|MadSci Network: Environment|
In the United States, severe thunderstorms and tornadoes are much less common west of the Rocky Mountains. Several ingredients come together east of this mountain range to help trigger severe weather. Some of those ingredients are:
All of these are important for the development of severe weather.
- low-level moisture
- surface low pressure which forms as air flows over the mountains, and as a result of this low pressure..
- a significant veering of the winds with height.
While moisture, low pressure and veering winds also occur west of the Rocky Mountains, they are less likely to be present at the same time. For example, moisture east of the mountains is typically transported northward from the Gulf of Mexico on a low-level jet (wind max), which forms in response to developing low pressure in eastern Colorado. With south or southeast winds in the lower levels of the atmosphere, and westerly winds in the upper levels, you have winds veering from, say, 160 degrees at the surface to 270 degrees aloft.
On the west coast, low-level moisture is more commonly supplied by the Pacific Ocean, and would increase with a westerly wind. With westerly winds at the surface and aloft, veering would be minimal. If the winds DID veer more, the low-level flow would come from the southeast and would be much drier, since the source region would be desert.
Kind of confusing, but I hope it helps answer your question.
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