How/why do pressure systems get cut off by the jet stream? What happens after the system gets cut off?

Question

This question is asked by my meteorology teacher. He dident give me a site or anything to find the answer. I also cant find a site with the info. Someone please help me and give me a link w/ a site i can read about it! THANK YOU.

Answer 1

Cut off lows usually form when an upper level trough digs sharply southward.  This is usually caused by a strong shortwave diving on the back side of this trough.  As the shortwave help stretch the main trough southward, the ridge ahead of this trough will build northward.  The trough will start to tilt more negatively as the shortwave energy will cause pressure to fall to the southeast or east side of the base of the trough.  

In some cases, the ridge ahead of the trough is too strong and will block the shortwave from ejecting outwards towards the east from the base of the trough.  So the shortwave will end up coming back northward once it rounds the bottom of the trough.  With the trough now tilting more negatively, the wave will run into higher pressure to the north and northeast and a developing deformation zone associated with the back side of the jet to the northwest.  This will help pinch off the base of the trough as additional shortwaves energy will ride over the top and will form a new path eastward as that will be the path of least resistence.  The new path will then be the main jet and will likely lift northward leaving behind a now closed low of what is left of lower end of the upper trough.  Soon, this low becomes a cut-off low as it loses complete influence of the upper jet.

Without any significant steering current, these lows will just be stationary or will drift around slowly.  The exact movement are hard to forcast as models tend to move them too quickly.  They will drift around until the either:

1. A jet comes close to the cut-off low.  A passing shortwave trough moving along the jet becomes in phase with the low.

2.  Another low pressure systems digs towards the cut-off low and "kicks" it out of the area.

3.  The low continues to fill and finally dissipates.

For more infomation, see links below:

https://www.metocwx.quantico.usmc.mil/me...

http://www.usatoday.com/weather/tg/wcuto...

http://www.mmm.ucar.edu/asr96/part_c.htm...

http://www-das.uwyo.edu/~geerts/atsc5150...

Answer 2

Have you ever heard of an oxbow lake?  That's what you get on a river like the Mississippi when it meanders to form a loop, and then the river finds a shorter passage and cuts off the loop.  The same thing happens with the jet stream--a loop forms in it, then the jet stream finds the shorter path and the loop gets "cut off." What you have then is an eddy that is disconnected from the jet stream.  When it is disconnected it doesn't get steered by it, and the cutoff low can just wander around.  It is essentially a pool of cold air at the upper levels, and when that cold air finds its way into your neighborhood it destabilizes the surface air it can bring rain, thunderstorms, and even snow during the winter.  Because they are disconnected from the jet stream their path is hard to predict, hence the old adage "Cutoff low, weatherman's woe."