The presence of three factors is necessary for a thunderstorm to occur.Those are: MOISTURE, INSTABILITY, and LIFT.Additionally, there is a fourth ingredient (WIND SHEAR) related to severe thunderstorms that is discussed separately and in-depth further down:

.Dewpoints below this level do not favor thunderstorms because the moist adiabatic lapse rate has a more stable parcel lapse rate at cooler temperatures.If elevated thunderstorms occur, surface dewpoints can be below 55 degrees Fahrenheit.

With lower moisture levels, stability also decreases.Instability occurs when a parcel of air is warmer than its surrounding air and rises on its own.The CAPE or LI values of stability are often expressed as positive numbers.A lack of stability in the lower atmosphere enables air in the upper layers of the atmosphere to rise.Deep convection and thunderstorms can't occur in an instable atmosphere.Daytime heating can increase instability.As a parcel of air rises from the low levels of the atmosphere to the position of positive buoyancy, it gains lift.Most often, instabilities occur at the upper and middle troposphere levels but not at the lower troposphere.Negative CAPE, convective inhibition, or the cap are all terms used to describe low level stability.

In the low troposphere, lift enables air to move over low-level convective barriers.This mechanism is frequently referred to as trigger.Various lift mechanisms are available. .The rising air is caused by these processes.Storms often first develop in regions with the most of these lift mechanisms.Also, moisture and instability must be taken into account.First, a thunderstorm will form in the region with a good combination of: PBL moisture, low convective inhibition, CAPE and lifting mechanisms.

The wind field distinguishes a thunderstorm from a severe thunderstorm.Several elements are necessary for a severe thunderstorm to form, including moisture, instability, lift, high speeds, and strong relative wind shear.It is ideal for the wind to have a veering directional change of 60 degrees or more from the surface to 700 millibars, that upper level winds be greater than 70 knots, and that the low level jet winds are 25 knots or greater.A tornado can form as a result of wind shear tilting a storm (dislodging updrafts from downdrafts), allowing us to develop mesocyclones for a longer period of time, and allowing rotating air to infiltrate the updraft (tornadogenesis).

A severe thunderstorm has these characteristics over an ordinary thunderstorm: a higher CAPE, drier air in the middle levels of the atmosphere (convective instability), better moisture convergence, a baroclinic atmosphere, and more lift.


Low level moisture can be determined by examining boundary layer dewpoints.When temperature and other factors are equal, thunderstorms are more likely when there is a dewpoint above 55 F.Temperatures below the dewpoint inhibit sufficient heat release, which reduces tornado dangers.

Tornadoes are more likely to occur when the LCL is relatively low compared to relatively high.We should also assess the amount of moisture in the lower troposphere and the rate of advection of moisture.

The lower troposphere lack of moisture reduces the chances of severe storms, but a lack of moisture in the middle troposphere can prevent severe storms if the lower troposphere is abundant with moisture.This situation is characterized by convective (potential) instability.

In severe conditions, the advection of higher dew point values into the boundary layer can enhance instability.A warm ocean source is often used to advect moisture.


Each type of instability will be discussed.Upon release of instability, air speed increases vertically.Hence, thunderstorms are formed when air quickly rises.The rise of air is caused by positive buoyancy, which means instability.Picture a basketball on the bottom of a swimming pool, for example.Upon release, the basketball shoots upwards to reach the top of the pool.Despite being less dense than the water surrounding it, the basketball rises.This phenomenon can also occur in the atmosphere.During the lift process, the lower troposphere is lifted until it becomes less dense than the air below.After a while, it will rise by itself.Its speed of rise depends on the amount of density difference between it and the surrounding air.A thunderstorm's updraft is less dense than the surrounding air, so a rising motion occurs when the air rises in the updraft.


Assessing parcel stability (also referred to as static stability) requires an examination of CAPE as well as Lifted Index calculations.CAPE is commonly measured by two methods: SBCAPE (surface based CAPE) and MUCAPE (most unstable CAPE).With values above 2,500 J/kg, CAPE is extremely unstable.VALUES LESSEN THAN -4 represent large LI values with values LESSEN THAN -7 as experiencing extreme instability.A high level of instability can result in high accelerations inside the updraft.The strength of the updraft is crucial to the generation of hail.


This is a result of latent heat being released.As the dewpoint in the PBL, or in the region where lifting begins, increases, so does latent instability.In general, the more latent heat that is released, the warmer the air parcel will become.Moisture and humidity in the PBL will cause a rise of air in the low levels of the atmosphere to cool at a slow rate (approximately 4 C/km) because the moist adiabatic lapse rate will be the dominant factor.When a storm has a large amount of moisture to lift, it will be more unstable than one that is ingesting dry air.When storms reach east of the Rockies, there is more low level moisture that can lift the storms.The classic example is a Nor'easter.A warm and moist air mass coming from either the Gulf Stream or the Gulf of Mexico can increase latent instability.


When dry mid-level air moves over warm and moist lower troposphere air, condensation (also called potential) instability occurs.When dynamic lifting, from the surface to mid-levels, produces moist adiabatic lapse rates of air lifted from the lower troposphere and dry adiabatic lapse rates of air lifted from the middle troposphere, then convective instability is released.It is thought that this affects the lapse rate in the atmosphere. Eventually, an atmosphere with little or no Surface Based CAPE can become one with a large SBCAPE (as compared to a parcel of air lifted from the surface).A dry air is able to cool faster than a moist air that is saturated with moisture.

When relatively dry mid-levels of the atmosphere exist, and high dewpoints (and near saturated conditions) exist in the PBL, convection is unstable.Using water vapor imagery, atmospheric moisture can be measured between 600 millibars and 300 millibars.Dark colors on water-vapor imagery indicate dry air in the mid- and upper-atmosphere.The surface, 850 mb, and 700 mb charts provide information on the low-level moisture profile.Skew-T diagrams are the most effective method for analyzing convective instability.There will be hydrolapse (rapid decrease of dewpoint with altitude) at the boundary between the near saturated lower troposphere and the dry middle atmosphere.

The dry air in the upper atmosphere and the moist air near the surface will often be separated by an inversion.The dry air aloft is commonly known as the elevated mixed layer (EML).Under this "capping" inversion during the day, heat, moisture, and instability can build.Convection-related explosions can occur when the cap breaks.



Unstable release occurs when the bottom of the swimming pool is raised and lifting is caused by the air being forced upward.When you lift a forced object, you are picking up a bowling ball or bench pressing.Objects cannot be lifted by themselves; they need a force.This is not the result of air rising by itself.

.If instability exists, it cannot be relieved without an appropriate amount of forced lifting for the specific case.

LIFTING MECHANISMS1. Frontal boundaries, dry lines, and outflow boundaries (low level convergence)2. Low level warm air advection3. Upslope flow4. Low pressure system (synoptic and mesoscale)5. Differential heating along soil, vegetation, soil moisture, land cover boundaries (low level convergence)6. Low level moisture advection7. Differential Positive Vorticity Advection, jet streak divergence (upper level divergence)8. Gravity wavePRECIPITATION FROM LIFTING

Dynamic precipitation is also known as stratiform precipitation. Dynamic precipitation results from aforced lifting of air. These forcing mechanisms include processes that cause low level convergence and upper level divergence. As unsaturated air rises the relative humidity of the air will increase. Once the air saturates, continued lifting will produce clouds and eventually precipitation. Dynamic precipitation tends to have a less intense rain rate than convective precipitation and also tends to last longer. While stratiform rain is the product of lifting, convective precipitation is the product of both lifting and instability release.


Severe thunderstorms develop when there is strong vertical wind shear.There are several ways wind shear influences a storm:


1. A significant increase of wind speed with height will tilt a storm"s updraft. This allows the updraft and downdraft to occur in separate regions of the storm the reduces water loading in the updraft. The downdraft will not cut-off the updraft and actually it will even enforce it.

2. Strong upper tropospheric winds evacuates mass from the top of the updraft. This reduces precipitation loading and allows the updraft to sustain itself.

3. Directional shear in the lower troposphere helps initiate the development of a rotating updraft. This is one component that is important to the development of a mesocyclone and the development of tornadogenesis. Stronglower tropospheric winds and directional shear together will generate high values of Helicity and thus this increases the tornado threat when severe storms develop.

4. The shear environment is important in determining the thunderstorm type. Both the vertical speed shear and directional wind shear have varying magnitudes. To simplify, we will have two categories: weak and strong. Thus, we have four combinations. Let"s discuss each combination (assuming the updraft is of moderate strength for each case (moderate instability).


A storm in this environment will move slowly and will be short lived. Since the storm moves slowly, the downdraft will cut-off the updraft and will thus diminish the storm. Storms in this environment are often termed "air mass thunderstorms" or "garden variety thunderstorms". If storms form in a moisture rich environment, rain can be heavy for brief periods of time. Severe is not likely.


This situation is often termed "unidirectional shear". The speed shear will allow the storm to move. The movement insures the storm will last longer than an airmass thunderstorm. Unidirectional shear often produces storms that form into lines (Mesoscale Convective Systems (MCS"s)). Since the storm moves, outflow produces lift that enables new storms to grow on the storm"s periphery. Over time, a line a storms result. These storms primarily produce small hail, weak tornadoes and heavy rain when they are associated with severe


When speed shear is weak the directional shear is not of significance. Storms in this environment will take on the characteristics of those in CASE 1. Hodograph wind speed will have similar pattern to CASE 1 and wind direction change with height will be high but often unorganized.


This situation can produce single-cell super-cells. This is the best situation in order to produce a rotating updraft. The speed shear enables the storm to move quickly and helps keep the updraft and downdraft separated while the directional shear helps rotate the updraft into the storm. These storms can produce large hail, strong tornadoes and heavy rain

Click here for a more in-depth presentation on typical hodographs associated with various storm types.

WAA, CAA and Hodographs

In relation to height, wind direction and wind speed give clues to synoptic temperature advection.Wind veering occurs when it turns clockwise with height.At the surface of the atmosphere, winds change from southeasterly to westerly in a veering case.A veering wind causes warm air to be advected.A strong advection of warm air will be determined by the strength of the wind and the amount of veering with height.At 700 mb, wind direction might be from the southwest at the surface with southerly winds at 700 mb, which through time would cause the low levels of the atmosphere to warm while the upper levels remain at the same temperature.Instability will result.Amounts of thermal advection and veering from the surface to the mid-levels will determine how unstable the low levels are.In the warm sector of a mid-latitude cyclone, veering is generally expected.Toward a warm front, the wind is likely to veer with height.Winds are usually light and northerly before warm fronts pass, then gradually shift to the east, before finally shifting to the south.A backing wind turns counterclockwise with height.Backing winds are associated with cold fronts.The wind behind a cold front will be northerly, then shift counterclockwise into a westerly direction with height.The prevailing westerlies at planetary scale generally cause the mid and upper levels of the atmosphere to be more westerly than easterly.Reassuring backing winds occur during cold air advection.The backing wind in the lower atmosphere is favorable for sinking motion at the synoptic scale.Storms are typically found out ahead of cold fronts.It is usually lighter or completely absent behind cold fronts.A hodograph shows wind speed and direction in relation to height.Through the diagram, it is very easy to figure out the veering and backing of wind.This will help you determine most likely types of thunderstorms.Surface levels to 850 mb, mid-levels 500 to 500 mb, and upper levels 150 to 850 mb are the lowest levels in the atmosphere.

MULTICELLS: Wind direction remains unidirectional from mid-levels into upper levels (850 to 300 mb), but more pronounced than it is for supercells; Speed shear is present (enhancement of wind speed with height)

AIR MASS STORMS Wind speed changes with height are relatively small Wind direction is fairly constant with height or unorganized Winds at the upper level are weaker than in a supercell or multicell.

Click here for a more in-depth presentation on typical hodographs associated with various storm types.TOP

Here are some conditions favorable to severe and an explanation of each:

Drier air in the mid-latitudes causes convective instability and a significant negative buoyancy in thunderstorms.By entrapping the dry air into the moist air of the cloud, evaporation, negative buoyancy, and a strong downdraft are caused.In this way, evaporative cooling reduces the amount of melting hail as it falls. HIGH INSTABILITY: High CAPE, unstable LI, unstable KI and TT; Strength of updraft is determined by positive buoyancy.There are large updrafts when there is a lot of instability.Updraft strength largely determines hail size.CAPE also causes the stretching that results in tornadogenesis. (Wind shear must also be present).

HIGH UPPER LEVEL TROUGH: Produces a strong positive vortex advection; causes differential temperatures (i.e. upper and lower level fronts)

High dewpoints in PBL:You need low levels of moisture to see storm development.Low-level moisture tends to increase latent instability.

A DYNAMIC TRIGGER MECHANISMS is needed for storms to form without a trigger mechanism, such as when a strong cap is present.Below are some examples of dynamic trigger mechanisms: 1.1.the cold and warm fronts3.boundary streak5.High upper level vorticity 6.orographic lift 7.downward advection of warm air at low levels (a gradient of hotter temperature moving toward a fixed point) 8.the low level jet 9.The gravity waves 10.Low level jet9. Gravity waves10. 9. Low-level jets10. Meso-lows TOUGH THUNDERSTORMS VS. FRONTAL STORMS Meteorologist Jeff Haby says that certain types of severe weather differ in association with different types of fronts.With cold fronts, warm fronts, or drylines, severe moisture can occur.The severity of the is similar to that of a warm front in the case of a stationary front.The convergence along the front, moisture along and ahead of the front, and the movements of the front and upper-level winds should all be determined first.An increase in convergence along a front will result in greater uplift.A strong convergence would be winds from southeast at 25 mph south of the front and north at 20 mph north of it.Higher dewpoints mean more moisture must be lifted as a front moves.There will be little precipitation if moisture is lacking on both sides of the front.When you know the movement of the front, you can forecast how long the precipitation will last.Slower moving fronts will produce more persistent rain.If a supercell forms, it moves faster depending on how strong the upper level winds are.As supercells mature, they turn slightly to the right (about 30 degrees) of the mean 700 to 500 millibar wind flow.

In comparison to the other front types, cold fronts tend to move the fastest.It leads to faster storm movement if storms do develop along the front as a result of this fast motion.There is a greater slope on a cold front than on other types of fronts.In this way, convection is vertically oriented (lifting associated with warm fronts is mostly horizontal). .As a rule, storms tend to be strongest to the southwest of the frontal boundary as a combination of higher dewpoints, more convective instability, cap breaks last, uninhibited inflow into storms, and storms are generally more isolated.

Severe generally settles on the warm side of the warm front, although it is most favorable near the warm front boundary.The largest area of wind shear is located along the warm front boundary.A good location for storm chasing warm fronts would be to stay close to warm front boundaries while at the same time being relatively close to the cyclone that connects to the warm front.Wordtune couldn't crunch your text. Try it on a shorter text.

DEWPOINT GRADIENTS: The higher the gradient of dewpoints between the drylines, the more intense the dryline is.A dryline without converging storms becomes useless.Usually, drylines are found in the high plains during the spring and early summer.A dryline must meet certain conditions to produce severe convection.In particular, convergence is essential. .Storms associated with drylines are often classic or LP supercells.As the moist air becomes shallower ahead of the dryline boundary, the amount of rain and moisture the storms can condense is limited.It is crucial to determine whether a dryline will produce thunderstorms based on the cap.


The following are the main ingredients for supercell thunderstorms. The more ingredients available, the morespectacular the storm will be once it is taken out of the oven.

1) Instability - Defined by the stratification of the temperature of the atmosphere.Temperature rises at low levels (PBL), and/or low levels (700-3000 mb) cool.Thermodynamic parameters are the best indicator of instability.There are several important measures, including CAPE, LI, cap, and dewpoint depression between 700 and 500 mb.Dry air in the mid-levels combined with warm and moist air in the PBL will lead to convective instability.

(1) Moisture (high dewpoints) - When storms form, the more latent heat can be released.On a day severe moisture advection may occur if moisture advection is not monitored hour by hour.In regions of maximum dewpoints, the air is more unstable.Here is a guide to dewpoint values and what they can tell us about their instability and latent heat: Over 75 Incredibly juicy The Juicy Years 65-74 I'm 55-64 years old, semi-juicy Moisture content less than 55 Warm PBL temperatures - As the temperature increases, the air density drops.An atmosphere that is more unstable is caused by increased daytime heating.Sunny days will have higher convective instability than days with continuous cloud cover.Increasing the likelihood of severe by breaking clouds on a day when severe has been forecast.Here's a temperature chart for buoyancy (lift determines whether it will be allowed): Exceptionally buoyant (if dewpoint is greater than 55)

.A low level jet can bring with it rapid temperature and dewpoint changes during the day.In the PBL, winds will be light, meaning severe moisture will be less likely.Consider these low level jet wind values at 850 when analyzing the data: It's more than 75 pounds of deliciousness 65 to 74 Juicy

Shear at 700 millibars at the surface - A shift in direction can lead to horizontal vorticity and tornadic activity.This also has differential advection effects.It is ideal to have a southeasterly wind at the surface transporting warm and moist air, a southwest or west wind at 700 millibar transporting dry air, and a northwesterly wind in the upper levels of the atmosphere.

(6) Strong speed shear with height - This will cause updrafts to tilt vertically, causing supercell thunderstorms.Thunderstorms can also be ingested by horizontal vorticity tubes caused by speed shear.

(1) Upper level jet stream - Determine the strength of the jet stream by using forecast models.In general, the stronger the jet stream, the stronger the upper level forcing.This chart illustrates how a jet streak's wind and upper level divergence (which occurs in the right rear and left front quadrants) look. An incredible divergence over 200 knots Divergence of 150 to 200 knots Large Divergence of 100 to 149 knots Good Divergence of 70 to 99 knots Marginal divergence of less than 70 knots Small divergence

A vortex is defined as a function of its trough curvature, earth vorticity, and speed gradients.You will notice that the VORT MAX is given when using models to assess strength of vorticity. The higher the value, the stronger the potential upper level divergence will be.The following chart shows 500 millibar vorticity and upper level divergence.The divergence "in the real world" will be much greater if the winds are rapidly advancing toward the vorticity maximum than if they are stationary or moving slowly.The divergence is greater than 200 knots Differenciation between 150 and 200 knots Loud divergence 100 to 149 knots Good divergence 70 to 99 knots Marginal divergence Less than 70 knots Small divergence

Click here for a more in-depth presentation on supercell thunderstorm structure and evolution.

Right and Left moving Supercells

What is the cause of splitting supercells? How can they move deviant to the deep-layer flow? And finally, why do left movers move more swiftly than right movers?

The cause of supercell splitting lies in vorticity dynamics. The tilting and stretching of horizontal vorticity into the vertical yields a positive and negative vertical vorticity center on the south and north side of a supercell (given a wind profile characterized by easterly surface winds becoming, linearly, westerly and increasing in intensity with height). Buoyancy gradients along the edge of the updraft also play a role.. The vertical pressure perturbation structure results in renewed development to the south of the cyclonic center and to the north of the anticyclonic center. Developing downdraft in the "center" of the updraft, in concert with the outward (south/north) development leads to the "splitting" of the single updraft into two discrete updrafts.. This all depends on the wind profile (and more specifically, the wind SHEAR profile).

A "right-mover" denotes a storm which has turn right of the mean wind, often by 20-30 degrees, though sometimes signficantly more. Cyclonic supercells also tend to move slower than the mean wind (while left-moves tend to move left AND faster than the mean wind). For many, the term "30R75" may ring a bell -- "30 degrees right and 75% of the mean wind". Different storms may not obey this rule-of-thumb, however! Low-topped or mini-supercells tend to be less developed in the vertical (thus the term low-topped LOL), and thus the "steering wind" (so to say) for those storms may be the 850-700mb layer), while more classic supercells that extend to the tropopause may be most heavily influence by the 700-400mb mean wind. Regardless, this kind of get muddied up with supercells develop strong pressure perturbation gradients, which is largely the cause of the deviant motion to begin with.

For those that are curious, you can find other good lectures regarding supercells and tornado dynamics (e.g. how helicity aids thunderstorm rotation, how rotation in an updraft enhances the updraft well beyond the effects possible with buoyancy alone, etc) by just going here.

51-52Isolated severe storms53-56 Widely scattered severe >56 Scattered severe storms
0-3 Weak
15-25Small convective potential
26-39 Moderate convective potential 40+High convective potential
1 - 1,500 Positive
1,500 - 2,500Large 2,500+Extreme
150-300Slight severe
300-400Severe possible 400+Tornadic possible
150-300 Possible supercell
300-400 Supercells favorable 400+ Tornadic possible
-1 to -4Marginal instability
-4 to -7Large instability -8 or lessExtreme instability
EHI >1 Supercells likely
1 to 5 F2, F3 tornadoes possible 5+ F4, F5 tornadoes possible
NOTES:*Max uvv = square root of 2 * CAPE*BRN (Bulk Richardson Number) = CAPE / (0-6 km) Shear*Showalter (SWI) = used when elevated convection is most likely*EHI = (SR HEL * CAPE) /160,000*SWEAT = 12(850Td) +20(TT-49) +2(V850) + (V500) +125(sin(dd500-dd850) + 0.2)*Total Totals = (T850- T500) + (Td850 - T500)= vertical totals plus cross totals*K index = (T850 -T500) + (Td850 - Tdd700)*SR Helicity : determines amount of horizontal streamwise vorticity available for storm ingestion*streamwise = parallel to storm inflow*Important to look for thermal and dewpoint ridges (THETA-E)*For tornado, inflow must be greater than 20 knots*20 to 30% of mesocyclones produce tornadoes*Tornado types: rope, needle, tube, wedge*Look for differential advection; warm/ moist at surface, dry air in mid levels*Severe hodograph: veering, strong sfc to 850 directional shear* >100 J/kg negative buoyancy is significant*Good match: BRN 2,000 J/kg*Strong cap when > 2 degrees Celsius*Study depth of moisture, TT unreasonable when low level moisture is lacking*KI used for heavy convective rain, values vary with location/season*Instability enhanced by .. daytime heating, outflow boundaries*Models generally have weak handle on return flow from Gulf, low level jet, convective rainfall, orography, mesoscale boundaries, and boundary conditions*Large hail when freezing level >675 mb, high CAPE, supercell*Synoptic scale uplift from either surface WAA or upper level divergence*Fair cumulus: cumulus humulus, cumulus mediocrus*T-storm warning when Hail > 3/4", wind > 58 mph, gate to gate shear > 90 knots*Sounding types: Inverted V, goal post, Type C, wet microburstTOP