation process will give an extra kick to the rising motion, further enhancing our first ingredient.
Ingredient #3 for severe thunderstorms, and especially for tornadoes, is large vertical wind shear, which is simply large changes in wind speed and/or direction between different heights in the atmosphere.
We often hear how vertical wind shear tears tropical systems apart and weakens them. That’s not the case for severe thunderstorms produced by non-tropical systems such as Sunday’s strong low pressure system to our north and the screaming upper level jet stream miles above us.
Instead, large vertical wind shear helps get thunderstorms rotating and can spin up tornadoes. Sunday, the surface low pressure to our north will produce a streak of strong winds known as a “low-level jet” with south-southwest winds of 50 miles per hour just a thousand feet above ground level in south Mississippi. At the same time, four miles higher in the atmosphere, the winds of the upper-level jet stream will be roaring from the west-southwest at over 100 mph.
Sunday’s storms will be followed by not one, but two cool fronts, Sunday night and Monday. The stable air behind the cool fronts will make for pretty uneventful weather for most of the week with cool mornings and mild afternoons.

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