Flash Flooding

>> The number one direct weather-related killer in the United States

Flash flooding is the number one weather-related killer in the United States.  These floods, which generally occur within six hours of the onset of precipitation, killed over 10,000 people in the United States since 1900.  Figure 1, found to the right, compares the deaths due to flooding with other fatal weather events.

>> Occurs when more rain falls than the ground can absorb and when it falls heavily enough to cause sudden, rapid runoff
>> Atmospheric factors

There are three primary ways flash flooding episodes develop.  One important factor is the strength of the upper-level winds.  Very weak upper-level steering winds can work to cause slow-moving and/or stationary thunderstorms.  This will cause the thunderstorms to dump a large amount of rain over one specific area for an extended period of time.

Flash flooding can also be caused by multiple thunderstorms continuously forming and moving over the same spot.  This atmospheric process is called training.

Finally, flash flooding can be caused by orographic lifting, when a moist and unstable airmass ascends the side of a mountain and the prevailing winds continue to blow additional moist, unstable air to the same location.

Microbursts

>> Exceptionally strong thunderstorm downdraft caused by unusually strong evaporational cooling

A microburst is an exceptionally strong thunderstorm downdraft that is caused by unusually strong evaporational cooling within the storm.  These microbursts can cause strong straight-line winds that are often in excess of 100 miles per hour.  They are also a major threat to landing aircraft as they cause a large difference in wind direction over a very small distance.  The impact of microbursts on airplanes can be seen in Figure 2 to the left.

Hail

>> Pieces of ice formed in the thunderstorm's updraft
>> Also known as the "white plague"
>> Ingredients needed for hail formation

A few key ingredients are needed to produce hail.  The first: freezing air needs to be located relatively close to the ground, but not TOO close.

The second and most important ingredient for large hail formation is a strong updraft.  The updraft needs to be strong enough to support the ever-growing hailstones.  Once the updraft can't support the hailstones any longer, they fall out of the thunderstorm toward the ground.  The process of hail formation can be seen in Figure 3 to the right.

Organized Thunderstorm Complexes

>> Squall Line: A band of thunderstorms that often forms well ahead of a cold front

A squall line is a band of thunderstorms that often forms well ahead of a cold front.  In order to produce a squall line a rich supply of warm, moist air is needed.  This requirement is often met if there is a low-level jet stream present in the area.  This low-level jet will work to bring warm, moist air in from the south. 

In addition to the presence of warm, moist air, these squall lines often form in association with large upper-level divergence and large scale lifting.  This is why squall lines are most often found ahead of a frontal system.  A squall line can be seen moving through Pennsylvania in the radar loop below.

>> Mesoscale Convective Complex (MCC): Primarily nocturnal bundle of thunderstorms that stretch across hundreds of miles

The mesoscale convective complex is a nocturnal bundle of thunderstorms that stretch across hundreds of miles.  Much like a squall line, the low-level jet stream feeds warm, moist air into these thunderstorm complexes.  In this case, however, a mid-level low pressure forms within the group of storms.  This low pressure helps to sustain this large group of thunderstorms.  Mesoscale convective complexes are very common in the Plains states and help to provide a good majority of the rain throughout that area.  An infrared satellite image of a MCC can be seen to the left.