Thursday, June 19, 2008

Some ingredients for Severe Thunderstorms. Weather 101 Study

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 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

65-74
Juicy

55-64
Semi-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

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