The Florida Climate Center and the Center for Ocean-Atmospheric Prediction Studies (COAPS) have been leaders in the research of climate variability in North America, particularly as related to the El Niño - Southern Oscillation (ENSO). Given that wildfire activity is intimately linked with temperature and precipitation patterns, it naturally follows that ENSO would also have an impact on annual wildfire occurrences. This relationship holds especially true for Florida and the Southeast United States, where there is a strong connection between ENSO and climate during the winter and spring months which overlaps the traditional wildfire season in this region.

COAPS first cooperated with the Florida Division of Forestry in a research project identifying the connection between ENSO and wildfire burn acreage in Florida. COAPS has since extended that research and developed a prototype wildfire risk forecast system for Florida and the Southeast under research grants from the Florida Division of Forestry and the USDA Forest Service.

The wildfire risk forecast is based on ENSO phase and the Keetch-Byram Drought Index (KBDI). The KBDI was developed by foresters in the Southeast and represents water available in the upper layers of the soil. The KBDI is used extensively in forestry and wildfire applications. The wildfire risk forecast gives monthly probabilities of the KBDI reaching critical values from January through July, coinciding with the traditional wildfire season in the Southeast. The forecast is released in January each year, and updated throughout the season. Results are available on www.agroclimate.org and are presented each year at the National Seasonal Assessment Workshop, sponsored by the National Interagency Coordination Center. Ongoing support for this project comes from the NOAA RISA program and USDA Risk Management Agency.

Contact David Zierden at This email address is being protected from spambots. You need JavaScript enabled to view it. for more information.

Images of snow and freeze events in Florida. Courtesy of Florida Memory.

Although many people head south to escape the cold in the winter, it isn't always warmer in Florida. When an intense low pressure system is followed by a strong high pressure system, particularly powerful invasions of cold air may occur in Florida. These cold air outbreaks can produce below-freezing temperatures and are usually accompanied by strong winds that can produce bitterly cold wind chills.

Over the past 150 years, numerous severe cold outbreaks have affected Florida. In February 1899, a cold wave that became known as the Great Arctic Outbreak pushed frigid Canadian arctic air into the state. During this event, the lowest temperature ever recorded in Florida (-2°F) occurred on February 13, 1899. Since this outbreak, a number of "impact freezes" have influenced the retreat of the famous Florida orange groves from areas around Jacksonville and St. Augustine to their current locations in south Florida. 

Contrary to popular belief, it does snow in Florida. The first documented account of snow in Florida was reported in 1774. During the Great Arctic Outbreak, which was responsible for one of the biggest widespread snow events in the state, snow measured up to 3 inches deep in numerous Panhandle communities, and snow flurries were reported as far south as Fort Myers. Since 1886, there have been more than 80 months in which at least a trace of snow has been reported somewhere in the state. 

Images of tornado damage in Florida. Courtesy of Florida Memory.

Truly destructive tornadoes are most frequently reported in Florida during the spring and summer; the most powerful usually strike in spring. Florida has the dubious distinction of having a higher frequency of tornadoes per 10,000 square miles than any other state, including Oklahoma! In Florida, measured in frequency of tornadoes for every 10,000 square miles, the coast between Tampa Bay and Fort Myers has a particularly high incidence, as do the western panhandle and parts of the Atlantic Coast.

So, what is a tornado? Simply put, a tornado is a violently rotating column of air that is spawned by a severe thunderstorm. It connects from the thunderstorm to the ground and often appears to have a funnel or column shape. A tornado, one of nature's most violent storms, can develop suddenly, and have winds in excess of 250 mph.

Generally, tornadoes in Florida form

1. along a squall line ahead of an advancing spring cold front from the North,
2. along the squall lines in areas where masses of warm air converge,
3. from isolated local summer thunderstorms, and/or
4. within a hurricane.

What triggers a tornado is still not altogether clear. Tornadoes spawn inside clouds when there is great turbulence and winds of various speeds and velocities come in contact with one another. Although there is uncertainty about the triggers, meteorologists are able to identify atmospheric conditions conducive to tornadic activity.

The intensity of a tornado can be determined only after the event when survey teams from the National Weather Service can examine structural damage. After the survey is complete, the tornado is given a value from the Fujita Scale. The Fujita Scale was developed by Dr. T. Theodore Fujita and categorizes tornadoes on the basis of their intensity and area. The scale relates a tornado's damage to the fastest quarter-mile wind speed at the height of the damaged structure to determine its intensity. Over the years, problems with the initial Fujita Scale had arisen and in 2007 the new Enhanced Fujita (EF) Scale was adopted. The EF Scale helps determine the intensity of a tornado when there is no structural damage (as can happen when, for example, a tornado passes through a corn field). 

Virtually all summer rainstorms are accompanied by thunder and lighting. No other part of the nation has more thunderstorm activity than Florida. In the western half of the peninsula in a typical year, there are over 80 days with thunder and lightning. Central Florida's frequency of summer thunderstorms equals that of the world's maximum thunderstorm areas: Lake Victoria region of equatorial Africa and the middle of the Amazon basin. The Amazon and east African areas maintain their frequency of thunderstorm activity throughout most of the year, whereas the number of thunderstorms in Florida drops off sharply in the fall and does not pick back up until spring.

The simplest definition of a thunderstorm is a local storm that produces lightning and thunder. The storm itself can either be a single cumulonimbus cloud, a cluster of several thunderstorms, or a line of thunderstorms. In order for thunderstorms to form, there needs to be:

1. Moisture - to form clouds and rain.
2. Unstable air - warm air that can rise rapidly.
3. Lift - cold or warm fronts, sea breezes, mountains or the sun's heat are capable of lifting air to help form thunderstorms.

Once all of these components are brought together, the thunderstorm then goes through a 3-stage life cycle:

Development or Cumulus Stage: A cumulus cloud forms and begins to grow vertical, usually above 20,000 ft. Usually, little, if any, rain occurs during this stage. There might be occasional lightning.	Mature Stage: The cloud has grown in considerable height, now in the range to 40,000 to 60,000 ft. Strong updrafts and downdrafts coexist within the storm. This is the most dangerous stage of the storm and is the most likely time for hail, heavy rain, lightning, strong winds and tornadoes. Storms occasionally have a black or dark green appearance.	Dissipating Stage: The downdraft cuts off the updraft, which cuts off the supply of warm moist air to the storm and therefore, it dissipates. Rainfall decreases in intensity, along with winds, though strong gusts are still possible. Usually, the anvil top of the cloud is all that remains of the initial cumulus cloud.

Types of Thunderstorms

• Ordinary Cell: As the name implies, this is a thunderstorm with only one cell. It's commonly referred to a 'pulse' thunderstorm.
• Multi-cell Cluster: These are thunderstorms that are organized in clusters of 2-4 short-lived cells.
• Multi-cell Line: Some thunderstorms will form in a line which can extend laterally for hundreds or miles. These 'squall lines' can persist for hours and extend for hundreds of miles. Squall lines can be continuous or with breaks and include contiguous precipitation. Long-lived squall lines are known as "derechos" and can travel hundreds of miles, causing considerable damage along their path.
• Supercell Thunderstorms: These are potentially the most dangerous form of all thunderstorms types. Supercell thunderstorms have produced numerous long-lived strong and violent (EF2-EF5) tornaodes, along with damaging wind, hail and flash floods.

Thunderstorm Hazards

• Hail: Hail is a showery precipitation in the form of irregular pellets or balls of ice more than 5mm in diameter, falling from a cumulonimbus cloud. Hailstones are formed when updrafts carry raindrops up into the highest parts of the cloud and the super-cooled liquid droplets collide. Hail drops back down into the warmer part of the cloud and carried back up, until the internal up and downdrafts can no longer support the size of the hailstone, then it falls to the ground. 

  Hail size typically refers to the diameter of the hailstones. To make it easier to report, the following descriptions are often used:

  Description	Diameter (inches)
  Pea	0.25
  Marble or Mothball	0.50
  Penny or Dime	0.75
  Nickel	0.88
  Quarter	1.00
  Half Dollar	1.25
  Ping Pong Ball	1.50
  Golfball	1.75
  Hen's Egg	2.00
  Tennis Ball	2.50
  Baseball	2.75
  Tea Cup	3.00
  Grapefruit	4.00
  Softball	4.50

   

  So, why does Florida have so many thunderstorms, but not that many instances of hail? The freezing level in a Florida thunderstorm is so high; hail often melts before it reaches the ground. Even though hail is not common to the state, there have been about a dozen events of hail of over 3 inches being reported in Florida.

  One event in 1996 in Lake Wales, hail as big as softballs was reported. Damage to the area was done to windows, roves and cars totaling $24 million. In 2007, the area of Kendrick (North of Ocala) reported hailstones ranging in size from 2 to 4 inches.

• Wind: Damaging winds are more likely to be associated with thunderstorms than tornadoes. In fact, many confuse damage produced by "straight-line" winds and often erroneously attribute it to tornadoes. The source of the damaging winds is the downdraft within the thunderstorm. A downdraft is a column of cool air that rapidly sinks to the ground that is usually accompanied by precipitation in a thunderstorm.

  Downdrafts can cause downburst, which can be further classified as either microbursts or macro bursts.

  A downburst is a strong downdraft current of air from a cumulonimbus clouds and is often associated with intense thunderstorms.

  • Microburst: A downdraft that can affect an area of less than 2½ miles wide with peak winds lasting less than 5 minutes.
  • Macroburst: A downdraft that can affect an area of at least 2½ miles wide and with peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause tornado-force damage of up to F3 intensity.
• Tornadoes: See the Florida tornadoes section.

• Flash Floods: Except for heat related fatalities, more deaths occur due to flooding than from any other hazard. The main reason is because people underestimate the power and force of water. The effects of flooding can be felt on local, state and even regional scales. In Florida, flooding occurs frequently, but most of the floods are minor. However, Floridians must be careful because even minor floods can cause many deaths.

  Floods are caused by rain, but flooding is more about how much rain falls, how fast it falls, and what happens to the rain after it hits the ground. The most common type of flood that happens during a thunderstorm is a flash flood. Most flash floods are caused be slow moving thunderstorms, thunderstorms that repeatedly move over the same area, or heavy rains from a tropical storm or hurricane. These floods can develop quickly depending on the intensity and duration of the storm, the topography of the area, soil conditions and ground cover.

• Lightning: Lightning is the most lethal component of the thunderstorm. While the conditions needed to produce lightning are understood, how lightning forms has never been verified. Forecasters may never able to forecast when and where a lightning strike will take place.
  

  Florida is the lightning capital of the country, mainly due to our geography. The very elements that make our state a great place for outdoor activities -- warm temperatures and plenty of water -- also make the environment primed for the production of thunderstorms, which generate lightning.

Lightning has both negative and positive polarities. Most lightning forms at the bottom of the cloud, though less than 5% of all lightning occurs from the top of the anvil, making it a positive lightning strike. Positive lightning is very dangerous for several reasons. Since it comes from the top of the anvil cloud, the electric field is much stronger than a negative strike (almost ten times greater!). Some positive strikes can strike the ground beneath the cloud; however, most positive strikes occur near the edge of the cloud or can strike more than 10 miles away. Positive lightning is often responsible for the phenomenon commonly referred to as a "bolt from the blue". Positive strikes are more lethal and cause greater damage than negative lightning.