Wilmington's Spotter Guide Updated: December, 2004

The Role of a Storm Spotter

The National Weather Service (NWS) office in Wilmington, Ohio uses Doppler radar and satellite pictures as the primary technology to detect severe weather.  However, the most important tool for observing thunderstorms is the trained eye of the storm spotter.  Doppler radar provides the motion of precipitation and of air inside a storm, but will not show the wind at ground level.  It is impossible for any radar to detect every severe weather event in its coverage area, and radar occasionally suggests severe weather when, in fact, none is present. As a trained spotter, you perform an invaluable service for the NWS.  Your real-time "ground truth" observations of tornadoes, hail, wind, and significant cloud formations provide a truly reliable information base for severe weather detection and verification.  By providing observations, you are assisting the meteorologists at Wilmington in their warning decisions, and enabling the NWS to fulfill its mission of protecting life and property.  Your spotter report can literally mean the difference between life and death!

Background and Safety tips

The NWS in Wilmington does not recommend chasing storms!!!  Our goal is to have enough storm spotters in each county to relay frequent reliable information from the safety of their homes or business directly to our office. We issue severe weather warnings for 52 counties across the southwest half of Ohio, northern Kentucky, and southeast and east central Indiana.  We have over 3000 spotters, which is an average of about 60 per county.  Of course, there are more spotters in the metropolitan areas of Cincinnati, Dayton, and Columbus.  Figure 1 shows the distribution of most of the spotters across Wilmington's county warning area.

If you happen to be caught outside during a thunderstorm, lightning is your number one threat.  Lightning is very dangerous due to its unpredictable behavior and its frequency.  Keep in mind that although lightning is very dangerous, it is not part of the definition of a severe thunderstorm.  Remember that every thunderstorm contains lightning, like figure 2, but typically the stronger storms will contain more frequent lightning.  The safest place to be is in a building.  The next safest place to be is a vehicle.  The vehicle carries the electrical current through the frame of your car, including your tires, to the ground.  If caught outside away from shelter during a thunderstorm, find a low spot away from trees, fences, and poles.  If you are in the woods, take shelter under the shorter trees.  If you feel your skin tingle or your hair stand on end, squat low to the ground on the balls of your feet similar to a baseball catcher (figure 3).  The soles of your shoes will provide some (minimal) protection from the potential electric current flowing through the earth.  Place your hands on your knees with your head between them.  Make yourself the smallest target, and minimize contact with the ground.

Figure 1 - Green (*) show the distribution of Spotters across NWS Wilmington, Ohio Forecast Area. 

Figure 2 - Time-lapse photo captures multiple cloud-to-ground lightning strokes during a night-time thunderstorm. Photo courtesy of  NSSL Photo library.

Figure 3 - Position you should take if caught out doors during a thunderstorm.  Photo courtesy of  NSSL Photo library.

Weather Information to Report

Please report any of the following events to the Wilmington National Weather Service Office . (Please use the (unlisted) severe weather reporting number.)

Significant or severe weather:

1. Tornadoes (figure 4)
2. Funnel Clouds
3. Wall Clouds
4. Persistent rotation in clouds
5. Hail 1/2 inch in diameter or larger
6. One tree uprooted or downed
7. More than 1 large limb downed
8. One inch or more of rain in 1 hour (measured)

Flood/Flash flooding:

1. Inch or more measured in an hour or less.
2. Flooding that results in evacuations
3. High water rescues from roofs/trees
4. Cars and trailers carried away by flood waters
5. Water covering roads.  Please indicate whether it was moving. (Figure 5 shows the aftermath of a flash flood in Clermont county)
6. Water entering homes (not just basements)

Any urban and/or small stream flooding:

1. Roads closed due to high water
2. Roads impassible due to high water
3. Small streams overflowing their banks

Report the following winter weather:

1. > 1" of snow per hour
2. > 1" of snow during the past 24 hours
3. Total Snow Depth (Figure 6 shows 13" of snow from Valentines weekend storm(2003).)
4. When you first measure 2”...4” and 6”
5. Any freezing rain/drizzle
6. Thunder/lightning associated with snow/sleet/freezing rain

When making a report, include the following information:

1. Your name and spotter ID (don't use if reporting via Ham Radio)
2. Location and time of event
3. What you saw\damage witnessed

Figure 4 - Oklahoma tornado (May, 1999).  Photo courtesy of  NSSL Photo library.

Figure 5 - Flash flooding in Clermont county (July, 2001), caused this bridge to collapse.  

Figure 6 - Valentine's weekend storm (February, 2003) produced thirteen inches of snow in Warren county, Ohio.   Photo courtesy of Robin Gerhardt.

Thunderstorm Types

The single cell

The single cell thunderstorm has a lifetime of only 20-30 minutes.  They usually do not produce severe weather and can be thought of as a "garden variety" type storm, such as Figure 7.  They typically occur on a day with little wind shear.  Some single cell storms with stronger updrafts may produce brief severe weather events.  These storms are capable of producing marginal severe hail and/or brief downbursts. 

 

The multi-cell storm

Multi-cell storms are the most common type of thunderstorm.  It is simply a collection of single cell storms.  Sometimes these multi-cells are in clusters,  while other times they are organized in lines (commonly referred to as a squall line). Figure 8 shows a squall line approaching from the west.  The squall line is approximately 50 miles away.  Each cell in a multi-cell storm may last only 20 minutes but the organized multi-cell storm may persist for many hours.  These storms are more intense than single cell storms but are weaker than super cells.  Multi-cell storms can produce very heavy rainfall, downburst winds up to 80 mph, hail the size of golf balls, and occasionally weak tornadoes.  Figure 9 shows a squall line about 5 miles away, approaching from the west (right) with a shelf cloud denoting the gust front.   The gust front marks the leading edge of gusty winds from the thunderstorm downdraft.  Near and just behind the gust front is the location for severe straight-line winds. The heaviest rain and largest hail also occurs  shortly behind the gust front.  The severe weather most associated with a squall line is damaging straight-line winds (downburst). 

 

The supercell storm

Supercells are quite rare across northern Kentucky, the southwest half of Ohio, and southeast and east central Indiana.  However, when they do occur they pose an extremely high threat to life and property.  What sets the supercell storm apart from the others is its strong rotation.  The intense updraft of the storm rotates and is called a mesocyclone.  This is the most likely location for tornado formation.  A supercell storm can produce all types of severe weather with giant hail more than 2 inches in diameter, powerful downbursts of 100 MPH or more, and strong to violent tornadoes all being possible.  Figure 10 shows what a supercell look like from about 50 miles away.  Two visual clues of an approaching supercell are the anvil and overshooting tops (both are denoted in yellow).  The storm is located to the west. Figure 11 shows what it looks like under the rain-free base, from about a half mile away, of a supercell with a funnel, wall cloud, and inflow (looking southwest).

Figure 7 -  Single cell thunderstorm.

Figure 8 - Multi-cell thunderstorms (Squall Line).  Photo courtesy of  NSSL Photo library.

Figure 9 - Squall line approaching from the west.  Shelf cloud denotes where the gust front is located (right above yellow line) approaching from the right (west). Image is looking south.  Photo courtesy of  NSSL Photo library.

Figure 10 - Supercell is located 50 miles to the west.  Note, anvil and overshooting tops denoted in yellow.  Photo courtesy of  NSSL Photo library.

Figure 11 - Likely picture from the rain free base underneath Supercell above.  Photo courtesy of Tim Marshall.

Important Clouds

Wall cloud

A wall cloud (figure 12) is defined as an isolated cloud lowering attached to the rain-free base.  The wall cloud is generally to the rear (mainly south or southwest) of the visible precipitation area.  Wall clouds are usually about 2 miles in diameter (can range from a  fraction of a mile up to 5 miles), and mark the strongest updraft in the storm.  When seen from within several miles, many wall clouds exhibit rapid upward motion and counterclockwise rotation.  However, not all wall clouds rotate.  Rotating wall clouds usually precede  strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour.  Wall clouds should be watched for signs of persistent, sustained rotation and/or rapid vertical motion. 

Shelf clouds and roll clouds

Shelf clouds and roll clouds are examples of "accessory clouds" that you may see beneath the cloud base of a storm (figure 13).  Shelf clouds are long wedge-shaped clouds attached to the parent cloud above (i.e. thunderstorm).  The shelf cloud is associated with the gust front.  Rising cloud motion often can be seen in the outer part of the shelf cloud, while the underside often appears turbulent, boiling, and wind torn.  A roll cloud is a low horizontal tube-shaped cloud associated with a gust front.  Roll clouds are relatively rare.  They are completely detached from the thunderstorm base or other cloud features.  Roll clouds usually appear to be "rolling" about a horizontal axis, but should not be confused with funnel clouds.  The possibility of a tornado forming beneath a shelf or roll cloud is quite small.  The main threat is damaging straight-line winds.

Wall cloud vs. shelf cloud

A big challenge as a spotter will be to discern  between a shelf cloud and a bonafide wall cloud. The table below will help to distinguish between the two.
 

Figure 12 - Wall cloud over central Minnesota.  Yellow line denotes wall cloud.  Photo courtesy of  NWS Minneapolis.

Figure 13 - Shelf Cloud (Yellow line).  The Shelf Cloud denotes the difference between cool moist air associated with the downdraft (rain cooled air) and the warm muggy air surrounding the thunderstorm which is feeding the updraft. Photo courtesy of  NSSL Photo library.

Figure 14 - Shelf Cloud with streamlines showing air flow in and around the Shelf Cloud.  Red denoting upward motion ahead of Shelf cloud.  Blue denoting downward motion behind Shelf cloud.  Remember a Shelf Cloud suggest downdraft and outflow.  It moves away from the precipitation area and slopes downward and away from the precipitation area.  Photo courtesy of  NSSL Photo library.

Figure 15 - Wall Cloud with streamlines (red) showing air flow in and around the Wall Cloud.  Remember a Wall Cloud suggests updraft and inflow.  It maintains its position with respect to the precipitation area and slopes upward and away from the precipitation area.  Photo courtesy of  NSSL Photo library.

• Suggest downdraft and outflow
• Move away from precipitation area
• Slope downward and away from precipitation

• Suggest updraft and inflow
• Maintain position with respect to precipitation area
• Slope upward and away from precipitation

Many spotters in this part of the country may not see a legitimate wall cloud for a few years or longer, and only a few of these wall clouds will actually produce tornadoes.  Your main clue as to the tornado potential of the wall cloud will be its rotation, especially if it is persistent.  Another clue of tornadic potential will be if you notice vertical motion, especially rapid vertical motion.

Tornado/Funnel Cloud Look-alikes

Experienced spotters are probably aware that a number of features (both natural and man-made) can bear a resemblance to a tornado or a funnel cloud.  Some of these features include virga, rain shafts, scud clouds, mammatus clouds, and smoke.  By far, the most commonly mistaken funnel clouds are scud clouds.  Shelf clouds and roll clouds can also be mistaken for funnel clouds and/or tornadoes.

Scud clouds 

Scud clouds are small, ragged, low cloud fragments that are unattached to a larger cloud base (Figure 16).  These clouds are often seen behind thunderstorm gust fronts.  These clouds are generally associated with cool moist air, such as thunderstorm outflow.

Virga

Virga is streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.  In certain cases, shafts of virga may precede a downburst.

Rain Shaft

A rain shaft is streaks of precipitation falling from a cloud and reaching the ground (Figure 17).

Mammatus clouds

Mammatus clouds (Figure 18) are rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil).  Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather.  They may accompany non-severe storms as well.

Smoke

A smokestack or fire can produce a plume of smoke that sometimes resembles a tornado, especially from a distance.  Unlike a tornado, the bottom of the smoke will not be rotating. 

To distinguish a tornado look-alike from a legitimate tornado, a spotter must look for organized and sustained rotation about a nearly vertical axis.

Figure 16 - Scud Cloud is the lower hanging cloud located just below the yellow text.  It could easily be mistaken for a wall cloud or a tornado.   Study the cloud feature to make sure it is what you think it is.  Photo courtesy of  NSSL Photo library.

Figure 17 - A rain shaft is located just above the yellow text.  These features can resemble a tornado.  Rain Shafts will change shape and move around.  If you see a feature like this study it for a few minutes and see if it changes shape. Photo courtesy of  NSSL Photo library.

Figure 18 - Photo of Mammatus Clouds near Wilmington after thunderstorms.  Photo courtesy of Mark Duquette.

Regional severe weather statistics

Since 2000, severe straight-line winds and large hail have proved to be much more common than tornadoes. The area has seen 1338 severe wind and hail events and only 24 tornadoes. So tornadoes account for only about 2 percent of our severe weather. Over the same time frame, the area saw 441 flash flood events. This shows we experience flooding about 15 times as often as tornadoes. On many occasions, straight-line wind damage is mistaken for a tornado.  Whenever possible, after widespread damage or suspected tornadic damage the staff at our office will perform a damage survey in the area hardest hit.  We will do an aerial survey of the damage when an aircraft is available, but  in most cases a  ground survey is done.

Watch versus Warning

Watch:
A NWS product which specifies locations in which conditions are favorable for hazardous (i.e. severe weather, winter storm, etc.) to develop.  A watch is a recommendation for planning, preparation, and increased awareness.  Watches typically cover periods of 6 to 12 hours for many counties.  Watches are issued for tornadoes, severe thunderstorms, floods, flash floods, winter storms, and high wind.

Warning:
A NWS product indicates that a particular weather hazard is either imminent or has been reported.  A warning indicates the need to take action to protect life and property.  The type of hazard may be a tornado, severe thunderstorm, flood, or flash flood.  Warnings typically cover periods of an hour or less for only one county.   Warnings are issued for tornadoes, severe thunderstorms, floods, flash floods, winter storms, and high wind.

***NOTE: Whenever a watch or warning is issued, the information will be toned and broadcasted over the NOAA Weather Radio.***