Severe

AT 351 Lab 10 April 2, 2008

Weather Fatalities - 2006

1 Types of

 Hail   Flood  Tornado  Severe (Straight-Line )

Thunderstorm distribution

2 Favorable Conditions

 Instability  Restricting Cap  Initial Lift  Shear  Fuel

Instability

 You know this part already  Steep  Means warm, moist air near the surface  Colder air above it  Needs to be pulled from a sounding

3 Sources of Lift

 Convective lifting  Boundaries  Fronts  Drylines  Outflow boundaries  Orographic  Convergence

Shear

 Because of the way a works, it needs to be tilted to remain strong  Therefore, winds need to change with height  Two kinds of shear  Speed Shear: Wind is faster as you go up  Directional Shear: Wind changes direction with height

4 Vertical

 Change of wind speed and/or direction with height  Weak vertical wind shear: short-lived since rainy downdraft quickly undercuts and chokes off the updraft  Sheared environments are associated with organized

Vertical Wind Shear

5 Fuel

 Just like anything else, a storm needs fuel to survive  The fuel for a storm is just a continued supply of what started it  Storm needs to remain in areas of warm, moist air  If storm moves into a colder region, it will die

Restricting Cap

 If the atmosphere is unstable all the way up, you get a constant updraft  More effective is when the energy is held back and released all at once  Stable layer near the surface that supresses convection  As ground heats during the day, energy builds up until it can “break the cap”  Also called “capping ”  Remember CIN?

6 Back to the Skew-T

 Meteorologists have made up a lot of numbers that are designed to tell how favorable the weather is for a storm  Some deal with instability  Some deal with shear  Some deal with both  Let’s discuss a few

Remember CAPE?

 Convective Available Potential Energy  Instability is great for convection  CAPE measures how unstable it is, and over what depth  It is an integrated quantity that tells how much energy is available for the storm

7

 A very simple number  Compares actual at 500 mb to theoretical parcel temperature  If actual temperature is colder: unstable  LI = Parcel – Environmental  If LI < -6, considered very favorable

8 Severe WEAther Threat

 A single number that takes a lot of other things into account  Temperature  Dewpoint  Stability  Wind Shear  SWT > 400 = very favorable

Thunderstorms

 Form in unstable environment (typically in mT air mass)  Triggered by  Surface heating  Terrain  Frontal boundaries  Important for heat and energy balance  Act to stabilize the atmosphere

9 Types of Thunderstorms

 Thunderstorms come in many varieties  Likelihood of severity proportional to storm lifetime  NWS definition of severe (one or more of the following elements  ¾” or larger diameter hail  50 kt or greater winds  tornadoes

Thunderstorm ingredients

 Triggering mechanism  Large  Low-level moisture  Vertical shear

10 Single cell thunderstorms  Also referred to as ordinary, pulse, or air mass thunderstorms  Typically do not produce severe weather  Three stages  Cumulus  Mature  Dissipating  Life span: ~45-60 min.

Cumulus Stage

 Warm moist air rises, condenses  Latent heat release keeps air in warmer than environment  Grows to a towering Cu  Cloud particles grow larger, begin to fall  No at surface

11 Mature Stage

 Marked by appearance of downdraft  Falling cloud drops evaporate, cooling the air  Storm is most intense during this stage  Cloud begins to form anvil  May have an overshooting top  Lightning and thunder may be present  Gust front forms  Downdraft reaches the surface and spreads out in all directions  Gust front forces more warm, humid air into the storm

Downdraft

 Like the exhaust of a car, the storm needs to get rid of the cold air that it took the energy from  Cold air plunges to the ground and spreads out  Creates “gust front” or “outflow boundary”  Can provide lift for new storms

12 Entrainment

 Downdrafts are cool not only because of evaporation of precipitation but also due to entrainment of cooler, unsaturated air from outside of the thunderstorm

13 Downbursts  Strong downdrafts  Microburst: diameter of downdraft is 4 km or less (sub-thunderstorm scale outflow feature)  Macroburst: diameter greater than 4 km (on scale of entire cold air outflow field)  Produce winds comparable to weak tornadoes

Downbursts

14 Gust fronts

 An area of high created at the surface by cold heavy pool of air from downdraft called a mesohigh  Gust front: leading edge of cold air from downdraft  Passage noted by calm winds followed by gusty winds and a temperature drop then precipitation  Convergence region between cold outflow and warm, moist inflow

Gust fronts

 Can generate new cells  Leads to multi-cell storms  Production of shelf and roll

15 Overshooting tops

16 Overshooting tops

Dissipation Stage

 Usually follows mature stage by ~15-30 min  Gust front moves out away from the storm, and most air is no longer lifted into the storm.  Downdrafts become dominant  Low level cloud drops can evaporate rapidly, leaving only the anvil as evidence of the storms existence

17 Types of Storms

 Single Cell  Multi-Cell  Bow Echo  Squall Line 

Single Cell Storms

 Normally weak and short lived  Rarely severe  Pose a relatively low risk to the public

18 Multi-Cell Storms

 Cluster of storms moving as a single unit  Stronger wind shear than the ordinary cell case  More organized multi-cells  Bow Echoes  Squall Lines  New cells tend to form on the upwind (W or SW) edge of the cluster, with mature cells located at center and dissipating cells found along the downwind (E or NE) portion of the cluster  Updraft competition for warm, moist low-level air so not incredibly strong and have short life spans

Multicell Storms  Cell 1 dissipates at time=0 while cell 2 matures and becomes dominant  Cell 2 drops heaviest precipitation at time=10 as cell 3 strengthens  Severe multicell storms typically produce a brief period of hail and/or downbursts during and immediately after the strongest updraft stage

19 Dissipation Mature Growing

Multicell Thunderstorm

Flash Floods Producer

20 Flash Floods  Except for heat related fatalities, more deaths occur from flooding than any other weather hazard  Most are vehicle-related  Occur from thunderstorms that repeatedly move over the same area or heavy rains from tropical cyclones

Flood Safety  DROWNING IS THE NUMBER ONE CAUSE OF FLOOD DEATHS  DO NOT walk through flowing water  DO NOT drive through a flooded area  STAY AWAY from power lines

21 Big Thompson Flood

 July 31, 1976  In canyon of Big Thompson River, received as much as 30.5 cm (12”) of rain in 4 hours (from 6:30 – 10:30pm)  This area usually only receives about 40.5 cm (16”) of rain in an entire year  Killed 145 people

Big Thompson Flood  Synoptic conditions  Cold front moved through earlier in day, remaining to the S near Denver  Weak inversion associated with the front kept cumulus from growing earlier in the afternoon  Strong SE flow behind cold front pushed unusually moist air upslope along mountain range  Moist, conditionally unstable air penetrated inversion and grew to large multicell storms  Extremely weak upper level winds

22 Big Thompson Flood

Squall Lines  Multicell storms can form as a line of storms extending for hundreds of km, called a squall line  Squall lines often form along or just ahead of a cold frontal boundary (called pre-frontal squall lines)  may be embedded within prefrontal squall lines  Leading line of thunderstorms may be followed by large region of stratiform precipitation where the anvil cloud trails behind the main storm.

23 Squall Lines  Solid or broken line of thunderstorms with a continuous, well-developed gust front on the leading edge  New updrafts form on downwind side, where line is moving into unstable inflow air

Squall Lines

• May be triggered as a line or may organize into a line from a cluster of cells • For a given CAPE, strength and longevity increases with increasing depth and strength of vertical wind shear

24 Squall Lines

 When vertical wind shear is strong and deep, the squall line may be composed primarily of supercells  Southern end of a mature squall line and breaks in the leading line of convection are favored locations for supercells

Bow Echoes

 Bow Echo – a bowed convective line (25 – 150 km long) with a cyclonic circulation at the northern end and an anticyclonic circulation at the southern end (Fujita)  Can produce long swaths of damaging winds  Form in conditions of large instability and strong low level shear  Observed both as isolated convective systems or as substructures within much larger convective systems (such as a squall line)

25 Bow Echoes

 Storm develops with strong jet streak coming in from behind  Bows the storm out forward  Can contain very strong winds and potentially tornadoes

Bow Echoes

26 Squall Lines

 A bow echo is part of a larger group called a squall line  Refers to a strong line of storms that have outrun the cold front or dry line that initiated them

Mesoscale Convective Complexes

• Individual storms can grow and organize into a large convective system (weak upper level winds) • can grow to the size of a state • moves slowly (less than 20 kt) • life span greater than 12 hours • New storms grow as older ones dissipate • Provide widespread precipitation • Can spawn severe weather • hail, high winds, flash floods, tornadoes • Formation • usually during summer when a cold front stalls beneath an upper level ridge of high pressure • surface heating and moisture can generate thunderstorms on the cool side of the front

27 Severe Thunderstorms

 You often hear about “severe thunderstorms”, but what makes them severe?  Well, there actually are criteria:  Winds of 50 kts (58 mph) or greater  Hail at least 0.75” in diameter (dime sized)  Tornado  Any one of these makes the storm severe

Watch/Warning

 Surely you already know this, but I’ll tell you anyway  Watch – Conditions are ripe for something to happen  Warning – Something is already happening or very likely to happen soon

28 Lightning

 Inside a cloud, updrafts and turbulence toss ice particles around  Each collision creates a small amount of electric charge  After a few million of those, the charge is too much to be held back by the air  Discharges all at once in a flash of lightning

29 Thunder

 If air is heated from 75 to 90 degrees, it will expand  If air is heated from 75 to 50,000 degrees, it will expand quickly  Thunder is a compression wave due to this rapid heating  The thunder you hear is not lightning “hitting the ground” but actually a sonic boom

A couple of lightning misconceptions  Lightning comes down from the clouds  It actually comes down AND goes up.  As a bolt begins the trip down, a “streamer” from the ground shoots upward toward the oppositely charged cloud.  The flash happens when they meet in the middle.  Entire process happens in under 0.001 seconds

30 A couple of lightning misconceptions  Lightning always hits the tallest object  Not true. It may seem that way, but lightning simply takes the “path of least resistance”.  If you conduct electricity better than the 30 ft. tall tree next to you, you will get hit

A couple of lightning misconceptions  Lightning never hits the same place twice  That’s just wrong.  There are many documented cases of lightning hitting twice in the same spot  Sometimes only a few seconds apart!

31 Lightning Facts

 The temperature of lightning is roughly 30,000 degrees C  The surface of the sun is only about 5700 degrees C  One bolt of lightning carries enough electricity to power the entire United States for 0.1 seconds  Lightning has been known to strike up to 15 miles from the actual storm

Why does lightning “flicker”

 Channel where flash occurred favorable location for another stroke  Already ionized  Already broke down the air  Second leader propagating downward in this channel  Dart leader because it proceeds more quickly, resistance of path lower  Second return stroke in this path  Happen over less than a second, our eyes cannot resolve these individual strokes

32 Why does Thunder sometimes occur after lighting, or not at all?  Speed of sound slower than speed of light  If strike is not right next to us, thunder will occur later  Atmosphere may bend the sound wave  Eddies may scatter sound waves  Depending on how far away from the strike you are, you may not hear the thunder

Some Math

 There are about 20,000,000 cloud to ground lightning strokes every year in the US  That’s 2,000,000 seconds of power for the whole country = 23 days  If we could harness it, we could thus become 6% more energy independent  Figure it out, and you’ll be rich beyond belief

33 Lightning Fatalities

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