Thunderstorm Basics Updraft Characteristics (Rising Air)

Cumulus Tower Upward Cloud Motion

Updraft Dominant Downdraft/Updraft Downdraft Dominant Rain-Free Base

Downdraft Characteristics (Air Descends to the Ground)

Known as the “Front” Side of Storm

Wall Cloud located beneath Updraft Downward Motion Rain-Free Base

Wall Cloud

Rainfall Area/Dark Area of Storm

Wall Clouds Can Occur with

Severe and Non-Severe Storms Downburst/Hail Threat in Downdraft Area Severe Thunderstorm Updrafts at a Distance

Overshooting tops are often

associated with severe

thunderstorm anvils

Back-sheared Anvil Overshooting Top

Hard, Solid appearance, not thin or wispy

Thunderstorm anvils (top of the storm) are

often back-sheared

ThunderstormThunderstorm Types which Fundamentals impact Iowa Multi-Cell Clusters • Severe or non-severe • Common in Iowa • Updrafts tend to be concentrated on the south side of the complex MultiThunderstorm-Cell Cluster Diagram Fundamentals• Produce the most flash flood events

National Weather Service Squall LinesDes Moines, IA • Damaging Thunderstorm Fundamentals winds (Primary threat) • Heavy rain • Hail • Weak Storm Movement tornadoes

Courtesy Rod Donavon Courtesy Mindy Beerends National Weather Service Shelf Cloud Des Moines, IA • Wedge shape cloud (attached) • Leading edge of storm Shelf Cloud • Suggests outflow (downdraft) National Weather Service Des Moines, IA ThunderstormThunderstorm Types which impactFundamentals Iowa (cont.) Supercell Characteristics NE • Persistent rotating updraft through a great depth of the storm

updraft downdraft • Typically counter- clockwise rotation looking into storm from some distance • Responsible for a high Courtesy Extreme Instability percentage of severe SupercellThunderstormThunderstorm Fundamentalsweather Thunderstorm Fundamentals National Weather Service N Light Rain Des Moines, IA

Moderate/Heavy Rain & Small Hail Large hail Gust Front Tornado Threat Strong Wind Hook echo Possible Damaging Wind Courtesy extremeinstability.com Large hail Visual Rotation Clues Anvil Edge

Spiral bands and Tornado Threat Supercell Thunderstorm striations in updraft (top view)

tower are clues that the Hook echo 0 5 10 updraft is rotating Nautical miles WSR-88D Radar Image National Weather Service Des Moines, IA “Classic” Supercell Rotating Updraft on the LEFT REAR Flank of the Storm National Weather Service Des Moines, IA Thunderstorm Fundamentals High Precipitation (HP) Supercell • HP Supercells are common in Iowa • HPs are difficult to spot due to large amount of precipitation Courtesy extremeinstability.com • Close observation Mid-level updraft striations dangerousThunderstorm due to Fundamentals are common in HP supercells obscuration National Weather Service HP Supercell Des Moines, IA N

Light Rain Tornado is often rain- Looking west wrapped

Heavy Rain

Updraft

Anvil Edge

Top view

Generally, there is not a good spot to view this type of storm since important features are often rain-wrapped National Weather Service Des Moines, IA Thunderstorm Fundamentals Tornado Pre-cursors: Clouds and Conditions Supercell Wall Cloud Rotating Wall Clouds Characteristics •Localized, persistent Clear Slot lowering of the rain-free base • Usually long lived and Funnel Clouds Courtesy Storm Effects rotating • Upward vertical motion often present • Usually forms close to the updraft/downdraft interface RotatingTornadoes Wall Cloud Rain-free Base National Weather Service Rear FlankDes Moines, IA Downdraft or “Clear Slot” Visually the RFD Funnel Clouds shows up as a A rotating, funnel-shaped cloud extending “clear slot” downward from a thunderstorm base southwest of the Attached to cloud base wall cloud Exhibit rapid rotation and are most often laminar or smooth in appearance Courtesy Simon Brewer Do not reach the ground Often form just before a tornado! Usually vertically-oriented National Weather Service Des Moines, IA Large Tornado - Large, violent tornadoes often resemble Classic Tornado - easier to spot, these tornadoes Landspout Tornado - common in Iowa. These tor- a “wedge.” They can be mistaken for a large cloud at are not rain-wrapped. They often resemble an nadoes tend to be small and short-lived. They are close distances. (Parkersburg - May 2008) elephant trunk.(Southern Iowa - June 2010) not associated with a supercell updraft.

Courtesy Nolan Atkins (Lyndon State College) Tornado Reporting Guidelines Who? (spotter number/source) What? (funnel or tornado) Where? (reference nearest city): - Can use Latitude/Longitude When? (time of event)

Movement (direction and speed) Invisible Condensation Funnel - it is still a Damage? tornado! Watch for dust or debris under the funnel cloud Use Proper Terms and be as specific as possible

Rain-Wrapped Tornadoes are especially dangerous. They are common with HP Supercell thunderstorms. False Tornadoes

Courtesy Allan Detrich

Gustnado - are not attached to the cloud base (extend to the cloud base), they are not tornadoes. They occur along gust fronts.

Scud - can occur under any thunderstorm base. Cloud fragments that can look ominous and can resemble a tornado.

“Virga” - Rain not reaching the ground Rain Shafts - Downdrafts which can resemble tornadoes Damaging Winds

Damaging Winds: Small Scale (winds can exceed 80 mph): Brief high winds events over a small area Associated with small, but intense downdrafts Large Scale (winds can exceed 90 to 100 mph): Extreme wind events over a large scale Can cause widespread damage Associated with “squall lines”

Courtesy Mike Zeamer Rain or Dust Foot A rain or dust footSevere suggest localized Hail intense winds

National Weather Service Des Moines, IA Severe Hail in a Supercell Thunderstorm Severe Hail •LargeOften Hail: occurs near the Courtesy Extreme Instability Large and destructive hail is almost updraft/downdraftalways associated with a supercell interface • Avoidthunderstorm this area if possible! Occurs in the downdraft Large hail is common near the updraft/downdraft interface

Hail Zone Rotating Wall Cloud A hail shaft can occasionallyCourtesy be Will Hark seen National Weather Serviceas a white area in the downdraft National Weather Service Des Moines, IA Des Moines, IA Hail Sizes:

1” or larger = Severe Hail

2.75” 3.00” Baseball Tea Cup

4.25”

Softball

2.50” 2.00” Tennis Ball Hen Egg

Non - Severe Hail 1.75” 1.50” 1.25” 1.00” 0.88” 0.75” Golf Ball Ping Pong Ball Half Dollar 0.50” Quarter Nickel Penny 0.25” Pea Severe Weather Reporting

Flash Flooding: Damaging Winds/Hail*: Additional Items to Report: Rainfall Measurements during heavy rain events Who? Snow Amounts Creeks, Rivers, or Streams out of their banks What? (damage and/or measured speeds over 40 mph) Ice Amounts Water covering roads or affecting property Where? (reference nearest city) Blizzard Conditions Additional Details: Can use latitude/longitude Bad Roads Is the water standing or flowing? When? (time of event) Ice Jams Estimated Depth of water * Do not use the term “marble” to report hail!

Spotter Safety

Overall Spotter Safety: Severe Thunderstorm Safety: Lightning Safety: Keep an eye to the sky Watch out for Severe Thunderstorm Winds Avoid tall objects Prepare for all hazards Watch out for Large Hail Be careful on ridge tops or in open areas Watch for lightning Avoid downdraft area of storm Stay in vehicle if mobile Watch for flash flooding Updraft/Downdraft interface is a dangerous Hear Thunder? You are at risk! Drive smart & safely location Use common sense

Tornado Safety: Flash Flood Safety: Situational Awareness #1 weather-related killer in the country! Seek sturdy structure Turn Around, Don’t Drown! Have an “escape” route Be Mindful of: Avoid “tunnel vision” Heavy thunderstorm rain Night spotting is very dangerous! Wet, saturated soils If your car is struck by even a weak tornado Quicker impact in steep terrain your life is in danger!