|SEVERE THUNDERSTORM COOKBOOK
METEOROLOGIST JEFF HABY
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
dewpoint depression between
700 and 500 mb.
Dry air in the mid-levels combined with warm and moist air in the PBL will produce
(2) Moisture (high dewpoints)- The more moisture available, the more
latent heat that 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:
|Greater than 75 ||Incredibly juicy|
|Less than 55 ||Low moisture content
(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):
|100+ ||Incredibly buoyant (if dewpoint greater than 55)||90-99+ ||Extremely buoyant (if dewpoint greater than 55)
||80-89+ ||Very buoyant (if dewpoint greater than 55)
||70-79 ||Fairly unstable (if dewpoint greater than 55)
||60-69 ||Marginal (if dewpoint greater than 55)
||Less than 60 ||Positive temperature and dewpoint advection needed
(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:
|Greater than 70 knots ||Incredibly fast advection||50 to 69 knots ||Very strong low level jet
||30 to 49 knots ||Descent low level jet
||20 to 29 knots ||Marginal low level jet
||Less than 20 knots ||Ill-defined low level jet
(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).
|Greater than 200 knots ||Incredible divergence||150 to 200 knots ||Large divergence
||100 to 149 knots ||Good divergence
||70 to 99 knots ||Marginal divergence
||Less than 70 knots ||Small divergence
(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.
|40+ ||Incredible divergence||30+ ||Very large divergence
||20-29 ||Large divergence
||Teens ||Descent divergence
||Less than 12 ||Low but positive divergence