Synoptically-forced Thunderstorms

Squall Line

• A narrow, often linear band of T-storms that develops in the warm sector of a mid-latitude low pressure system, usually 60-180 miles in advance of the cold front ... lifetime can be 6+ hours.

• Set off by the active low-level jet, not low-level convergence at front.

• Upper-level divergence is present ahead of upper-level trough.

• Low-level jet provides a "tongue" of warm, moist air about 1000-2300 feet above the surface; can form at night, when surface layer cools and stabilizes, causing a "de-coupling" above the inversion layer where the winds are typically weaker.

• The Plains states slope gently upward from Miss. River to the eastern slopes of the Rockies; this energizes the low-level jet, as air cools more at night in the mountains, thereby causing the pressure surface to slope downward from east to west ... the resulting pressure gradient force is directed from east to west and can enhance s southerly low-level jet to greater than 60 mph.

• The most intense T-storms in a squall line are usually at the southern end of the line, where the low-level jet is feeding in the moisture.

Supercell T-Storm

• The most intense synoptically-forced T-storm, readily capable of producing severe weather.

• A "tilted updraft" is present ... can approach speeds of 100 mph!

• There are two downdrafts ... one in front due to the heavy rain ... the other in back due to entrainment of dry, mid-tropospheric air.

• Can persist as long as ample supply of low-level moisture is maintained.

• For severe weather ... need a "capping inversion" or a "lid" of warm, dry air (700 mb) overlying the warm, moist air (850 mb or lower) ... this sets up the "pressure cooker", "loaded gun" situation.

• Peak time is in Spring and early Summer, especially in south-central Plains.

• A previously "capped" supercell T-storm can "explode" in less than 30 minutes!

Mesoscale Convective System (MCS)

• T-Storms merge into "clusters"... can last from 6 hours to as long as 16 hours.

• A concentric bundle of many individual T-storms that are unified by a regional-scale circulation.

• Areal coverage of at least 100,000 square km (approx. 2/3 the size of Iowa).

• Unique to mid-latitudes.

• Form in environment of weak upper-level winds in vicinity of a ridge.

• Requires a strong low-level jet, transporting extra-ordinarily warm, moist air into the region.

• Nocturnal ... reaches peak in early morning hours (1-3 AM) ... most common in Midwest.

• New T-Storms form in convective region facing the low-level jet.

• Remainder of MCS characterized by widespread, but generally less precip.

• Mesoscale circulation ("L") at 500 mb binds circulation together, due to warming at mid-troposphere due to large releases of latent heat, which in turn leads to convergence at center of MCS.

• Usually track ENE along upper-level ridge ... can turn Southeastward over top of ridge.

• MCS and low-level jet weaken toward dawn, but weak circulation and soggy ground can re-ignite an MCS the next night ... leftover outflow boundaries can set off daytime T-storms.

• Stationary MCS can be very dangerous ... Johnstown Flood (8" rain in 7 hours).

• A single MCS can produce 2 trillion gallons of water!

• MCS can also form in China, Autstralia and Argentina.

• Mesoscale Convective Complex (MCC): when T-Storm "clusters" merge into a "complex".