Squall Lines

  • Squall Lines are the most common form of MCS.
  • May be composed of ordinary cells and/or supercells. Supercells (if present) are found at the end of the lines. Supercells at the end of the line have the greatest potential to retain supercell status.
  • Common to find supercells at the southern end or at breaks in the line.
  • Bookend vortices may form along the north and south end of the line. These typically form 2 to 4 hours into the life cycle of the system. The dominant cyclonic vortex may persist beyond the life cycle of the squall line as a MCV.
  • Approximately 70% of squall lines forms from the merger of convective clusters.
  • Stronger sheared environments promote longer-lived, more severe lines.
  • Rear-Inflow Jet (RIT): the transport of convective cells rearward promotes the advection of warm air into the mid levels of the storms. At the same time, the cold pool builds rearward in response to increase precipitation. These two processes promote the development of the RIJ. The difference between the warmness of the updraft and the coolness of the cold pool influence the strength of the RIJ.
  • The descent of a portion of the rear inflow jet at the rear of the storm creates the wake low (mesolow). It is also responsible for the strong surface winds.
  • Increasing instability and mid-level dryness create a stronger cold pool and stronger winds.
  • Squall lines that form in environments of weak wind shear tend to weaken because the new cells tend to develop upshear of the gust front.
  • The component of the 0-3 km wind shear perpendicular to the line is the most important factor in the line's evolution.
  • LFC Height: When the LFC height is near 1.5 to 2.0 km, convection is harder to trigger. This requires greater wind shear. High LFC environments require more lifting at the leading edge of the cold pool. For any given wind shear and cold pool strength, a system is more likely to be stronger and longer-lived when the LFC height is low.
  • Squall Line Wind Shear: Weak < 20 knots, Moderate 20 to 35 knots, Strong > 35 knots.
  • Cold fronts can re-trigger a squall line even in an environment of modest wind shear.
  • Average cold pool speed is 40 knots
  • Squall Line Motion:
    • Individual cells tend to move consistent with the 0-6 km wind shear vector.
    • In lines longer than 200 km, individual cells move with the mean wind, but the line motion remains perpendicular to the line's initial orientation.
    • Shorter lines tend to reorient themselves with the low-level shear vector (and propagation direction of the individual cells).
  • Squall Line Life Cycle
    • first 1 to 2 hours the independent convective cells are arranged in a narrow line.
    • first 2 to 6 hours the solid line of convective cells along the leading edge are followed by a narrow region of weak precipitation followed by a broard area of stratiform precipitation.
    • first 4 to 8 hours the leading edge of convection weakens as the cold pool surges ahead (the stratiform region may persist for hours)
  • Progressive Derechos: form north of and parallel to a weak stationary boundary and assume a sligh angle towards the warm sector..
    • Nearly 85% of progressive derechos form in concert with 850 mb warm advection.
    • They move faster than the mean cloud wind because they are forward propagating at an average speed of 45 knots.
  • Serial Derechos: comprised of multiple bow echoes and typically located in the warm sectory of a synoptic-scale cyclone.
    • Serial derechos tend to occur in Spring/Fall and are associated with low pressure system/frontal boundaries.
    • They take the form of multiple bow echoes within a long squall line.
    • Average forward speed is approximately 30 knots or less.
  • Derechos rely upon the presence of sufficient instability for thunderstorms in a regime of significant warm air advection. A weak boundary parallel to the upper-level flow is another contributing factor. Development of MCS activity with derecho potential is most likely at the intersection of the low-level boundary and a low-level jet.

Bow Echoes

  • Non-transient crescent-shaped radar echo.
  • Bow echoes that form within a squall line are called line echo wave patterns (LEWP).
  • Form in environments of high low-level shear and CAPE > 2500 j/KG and 700mb winds > 17 m/s.
  • Characterized by tight leading-edge reflectivity gradient.
  • Rear Inflow Jet (RIJ) may be apparent as a weak reflectivity echo.
  • Book-end vortices typically form on each end of the structure. The features tend to focus and intensify the RIJ.
  • HP Supercells can evolve into a bow echo.
  • Severe bow echoes are most likely in moderate to strong low-level shear and high CAPE.
  • More likely to form when the Lifted Index (LI) is -8, and 700mb and 500mb winds are 17 to 18 m/s.
  • Bow echoes tend to propagate in the direction of the low-level shear and tend to move faster than nearby isolated convection.
  • Weakly-forced warm season systems that form along an east-west bounding with high CAPE.
  • Strongly-forced systems that form in the warm sector have a wider variety of CAPE values.
  • Environments that are favorable for derechos are characterized by large mid-level dewpoint depressions. (promotes strong cold pool formation).