Convective mode, whether storms are discrete cells, a squall line, or a derecho-producing MCS, is often the single biggest fork in a severe weather forecast. This page covers the storm modes and wind phenomena that come with them, from bow echoes to microbursts and haboobs. Mode debates before an event are half the fun of the XWD Discord’s forecast channels.
A multicell storm is a thunderstorm cluster where new cells continuously develop on the inflow flank as older cells weaken and dissipate. Less organized than supercells but can still produce brief tornadoes, large hail, and flooding. The most common storm type — most thunderstorms you see on radar are multicell clusters.
A squall is a strong wind characterized by a sudden onset, in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute. In nautical use, a squall refers to the whole severe local storm, including winds, clouds, precipitation, thunder, and lightning. Not to be confused with a squall line, which is a long line of thunderstorms.
Squall line: any line of thunderstorms. Bow echo: a bow-shaped squall line driven by a strong rear-inflow jet — the radar signature for widespread damaging winds. Derecho: a long-lived bow echo (or MCS) that produces a damage swath 240+ miles long with sustained 58+ mph winds. All derechos produce bow echoes; not all bow echoes become derechos.
A squall line is a continuous or near-continuous line of thunderstorms, often forming along or ahead of a cold front. Primary threats are damaging wind and embedded QLCS tornadoes.
A bow echo is a bow-shaped line of storms produced by a strong rear-inflow jet. They're a primary signature for damaging straight-line winds and can spawn QLCS tornadoes at the apex or northern tip.
A derecho is a widespread, long-lived windstorm associated with a fast-moving line of storms. To qualify it must produce a swath of wind damage 240+ miles long with widespread 58+ mph winds.
A QLCS (Quasi-Linear Convective System) is a line of thunderstorms — squall lines and bow echoes. QLCS tornadoes are typically brief, weak, and spin up with little warning along the leading edge.
A Mesoscale Convective System (MCS) is a complex of thunderstorms organized on a scale larger than individual cells but smaller than a synoptic system. Squall lines, bow echoes, and MCCs are all MCS types.
A Mesoscale Convective Vortex (MCV) is a mid-level rotation left behind by a decaying MCS. MCVs can persist for days and trigger fresh rounds of severe weather, sometimes including tornadoes.
A downburst is a strong downdraft producing damaging winds at the surface. Subcategorized as microbursts (< 2.5 mi) or macrobursts (> 2.5 mi).
A microburst is a localized, intense downdraft (under 2.5 mi wide) producing damaging straight-line winds of 100+ mph at the surface. Often mistaken for tornado damage.
A macroburst is a large-scale downburst — greater than 2.5 miles wide — capable of producing widespread straight-line wind damage for 5–30 minutes.
A gustnado is a brief, shallow spin-up along a storm's outflow gust front. Not technically a tornado — there's no connection to a parent storm's updraft — but can still cause minor damage.
A landspout is a non-supercell tornado that forms when a surface boundary's vorticity gets stretched vertically by a developing cumulus updraft. Generally weaker than supercell tornadoes, but still capable of damage.
A waterspout is a tornado-like vortex over water. Most are fair-weather (non-supercell) and weak, but tornadic waterspouts associated with supercells can be as destructive as land tornadoes if they move onshore.
A dust devil is a small, rotating column of rising air made visible by lofted dust, debris, or sand. Unlike tornadoes, dust devils form in fair weather from intense surface heating — no parent thunderstorm is needed. They are typically weak and short-lived but can occasionally reach 100+ mph and damage lightweight structures.
A haboob is a large, intense dust storm driven by thunderstorm outflow. The wall of dust can be thousands of feet tall and tens of miles wide, reducing visibility to near zero almost instantly. Common in Arizona (Phoenix metro), New Mexico, and parts of the Middle East. EAS code for Dust Storm Warning: DSW.
Virga is precipitation that evaporates or sublimates before reaching the ground — appearing as trails or streaks falling from a cloud base that don't hit the surface. Common in dry desert climates. Virga can produce dangerous microbursts as the evaporating precipitation cools and accelerates the air downward.
Steam fog (evaporation fog) forms when cold air moves over much warmer water. Water evaporates rapidly into the cold air, immediately condensing as it exceeds the air's saturation capacity. Looks like wisps of 'steam' rising from rivers, lakes, and the ocean. Common in fall when lakes are still warm but air turns cold.
Lightning is a massive electrostatic discharge between regions of opposite charge — either within a cloud (intracloud, most common) or between cloud and ground (CG). A typical bolt carries 300 million volts. The charge separation is driven by ice particle collisions in the mixed-phase zone of thunderstorm updrafts.
Thunder is the sound produced by the rapid expansion of air superheated by a lightning bolt. The lightning channel reaches ~50,000°F in microseconds, causing explosive expansion that propagates as a sound wave. You can estimate distance by counting seconds between lightning and thunder: 5 seconds ≈ 1 mile.
A lightning jump is a rapid increase (sigma ≥ 2) in total lightning flash rate over 1–2 minutes. Lightning jumps precede severe weather (tornadoes, large hail, damaging wind) by an average of 10–15 minutes and are integrated into ProbSevere and experimental NWS severe weather decision support tools.
Some National Weather Service offices and private weather services issue color-coded thunderstorm probability alerts for specific locations based on model guidance and local climatology. These are non-standard (not official NWS products) and vary by source. The SPC's Day 1–3 outlooks are the authoritative official source for thunderstorm and severe weather probability.