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Tornadoes, destructive wind possible Monday afternoon & evening

(WTOK)
Published: Dec. 15, 2019 at 8:27 AM CST
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Severe thunderstorms capable of producing tornadoes are possible for all of East Mississippi and West Alabama Monday afternoon and evening. Other threats from severe thunderstorms will be damaging straight-line winds that can cause damage like tornadoes. Locally heavy rainfall can also lead to localized street flooding.

TIMING

The earliest time of arrival of severe thunderstorms will be 11 AM to 1 PM. Any storms that early will be isolated, but they could be severe if they occur. The most likely arrival time is between 4-6 PM. From there, timing for specific locations gets tricky since thunderstorms could be developing ahead of a line of strong storms. That means we are all effectively fair game starting between 4 PM and 6 PM and ending between 8 PM and 10 PM.

THREATS
  • Tornadoes
  • Straight-line Winds causing damaging like tornadoes
  • Locally heavy rainfall
  • Frequent Lightning
  • Small Hail
WHAT YOU SHOULD DO NOW
  • Stay informed: have at least three ways other than social media and tornado sirens to get warnings. Social media algorithms don't often allow you to see warnings for a week or more after they are posted. Sometimes you won't see the warning for a year or more. It sounds crazy, but it happens. Tornado sirens are not designed to be heard indoors, and they don't always work reliably. Better information sources include Newscenter 11, NOAA Weather Radio, and local radio stations.
  • Review your tornado safety plan. Make sure everyone in your home knows what to do and where to go if a tornado threatens. If you live in a mobile home, have a plan to quickly move to a safe place that isn't a mobile home. For more information, click here.
  • Know the difference between a tornado watch and a tornado warning. A Tornado Watch means conditions are favorable for tornadoes to form. A Tornado Warning means a tornado has formed or is in the process of forming, and you should move to safety immediately if you are in the warning area.
THE SHORT DISCUSSION

An approaching storm system will draw warmth and humidity northward from the Gulf of Mexico. That warmth and humidity will energize thunderstorms. With favorable winds above the ground, some of the thunderstorms could rotate and produce tornadoes. That's especially true with thunderstorms that form ahead of a line of thunderstorms that will march eastward across East Mississippi and West Alabama between 4 PM and 10 PM Monday. This system has more energy available than previous systems, that were mostly tamed before they were able to produce widespread severe weather. The threat of severe thunderstorms is considerably higher from this system, as a result. That does not mean everyone will get a tornado. Our team of meteorologists will closely monitor conditions Monday afternoon and evening, and we will interrupt programming to bring you live updates on Newscenter 11 if dangerous weather threatens any part of the Newscenter 11 viewing area.

THE MORE TECHNICAL DISCUSSION, FOR THOSE INTERESTED
INSTABILITY

One of the ways we measure instability is with a product that is rooted in a long mathematical equation called Convective Available Potential Energy, or CAPE for short. The previous two storm systems struggled to get CAPE values over 200. Big tornado outbreaks in the spring often have CAPE values well over 1,000 - sometimes closer to 2,000. Our CAPE forecasts for this storm system vary between 500 and 800. That's more than high enough in the late fall and winter for severe thunderstorms to form. CAPE is determined from temperature and humidity of the environment and how those feature change by lifting air from the ground.

WIND SHEAR

Wind shear is simply a change of wind speed and/or direction over some distance. When we are analyzing a severe weather threat, we usually are looking at a vertical distance. In other words, we are looking at how the wind changes wind speed and direction as we ascend through the sky from the ground. Wind that can add spin to thunderstorm updrafts that have been energized by the instability previously discussed are ideal. In this case, the wind is from the south near the ground and turns gradually so that is from the west just a couple of thousand feet above the ground. That turning combined with an increase from 10-20 mph near the ground to nearly 50 mph 5,000 feet above the ground gives us a favorable wind profile for tornadoes.

PROBABILITIES

The Storm Prediction Center is the branch of the National Weather Service that issues severe weather forecasts. Based on their probability forecasts, they issue one of five severe weather risk categories: Marginal, Slight, Enhanced, Moderate, and High. Their forecasts early Sunday morning indicate probabilities of severe weather threats as follows:

  • Overall Severe Threat: 30%
  • Tornado: 5% (Slight Risk - Level 2 of 5)
  • Straight-line Wind: 30% (Enhanced Risk - Level 3 of 5)
  • Hail: 15% (Slight Risk - Level 2 of 5))

What do those probabilities mean? They indicate the threat level for severe weather to occur within 25 miles of a given point within the threat area.

Those probabilities seem low, but they generally don't mean much by themselves. For some context, we can compare these forecast probabilities to climatology - or the 30-year averages for Monday's date. Climatological averages for our area are as follows (with some variation depending on exact location):

  • Overall Severe Threat: 1%
  • Tornado: 0.20%
  • Straight-line Wind: 0.25%
  • Hail: 0.25% (west of us)

So in comparison to climatology, our forecast threat probabilities are

THIRTY TIMES HIGHER THAN NORMAL

for the overall severe weather threat,

TWENTY-FIVE TIMES HIGHER THAN NORMAL

for tornadoes,

120 TIMES HIGHER THAN NORMAL

for straight-line winds, and

SIXTY TIMES HIGHER THAN NORMAL

for hail.

In other words, severe weather is thirty times more likely than normal to occur within 25 miles of your home. Tornadoes are twenty-five times more likely than normal to occur with 25 miles of your home. Damaging winds are 120 times more likely than normal to occur within 25 miles of your home. Hail is sixty times more likely than normal to occur within 25 miles of your home.

Some context makes it more clear that this severe weather threat is significant, and it's the highest risk we have faced since the spring.

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