There are three ingredients that must be present for a thunderstorm to occur. They are: MOISTURE, INSTABILITY, and LIFTING. Additionally, there is a fourth ingredient (WIND SHEAR) for severe thunderstorms and each are covered separately and in-depth farther down:
As a general rule, the surface dewpoint needs to be 55 degrees Fahrenheit or greater for a surface based thunderstorm to occur. A dewpoint of less than this is unfavorable for thunderstorms because the moist adiabatic lapse rate has more stable parcel lapse rate at colder dewpoints. Dewpoints at the surface can be less than 55 degrees Fahrenheit in the case of elevated thunderstorms.
Instability also decreases as low-level moisture decreases. Instability occurs when a parcel of air is warmer than the environmental air and rises on its own due to positive buoyancy. Instability is often expressed using positive CAPE or negative LI values. Instability is what allows air in the low levels of the atmosphere to rise into the upper levels of the atmosphere. Without instability, the atmosphere will not support deep convection and thunderstorms. Instability can be increased through daytime heating. Lift is what gives a parcel of air the impetus to rise from the low levels of the atmosphere to the elevation where positive buoyancy is realized. Very often, instability will exist in the middle and upper levels of the troposphere but not in the lower troposphere. Low level stability is often referred to as negative CAPE, convective inhibition, or the cap.
It is lift that allows air in the low levels of the troposphere to overcome low level convective inhibition. Lift is often referred to as a trigger mechanism. There are many lift mechanisms. A list of many of them follows: fronts, low level convergence, low level WAA, low level moisture advection, mesoscale convergence boundaries such as outflow and sea breeze boundaries, orographic upslope, frictional convergence, vorticity, and jet streak. All these processes force the air to rise. The region that has the greatest combination of these lift mechanisms is often the location that storms first develop. Moisture and instability must also be considered. A thunderstorm will form first and develop toward the region that has the best combination of: high PBL moisture, low convective inhibition, CAPE and lifting mechanisms.
The difference between a thunderstorm and a severe thunderstorm is the wind field. For a severe thunderstorm, the ingredients that must be present are moisture, instability, lift and strong speed and directional storm relative wind shear. Ideally, wind will have a veering directional change of 60 degrees or more from the surface to 700 millibars, upper level winds will be greater than 70 knots, and the 850 to 700 mb winds (low level jet) will be 25 knots or greater. Wind shear aids in the following: Tilting a storm (displacing updraft from downdraft), allows the updraft to sustain itself for a longer period of time, allows the development of a mesocyclone, and allows rotating air to be ingested into the updraft (tornadogenesis).
Severe storms also tend to have these characteristics over ordinary thunderstorms: higher CAPE, drier air in the middle levels of the atmosphere (convective instability), better moisture convergence, baroclinic atmosphere, and more powerful lift.
Here are some conditions favorable to severe weather and an explanation of each:
DRY AIR IN THE MID-LEVELS OF THE ATMOSPHERE:
PBL WIND SHEAR:
STRONG UPPER LEVEL WINDS:
STRONG UPPER LEVEL TROUGH:
HIGH DEWPOINTS IN PBL:
DYNAMIC TRIGGER MECHANISMS:
Certain types of severe weather differ in association with different front types. Severe weather can occur with cold fronts, warm fronts, and drylines. In the case of a stationary front, the severe weather tends to be similar to that associated with a warm front. First, you need to determine the convergence along the front, moisture along and ahead of the front, the movement of the front, and the upper level winds. Stronger convergence along a front will result in an increased potential for uplift. An example of strong convergence along a cold front would be winds from the southeast at 25 mph south of the front and north at 20 mph north of the front. The higher the dewpoints, the more moisture a front will have to lift. If moisture is lacking on both sides of the front, do not expect significant precipitation. The movement of the front will help you determine how long the precipitation will last. Slower moving fronts are more prone to produce heavy persistent rain. The upper level winds determine how fast a supercell will move once it forms. Supercells tend to follow the mean 700 to 500 millibar wind flow and upon maturity will turn slightly to the right (about 30 degrees) of the mean 700 to 500 mb flow.
COLD FRONTS: Cold fronts tend to be the fastest movers compared to the other front types. This fast movement increases convergence along the front and results in faster storm movement, if storms do develop. The slope of a cold front is greater than that of the other frontal types. This results in convection that is more vertical (lifting associated with warm fronts has a large horizontal component). For severe weather to be associated with cold fronts, look for the following: high dewpoints ahead of the front (60 F or greater), strong upper level winds (300 mb wind greater than 120 knots), front movement between 10 and 20 mph, and convergence along the front. Storms tend to be strongest on the southwest edge of the frontal boundary due to a combination of the following: higher dewpoints, more convective instability, cap breaks there last, uninhibited inflow into storms, storms are generally more isolated and thus realize more convective energy.
WARM FRONTS: Severe weather generally occurs on the warm side of the warm front but is most favorable in the vicinity of the warm front boundary. This is due to the fact that the greatest directional wind shear is located along the warm front boundary. When storm chasing warm front convection, a good location would be to stay near the warm front boundary while at the same time being relatively close to the mid-latitude cyclone which connects to the warm front. As a general rule, severe weather is not as common along a warm front boundary as compared to out ahead of cold front boundaries for these reasons: A smaller frontal slope results in less frontal convergence, east of the Rockies convective instability (dry air in mid-levels) is not as well defined with warm fronts, convection tends to be more horizontally slanted, the temperature gradient from one side of the frontal boundary to the other is generally less in association with warm fronts.
DRYLINES: The higher the dewpoint gradient from one side of the dryline to the other is a good indication of dryline intensity. Critical point: No convergence along the dryline results in NO storms. Drylines are most common in the high plains in the Spring and early Summer. Certain factors must be in place for a dryline to produce severe convection. As mentioned, the most critical is convergence. This convergence can be intensified by a combination of the following: Strong upper level winds overriding the dryline (can produce dryline bulge), warm moisture rich air being advected directly toward the dryline boundary (i.e. 850 mb Southeast wind at 30 knots ahead of the dryline, West wind at 35 knots behind dryline), and a upper level trough. Severe storms in association with drylines tend to be classic or LP supercells. The shallowness of moist air ahead of the dryline boundary limits the amount of PW and moisture the storms can convect. The cap is critical to determining if a dryline will produce storms. If convergence is not strong enough, the cap (inversion above PBL will prevent convection from occurring. Strong convergence will break the cap. Generally, drylines are most intense and significant when a mid-latitude cyclone over the High or Great Plains forces warm moist air from the Gulf and dry air from the high plains to advect over the top of the warm moist air.
The following are the main ingredients for supercell thunderstorms. The more ingredients available, the more spectacular the storm will be once it is taken out of the oven.
(1) Instability - Defined by the temperature stratification of the atmosphere. Instability increases by warming the low levels (PBL) and/or cooling the mid and upper levels (700 to 300 mb). It is most easily assessed by looking at thermodynamic parameters. The most important include the 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 produce convective instability.
(2) Moisture (high dewpoints) - The more moisture available, the more Latent heat can be released once storms develop. It is important to look for moisture advection hour by hour on a day severe weather is possible. The air is more unstable in regions of dewpoint maxima. Here is a guide to dewpoint values
and the instability and latent heat they can provide:
(3) Warm PBL temperatures - Air density decreases with increasing temperature. The greater the heating is during the day, the greater the instability of the atmosphere. Days with sunshine will be more convectively unstable than days with continuous cloud cover. The breaking of clouds on a day when severe weather has been forecast will increase the likelihood of severe weather. A temperature guide for buoyancy follows below (lift will determine if bouyancy is allowed to occur):
(4) Low level jet/ inflow - Strong low level winds will quickly advect warm and moist air into a region if it is associated with the low level jet. Unimpressive temperatures and
dewpoints can change rapidly during the day via the low level jet. If winds are light in the PBL, severe weather is not as likely. Here are some low level jet wind values at 850 to keep in mind when analyzing:
(5) Strong surface to 700 millibar directional shear - Change in direction with height will cause horizontal vorticity which can lead to tornadic development. It also produces differential advection. Best case would be to have southeast 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 in the vertical thus leading to supercell storms. Speed shear also causes tubes of horizontal vorticity, which can be ingested into thunderstorms.
(7) Upper level Jet Stream - Use forecast models to determine the strength of the jet stream. The stronger the jet, the stronger the upper level forcing. Below is a guide to jet stream wind and upper level divergence (occurs in right rear and left front quadrant of a jet streak).
(8) 500 millibar vorticity - Vorticity is a function of trough curvature, earth vorticity, and speed gradients. When using models to assess strength of vorticity you will notice a value is given for the VORT MAX. The higher the value, the higher the potential upper level divergence. Below is a guide to 500 millibar vorticity and upper level divergence. If the values of vorticity are being rapidly advected, divergence will "in the real world" be much more than if the winds through the vorticity maximum are stationary or moving slowly.
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.
*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 weather hodograph: veering, strong sfc to 850 directional shear
* >100 J/kg negative buoyancy is significant
*Good match: BRN < 20 and CAPE >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 weather 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 microburst