Chapter 11: Thunderstorms and Tornadoes

Chapter 15: Thunderstorms and Tornadoes

 40,000 thunderstorms occur everyday somewhere in the world.

 Thunderstorms produce  Heavy rain  High winds  Lightning & Thunder

 Hail  Tornadoes

 Thunderstorms occur as cumulonimbus clouds. Two kinds of thunderstorms

1. Air Mass 2. Severe

1. Air Mass Thunderstorm

 most common  least destructive  “self-extinguishing”  fairly upright  downdrafts cut off warm moist air to updraft that initially created thunderstorm Characteristics

 Usually live less than one hour  Form within air mass  Form away from frontal boundaries

Some definitions…

Cell – a single, individual thunderstorm

Updraft – a column of warm, moist, rising air in a thunderstorm

Downdraft – a column of cold, descending air in a thunderstorm Air Mass Thunderstorm Life Cycle

3 stages

I.cumulus stage II. mature stage III. dissipating stage I. Cumulus Stage

 Unstable, warm parcels rise

 Cu clouds form

 Cu 7-9 km deep

 Updraft speeds up to 20ms-1 (45 mph)

 Cu stage ends when precipitation begins to fall. I. Mature Stage  Begins when precipitation starts to fall  Falling precipitation drags on air around it to initiate downdraft  Updrafts and downdrafts, both distinct, dominate cloud  Lightning, thunder, precipitation most intense  Anvil forms, giving the thunderstorm its characteristic appearance. II. Dissipating Stage III. An increasing amount of the cloud ends up precipitating  So…an increasing amount of the cloud is represented by downdrafts  Dominant downdraft cuts off inflow to updraft  Bottom half precipitates out of evaporates  Top may persist a while

Severe Thunderstorm A thunderstorm that produces  Winds of at least 58 mph (50 knots, 93 km/h)  Hail of at least 3/4" (1.9 cm)  Tornadoes

This is the official definition of a severe thunderstorm…

***No accounting for rainfall***

Extreme flash flooding can eliminate a town or city - but it's not "severe." Any thunderstorm can become severe Criteria may be met for only a few minutes---hail, winds, even small tornadoes (“gustinadoes”)

BUT…

Severe thunderstorms usually have more organization that the average air mass thunderstorm.

Updraft and downdraft typically work together in a severe thunderstorm

Supercell Thunderstorm - a thunderstorm with a single, rotating updraft. Distinguished from air mass thunder-storm by organized updraft

Diameters: 20-50 km (12-30 miles) (Air mass thunderstorms typically smaller)

Smaller than organized thunderstorm systems (squall lines, mesoscale convection systems)

Lifetime: 2 to 4 hours (Air mass thunderstorms last < 1 hour)

Organized Thunderstorm Systems

Far larger than single air mass or supercell thunderstorm Mesoscale Convective Systems (MCS) - groups of individual thunderstorms clustered together

Individual thunderstorms come from a common origin (warm, moist, unstable air)

But consider when…

These favorable condition to form thunderstorms often occurs over large areas (10-1000 km)

In these regions, thunderstorms often lead directly to the formation of others

Two primary MCS types: 1. Squall line (line-type MCS) 2. Mesoscale Convective Complex (circular MCS)

1. Squall Line - Large number of individual thunderstorm cells arranged in a band.

 500 km (300 mi) long band (average)  10 hour life span (average)  usually parallel to and 300-500 km (180-300 mi) ahead of cold front.

Gust front - a cold, dense wedge of air that surges ahead of the squall line. . Formed by spreading out of downdraft air . Can initiate new or maintain current updrafts

2. The Mesoscale Convective Complex (MCC) - An MCS having certain satellite-based size & shape criteria

Shape - not a line---criteria require round or oval shape

Structure - more complex than squall Line

Downburst - a strong downdraft which induces an outburst of damaging winds on or near the ground  may be straight or curved, but always highly divergent

Macroburst - large downburst with outburst winds extending in excess of 4 km (>2.5 miles) in horizontal dimension.  Intense macrobursts can cause widespread, tornado-like damage  Damaging winds last 5 to 30 minutes  Can be as high as 60 m/sec (134 mph).

Microburst - small downburst with its outburst, damaging winds extending only 4 km (≤ 2.5 miles) or less.  intense microburst  winds as high as 75 m/sec (168 mph)

From UIUC

Microbursts pose a distinct danger to aircraft (esp. on landing)

From UIUC

Tornadoes Def - a rapidly rotating column of air with extreme horizontal winds o most often beneath a Cb cloud

Average Characteristics

Diameter 100 yards Path Length 2.5 miles Lifespan A few minutes Wind Speeds 40 – 280 mph Forward Speed 30 mph

Waterspouts Similar to a tornado, but over a water body - Usually in tropics, over warm water body

- Usually smaller than tornadoes . Diameter - 17-330 feet . Wind speeds ~ 90 mph

- Develop from Towering Cu

- Tropical waterspouts usually develop up from water surface

Funnel Cloud A tornado that has not yet reached the surface Tornadoes: When and Where

Everywhere in the world, all months, all hours, BUT…

- More in the U.S. than anywhere **Due largely to topography**

- > 1,000 tornadoes in U.S. annually

Florida and Oklahoma are the tornado capitals of the U.S.

Florida  Hurricane induced

May  largest number of tornadoes

Late afternoon generally preferred Missouri Tornadoes

 Late May – June  Western MO more prone

 Average 7 per year

 La Nina year, positive phase of Pacific Decadal Oscillation  up to 12 per year

Tornado Formation

Gustinadoes – outflow driven; bottom up Supercells – well-developed from wind shear changes; top down

Remember wind shear???

55 5000 feet

Rotation from shear becomes tilted into the vertical

Mesocyclone – large vortex up to 12 miles in diameter

 deep inside cloud

 several km above surface

 can precede tornado development up to 30 minutes

Tornado Damage “Equalizing pressure” by opening windows is a MYTH Fujita Scale - a damage scale used to rank tornado intensity

- most applicable to damage to residential homes

“F-4 tornado”  no such thing “A tornado that does F-4 damage”  that I’ll buy…

Severe Weather Safety

Watch – a statement from the National Weather Service (NWS) that says weather conditions are right for the formation of severe weather - covers large areas

- issued by the Storm Prediction Center

Q: What’s your job?

A: Keep an eye on the sky--- keep a radio or TV on.

Warning – a statement from the National Weather Service that says severe weather has developed or is imminent. - Usually covers one or several counties

- Issued by your local NWS Warning and Forecast Office (WFO) ***FIND SHELTER***

- Don’t open windows

- Go to an internal room on the lowest floor . Basement . Bathroom – pipes form a “cage” around you

- In a car, don’t try to outrun the storm

. A ditch is probably ideal . Underpasses are NOT necessarily safe