Significant Severe Weather Event Next Week

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The ImpactWeather StormWatch team has identified a significant threat of severe weather beginning late this weekend and into early next week. Keyword: Significant.

This, on the heels of last week’s webinar on the spring severe weather outlook which included our forecast of a record-breaking year of severe weather. Read last week’s YourWeatherBlog entry here, which includes a link to the recorded webinar.

April is typically ripe with severe weather. Nearly every frontal system plowing across the Plains or the South is capable of producing a severe thunderstorm. And because it’s April the last of the winter storm systems can kick up their heels in a fantastic way as the beginning of the summer pattern seeds the eastern half of the country with warm, humid and unstable air. Combine these ingredients with a brisk stir to yield severe thunderstorms.

The jet stream on April 3, 1974 was a significant contributing factor to the Super Outbreak of tornadoes. Image: Wikipedia

However not all severe thunderstorms are created equal. Last week’s severe thunderstorms in Dallas had all the ingredients on hand to produce tremendous storms and tornadoes, but as the system moved east the ingredients lost some of their punch and the storms became less frequent and less severe. More often than not, last week’s type of mega-storms are the exception rather than the rule as it takes many ingredients to align in an exact way to produce such violence.

Indications are good that next week's surface analysis will look very similar to this analysis from April 3, 1974. Image: Wikipedia

It looks like next week all the ingredients will be in exactly the right place. So impressive is the initial data, that we’re seeing similarities to the April 3rd, 1974 Super Outbreak that produced the second largest tornado outbreak within a 24-hour period in the U.S. (the largest 24-hour outbreak is last year’s April 25-28 outbreak which produced 208 tornadoes on April 27).

Meteorologists, students of meteorology and even serious hobbyists know that severe weather is about much more than just a cold air mass slamming into a warm air mass. Severe weather is as much about what’s happening in the upper atmosphere as what’s happening on the surface. Key upper ingredients include a massive upper-level trough of low pressure. These troughs will often be reflected on pressure charts above 30,000 feet. Also needed, cold air aloft. In elementary school we learned that warm air rises, but if that warm air is suddenly surrounded by cold air it will rise faster, sometimes explosively so. Finally, our short lesson of severe weather criteria includes one more main ingredient: strong upper-level winds. These winds act like an exhaust fan at the top of a hot stove, helping air already moving up to continue moving up and even accelerate.

Valid early Monday morning, April 16 this mid-altitude chart shows a mature low pressure trough over the Rockies with the most active and strongest section of the jet stream over the Plains. Image: WSI

What’s the big deal about air moving up? After all, it’s the air moving downward that causes all the damage on the ground and even helps initiate a tornado. Upward-moving air cools and condenses as it rises. This action forms clouds and eventually precipitation. Fast-moving air takes that scenario ever higher into the atmosphere where the conditions between warm/cold and moist/dry become even more extreme. With these conditions, what would otherwise be an ordinary thunderstorm crosses over into the realm of severe.

Next week begins with all of these ingredients in place. Early on Sunday a large upper-level trough moves across the desert Southwest and Great Basin of the United States. This trough reflects downward to a strong surface cold front moving into the Plains. Already in the Plains: an unstable air mass that begins to take shape tomorrow with a pronounced southerly wind from the Gulf of Mexico and Texas. Five days of that kind of wind is like filling an Olympic-sized swimming pool with gasoline, then just waiting for a spark. That spark arrives with the cold front and then continues to fire as the upper-level trough swings through during the following couple of days.

By this coming Saturday, the strong cold front is oriented from the western Great Lakes to western Texas and northern Mexico. North Texas, Oklahoma and Missouri may experience severe weather as this cold front moves through the region. How similar is this panel to the one from April 3, 1974? Image: ImpactWeather StormWatch

Where exactly will the outbreak be? Too early to say as our confidence in a forecast more than five days out is low. However, the central Plains, even the northern regions of the southern Plains, seem most at risk initially. As the system moves east, the threat should move east with it.

ImpactWeather stresses active preparation with respect to severe weather. Hurricanes, ice storms and tornadoes all produce threats to life, business and property but waiting until the tornado is on the ground or the ice storm has robbed you of power or the storm surge is lapping at your door step is, by then, too late. Active preparation means preparing now for a storm that may not come tomorrow or may not come next week or even next month, but for the storm that looms — eventually and inevitably — on the horizon for almost every person and business in the United States and so many other countries around the world. Many of you are already prepared, no matter what the weather forecast indicates. For the rest of you, now is the time.

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