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Module 02

Thunderstorm fundamentals

What makes a storm, what makes it organized, and what kind you are standing under. Eight lessons from the three ingredients to the bow echo, with equal weight on the storm modes that dominate the Southeast and the ones that photograph well on the Plains.

Start lesson 01

Lessons8

Reading timeabout 4.5 hrs

Objectives42

The lessons

Take them in order, or don't

01Moisture, instability, lift, and the numbers behind themThree ingredients make a thunderstorm and a fourth decides what kind you get. Each of them has a number attached, and the numbers are on the same forecast pages you already look at.Storm sciencePuts numbers on the ingredients, which is what lets you read a day before the first tower goes up.35 min 02The life of an ordinary cellAn ordinary thunderstorm builds the thing that kills it. Watch the sequence once with that in mind and the timings stop being trivia.Storm scienceThe three stages, plus why each transition happens and what you can actually see from the ground at each one.30 min 03Wind shear decides what kind of storm you getSame moisture, same instability, same lift, different wind profile, completely different day. Shear is the parameter that turns a forecast of storms into a forecast of a storm mode.Storm scienceGives the shear thresholds, and shows how the shear vector relative to a front predicts discrete storms versus a line.35 min 04Pulse storms and downburstsThe storm that will not look severe, has no rotation to report, lasts half an hour, and puts a tree through somebody's roof. Most of our summer severe weather comes from this.Storm scienceThis is the dominant summer severe threat in warm humid climates. The specific Southeast wet-microburst setup was studied in north Alabama, and the numbers are usable.25 min 05Multicells: how storms move by building new stormsA cluster can move northeast while every cell in it moves east, or sit still for three hours while its cells race away downstream. Once you can separate the two motions, radar loops stop being confusing.Storm scienceStorm motion is not cell motion. This is the most useful concept in the module, and it underlies both backbuilding flash floods and fast-moving bow echoes.25 min 06The supercell: why it rotates and how it is builtA supercell is not a big thunderstorm. It is a thunderstorm that has solved the problem in lesson 2, and rotation is how it solves it.Storm scienceWhere a supercell's rotation comes from, why it strengthens the updraft, and how the storm is laid out, so the visual features in modules 3 and 4 have a map to hang on.35 min 07LP, classic, HP, and the regional variantsThe supercell you were shown in class is a photograph from Kansas. This lesson is about the ones that actually arrive here, most of which you cannot see.Storm scienceLP, classic and HP are usually drawn from Plains photography. Our spotters mostly see HP storms, embedded supercells, tropical mini-supercells and cool-season storms at night.30 min 08QLCS, bow echoes, derechos, and mode changesA line of storms is not a big multicell cluster. It has its own dynamics, its own tornado problem, and a definition dispute running through the middle of it that is worth understanding rather than memorising.Storm scienceBow echoes, derechos and the QLCS. Across the Southeast and the Ohio Valley the QLCS is how severe weather usually arrives in the cool season, and its tornadoes are the hardest in the country to warn for.40 min

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