Meteo 3: Chapter 14 Spawning severe weather Synoptically-forced storms Read Chapter 14

Thunderstorms arise from “deep convection”  INGREDIENTS FOR SEVERE WX  1) Warm, moist air in lower troposphere (sunshine usually precedes severe wx)  2) Instability (aided by cold air aloft)  3) Strong triggering mechanism (low pressure, fronts, upper-level trough)

Watches-  Severe Thunderstorm Watch: Issued by Storm Prediction Center when conditions are favorable for severe weather  Tornado Watch: Issued by SPC if conditions favor tornadoes  Warnings issued by local NWS offices if severe wx/tornadoes are observed or indicated by radar

Vertical Wind Shear  Vertical Wind Shear: A change in the speed and/or direction of the wind with height  If wind in direction of t-storm motion increases with height, the updraft tilts forward  Updraft and downdraft separate, prolonging t-storm life & allowing it to strengthen

Squall Line  Narrow, linear line of storms that develop to east of cold front in warm sector  Look for moisture convergence (fuel)…upper-level trough is the “spark”  Lasts 6-8 hours…weaken as they move in more stable environment

“State College Effect”

Derecho – damaging wind storm  Clusters of downbursts that produce a widespread, damaging wind storm –Associated with squall lines –Much larger area of wind damage than with microbursts  Bow echo on radar…intense wind at “surge region”

Derecho - St Louis July 19 th, 2006  90 mph wind gusts knocked out power to half a million St. Louis metro area residents  Radar loop showing gust front Radar loop showing gust front Radar loop showing gust front  Heat indices of degrees next day (what temperature feels like when moisture content is accounted for)

More on derechos  What makes a squall line w/ bow echoes a derecho? –1) Many reports of severe wind damage over an area whose major axis is >= 250 mi –2) Some damage as bad as seen with a weak tornado or reported gusts > 75 mph  Most triggered by mid-latitude cyclones in warm season…usually form along stationary front  Fast moving

Meteo 3: Chapter 15 Tornadoes Some material pulled from Zack Byko’s website website

Tornadoes!  Tornado: A violently rotating column of air extending from a cloud to the ground.  Tornado: A violently rotating column of air extending from a cloud to the ground.  Sometimes there’s a classic condensation funnel, other times only rotating dirt/debris on ground  Appearance depends on moisture & ground surface  Powerful tornadoes usually have condensation funnel

Importance of capping inversion

Synoptic setup for tornadic thunderstorms Tornado Alley example

Tornado formation  Typically form from Supercells - large, solitary, long-lasting thunderstorms with rotating updrafts (mesocyclone)

Tornado formation  Strong vertical wind shear creates ‘horizontal rolls’  Strong updrafts at rear of storm cell tilt horizontal roll vertically  Roll is tilted into mesocyclone, a rotating column of air sometimes found in severe storms

Tornado formation  Roll stretches vertically, spinning faster and faster (think conservation of angular momentum again!)

Tornado formation  If rotating column of air reaches ground, a tornado is born  Photos courtesy of the National Severe Storms Center Photos courtesy of the National Severe Storms Center Photos courtesy of the National Severe Storms Center

Cycloid damage path caused by suction vortices: how a tornado can destroy one house and spare the neighbors altogether

Suction Vortices

Fujita Scale  A scale measuring intensity of tornadoes  F0 – weakest -> F5 - strongest 

OK, REAL Tornado Safety   Final note: Always take heed of watches and warning, even though severe weather usually impacts a very small fraction of a watch area.

Whirlwinds that are not tornadoes  Dust Devils ▪ Waterspouts

Now, some video  h4Vo h4Vo h4Vo  tHEM tHEM tHEM  ctpFBz8 ctpFBz8 ctpFBz8  lSw lSw lSw