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Ingredients for Severe Weather

Posted at: 06/19/2013 7:54 PM
Updated at: 06/19/2013 7:55 PM

The chance for thunderstorms has increased on almost every day from Thursday to the beginning of next week. This thunderstorm chance also comes with a chance for strong storms and even severe weather, something your Storm Tracker 6 Weather Team will be closely tracking. So, you may be wondering why so many chances for thunderstorms? Well, I’m glad you asked!

Any thunderstorm needs three critical ingredients: a source of lift, instability, and moisture. A strong or severe thunderstorm needs one more element – a source of rotation known as shear.

Our source of lift for every storm over the next week can be sourced in part to an upper level low (ULL) currently located over the Pacific Northwest. You may be familiar with our surface low pressure systems that bring us rain, snow, etc. well an upper level low pressure system is almost the same just in the upper levels of the atmosphere. When this ULL spins counterclockwise, like all low pressure systems, do it spins off waves of energy known as vorticity. Each of these energy waves gives us the lift needed for rain showers and thunderstorms.

Instability, sometimes referred to as “thunderstorm fuel”, will be in ample supply. The atmosphere commonly becomes unstable when warmed. With temperatures at the surface likely reaching the upper 80s and possibly 90º this weekend the supply of instability will be essentially limitless especially during the daytime heating periods. A catch 22 of sorts with instability is something referred to as a cap or lid. This occurs when too much warm air is present in the mid-levels of the atmosphere causing thunderstorms to be cooler than their environment and stunting their growth. A strong cap will likely be in place on some days over the next week but instability looks likely to win out.

Moisture in the air is measured as dewpoint. The dewpoint is literally the temperature the air needs to be in order for dew to form. For thunderstorms we usually look for dewpoints at least reaching the 60º, over the next week dewpoints look to reach the middle 70s giving us plenty of moisture. This will also make the air very humid and downright uncomfortable at times.

This leaves us with our last ingredient: a source of rotation or shear. This is also the ingredient that is lacking the most. For severe storms we typically need shear values near 40 kts or more. On most days over the next week shear values will be near 30 kts and only at a few times reach 40 kts. This ingredient will likely be the determining factor between thunderstorms and strong thunderstorms. Something that will likely push our thunderstorm risk to strong levels will be the arrival of the Low Level Jet or LLJ. Similar to the Jet Stream, the LLJ is a region of strong winds in the lower levels of the atmosphere. Typically the LLJ will not kick in until evening increasing our storm chances. The LLJ, if in the right spot, can increase our shear and therefore our potential for severe weather.

Looking at these ingredients over the next week there is certainly a chance for stronger storms almost every day especially in the evening when the LLJ kicks in. We will be tracking the risk for strong and severe weather without stop over the next few days.

Justin Thompson-Gee
Storm Tracker 6 Meteorologist