'Positive' Thoughts in Weather

It's helpful to think positive thoughts throughout life, but it's also helpful to think positive with the weather, too.

Big thunderstorm outbreaks can occur anytime of the year, but are most typical during the spring and early summer months.  As we begin to wind down with summer and enter into fall, our severe weather outbreaks become a little less.  Although, there is a slight increase in the number of tornadoes during the fall due to an increase in jet stream winds as the seasons transition from warm to cool.

We've had a few feisty thunderstorms over northern Illinois and southern Wisconsin this year, but overall it hasn't been too terribly bad.  When thunderstorms are in the forecast, it's important to look at what's happening up above our heads.  We have to connect everything together in weather - from the surface up to the top of the atmosphere.  And this includes looking at the position of the jet stream.

There are many different signals meteorologists look at when forecasting to give us an indication on whether or not thunderstorms have the potential to be severe.  One such important forecasting factor is whether or not the dip, or trough, in the jet stream will be positively tilted or negatively tilted.

A negative tilt in the jet stream indicates a maturing area of low pressure and a higher possibility of severe weather.  The axis of the trough points NW-SE and allows colder air aloft to be placed over warm air at the surface.  This type of set up allows for an increase in instability because air parcels that rise from the surface due to heating from the sun, continue to rise because they are warmer than the surrounding air mass they're moving into.  When warm air aloft is located over warm air at the surface the parcels can't rise as high or as fast.

There are several factors in the atmosphere that can cause a trough of low pressure to become negatively tilted:  a ridge of high pressure located east of the trough or a small area of strong winds at the base of the trough, called a jet streak.

A positive tilt in the jet stream indicates that the upper level winds are not as strong which may reduce the strength of wind shear throughout the atmosphere.  The air located above the surface isn't as cold, so air parcels may have a hard time rising keeping instability a bit lower.  All of this suggests severe weather is less likely.

Why am I telling you all of this?  Because there is a chance for thunderstorms over the area this Friday.  But when looking at what could possibly happen in the upper levels of the atmosphere, the tilt in the jet stream will be slightly more positive.  Now, that's not to say some breaks in the cloud cover during the day won't help promote a few stronger thunderstorms, but it's not going to be a big severe weather day.