There is a really cool description of a flow storm in the shadow world players guide and in Terry's Loremaster Legacy the central characters get to watch a flowstorm from the deck of a ship.
This is from the players guide.
In nature, a Flowstorm takes the form of a great whirlwind or thunderstorm, but instead of dust or water vapor, it is composed of translucent bands of multicolored light (often a lurid green hue) with energy discharges arcing between them.
The greatest of these monstrosities behave like hurricanes and can last for weeks or months, wreaking havoc with weather, Essænce, and life. While to the untrained observer the impact of a Flowstorm is very similar to that of a normal (albeit prismatic and extremely violent) meteorological event, this is but a side effect of far more devastating damage to the invisible Essænce.
Almost all Flowstorms spawn whirlwinds of varying intensity. A small storm (sometimes referred to as a flow squall) may only support one small whirlwind, while larger ones may create dozens of powerful effects, some similar in might to minor tornadoes.
