Great temperature swings during spring and fall

From near 90 degrees last week, to highs in the 60's this past weekend, to highs back in the 80's this week it's been one wild ride in the temperature department.  And believe it or not, these large temperature swings are common during the transition seasons of spring and fall.

The reason for this is because the atmosphere doesn't like anything off balance.  The seasons are caused by the earth's rotation around the sun.  This we know.  But as the temperature difference grows across North America that's when larger storm systems form, bringing severe weather on the warm side, and blizzard conditions just 50 miles away!  These low pressure systems are Mother Nature's way of trying to balance out the atmosphere.

The flow of the jet stream can be broken down into two patterns:  zonal flow and meridional flow.  When the jet stream winds are parallel, or close to parallel, to the lines of latitude the wind pattern is called zonal.  Upper level winds will flow from west to east.  When jet stream winds cross the latitude lines at angles, the wind pattern is called meridional.  This type of wind pattern is what produces the big troughs and ridges within the jet stream.  Sometimes a highly amplified meridional flow can cause a blocking pattern in the atmosphere, bringing much above or much below normal temperatures.

During the summer months, the jet stream winds are typically not as strong as they are during spring and fall.  Temperatures across the United States are more uniform.  However, in the fall the polar branch of the jet stream begins to dip further and further south.  This in turn pulls down colder air from the Arctic.  That cold air clashes with the warmer air in place causing low pressure systems to form.  Eventually, the jet stream won't be retreating north as much and the cold air that's been bottled up in the Arctic will stick around during the late fall and winter months.  Then, as spring arrives, the jet stream begins to be pulled back north allowing warmer air to be pulled north with it.  The middle of spring is usually the height of severe weather season - late April to May - across the center of the country.  Now, this varies as the jet stream moves, so different parts of the U.S. will experience different peaks of severe weather.  But there is also a slight increase (or mini severe weather season) towards the end of October and November with stronger jet stream winds during late fall.