Two numbers, the freezing level and the boundary-layer moisture, decide which of these two hazards your region actually hands you. They are not the same everywhere, and the training photographs are from somewhere else.

In this lesson you will

  • Explain how freezing-level height governs whether hail survives to the ground.
  • Distinguish wet-microburst from dry-microburst environments and identify which your region produces.
  • Characterise the dominant hail and wind regimes across the main US regions.
  • Describe reporting nuances in tropical cyclone feeder bands.

Two dials, set differently everywhere

Everything in the previous seven lessons is physics and applies wherever you are standing. What is not the same everywhere is which hazard that physics delivers to you, and it comes down to two quantities.

Freezing level height
How far a stone has to fall through air above freezing before it reaches the ground. Lesson 1 noted that between 40 and 70% of what forms aloft can melt on the way down. A high freezing level pushes that fraction up and takes the hail away.
Boundary-layer moisture
Whether a descending downdraft arrives wet or dry. A deep dry sub-cloud layer lets evaporation accelerate it, producing a dry microburst under a storm that may not be raining at the ground at all. A moist one produces a wet microburst, buried in rain.

What that produces, region by region

RegionHailDamaging wind
Central and southern PlainsThe maximum. Deep instability with a freezing level low enough that large stones survive. This is where the textbook photographs are taken.Supercell RFD and organized squall lines, on days that were forecast.
High Plains and interior WestFrequent, and often small. High terrain puts the ground closer to the freezing level.Classic dry microbursts under high-based storms, with virga and blowing dust and little or no rain at the surface.
Southeast and Gulf coastGenuinely uncommon at large sizes. A warm marine air mass puts the freezing level high and most of the stone melts.Wet microbursts from ordinary summer pulse storms, in heavy rain, frequently with no watch in effect.
Midwest and Ohio ValleyCommon in the warm season, with the largest stones on the western side.Derechos and long-lived MCSs, the regime lesson 4 describes.
Northeast and New EnglandLess frequent and generally smaller.Pulse and multicell downbursts in summer, plus damaging wind from decaying tropical systems.

Boundaries are soft and every region gets every hazard occasionally. The column that matters is your own, and the point of reading the others is knowing which training material was written for somebody else.

The coastal case, with numbers

The Gulf coast row is worth expanding because it is the one where the figures are published and the contrast is sharpest. Two comparisons make the scale concrete better than an average would. On the Texas Gulf coast, one coastal county recorded 18 severe hail days since 2000 against 80 for the county containing San Antonio, a few hours inland. And when NWS Jacksonville logged two-inch hail in August 2020, it was only the third August two-inch report in Florida since 1955.

And then it happens anyway
The exception, and it exists in every region, is the setup with anomalously cold air aloft and strong shear, where a modest storm produces large hail because the freezing level came down to meet it. NWS Melbourne documented an event with only around 500 J/kg of CAPE but 500 mb temperatures near -16 C and 64 knots of deep-layer shear. Low instability does not mean no hail when the temperature profile is right, and a climatology is not a forecast.

The wind you are most likely to actually witness

This is the practical half of the lesson. Ask which damaging-wind event is commonest where you live, rather than which is most dramatic, because the commonest one is what you will be standing under.

The unglamorous answer, in much of the country
Not the squall line on a watch day. A single-cell summer storm with no watch in effect, over in twenty minutes, dropping a downburst on one neighborhood. In a moist region it arrives buried in rain; in a dry one it arrives with dust and almost no rain at all. Neither looks like the training photographs. Lesson 4's cues, the rain foot and the shelf cloud, are what you get, and you get them late.

Tropical systems

Landfalling tropical cyclones bring their own reporting problem, and they bring it a long way inland: feeder bands produce brief convective gusts and spin-up tornadoes on top of a background of sustained tropical wind. Module 4 lesson 9 walks the rotation side of that; the wind side is subtler.

  • Separate the gust from the background. A 55 mph sustained wind with 70 mph gusts is a different report from a 70 mph convective gust in a 30 mph background, and the second is the one a band produced.
  • Timing matters more than usual, because bands arrive in a sequence somebody is tracking.
  • The measured or estimated flag matters more than usual too, because anemometers in a landfalling cyclone are often the only instruments still reporting, and often not exposed the way lesson 5 requires.

A tropical rainband is not just heavy rain, and treating it that way is how people are surprised by a gust or a brief tornado hours from the center and hundreds of miles from the coast.

Check yourself

An August afternoon in a warm, deeply moist coastal air mass. A single storm with no watch in effect produces damaging wind over a small area. Which hazard was this most likely, and why?

  1. A wet microburst, because a warm moist boundary layer makes that the dominant damaging-wind mode there
  2. Large hail, because summer storms in that air mass have the strongest updrafts
  3. A tornado, because damaging wind from a single cell implies rotation
  4. A dry microburst, because single-cell downbursts are dry by definition
Hint

Two dials. One of the wrong answers is the hazard a high freezing level takes away, and one confuses a dial setting with a definition.

Answer

A wet microburst, because a warm moist boundary layer makes that the dominant damaging-wind mode there

Wet microbursts from summer pulse storms dominate damaging wind in warm moist air masses, and they arrive without a watch, in heavy rain, from a storm that looks unremarkable. Large hail is the hazard a high freezing level suppresses. And wet against dry is set by the sub-cloud moisture, not by whether the storm was a single cell: the same cell over the High Plains would give you the dry version.

Sources

  1. Atkins & Wakimoto 1991, Wea. Forecasting 6, 470–482: wet microburst activity over the southeastern United States
  2. Fujita 1985, SMRP Research Paper 210: The Downburst, Microburst and Macroburst
  3. NWS Melbourne and Jacksonville event write-ups
  4. SPC severe weather climatology
  5. NSSL Severe Weather 101, hail basics