A Brief Climatology of Tropical Cyclones
in Louisiana (continued)

Rainfall.  Heavy rains and flooding are the primary problem associated with tropical cyclones across the Pelican State.  The worst aspect of tropical cyclones is that the weaker a system is, the more effective a rainmaker it tends to be.  Also, tropical systems with large circulation patterns are most efficient at producing excessive rains that more intense hurricanes, mainly because intense hurricanes have decreased circulation patterns as they become more efficient at harnessing the warm and moisture for the ocean.

Recent examples of flooding across the state from tropical cyclones include Frances of 1998, Allison in 1989, Bonnie (NW Louisiana) in 1986, Juan in 1985, and Claudette in 1979.  However, if it were not for the intermittent invasions from tropical cyclones, rainfall during the months of August, September, and October would average about 25% less that it currently does.

In most cases, the heaviest rain around a tropical storm or hurricane is dumped to the right of its track.  Naturally, the slower a tropical system moves, the greater the rainfall a location can expect.  When a cyclone interacts with old frontal boundary, a secondary rainfall maximum will occur at and to the north of the boundary in question.  Luckily, Louisiana has very little topography, which makes rainfall amounts above 30" and landslides exceedingly rare events.

When a tropical cyclone transitions to a non-tropical low, dry air will wrap around the southern and eventually eastern side of the storm.  Maxima in rainfall will occur just west and distant to the east of the track; the least amount of rain will fall in the system's "dry slot", a relatively cloud-free area south/southwest of a non-tropical low.  Severe weather such as downbursts, tornadoes, and hail are more commonly seen as dry air intrudes into the cyclone's circulation.

In any event, rain will fall more distantly to the east of the center than it will to the west.  Upward vertical motions/warm air advection will occur to the east of the center, while subsidence/cool air advection will occur to the west.  This leads to a sharp contrast in weather to the west of the point of landfall, between continuous heavy rain with high temperatures hovering near 80, and sunny skies seen just to their west with high temperatures at or above 100 degrees.  Below is a table showing the ten highest rainfall amounts ever reported in Louisiana with tropical cyclones.
 

Fatalities.  The further you go back in time, the more people used to die from hurricanes.  This century, there has been a steep dropoff in the number of deaths from cyclones due to timely warnings by the National Weather Service, and also by information campaigns by the public and private sector about the dangers of these storms.  Below shows the ten most deadly storms ever to effect Louisiana.

Movement.  During most months of the hurricane season, tropical cyclones move northwest into the Pelican State.  This is mainly due to the wind flow around the Azores/ Bermuda high pressure system, which is strongest in July.  In June and October, storms are more likely to move in from the south and southwest.  Cold fronts invading the state from the north and west would cause winds across the bayou to become south and southwesterly, quickly picking up any low pressure system lurking in the Gulf at the time.

Storms are most likely to stall across southwest Louisiana than any other portion of the state.  A climatological "col area" referring to an area between distant low/high pressure cells where winds are nearly calm, exists across southwest Louisiana and southeast Texas during the summer.  The Bermuda high is usually too far east to exert much of a north/northwesterly motion on a storm while the Mexican Plateau low/trough is normally too far south to steer a system to the west.  This can cause storms to stall for days across that region; Juan (1985), Claudette (1979), and Allison (1989) are the most recent storms to stall near the Sabine River.