Squall Lines: Meteorology, Skywarn Spotting, & A Brief Look At The 18 June 2010 Derecho Gino Izzi National Weather Service, Chicago IL Outline • Meteorology 301: Squall lines – Brief review of thunderstorm basics – Squall lines – Squall line tornadoes – Mesovorticies • Storm spotting for squall lines • Brief Case Study of 18 June 2010 Event Thunderstorm Ingredients • Moisture – Gulf of Mexico most common source locally Thunderstorm Ingredients • Lifting Mechanism(s) – Fronts – Jet Streams – “other” boundaries – topography Thunderstorm Ingredients • Instability – Measure of potential for air to accelerate upward – CAPE: common variable used to quantify magnitude of instability < 1000: weak 1000-2000: moderate 2000-4000: strong 4000+: extreme Thunderstorms Thunderstorms • Moisture + Instability + Lift = Thunderstorms • What kind of thunderstorms? – Single Cell – Multicell/Squall Line – Supercells Thunderstorm Types • What determines T-storm Type? – Short/simplistic answer: CAPE vs Shear Thunderstorm Types • What determines T-storm Type? (Longer/more complex answer) – Lot we don’t know, other factors (besides CAPE/shear) include • Strength of forcing • Strength of CAP • Shear WRT to boundary • Other stuff Thunderstorm Types • Multi-cell squall lines most common type of severe thunderstorm type locally • Most common type of severe weather is damaging winds • Hail and brief tornadoes can occur with most the intense squall lines Squall Lines & Spotting Squall Line Terminology • Squall Line : a relatively narrow line of thunderstorms, often fast moving • Bow Echo: line of thunderstorms shaped like a “bow”, frequently produces damaging winds • Derecho: a fast moving line of thunderstorms that produces widespread, significant wind damage (long track, duration Squall Lines • Cold Frontal: – can extend across great distances (many states) – most common spring and fall Squall Lines • Cold pool driven (MCS) – most common late spring into summer – tends to occur in weaker forcing, thus often harder to forecast Squall Lines Shelf Cloud Squall Line Spotting • Big visual clue: Shelf Cloud • Shelf Clouds: – extend across entire sky – likely location of damaging winds Squall Line Spotting Squall Line Spotting Squall Line Spotting • Important to distinguish between shelf cloud and wall clouds • Difficult to remember the difference, very common mistake spotters make reporting shelf clouds as wall clouds • Shelf clouds Stretch across the horizon Squall Line Spotting Shelf Clouds Wall Clouds • Stretches across the • Localized lowering entire horizon • Associated with • Associated with supercells squall lines • If tornado forms, this • Primary hazard is is where damaging winds • Could rotate • Could rotate vertically horizontally Quiz Time! Wall or Shelf? Wall or Shelf? Wall or Shelf? Wall or Shelf? Wall or Shelf? Wall or Shelf? Squall Line Spotting So you have shelf cloud now what… • Expect the potential for damaging winds • Be sure you are in a safe location, your safety is important • If you can do so safely, measure winds • If not, do your best to estimate winds • Keep in mind tornado threat isn’t zero… Squall Line Tornadoes Squall Line Tornadoes • Tend to have shorter live spans than supercell counterparts • Very rarely can reach EF -2/EF -3 intensity, but majority are EF-0/EF-1 Squall Line Tornadoes • Very difficult to spot for multiple reasons: – Form quickly – Short lived – Move rapidly – Typically poor contrast and/or rain-wrapped • Not only difficult to spot, but can also put spotter potentially in harms way Mesovortices • Mesovortex: a circulation, usually along leading edge of bow echoes and squall lines, that often causes enhanced damaging winds and sometimes tornadoes • Relatively newly discovered phenomena • Our region has a high regional frequency Mesovortices • Most typically form along boundaries intersecting bows or in strong low level shear environment • Requires a “balanced” bow echo/squall line, not too cold pool dominant: – Fairly upright updrafts – Outflow not out-running leading edge Mesovortex vs. Mesocyclone •• In Supercells –– the rotating updraft is the mesocyclone and it’s located at back edge (SW) portion of storm –– Tornadoes in the back! Mesovortex vs. Mesocyclone •• QLCS’s ––rotationrotation (a mesovortex ) is located along the leading edge (E) portion of storm InflowMesovortices Notches –– Tornadoes in the front! Gust Front – Shelf Cloud Case studies of Squall Lines tornadoes & examples of mesovortices ((MVsMVs)… July 4, 2004 –Southwest Missouri • Two leading line MVs… • This was the first case that got me interested in MVs • 1 tornado, but extreme wind damage in a “thin” swath February 6, 2008 –Central Kentucky • Another example of a leading-line MV • This squall line produced 16 tornadoes across central Kentucky (overnight in the winter!) • 12 tornadoes in 1 hour! October 24, 2001 –Northern Indiana • Normally, MVs are leading-line, low-level circulations • But a few MVs during this event were well behind leading edge, wrapped in heavy rain • No visual “heads up” = much more dangerous • The 2 nd largest TOR outbreak in IWX CWA! Base reflectivity – rear Base storm-relative inflow identified velocity – MVs identified Challenges With Mesovortices • Still new to us, so meteorologists have a lot more to learn: – How do they form? – Why do some last so long, others brief? – Why do some produce tornadoes while most don’t? • Tend to form (and dissipate) very quickly – Can go from non-detectable on WSR-88D one volume scan to tornado producing the next Challenges With Mesovortices • Often smaller/shallower than supercell mesocyclones hard to see at > distances • Not all mesovortices produce tornadoes and/or wind damage • Hard to tell on radar which will produce damaging winds, tornadoes, or neither Challenges With Mesovortices • Pose big challenges, still much debate on how to warn for them: – Big tornado warning for entire squall line or bow echo? – Severe thunderstorm warning w/hard hitting wording? – Multiple small/short tornado warnings? QLCS Damage The 18 June 2010 Derecho What Happened • Bow echo produced widespread wind damage from MS River across all N IL/N IN/S Lower MI – Numerous trees (1000s in Chicago alone), powerlines downed, w/some structural damage – Hundreds of thousands households lost power – Many reports 70+ mph winds, including M77 mph @ Chicago Crib (lakefront) – Multiple injuries, 1 fatality in Chicago area Meteorological Set-up • Strong heating of moist airmass led to strong potential instability in advance of MCV • Somewhat modest deep layer shear likely augmented by stronger flow w/MCV • Somewhat marginal shear (esp. LL) weaker, more transient MV’s, few tornadoes RUC MLCAPE 20z 0 Hour Forecast >2500 J/Kg Mesovorticies • Relatively few/weak TMDW VWP MV’s with bow – 0-3km shear was weak – No well defined boundaries intersecting bow – Appeared to be more RIJ dominant Summary • RIJ dominant bow produced widespread wind damage, locally significant, despite lack of MVs • Timing & antecedent Wx led to potential danger of bow for Chicago Metro Summary • Synoptically evident event, led to accurate short term forecasts • Challenge leading up to event was conveying potential danger to decision makers/public… Questions? [email protected] © Walker Ashley.