Summer Weather:
Convection Information & Safety

 

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Nearly 2,000 thunderstorm cells are estimated to be present over the planet at any given time.  It is estimated that globally there are 16 million thunderstorms each year.  In the United States, central Florida has almost 100 thunderstorm days annually.  By comparison the number of thunderstorm days in Wyoming varies across the state from about 20 to 60 per year.  

 

 

 

Ingredients

What is needed to create thunderstorms?

Three Main Ingredients for thunderstorm development:

  • Lift
  • Moisture
  • Instability

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Convective Indices

What do forecasters look for when forecasting thunderstorms?

  • Lifted Index (LI)
    Definition: It is a stability index used to determine thunderstorm potential. The LI is calculated by taking a representative low level air parcel and lifting it adiabatically to 500 mb. The algebraic difference between this air parcel and the sounding temperature at 500 mb (around 18,000 feet) denotes the LI. Since the LI accounts for moisture below 850 mb, it provides more reliable stability information than the Showalter Index (SWI). The greater negative values of LI indicate energy available for parcel ascent.
    Lifted Index (LI) Thunderstorm Indication
    < -5 Very Unstable, Heavy/strong thunderstorm potential
    -3 to -5 Unstable, Thunderstorms probable
    0 to -2 Marginally Unstable, Thunderstorms possible
    Current Analysis
  • Dewpoint Temperatures
    Definition: A measure of atmospheric moisture. The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur. The higher the dew point, the greater amount of water vapor in the air mass.
  • Convective Available Potential Energy (CAPE)
    Definition: It defines the vertically integrated positive buoyancy of an adiabatically rising air parcel on a sounding. This is proportional to the amount kinetic energy that the air parcel gains while it is warmer that its surrounding environment. As a result, CAPE provides the best measure of the potential instability available in the atmosphere. Increasing values of CAPE generally lead to progressively vigorous convection. However, severe thunderstorms can form in environments showing weak to moderate CAPE, especially if the Storm Relative Helicity values are high.
    Current Analysis
  • Convective Inhibition (CIN)
    Definition: It represents the cumulative effect of atmospheric layers the are warmer than the parcel moving vertically along the adiabat. Low level parcel ascent is often inhibited by such stable layers near the surface. If natural processes fail to destabilize the lower levels, an input of energy from forced lift (a front, an upper level shortwave, etc.) will be required to move the negatively buoyant air parcels to the point where they will rise freely. Since CIN is proportional to the amount of kinetic energy that a parcel loses to buoyancy while it is colder than the surrounding environment, it contributes to the downward momentum.
    Current Analysis
  • K-Index
    Definition: It is a measure of the thunderstorm potential based on vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer. The temperature difference between 850 mb and 500 mb is used to parameterize the vertical temperature lapse rate. The 850 dew point provides information on the moisture content of the lower atmosphere. The vertical extent of the moist layer is represented by the difference of the 700 mb temperature and 700 mb dewpoint. This is called the 700 mb temperature-dew point depression.

    The index is derived arithmetically and does not require a plotted sounding. K-index = [(850 mb temperature - 500 mb temperature) + 850 mb dew point - 700 dew point depression]. The K-index favors non-severe convection, especially heavy rain producing convection. Threshold values vary with season, location, and synoptic situation.

    K-indices are also used to determine the potential of flooding. When your K-index is high (above 35), it means that you will likely see numerous thunderstorms develop. If these thunderstorms track across the same area, you may have a various serious flooding situation on your hands.
    K-index value Thunderstorm Probability
    Above 35 Numerous thunderstorms
    31 to 35 Scattered thunderstorms
    26 to 30 Widely scattered thunderstorms
    20 to 25 Isolated thunderstorms
    < 20 None
  • High Level Total Totals
    Definition: This index estimates the potential for severe convection. It combines the effects of vertical temperature lapse rate (Vertical Totals or VT) and low level moisture (Cross Totals or CT) in a given environment. The following formulas are used to create the Total Totals index:

                   Vertical Totals (VT) = 850 mb temperature - 500 mb temperature
                   Cross Totals (CT) = 850 mb dew point - 500 mb temperature

    When the two are combined, you have the following formula: Total Totals (TT) = Vertical Totals (VT) + Cross Totals (CT). The following table shows what these relationships typically mean east of the Rockies:
    Cross Totals Vertical Totals Total Totals Forecast
    30 26 or more 56 Numerous thunderstorms, scattered severe, scattered tornadoes
    26-29 26 or more 52 Scattered to numerous thunderstorms, few to scattered severe, few tornadoes
    24-25 26 or more 50 Scattered thunderstorms, few severe, isolated tornadoes
    22-23 26 or more 48 Scattered thunderstorms, isolated severe
    20-21 26 or more 46 Scattered thunderstorms
    18-19 26 or more 44 Isolated or few thunderstorms
    High lapse rates or a source of low level moisture will yield large values of TT. However, high lapse rates can produce large TT, with little supporting low level moisture. The sounding must be examined carefully to ascertain the validity of the TT for a given environment. Also as with any index, you must carefully examine your environment.
  • Lapse Rates
    Definition: The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring. The global average rate of temperature change with height in the atmosphere is 6.5°C/km. The adiabatic lapse rate (or dry adiabatic lapse rate) is the normal rate of change (9.8°C/km) for a dry parcel of air that is moved up or down and cools or warms as the pressure changes. The wet (moist) adiabatic lapse rate (4.9°C/km) is the rate at which saturated air cools as it ascends.

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 Wyoming

The map below shows the average annual number of thunderstorm days in Wyoming as reported by the National Oceanic and Atmospheric Administration:

Average Annual Thunderstorm Days - Wyoming

Thunderstorms occur with the greatest frequency in the eastern counties.  This is partly caused by the warmer surface temperatures allowed by the relatively low elevations in this region, but it is primarily the result of the more humid surface air there.  Moist air moving across the Great Plains is often too shallow to cross the Laramie Range and the Big Horn Mountains.  As a result, the surface air is drier on the west side of these topographic barriers and thunderstorms are correspondingly less common.  Moisture from the Pacific Ocean fuels thunderstorms in western Wyoming, but the intervening middle third of the state, and especially the Big Horn Basin area, is relatively protected from thunderstorms by topographic barriers to low-level moisture airflow.  Thunderstorms occur most often during the summer months of June, July and August when surface temperatures are highest, reaching a peak in the number of thunderstorm days in July.  Almost no thunderstorms occur from November through February.


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Watches & Warnings

What's the difference between a Watch and Warning?

In general, a watch means the atmosphere is capable of producing hazardous weather; a warning means that specific hazardous weather element is imminent. Below is a table summarizing needs for the different types of summertime watches and warnings:

 


Warning Verification
 Severe Thunderstorm


 1" Hail - Size of a quarter
58 mph Wind
Tornado

 Tornado  
Tornado Spotted

 Flash Flood
 Flooding reported within 6 hours of event.

 Flood
 Flooding as a result of heavy rains 6 hours or more after storm.

 

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