What is needed for thunderstorm development?

Question

I've studied weather as a hobby for a few years now (i'm 20) but I have some questons.

from what I know you need high dew point(moisture), heat, instability (CAPE value of 1200 or more), but what else? Does high pressure prevent thunderstorm development? What pressure numbers are ideal for development? What is the best indicator that thunderstorms will happen?

Here in virginia we had pretty sever thunderstorms yesterday but today it was clear with a few small clouds and thats it but under the same conditions (high dewpoint, 95 degrees,) but the CAPE value was 800 which is low. What causes low CAPE values? Why do thunderstorms form on some days and not on others?

Answer 1

Thunderstorms only require abundant moisture, instability, and a lifting mechanism for development.

High pressure will prohibit thunderstorm development because its a sinking mechanism, not a rising mechanism.

A pressure lower than 1000 mb or 30 inches is ideal.

The best indications are the flow of moisture from the Gulf of Mexico at the 850 mb level, a CAPE value greater than 2000 J/kg, and a good lifting mechanism, such as a frontal boundary.

Temperature inversions are known to s***w up the CAPE values.

Thunderstorms form on different days because of the conditions. Unless requirements are met for development, thunderstorms will not form.

Answer 2

I'll touch on some things that the user above didn't exaggerate on.

Thunderstorms are a very fickle forecast. The reason being is that basically anything can cause them at any time. I live in Norfolk, VA, so we will used these storms you were speaking of yesterday as an example.

The storms that you saw yesterday were a variant of an "air mass" thunderstorm. This form of thunderstorm forms when a cooler airmass meets a warmer, thus producing life. There is a trough, or area of lower pressure (don't confuse with a front) that is located over us as we speak because of the temperature dynamics. The areas ocean is a good 30 degrees cooler than the air over the land, so the high humidity and temperatures, along with a lifting mechanism to form clouds (i.e. the trough) will cause moderate to severe storms.

The way this works is the cooler air wedges underneath the warm, very humid air and causes the lifting of the warm air (where you get your CAPE value). Once the air is being lifted, it's adiabatic lapse rate will continue to decrease until zero. After that, once the particle of air and moisture has cooled and expanded into what we call an inversion, you then have a thunderstorm.

Thunderstorms can form in High Pressure, and a lot of people to misconcieve that notion. On every pressure system there is a positive and a negative side. These positive and negatives are vorticies values, and they determine basically the direction particles in the air are spinning.

Now, knowing that weather travels in patterns, you know that Lows come after Highs and vice versa. It's a never ending pattern. So on the front of every high, you will have negative vorticity (suppressing vertical development of clouds) and on the back you will have positive (connecting with the next incoming low, and increasing the vertical development of clouds). On the front of lows you will have positive and on the rear, negative. Put the two together and you have a worldwide map of good weather vs. bad weather.

So, in theory you CAN have thunderstorms in high pressure, but only on the positive (back) side of a high, which we are currently in.

To answer your other questions, you are correct when you say you need moisture, heat and instability (lifting mechanism). The prime conditions for thunderstorms are when the temperature and the dewpoint are within 2-5 degrees of each other all the way up towards the 500mb region of your skew-t. From there you should begin looking for an inversion that could extend from 500mb-300mb, depending on the strength of the thunderstorm.

Your Cape, Showalter and SWEAT indices are just that, indices. They are no more reliable than the TV weatherman.

When you are looking for thunderstorms, just remember the types:

Airmass:When two airmasses collide (i.e. a trough, frontal system) and provide the amound of lift needed to form a storm.

Convective: When sheer heat and moisture form a thunderstorm. Major cause of Summer "Popcorn" thunderstorms.

Advective: These storms will form over an area affected by sea or land breezes.

I hope my garble here has helped. It's rather hard to really put a finger on thunderstorms because there are so many variables to their development. There are also many false notions out there that originate from crow-tales farmers used to use.

Don't trust everyone you meet.

Take care buddy. If you ever need help with your weather studies, shoot me an email at [e-mail]