Storms that form overnight can often be efficient rain producers

Thunderstorms that develop during the late-afternoon sometimes can form into a cluster of storms that can produce significant rainfall over a large area.  We had a couple move through during the early morning hours late last week and southern Iowa sure did get its healthy dose of rainfall within the past 24 hours.  So, just how do these storms form?

Storms basically need three ingredients to develop and thrive: warm temperatures, moisture and lift.  The warmth comes from the south, the moisture comes from the gulf and the lift comes from either a front or some type of mid-level disturbance.  Often times the storms will develop as individual cells during the late afternoon/evening as either a front/disturbance comes through and we're often at the peak heating time of day.  Now once the sun begins to go down we begin to lose our heat source.  So how can these storms still produce severe weather and heavy rain.  A lot of times it's the low level jet and/or the upper level disturbance. 

The low level jet is an area of stronger winds in the lower levels of the atmosphere (usually about 5000ft above) that forms overnight.  In other words, it's a nocturnal jet.  This is what helps to supply the moisture needed to keep the storms going.  Also, you'll sometimes find that either a 'front' in the mid-levels of the atmosphere or an upper level disturbance will provide the lift needed.  As long as there is a continuous supply of moisture, the storms will continue to develop.  Unlike our main jet stream, the low level jet usually stays in one location overnight and will weaken shortly after sunrise.  Because the storms typically don't have the forward momentum like they would if there were to form during the day, they can form over the same location which then leads to flooding concerns. 

Tonight I anticipate storms to develop to our west, but because the main system will have shifted eastward we hold a better chance of picking up on rainfall overnight.